The Halicopter

The it

Helicopters

Helicopters operated in expected or actual icing conditions at night shall be equipped with a means to illuminate or detect the formation of ice.

From: Airworthiness (Third Edition), 2016

Related terms:

Energy Engineering

Fuselages

Rotors

Solar Cells

Rotor Blade

Gearbox

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Physiological and Cognitive Changes during Helicopter Underwater Egress Training

Sarita J. Robinson, in Handbook of Offshore Helicopter Transport Safety, 2016

6.2 Training Reluctance

HUET is a standard requirement for working offshore. However, some offshore workers may not fully engage in the training. This can be for several reasons.

•

One reason that offshore workers may not engage in HUET is that they either do not understand the risk posed by flying over water or believe that a helicopter crash over water would be nonsurvivable. The lack of a realistic understanding of the risks posed by a potential disaster situation has been shown to lead to a failure to engage with emergency procedures. For example, residents living near Mount St. Helens were found not to have evacuated the local area as they did not understand the risks associated with a volcanic eruption (Greene, Perry, & Lindell, 1981). Although the survival rates for helicopter crashes over water are not good, with the most current survival rates being suggested to be around 68% (Taber, 2014; Taber & McCabe, 2006), training has been shown to enhance survival (Ryack et al., 1986). Therefore it is important that HUET outlines the potential risks of helicopter flights over water and the benefits of training on enhancing survival rates to facilitate engagement.

•

Another reason people may not engage fully with the HUET is that they are using the coping strategy of denial to deal with their anxiety regarding helicopter flights. Refusing to believe that a helicopter crash is possible can be psychologically protective as it reduces the anxiety associated with flying (Tobin, Holroyd, Reynolds, & Wigal, 1989). If a person does not accept that helicopters can crash then there is no need to worry about the possibility. However, denial can also lead to maladaptive coping, as people who deny risks may then not fully engage in useful proactive activities (Grothmann & Patt, 2005) such as HUET. Therefore at the start of HUET courses it is important to assess the participants' thoughts regarding the likelihood of helicopter ditching and ensure that participants understand the benefits of safety training.

•

A final reason people may fail to engage with HUET is the increased anxiety levels that the training may induce. People who have a preexisting phobia of water or fear of enclosed spaces may find HUET especially challenging. If people perceive that they are at risk of physical harm from undertaking an activity it is likely that they will not be able to fully engage with that activity (Lazarus, 1966). Therefore, training courses should not underestimate the negative effects of anxiety on HUET. Interventions to reduce the anxiety that HUET can evoke are possible. For example, systematic desensitization (a common treatment for phobias) has been found to reduce phobic anxiety associated with HUET (Brooks, Gibbs, Jenkins, & McLeod, 2007). If the anxiety is less marked, interventions such as prepool training using virtual reality technologies (Reznek, Harter, & Krummel, 2002) or techniques such as goal-setting and positive self-talk may be helpful (Barwood, Dalzell, Datta, Thelwell, & Tipton, 2006).

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Helicopter Vibration Reduction

William A. Welsh, in Morphing Wing Technologies, 2018

1 Introduction

Helicopter fuselage vibration degrades ride quality, causes crew fatigue, and damages components necessitating expensive part replacement. Average cabin floor vibration levels in the 1960s were often around 0.3 g resulting in a truly uncomfortable experience for crew and passengers and frequent replacement of damaged parts [1]. With a consistent demand from the helicopter-user community, levels have improved over the years as improved technology has been developed and applied. Now levels are almost always between 0.1 and 0.2 g at most places in the cockpit and cabin; still not a "Jet smooth ride," but a significant improvement.

In this chapter, existing approaches to helicopter vibration reduction as well as new technologies being pursued to achieve a jet-smooth ride are reviewed. (Note that the designations S-76 helicopter and S-92 helicopter are registered trademarks of Sikorsky a Lockheed Martin Company.)

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Helicopter Human Factors

Sandra G. Hart, in Human Factors in Aviation, 1988

Landing

Helicopter approaches can be made from any direction (although they land into the wind unless operational necessity dictates otherwise), and approach angles can be very steep. Traffic flow management systems, tailored to the needs of fixed-wing aircraft, do not take advantage of the maneuverability of helicopters or their ability to land with minimal runway length (Livingston, 1985). Furthermore, skids, standard on most small helicopters, allow landings on soft or sloping terrain, but they limit mobility on the ground. Helicopters with skids must achieve a hover and then gain forward speed to travel across the ground.

Although many helicopters are at least minimally equipped for operations in instrument conditions, the information provided (which is adequate for fixed-wing landings) is not sufficiently sensitive or accurate for the slow speeds flown (Verdi & Henderson, 1975). Although there are over 4,000 heliports and helistops in the United States and Canada (Livingston, 1985), they are not sufficient for the growing number of civil helicopters (more than 6,000 by 1986). Only recently have public use facilities equipped for instrument flight conditions been developed in urban areas. Many heliports do not provide adequate lighting or visual approach guidance, thus helicopters must rely on their own external spotlights at night. The current lack of landing facilities tailored to helicopter needs forces pilots to adapt to regulations and flight profiles that are suboptimal and reduces their operational flexibility.

Helicopters often land on unprepared sites, where no ground-based approach aids are available, during search and rescue, medivac, law enforcement, agricultural, and fire-fighting missions. Relatively limited funds have been devoted to developing adequate landing aids for these circumstances. However, low-cost systems have been tested that allow the creation of lighted helipads in even remote areas (Hodgkins, 1984; Kocks, 1985). These units can be placed around the perimeter of a landing site to provide the visual cues necessary for safe landings at night and under reduced visibility.

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Immersion Suits for Helicopter Transportation

Dana H. Sweeney, in Handbook of Offshore Helicopter Transport Safety, 2016

9.6.3 Equipment Integration and Snagging

HTS systems are typically fitted with accessories to provide the wearer with survival resources should they become immersed and/or submerged in the water. A relatively standard set of HTS survival accessories includes life jacket, gloves, hood (if required), emergency breathing system, personal locator beacon, strobe light, buddy line, whistle, and spray shield. Other accessories may include goggles and/or nose plugs. A key consideration for the design of HTS systems is that accessories are visible to the wearer both in air and in the water (with labeling that is easily read under dimly lit conditions) (Figure 9.9), accessible (preferably by either hand), and designed so that they can be used with reduced dexterity that results from gloves or cold hands (MacKinnon & Mallam, 2010). It is important that an HTS system is designed so that the suit and accessories create minimal snagging hazards during egress. All accessories must be adequately secured and be positioned to minimize bulk and preserve mobility.

Figure 9.9. Example of pictorial HTS instructions.

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Parallel Calculation of Helicopter BVI Noise by Moving Overlapped Grid Method

Takashi Aoyama, ... Eiji Shima, in Parallel Computational Fluid Dynamics 1999, 2000

1 INTRODUCTION

Helicopters have the great capability of hovering and vertical takeoff and landing (VTOL). The importance of this capability has been realized again especially in Japan after the great earthquake in Kobe where it was shown that helicopters were effective as a means of disaster relief. It is worthy of mention that an international meeting of American Helicopter Society (AHS) on advanced rotorcraft technology and disaster relief was held in Japan in 1998. However, it cannot be said that helicopters are widely used as a mean of civil transportation. Although their capability is effective in the civil transportation, noise is a major problem.

Helicopters produce many kinds of noise, such as blade-vortex interaction (BVI) noise, high-speed impulsive (HSI) noise, engine noise, transmission noise, tail rotor noise, blade-wake interaction (BWI) noise, main-rotor/tail-rotor interaction noise, and so on. BVI noise is most severe for the civil helicopters which are used in densely populated areas because it is mainly generated in descending flight conditions to heliports and radiates mostly below the helicopter's tip-path plane in the direction of forward flight. What makes it even worse is that its acoustic signal is generally in the frequency range of most sensitive to human subjective response (500 to 5000Hz).

Many researchers have been devoting themselves to developing prediction methods for BVI noise. Tadghighi et al. developed a procedure for BVI noise prediction [1]. It is based on a coupling method of a comprehensive trim code of helicopter, a three-dimensional unsteady full potential code, and an acoustic code using the Farassat's 1A formulation of the Ffowcs Williams and Hawking (FW-H) equation. National Aerospace Laboratory (NAL) and Advanced Technology Institute of Commuter-helicopter, Ltd. (ATIC) also developed a combined prediction method [2] of a comprehensive trim code (CAMRAD II), a three-dimensional unsteady Euler code, and an acoustic code based on the FW-H formulation. The method was effectively used in the design of a new blade [3] in ATIC. However, one of the disadvantages of the method is that users must specify indefinite modeling parameters such as the core size of tip vortex.

The recent progress of computer technology prompts us to directly analyze the complicated phenomenon of BVI by CFD techniques. The great advantage of the direct calculations by Euler or Navier-Stokes codes is that they capture the tip vortex generated from blades without using indefinite parameters. Ahmad et al. [4] predicted the impulsive noise of OLS model rotor using an overset grid Navier-Stokes Kirchhoff-surface method. Although the calculated wave forms of high-speed impulsive (HSI) noise were in reasonable agreement with experimental data, the distinct spikes in the acoustic waveform of blade-vortex interaction noise could not be successfully captured. This is because the intermediate and background grids used in their method are too coarse to maintain the strength of tip vortex.

In order to solve the problem, NAL and ATIC developed a new prediction method [5] of BVI noise. The method combines an unsteady Euler code using a moving overlapped grid method and an aeroacoustic code based on the FW-H formulation. After making some efforts on the refinement of grid topology and numerical accuracy [6–8], we have successfully predicted the distinct spikes in the waveform of BVI noise. We validated our method by comparing numerical results with experimental data [9–11] obtained by ATIC. Our calculations are conducted using a vector parallel super computer in NAL. A new algorithm of search and interpolation suitable for vector parallel computations was developed for the efficient exchange of flow solution between grids. The purpose of this paper is to summarize the progress of the prediction method developed under the cooperative research between NAL and ATIC.

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Introduction

In Human Performance Models for Computer-Aided Engineering, 1990

HELICOPTER FLIGHT PROBLEMS AND APPLICATIONS OF HUMAN PERFORMANCE MODELS

Helicopter operation is difficult, and performing low-altitude, low-visibility missions with a single-person crew places very severe demands on the pilot. Analysis and design of the helicopter cockpit system and the missions it is to perform must be thorough to ensure that the missions are indeed possible and that the cockpit system, especially visual aids and displays, facilitates successful accomplishment of required flight tasks. If the A3I project and others based on similar concepts are successful, this analysis and design will be accomplished by using CAE facilities and design methodologies based on the use of human performance models of the type discussed in this report.

This chapter attempts to give the reader a concrete, intuitive feel for the application of human performance models to the design of advanced helicopters and other highly automated vehicles. A sequence of vignettes is presented, each of which is a brief episode illustrating an important practical problem that can arise from a limitation in the perceptual and cognitive capabilities of the pilot, which might be solved through design based on human performance models. The kinds of models that might be used to characterize pilot capabilities are described, along with the way in which they might be used for design. Reference is made to chapters of this report in which these models and their application are discussed more fully.

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Rotary Wings Morphing Technologies: State of the Art and Perspectives

Matthew L. Wilbur, ... Uwe T.P. Arnold, in Morphing Wing Technologies, 2018

Abstract

Helicopters play significant roles in aviation for search and rescue operations and for transportation of people or goods in remote or inaccessible places. They can fulfill these roles because of their capability to take off and land vertically and to hover for periods of time.

A rotary-wing aircraft is exposed to a complex operating aerodynamic environment when compared to fixed-wing aircraft. This complexity introduces a plethora of issues, ranging from limited performance in forward flight to a high vibration and noise environment, which negates a "jet smooth" ride.

This chapter presents an overview of the solutions investigated to overcome these issues, with a particular focus on state-of-the-art of morphing technologies applied to rotor blades. Different technologies are presented and discussed, ranging from proof-of-concept prototypes to solutions currently in production. Following the overview, a critical review of some significant efforts is presented.

To date, the only morphing technology that has made it to a production helicopter is servo flaps, first used in the Kaman K-125. Other morphing solutions, such as active twist of rotor blades, are mature enough for use on unmanned aerial vehicles (UAVs).

Individual blade control (IBC) is being investigated in several projects on full-scale helicopters and is possibly the next morphing technology closest to production. The benefits of higher harmonic control have been investigated and tested by multiple authors.

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Development and Implementation of Helicopter Underwater Egress Training Programs

Sean Fitzpatrick, in Handbook of Offshore Helicopter Transport Safety, 2016

2.1.1 History of Ditching Training

HUET programs and simulators have been developing and evolving since the first instances of aircraft ditchings during takeoff or landing from aircraft carriers in the early 1900s. One of the earliest known ditchings took place on July 31, 1912. Lieutenant Theodore G. Ellyson (the US Navy's first aviator) ditched during experiments with an early catapult system at Annapolis, Maryland (http://www.nationalaviation.org/ellyson-theodore/). The A-1 Triad that Ellyson was flying was caught in a crosswind, which sent him and the airplane plunging into the Severn River. At the time, the US Navy focused on equipment and procedures designed to facilitate the recovery of the aircraft if it ditched into the water. There was little emphasis placed on techniques needed to assist aircrew or passengers in escaping from a flooded and, potentially, sinking aircraft.

The US Naval Aviation Museum (n.d.) reports that during World War II, several F4F Wildcat fighters flying from the carrier Hornet (CV 8) were forced to ditch as a result of fuel depletion following a mission (http://www.navalaviationmuseum.org/history-up-close/objects-of-history/birth-dilbert-dunker/). Lieutenant John Magda wrote the following in an after-action report: "There should be a landing in water 'check-off list' in every plane, because at a time like that there are a few things you may forget that may prove to be a very dear mistake. There is very little time to do anything after the plane hits the water—thirty seconds at the most" (US Naval Aviation Museum, n.d.). Another VF-8 pilot, Lieutenant H.L. Tallman, wrote, "Shock of landing is not bad, but the water that gushes into the cockpit and the splash caused by the impact leads one to believe at the moment that the plane is going right on down. Actually, by the time you've recovered your senses (1–2 seconds) the water is up to your neck" (US Naval Aviation Museum, n.d.).

Based on these early experiences, HUET programs, as we know them today, originated using the Dilbert Dunker. The Dilbert Dunker (Figure 2.1), which was engineered and built by Ensign Wilfred Kaneb, provided seating for a single pilot. When the simulated cockpit was released, it traveled down a pair of steel rails as fast as gravity would take it and then rolled nose-over as it impacted the water. The pilot was then required to overcome in-rushing water and disorientation and make his/her escape.

Figure 2.1. Dilbert Dunker at 12 Wing Shearwater c. 1980.

From Taber and Bohemier (2014) with permission.

Kaneb recalled that one senior officer's recommendation during the Dilbert Dunker's development was to design a machine that would "teach them what it is like to be drowning" (US Naval Aviation Museum, n.d.). In an overview of the Dilbert Dunker history, the US Naval Aviation Museum (n.d.) posits that Kaneb's objective was not to teach trainees what it was like to be drowning, rather he wanted to teach them how to orient themselves under water and make a successful escape, or egress, to the surface.

As the Dilbert Dunker was the only underwater egress simulator (UES) in existence at the time, both helicopter and fixed-wing aircrew personnel used it for training. One of the primary training issues, however, was the fact that the Dilbert Dunker did not roll onto its side as a helicopter does when it ditches into the water. To address this issue, as well as to accommodate multiple trainees at once, the Burtech 9D5 helicopter UES was introduced in 1974 (Figure 2.2). The 9D5 was created to be a simulator that would familiarize aircrew and passengers with escape techniques from a ditched helicopter. The 9D5 used a rudimentary cockpit and generic back end, which could accommodate up to two trainees in the front and four in the back (see Cunningham, 1978 for a future overview of the 9D5). It was not intended to simulate any particular helicopter; instead it was considered to be a composite of all US Navy and Marine Corps helicopters being operated at the time (see Chapter 7 for a discussion on generic UESs). This was partially accomplished by the use of a universal-type exit release mechanism (i.e., pull up, pull down, push forward, pull back, etc.).

Figure 2.2. US Navy Burtech 9D5, NAS, Pensacola, Florida.

Image courtesy of Smith, Ray E. Naval Aviation Survival Training Program, U.S. Navy.

Following the 9D5, the McLean and Gibson UES was introduced in 1985. Like the 9D5 the McLean and Gibson was designed for use by helicopter aircrews and passengers, rolled onto its side, and included a replicated cockpit on both the front and the back of the simulator, which allowed up to four pilots to train at a time in addition to passengers or flight crews in the cabin area (Figure 2.3). Although an improvement on the 9D5, the McLean and Gibson initially lacked exits (they were added later); also the round shape of the simulator versus the flat sides of a helicopter reduced fidelity (Summers, 1996; see also Chapter 7), and the seats were not configurable.

Figure 2.3. Maclean and Gibson simulator in use to train Royal Canadian Air Force personnel in 1984.

From Taber and Bohemier (2014) with permission.

By the mid-1980s, HUET was becoming common for the oil and gas industry, the military, and civilian aviators around the world and resulted in the demand for a more advanced UES. In 1986, Survival Systems Ltd began construction on a completely new series of Modular Egress Training Simulators (METS™). The first METS™ Model 30 went into service in Dartmouth, Nova Scotia, in 1987 and was used to train both Canadian military aircrews and offshore oil and gas workers (Figure 2.4). The METS™ has continued to evolve and is now capable of replicating dozens of helicopter types, amphibious vehicles, fighter jets, fixed-wing aircraft, and rigid-hull inflatable boats. Although there are a number of other organizations that design and build UESs (Chapter 7), the METS™ is currently considered to be the world standard. For example, as of this writing, more than 100 models of the METS™ are in use all over the world.

Figure 2.4. Modular Egress Training Simulator (METS™) based on the CH124 airframe (1987).

From Taber and Bohemier (2014) with permission.

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Transportation

Ian Sutton, in Plant Design and Operations, 2015

Helicopters

Helicopters are used to transport personnel and light freight to and from offshore platforms. They are also used for the emergency evacuation of injured personnel (but cannot be used if the platform or rig is sinking or on fire). The crash of a helicopter is almost always a very serious event—often leading to fatalities and serious economic loss.

Guidance

The following general guidance to do with the operation of helicopters offshore is provided.

Personnel movement

Personnel boarding a helicopter must wait in a designated area before going on to the helideck itself. And they must then follow crew directions as to where to go. Similarly, personnel disembarking from the helicopter should always follow the instructions of the deck crew.

Personnel using helicopters should understand the following:

•

The authority of the pilot

•

How to communicate with the pilot

•

Whenever approaching or leaving a helicopter with blades rotating, all employees shall remain in full view of the pilot and keep in a crouched position

•

Employees shall avoid the area from the cockpit or cabin rearward unless authorized by the helicopter operator to work there

•

There shall be constant reliable communication between the pilot, and a designated employee of the ground crew who acts as a signalman during the period of loading and unloading. This signalman shall be distinctly recognizable from other ground personnel

•

No unauthorized person shall be allowed to approach within 15 meters of the helicopter when the rotor blades are turning

•

Manifest rules and weight limitations on personal effects

•

Handling of light objects that could be blown away by rotors

•

Use of lifejackets, seating arrangements, seatbelts, and the storing of cargo

•

Safe conduct during flight

•

Emergency procedures.

Air turbulence

A particular concern to do with helicopter operations associated with offshore oil and gas production is air turbulence—particularly the turbulence that comes from the generator turbines.

Rescue

Helicopters can be used to remove injured personnel from an offshore facility. However, if there is an onboard fire—or there is the potential for such a fire—helicopters should not approach the facility.

Regulations and Standards

The U.K. Civil Aviation Authority provides guidance for the safe operation of helicopters in its CAP 437—Standards for Offshore Helicopter Landing Areas. The Table of Contents for this standard is shown in Table 15.3 (it has been lightly edited to save space).

Table 15.3. Contents for CAP 437

Chapter 1: Introduction

History of Development of Criteria for Offshore Helicopter Landing Areas

Department of Energy and the Health and Safety Executive

Guidance on the Design and Construction of Offshore Installations, 1973 Onwards

Applicability of Standards in Other Cases

Worldwide Application

Chapter 2: Helicopter Performance Considerations

General Considerations

Safety Philosophy

Factors Affecting Performance Capability

Chapter 3: Helicopter Landing Areas—Physical Characteristics

General

Helideck Design Considerations—Environmental Effects

Structural Design

Loads—Helicopters Landing

Loads—Helicopters at Rest

Size and Obstacle Protected Surfaces

Surface

Helicopter Tie Down Points

Safety Net

Access Points

Winching Operations

Normally Unattended Installations

Chapter 4: Visual Aids

General

Helideck Landing Area Markings

Lighting

Obstacles—Marking and Lighting

Chapter 5: Helideck Rescue and Firefighting Facilities

Introduction

Key Design Characteristics—Principal Agent

Use and Maintenance of Foam Equipment

Complementary Media

Normally Unattended Installations

The Management of Extinguishing Media Stocks

Rescue Equipment

Personnel Levels

Personal Protective Equipment (PPE)

Training

Emergency Procedures

Further Advice

Chapter 6: Helicopter Landing Areas

Landing Area Height Above Water Level

Wind Direction (Vessels)

Helideck Movement

Meteorological Information

Location in Respect to Other Landing Areas in the Vicinity

Control of Crane Movement in the Vicinity of Landing Areas

General Precautions

Installation/Vessel Helideck Operations

Helicopter Operations Support Equipment

Chapter 7: Helicopter Fueling Facilities

General

Product Identification

Fuelling System Description

Chapter 8: Helicopter Fueling Facilities

General

Fuel Quality Sampling and Sample Retention

Recommended Maintenance Schedules

Filling of Transit Tanks

Receipt of Transit Tanks Offshore

Decanting from Transit Tanks to Static Storage

Fueling Direct from Transit Tanks

Long-Term Storage of Aviation Fuel

Aircraft Refueling

Quality Control Documentation

Chapter 9: Helicopter Landing Areas on Vessels

Vessels Supporting Offshore Mineral Workings and Specific Standards for Landing Areas on Merchant Vessels

Amidships Helicopter Landing Areas—Purpose-Built or Non-Purpose-Built Ship's Centreline

Helicopter Landing Area Marking and Lighting

Ship's Side Non-Purpose Built Landing Area

Ship's Side Non-Purpose Built Landing Area Markings

Night Operations

Poop Deck Operations

Chapter 10: Helicopter Winching Areas on Vessels and Wind Turbine Platforms

Winching Areas on Ships

Helicopter Winching Areas on Wind Turbine Platforms

Further guidance is provided in API RP 2L—Planning, Designing, and Constructing Heliports for Fixed Offshore Platforms, and in OSHA's 1926.551—Helicopters, Hoists, Elevators, and Conveyors.

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Maintenance and monitoring of composite helicopter structures and materials

M. Martinez, ... N. Bellinger, in Structural Integrity and Durability of Advanced Composites, 2015

21.3.1 Water absorption

Helicopters undertaking SAR operations will inevitably be in contact with water. It is essential that large amounts of water are not absorbed by the floorboard protection scheme, thereby damaging the protection scheme and increasing the weight of the aircraft. To select and maintain the floorboard panels, the protection schemes are usually tested for their water absorption property. These tests are usually fairly simple. Initially the test coupons, cut to size (75–100 mm square), are dried in an oven for 4 h at 27 °C before being weighed on a scale, with this weight being considered to be the dry weight of each floorboard protection scheme. Specimens are then submerged in water at room temperature for 24 h, patted dry with a lint-free cloth, and then weighed again. The percent of water absorption can then be calculated taking the difference between the wet and dry weight and dividing this difference by the dry weight. In some cases, protection schemes are then resubmerged for a second 24 h soak to give another data point to rank relative water absorption. This gives each protection scheme both 24 and 48 h percent water absorption values. Additional tests are also required to qualify cabin materials for helicopter and aircraft use, which are not mentioned in this chapter.

Helicopter Rotors

Related terms:

Fuselages

Helicopters

Propellers

Rotors

Wind Turbines

Hydrogen

Polymer Matrix Composite

Liquid Hydrogen

Rotor Blade

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Aerodynamic Analyses of Tiltrotor Morphing Blades

Antonio Pagano, in Morphing Wing Technologies, 2018

4.5.1 Planform

A helicopter rotor blade is herein generated by positioning, rotating, and scaling 2D airfoils (in nondimensional coordinates) along the blade span. The shape of each individual airfoil (including thickness) is studied separately because the aerodynamic and structural requirements are different depending on the airfoil spanwise position. When a set of airfoils is available, the designer act on some spanwise stations where he previously defined the blade constructive parameters such as chord length, geometric twist, and vertical and horizontal leading edge offset (with respect to a given reference axis) to give the blade the desired planform including sweep and dihedral angles.

The approach followed here to parameterize the blade planform geometry uses the traditional approach to design the blade, thus, the planform shape is separated by the sectional shape. This means that the blade shape is modified by means of constructive parameters affecting the blade planform whereas the sectional shape is modified by selecting the appropriate set of airfoils and distributing them along the blade span.

In order to perturb the design surface in a continuous way, six constructive parameters are identified for each spanwise station (see Fig. 10): chord length (c), geometric twist (t), vertical and horizontal leading edge offset (x,z), sectional sweep (γ), and dihedral angles (δ). Under the hypothesis of parallel planar sections, these parameters reduce to the first four. For simple blade shapes (e.g., rectangular shape), the constructive parameters at the inner and outer stations can be used. Nevertheless, the number of spanwise stations is expected to increase especially for complex blade shapes and highly non linear parameter variations. Of course, the number of sections needs to be limited anyway, otherwise the number of design variables becomes larger and larger. For this reason, the adopted parametric model is based on three or, at most, four sections.

Fig. 10. Design variables for blade planform parameterization.

Generally, the first and the last section correspond to the sections limiting the geometry to be optimized. On the contrary, the intermediate sections are chosen by the user. Indeed, the user chooses the position of the intermediate section and a software tool calculates the constructive parameters by interpolating on the closest spanwise stations. When more than four spanwise stations are necessary to appropriately characterize the blade, the constructive parameters associated to the sections in excess are not considered as design variables but they are modified according to predefined interpolation functions which distribute the deltas of the surrounding design variables.

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Delayed feedback control of pitch-flap instabilities in helicopter rotors

Rudy Cepeda-Gomez, in Stability, Control and Application of Time-delay Systems, 2019

2 Dynamics of the rotor

A helicopter rotor blade is usually mounted on a set of hinges which allow three angular degrees of freedom. A typical arrangement of the hinges is shown in Fig. 1. The flapping motion is defined as an up and down rotation, through an angle β, in a plane which contains both the blade and the shaft. This angle is considered positive when the blade flaps upwards. A flapping blade rotating at high speeds is subject to large Coriolis moments in the plane of rotation, and a hinge is introduced to alleviate these moments, allowing the motion of the blade in the same plane of rotation. This is denoted as lead-lag motion, and the lagging angle is denoted by ξ. The pitching or feathering motion, denoted by θ, is a rotation about an axis parallel to the blade span. The angle is considered positive when the leading edge of the blade moves upwards.

Fig. 1. Typical hinge arrangement and degrees of freedom of a helicopter rotor blade.

Wikimedia Commons.

The pitch-flap flutter is an unstable behavior produced by the coupling of the pitching and flapping motions of the rotor. It can lead to very high oscillatory loads in the pitch control circuit. Stammers [21] presented a detailed model to describe the coupling between θ and β for an uncontrolled rotor moving forward with a horizontal speed Vf. The basic assumptions made to obtain this model are that no lagging motion occurs, that the flapping hinge is horizontal at the blade root and on the axis of rotation, that the flexural axis of the nonrotating blade lies along the quarter-chord line, and that the blade is of constant chord and symmetrical cross-section. Stammer's model has the form

(1)Mθ..(ψ)β..(ψ)+C(ψ)θ.(ψ)β.(ψ)+K(ψ)θ(ψ)β(ψ)=0

with the inertia M, damping C, and stiffness K matrices defined as

(2)ŻŻŻM=1r′σk201,C(ψ)=nmθ.8k21+43μsin(ψ)00nlŻ1+43μsin(ψ),K(ψ)=ν12r′σk2nlθ45+2μsin(ψ)+23μ2(1−cos(2ψ))1+43nlŻμcos(ψ)+nlŻμ2sin(2ψ).

As it is customary in helicopter dynamics, the independent variable in Eqs. (1), (2) is not the time t, but the azimuth angle ψ = Ωt. Here, Ω is the angular speed of the rotor shaft, which is assumed constant during the operation of the aircraft. In Eq. (1), a dot over a variable denotes its derivative with respect to ψ.

The parameter μ in Eq. (2) is known as the tip speed ratio, and it corresponds to the ratio of the horizontal speed of helicopter to that of the tip of an advancing blade

(3)μ=VfΩR,

with R being the blade span. For a helicopter in hover or vertical flight μ = 0, whereas μ > 0 for any horizontal flight condition.

Among the parameters presented in Eq. (2), two are easily varied and thus the stability is studied with respect to them. These parameters are σ, which is the distance of the center of gravity (c.g.) of the blade aft from the center of pressure, expressed as a fraction of the chord; and the nonrotating torsional natural frequency ν1. The other parameters, described in Table 1, are taken as fixed. Notice that the ν1 is given as a fraction of Ω. Table 1 also shows the numeric values used for the fixed parameters in the numerical analysis sections. These values are taken from Ref. [11].

Table 1. Parameters of the dynamic model.

DescriptionValuer′Blade aspect ratio14.14Nondimensional radius ofkgyration of chordwise element0.05about its flexural axisn18 Lock number1.6667mθ.Aerodynamic derivative coefficient0.5lθAerodynamic derivative coefficient0.5ŻlŻAerodynamic derivative coefficient0.5

By means of two sticks in the cockpit, the pilot can exert a control moment which directly affects the feathering motion of the rotor. The uncontrolled model (1) is then expanded using an input signal u(ψ), which corresponds to the motion of the lower end of an actuation rod. Following again Ref. [11], the controlled model becomes

(4)Mθ..(ψ)β..(ψ)+C(ψ)θ.(ψ)β.(ψ)+K(ψ)θ(ψ)β(ψ)=Bu(ψ)=ν120u(ψ).

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Parallel Numerical Method for Compressible Flow Calculations of Hovering Rotor Flowfields

Jeu-Jiun Hu, San-Yih Lin, in Parallel Computational Fluid Dynamics 2002, 2003

1 INTRODUCTION

The numerical simulation of a helicopter rotor is a complex and great challenging problem. The flow is three-dimensional, unsteady and turbulent. The vortex wake shaded by the advancing blade interaction with retreating blade causes the complexity of the flow field. So, to accuracy of capture wake, one needs high order numerical scheme and high grid density points. This will cost a lot of CPU times, memory and is expensive for numerical simulations. As the personal computer processing speed improvement, the problem can be solved via PC cluster parallel computing.

Numerical solutions for hovering rotors have being investigation for pass years, for Euler calculations [1,2,3] and for Navier-Stokes calculations [4,5,6]. Generally speaking, the method can be divided into three groups: 1) rotating non-inertial reference frame with relative velocities as flows variables [6], 2) rotating non-inertial reference frame with absolute velocities as flows variables [1,2,3,5] and 3) dynamic grid system [4]. Methods 1 and 2 have to include the necessary source terms in the Euler or Navier-Stokes equations that are centrifugal and Coriolis forces. There are probably only two papers [6] in hovering rotor simulation that used the method 1 formulation which needs additional velocity transform to reduce the numerical errors due to the rotating frame.

In this paper, the Euler and Navier-Stokes, three-dimensional solutions of hovering rotor using modify Osher-Chakravarthy high order upwind scheme with structure grids [7] and comparison of method 1 with method 2 are purpose. The parallel software used is based on MPI version 1.1.2 and PGI Fortran 90 version 3.1. The OS is Red-Hat Linux IA-32. Local hub data commutation speed is 100 mega. CPU is PIII 600.

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Correlating structure of tip vortices and swirl flows induced by a low aspect ratio rotor blade

Yong Oun Han, Young Soo Kim, in Engineering Turbulence Modelling and Experiments 4, 1999

1 INTRODUCTION

At the tip of a helicopter rotor, the tiny and fast rotating tip vortices are generated by the 3-dimensional effect of the finite blade wing. Beneath the rotor disc big and slow swirl flows are also induced by the main portion of the rotor blade simultaneously. It can be illustrated that the slipstream made by the hovering rotor are wrapped by a bunch of helical trails drawn by tip vortex centers. As the wake age gets older, the swirl flow which is the rotating component of the slipstream with a small swirl energy correlates with the rotating (or, circumferential) component of the tip vortex which became weak. Note that the axes of two rotating flow components are perpendicular to each other. When both rotating energies become comparable, the resulting mixed rotating flow becomes an excellent flow model to visualize the three-dimensional vertical flow, one of the distinctive features of turbulence. It is, therefore, of great advantage to understand the basic characteristics of turbulence if we are to determine how strongly the tip vortex correlates to the swirl velocity component of the rotor in downstream.

Whereas the tip vortex has been dealt with rather as an unfavorable aerodynamic source which causes such problems as the profile drag, the BVI problem [1], the vortex- airframe body interaction[2] and an acoustic noise [3], its excellent vortical characteristics with respect to the turbulence study have been overlooked or, not recognized yet. Since it is very difficult to measure turbulent features of the tip vortex for technical reasons, veiy little turbulence data has appeared. Recently, the quantitative turbulent structures have started to be unveiled by tip vortex data obtained mainly from fixed wings[4, 5, 6]and rotor blades[7, 8],

In this study the basic mean field data of tip vortices and swirls with wake ages and then turbulence quantities will be supplied by LDV measurements to investigate the three-dimensional vortical motion. Correlations of both rotating components in far downstream will be discussed, also.

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Gas Turbines: An Introduction and Applications

Claire Soares, in Gas Turbines, 2008

Direct Drive and Mechanical Drive

With land based industries, gas turbines can be used in either direct drive or mechanical drive application.

With power generation, the gas turbine shaft is coupled to the generator shaft, either directly or via a gearbox: "direct drive" application. A gearbox is necessary in applications where the manufacturer offers the package for both 60 and 50 cycle (Hertz, Hz) applications. The gear box will use roughly 2 percent of the power developed by the turbine in these cases.

Power generation applications extend to offshore platform use. Minimizing weight is a major consideration for this service and the gas turbines used are generally "aeroderivatives" (derived from lighter gas turbines developed for aircraft use).

For mechanical drive applications, the turbine module arrangement is different. In these cases, the combination of compressor module, combustor module, and turbine module is termed the gas generator. Beyond the turbine end of the gas generator is a freely rotating turbine. It may be one or more stages. It is not mechanically connected to the gas generator, but instead is mechanically coupled, sometimes via a gearbox, to the equipment it is driving. Compressors and pumps are among the potential "driven" turbomachinery items (see Figure 1-4).

Figure 1-4. A typical free power turbine.

(Source: Bloch and Soares, Process Plant Machinery, Second Edition. Boston: Butterworth-Heinemann, 1998.)

In power generation applications, a gas turbine's power/size is measured by the power it develops in a generator (units watts, kilowatts, Megawatts). In mechanical drive applications, the gas turbine's power is measured in horsepower (HP), which is the torque developed multiplied by the turbine's rotational speed.

In aircraft engine applications, if the turbine is driving a rotor (helicopter) or propeller (turboprop aircraft), then its power is measured in horsepower. This means that the torque transmission from the gas turbine shaft is, in principle, a variation of mechanical drive application. If an aircraft gas turbine engine operates in turbothrust or ramjet mode, (i.e. the gas turbine expels its exhaust gases and the thrust of that expulsion propels the aircraft forward), its power is measured in pounds of thrust. What follows are examples of operational specifications for land-based gas turbines.

Alstom's GT 24/ GT 26 (188MW 60Hz, 281MW 50Hz). Both Used in Simple Cycle, Combined Cycle, and Other Cogen Applications.

GT24 (ISO 2314 : 1989)FuelNatural gasFrequency60 HzGross electrical output187.7 MW*Gross electrical efficiency36.9 %Gross heat rate9251 Btu/kWhTurbine speed3600 rpmCompressor pressure ratio32:1Exhaust gas flow445 kg/sExhaust gas temperature612 °CNOx emissions (corr. to 15% O2,dry)< 25 vppm

(Source: Alstom Power.)

GT26 (ISO 2314 :1989)FuelNatural gasFrequency50 HzGross electrical output281 MW*Gross electrical efficiency38.3 %Gross heat rate8910 Btu/kWhTurbine speed3000 rpmCompressor pressure ratio32:1Exhaust gas flow632 kg/sExhaust gas temperature615 °CNOx emissions (corr. to 15% O2, dry)< 25 vppm

*In combined cycle, approximately 12 MW (GT26) or 10 MW (GT24) is indirectly produced via the steam turbine through heat released in the gas turbine cooling air coolers, into the water steam cycle.

Alstom's GT 11N2, Either 60Hz or 50Hz (with a gear box). Used in Simple Cycle, Combined Cycle and Other Cogeneration Applications.

GT11N2 (50Hz)FuelNatural gasFrequency50 HzGross electrical output113.6 MWGross electrical efficiency33.1%Gross heat rate10,305 Btu/kWhTurbine speed3600 rpmCompressor pressure ratio15.5:1Exhaust gas flow399 kg/sExhaust gas temperature531 °CNOx emissions (corr. to 15% O2,dry)< 25 vppm

(Source: Alstom Power.)

GT11N2 (60Hz)FuelNatural gasFrequency60 HzGross electrical output115.4 MWGross electrical efficiency33.6%Gross heat rate10,150 Btu/kWhTurbine speed3600 rpmCompressor pressure ratio15.5 : 1Exhaust gas flow399 kg/sExhaust gas temperature531 °CNOx emissions< 25 vppm (corr. to 15% O2,dry)

SGT-600 Industrial Gas Turbine—25 MW (Former Designation, Alstom's GT10)

Technical specifications:Dual fuelNatural gas and liquidFrequency50/60 HzElectrical output24.8 MWElectrical efficiency34.2%Heat rate10,535 kJ/kWhTurbine speed7,700 rpmCompressor pressure ratio14.0:1Exhaust gas flow80.4 kg/sExhaust gas temperature543 °CNOx emissions (corr. to 15% O2, dry)<25 vppm

(Source: Siemens Westinghouse.)

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Multitime multigrid convergence acceleration for periodic problems with future applications to rotor simulations

H. van der Ven, ... B. Oskam, in Parallel Computational Fluid Dynamics 2001, 2002

1.1 Applications

Main application area for the DG method is the simulation of the flow around helicopter rotors, in both hover and forward flight conditions. Rotor flows are characterised by complicated aerodynamic phenomena, such as blade-vortex interaction and compressibility effects at the advancing blade, leading to high noise levels. Present day CFD technology is sufficiently mature to accurately resolve these aerodynamic phenomena (for details, see [2]), but at a prohibitively high computational cost. For a typical helicopter rotor in steady state forward flight the minimum frequency is the blade passing frequency, and the maximum frequency is dominated by the short duration Blade-Vortex Interaction events, given rise to a bandwidth of the order of one hundred.

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Alternative Fatigue Formulations for Variable Amplitude Loading of Fibre Composites for Wind Turbine Rotor Blades

R.P.L. Nussen, D.R.V. Van Delft, in European Structural Integrity Society, 2003

Loads

In terms of loads, the rotating wings of a wind turbine may best be compared to a helicopter rotor, which has been tilted 90°, but with the upper side of the airfoil shaped blade crosssection located downwind instead of upwind. However, where a helicopter blade is connected flexibly to the rotor axis, a wind turbine rotor blade is fixed rigidly to the hub. This results in significant bending moment loads in the blade. The loads can be separated into wind loads (constant wind speed + gusts), and mass loads (dynamic loads, gravity loads, centrifugal loads). The wind load causes high forces perpendicular to the rotor plane (comparable to rotor thrust in a helicopter) and a torque on the hub (facilitating energy production). An alternating load component is found in edgewise (or in-plane) direction due to the mass loading of the rotor blades. Also, the blade is loaded by centrifugal forces, (see Fig. 1).

Fig. 1. Loads on a wind turbine rotor blade

Large variations in loads occur due to the variable nature of the wind. The estimated amount of load cycles lies in the order of magnitude of 108 cycles for a modem (large) wind turbine, see Mandell et.al. [1].

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Transverse Thrusters

J.S. Carlton FREng, in Marine Propellers and Propulsion (Third Edition), 2012

14.1.1 Performance Characterization

The usual measure of propeller performance defined by the open water efficiency (η0) and given by equation (6.2) decreases to zero as the advance coefficient J tends to zero. However, at this condition thrust is still produced and as a consequence another measure of performance is needed to compare the thrust produced with the power supplied.

Several such parameters have been widely used in both marine and aeronautical applications; in the latter case to characterize the performance of helicopter rotors and VTOL aircraft. The most widely used are the static merit coefficient (C) and the Bendemann static thrust factor (ζ) which are defined by the following relationships:

(14.5)C=0.00182T3/2SHPρπD2/4=KT3/2π3/2KQξ=TPs2/3D2/3(ρπ/2)1/3=KTKQ2/3[π(2)1/3]}

In these equations the following nomenclature applies:

T is the total lateral thrust, taken as being equal to the vessel's reactive force (i.e., the impeller plus the induced force on the vessel).

SHP is the shaft horsepower.

Ps is the shaft power in consistent units.

D is the tunnel diameter.

ρ is the mass density of the fluid.

and KT and KQ are the usual thrust and torque coefficient definitions.

Both of the expressions given in equation (14.5) are derived from momentum theory and can be shown to attain ideal, non-viscous maximum values for C = 2 and ζ = 1.0 for normal, non-ducted propellers. In the case of a ducted propeller with no duct diffusion these coefficients become C = 2 and ζ=23.

Clearly it is possible to express the coefficient of merit (C) in terms of the Bendemann factor (ζ) and from equation (14.5) it can easily be shown that:

(14.6)C=ζ3/22

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Surface protection and coatings for wind turbine rotor blades

B. Kjærside Storm, in Advances in Wind Turbine Blade Design and Materials, 2013

12.5.1 Standards

There are no general standards for the testing of the surface protection of rotor blades. The standards for testing helicopter blades are useful guidelines, but helicopter rotor blades are examined after a certain number of hours in the air, whilst wind turbine rotor blades need to be protected in a way that ensures they can operate for years without maintenance.

The standards for both offshore constructions and the paints and varnishes used on them can be used as guidelines when creating a set of test parameters for the surface protection of wind turbine rotor blades.

Various other standards can also be used as guidelines, including the Norsok Standard M-501, 'Surface preparation and protective coating', which is a standard developed by the Norwegian petroleum industry, and the European standard ISO 20340 'Paints and varnishes – Performance and requirements for protective paint systems for offshore and related structures'.14,15 ISO 4628 'Paints and varnishes – Evaluation of degradation of coatings' and 'Designation of quantity and size of defects, and of intensity of uniform changes in appearance'16 can be used to create a programme that tests the influence of the different parameters that can attack the surface. Parts 4628–2, 4628–4 and 4628–5 are especially useful for wind turbine rotor blades.17–19

The standards for offshore painted steel structures are more relevant than the standards for helicopter blades when standardizing protection for wind turbine rotor blades, because wind turbines are similar to offshore installations for the petroleum industry. Offshore structures used in the petroleum industry are divided into four zones based on the environment that each structure is exposed to (ISO 20340).14One zone corresponds to the area exposed to atmospheric category C5-M, and the other three zones correspond to category Im2, which are the underwater zone, the tidal zone and the splash zone.14

All four zones are relevant for wind turbine towers, but only the zone corresponding to atmospheric category C5-M is regarded as relevant to rotor blades (see Reference 20 for ISO 12944–2:2000). Both category C5-M and Im2 are categories that describe a harsh environment. The categories are both designed for steel constructions, but most of the parameters can be applied to composites and structures with surface protection. C5-M and Im2 define corrosiveness based on the mass or thickness lost by standard specimens, and they also describe typical atmospheric environments.

This section will now conclude by describing a selection of useful tests that have been proved to be able to highlight the ability of different surfaces to withstand harsh atmospheres. These are modified from tests for paints and varnishes in offshore (marine) atmospheres, taking into consideration that the standard tests have been designed for metals, in particular painted steel.

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The Ffowcs Williams and Hawkings equation

Stewart Glegg, William Devenport, in Aeroacoustics of Low Mach Number Flows, 2017

Abstract

Lighthills acoustic analogy gives the general solution to the wave equation for a medium that includes turbulence and stationary scattering objects. In many important applications, such as for propellers and helicopter rotor noise, the surfaces are moving and so we need to modify the analysis to take full account of surface motion. Very powerful techniques to address this problem have been pioneered by Ffowcs Williams and Farassat, and the objective of this chapter is to introduce these techniques. We will start by reviewing the concept of a generalized derivative and then show how these may be used to give solutions to Lighthill's equation for a medium that includes moving surfaces and convected turbulent flow. This will be followed by a general discussion of the sound fields from moving sources and the extension of the results to sources in a moving fluid. Finally we will show how incompressible computational fluid dynamics codes can be used to calculate the sound radiated by stationary objects in the flow.Skip to Main content

Military Helicopters

Related terms:

Airframes

Fuselages

Helicopters

Rotors

Carbon Fiber

Rotor Blade

Aircraft Structure

Main Rotor

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Development and Implementation of Helicopter Underwater Egress Training Programs

Sean Fitzpatrick, in Handbook of Offshore Helicopter Transport Safety, 2016

2.1.3.1 Military Programs

Elite military HUET programs such as those delivered to the American or Canadian military are generally considered to be at the top of the training food chain. These programs are tailored to a specific aircraft type. The theoretical and practical course curriculum targets specific aspects of the event for each member of the aircrew. To enhance this specificity in the performance of critical tasks during an emergency, aircrews are encouraged to train together whenever possible. Based on accident investigation report findings, the program addresses the aircraft type, flying conditions, and common mission profiles during flight with an emphasis on critical phases of flight (e.g., takeoff, hover, landing, and approach). Night operations also receive extra attention because of the reduced visibility leading up to and during the ditching and the increased difficulties associated with performing postegress survival skills, and because rescue operations become more difficult at night (Taber, 2010, p. 211–290, 2014).

When training military personnel, HUET simulators are typically configured to replicate the operational aircraft type (Figure 2.5). Emergency exit locations are aligned as precisely as possible with regard to seating and equipment locations. Props, such as helmet bags, stretchers, life rafts, workstations, flying controls, overhead or center consoles, sensor operator stations, helicopter emergency egress lighting systems, internal egress guide bars, and other relevant egress landmarks are incorporated into the simulator to ensure that personnel develop an egress process in training that can be transferred directly to the real aircraft (Chapter 7). To further enhance the training environment, the aircrew wear representative flight suits and aviation vests, aircrew harnesses, survival backpacks or seat-packs, and flight helmets, which may be fitted with mock night vision goggles and communications cords. In the case of helicopter-borne troops or special operations personnel, equipment may include individual restraint harnesses, body armor, gas masks, rubber weapons, fast ropes, or rappel harnesses. Search and rescue personnel or flight engineers may also train using monkey tails or hoisting points.

Figure 2.5. Underwater egress training simulator interior military configuration example.

Trainees are taught to egress from primary, secondary, and tertiary crew positions. Exits may be in place, already jettisoned by instructors or other trainees, or set in the open position such as sliding doors or aircraft ramps. Trainees may be seated or kneeling, restrained or free standing. They may be sitting on the floor with their feet hanging from the sill, connected to ropes or rappel anchor points. Dependent on the aircraft type and personal equipment worn, trainees may perform egress techniques from crash-attenuating seats and they may be expected to disconnect communications cords or remove their flight helmets prior to egressing. Often crew will be required to wait for another crewperson to egress before it is their turn to move. Although this is a realistic situation for many aircrew, it is not currently an egress training sequence that is fully incorporated for offshore passengers.

As training exercises or evolutions progress and become more intense, wind, waves, rain, low-level light or complete darkness, and a range of sound effects will be added to increase the feeling of realism. All or some of this training may also involve the use of compressed-air EBSs (Civil Aviation Authority, 2013; see also Chapter 8). Following or prior to their inverted training exercises trainees will also perform surface or hover evacuations as a group. They will be required to inflate aviation vests, board a life raft, secure the raft canopy, and perform sea survival skills in a simulated storm until rescued.

This type of training complexity and integrated task requirements may appear complex at first. However, as the instructional staff become more familiar with the aircraft layout and configurations including the type of equipment being used by each trainee (including their roles and duties in the helicopter), it becomes easier to identify the specific interactions necessary to prepare individuals for a possible ditching. In the multifaceted and dynamic world of military aircraft ditching programs the key to delivering successful training is understanding the specific needs associated with a particular organization or unit.

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Fatigue failures of aeronautical items

Manuele Bernabei, ... Mikael Amura, in Handbook of Materials Failure Analysis with Case Studies from the Aerospace and Automotive Industries, 2016

1 Introduction

A search and rescue military helicopter crashed during a ferry flight to an airport where an exercise was scheduled. All crewmembers suffered fatal injuries. The helicopter veered out of control following the in-flight detachment of one of the five main rotor blades.

The Safety Investigation Board found that the largest part of the failed blade detached about 900 m before the wreck, considering the direction of the flight. The failure analysis of the part was conducted in the immediate aftermath: it was immediately apparent that part of the blade had detached first and this caused the helicopter to become uncontrollable. The main rotor blade is about 9 m long. The structural part is a hollow extruded spar made of 6061-T6 aluminum alloy (Figure 5.13, section view), 4.5 mm thick on average. The airfoil geometry is completed by 25 aerodynamic ends named pockets, made of 0.25 mm thick aluminum alloy foils. Each pocket is glued and sealed on the spar (Figure 5.13). Furthermore, the blade is equipped with an In-flight Blade Inspection System (IBIS) and the internal part of the hollow spar is filled with nitrogen. If the gas pressure decreases, a warning "blade press" light illuminating in the pilot control panel means that there is the possibility of a blade failure. At the time of the mishap and from the moment the alert signal began, the maximum flight time of the helicopter was 6 h at decreased max speed. It is uncertain whether the blade warning light was alight when the blade detached.

Figure 5.13. Helicopter main rotor blade structure.

However, it is known that the warning was alight during the second last flight and that the crew was able to reset the IBIS in accordance with the prescribed procedures.

Following, the failure analysis on the aforementioned blade will be illustrated. Fatigue failure initiated at the outer spar surface and propagated along the spar thickness. The observations showed multiple initiation points along an incision on the outer surface. Evidence of the presence of abnormal material was found in the incision: traces of iron. The damage acted as a stress concentration raiser for the normal operative loads.

In addition, fatigue life assessment was carried out to quantify the number of cycles to failure. This was analyzed to introduce NDT inspections along with IBIS and to achieve higher safety standards [2].

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Fibre–polymer composites for aerospace structures and engines

In Introduction to Aerospace Materials, 2012

15.3.3 Composites in helicopters

Composites are often used in the fuselage and rotor blades of helicopters. Carbon, glass and aramid fibre composites are regularly used in the main body and tail boom of many commercial and military helicopters to reduce weight, vibration and corrosion as well as to increase structural performance. Composites are being used increasingly to replace aluminium in the main rotor blades to prolong the operating life by improving resistance against fatigue. Most metal blades must be replaced after between 2000 and 5000 h of service to ensure fatigue-induced failure does not occur, and the operating life can be extended to 20 000 h or more with a composite blade. Figure 15.7 presents a cross-section view of a composite rotor blade, which is a sandwich construction containing both carbon and glass fibres in the face skins.

15.7. Helicopter rotor blade constructed of sandwich composite material

(image courtesy of Westland Helicopters).

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Design, evaluation, and applications of electronic textiles*

H.L. Wainwright, in Performance Testing of Textiles, 2016

9.1.2 Installing sensors, batteries, wiring, and other hardware challenges

Research has been ongoing on making functional inorganic nanocoatings for application onto the surfaces of textiles for the purpose of conducting power to discrete components. Amberstrand® is one of the pioneers that developed a conductive copper thread that could be used in conventional sewing machines for use in fabric structures installed in military helicopters for transmission of low current power to digital devices. However, the fabric containing the wires had to be mounted between insulating layers since the thread wires were unshielded. The fabric could not be sharply folded for fear of shorting, and the copper thread did not perform well under rugged exposure conditions.

Another challenge being undertaken by researchers is the creation of layered organic polymers that are flexible enough to withstand the punishment of being embedded in fabrics to form flexible transistors. These bear many long names such as fiber-embedded electrolyte-gated field effect transistors or organic field effect transistors (OFETs); electric double-layer capacitor-gated organic field effect transistors or EDLC-OFETs; electrochemical transistors; thin film transistors).

Although a fabric transistor may be considered the "Holy Grail" of components for E-Textiles, it will ultimately boil down to manufacturability on a mass scale, and initiating procedures that assure quality, consistency, and durability.

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Helicopter Human Factors

Sandra G. Hart, in Human Factors in Aviation, 1988

Visual Workload during Terrain Flight

During NOE flight, pilots devote most of their attention to the task of immediate flight control and guidance, moving opportunistically between and around obstacles. They spend as much as 80% of the time looking outside (Simmons & Blackwell, 1980). Thus, they have limited opportunities to scan cockpit instruments, navigate, or work radios. In military helicopters, these duties are performed by a copilot–navigator, who must remain spatially oriented at all times, visualizing how the terrain should appear from a map and correlating this information with momentarily viewed terrain features. This is difficult, as maps provide limited resolution (e.g., a 50,000:1 scale) and little information about cultural features. Copilots must plan ahead (to tell pilots the route and airspeed to be flown), work the radios, and monitor cockpit instruments. Navigation, map interpretation, and terrain analysis can require 90% of a copilot's visual attention, and communicating about navigation may occupy 25% of the crew's time. In single-pilot operations, pilots must divide their attention between the visual scene outside (62%) and inside the cockpit (21%), losing valuable time transitioning and experiencing high workload (Cote, Krueger, & Simmons, 1983; Sanders, Simmons, & Hoffman, 1979).

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Introduction to aerospace materials

In Introduction to Aerospace Materials, 2012

1.3.1 Aluminium

Aluminium is the material of choice for most aircraft structures, and has been since it superseded wood as the common airframe material in the 1920s/1930s. High-strength aluminium alloy is the most used material for the fuselage, wing and supporting structures of many commercial airliners and military aircraft, particularly those built before the year 2000. Aluminium accounts for 70–80% of the structural weight of most airliners and over 50% of many military aircraft and helicopters, although in recent years the use of aluminium has fallen owing to the growing use of fibre–polymer composite materials. The competition between the use of aluminium and composite is intense, although aluminium will remain an important aerospace structural material.

Aluminium is used extensively for several reasons, including its moderately low cost; ease of fabrication which allows it to be shaped and machined into structural components with complex shapes; light weight; and good stiffness, strength and fracture toughness. Similarly to any other aerospace material, there are several problems with using aluminium alloys, and these include susceptibility to damage by corrosion and fatigue.

There are many types of aluminium used in aircraft whose properties are controlled by their alloy composition and heat treatment. The properties of aluminium are tailored for specific structural applications; for example, high-strength aluminium alloys are used in the upper wing skins to support high bending loads during flight whereas other types of aluminium are used on the lower wing skins to provide high fatigue resistance.

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Manufacturing of fibre–polymer composite materials

In Introduction to Aerospace Materials, 2012

14.2.3 Aramid (Kevlar) fibres: production, structure and properties

Synthetic organic fibres are used in polymer composites for specific aerospace applications. Organic fibres are crystalline polymers with their molecular chains aligned along the fibre axis for high strength. Examples of organic fibres are Dyneema® and Spectra®, which are both ultra-high-molecular-weight polyethylene filaments with high-strength properties. Of the many types of organic fibres, the most important for aerospace is aramid, whose name is a shortened form of aromatic polyamide (poly-p-phenylene terephthalamide). Aramid is also called Kevlar which is produced by the chemical company Du Pont.

The most common aerospace application for aramid fibre composites is for components that require impact resistance against high-speed projectiles. Aramid fibres absorb a large amount of energy during fracture, thus providing high perforation resistance when hit by a fast projectile. For this reason, aramid fibre composites are used for ballistic protection on military aircraft and helicopters. They are also used for containment rings in jet engines in the event of blade failure. Aramid composites, similarly to glass-reinforced composites, have good dielectric properties, making them suitable for radomes. Aramid composites also have good vibration damping properties, and therefore are used in components such as helicopter engine casings to prevent vibrations from the main rotor blades reaching the cabin. Figure 14.8 shows the vibration damping loss factor for several aerospace materials; aramid–epoxy has ten times the loss decrement of carbon–epoxy and nearly 200 times higher than aluminium.

14.8. Vibration loss damping factor of aramid–epoxy composite compared with other aerospace materials.

The process of producing aramid fibres begins by dissolving the polymer in strong acid to produce a liquid chemical blend. The blend is extruded through a spinneret at about 100 °C which causes randomly oriented liquid crystal domains to develop and align in the flow direction. Fibres form during the extrusion process into highly crystalline, rod-like polymer chains with near perfect molecular orientation in the forming direction. The molecular chains are grouped into distinct domains called fibrils. The fibre essentially consists of bundles of fibrils that are stiff and strong along their axis but weakly bonded together, as shown in Fig. 14.9. As for carbon fibres, aramid fibres are highly anisotropic with their modulus and strength along the fibre axis being much greater than in the transverse direction. The properties of two common types of aramid fibre are given in Table 14.3, and they exceed the specific stiffness and strength of glass fibres. Aramid fibre composites are lightweight with high stiffness and strength in tension. However, these materials have poor compression strength (which is only about 10–20% the tensile strength) owing to low micro-buckling resistance of the aramid fibrils. Therefore, aramid composites should not be used in aircraft components subject to compression loads. Aramid fibres can absorb large amounts of water and are damaged by long-term exposure to ultraviolet radiation. Therefore, the surface of aramid composites must be protected to avoid environmental degradation.

14.9. Polymer chains aligned in the fibre direction in aramid.

Table 14.3. Comparison of average properties of aramid fibres against glass and carbon fibres

PropertyKevlar (Type 29)Kevlar (Type 49)E-glassIM CarbonDensity (g cm−3)1.441.442.61.8Young's modulus (GPa)70.5112.476230Specific modulus (GPa m3 kg−1)497829128Tensile strength (GPa)2.93.03.54.2Specific strength (GPa m3 kg−1)22.11.32.3Elongation-to-failure (%)3.62.44.81.9

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Design and testing of crashworthy aerospace composite components

A.F. Johnson, ... M.W. Joosten, in Polymer Composites in the Aerospace Industry, 2015

10.1 Introduction

This chapter presents the current state of the art in design, analysis and test of crashworthy aircraft structures manufactured from composite materials. Carbon fibre composites are well established for aircraft structures due to their high specific stiffness and strength, their inherent fatigue and corrosion resistance, and suitability for fabrication of large structures. After initial success in general aviation aircraft and executive jets (Beechcraft Starship, Hawker Beechcraft 390 Premier), followed by military helicopters (EC Tiger, NH90), composites are now being used for primary wing and fuselage structures in large transport aircraft, such as the Boeing 787 and Airbus A350 XWB. A critical safety issue for the design of primary aircraft structures is vulnerability and damage tolerance due to crash loads and foreign object impacts from bird strike, hail, tyre rubber and metal fragments. New composite aircraft structures are particularly vulnerable to impact damage, due to the thin composite skins and the generally brittle behaviour of carbon fibre reinforced epoxy resins. A major concern for the aircraft industry is to develop a viable strategy for design and certification of large composite aircraft structures subjected to crash and impact loads. Previous experience with design of metallic structures under crash loads shows that impact energy is absorbed by plastic deformation and plastic hinge formation in the structure, which is not usually relevant to the orthotropic elastic behaviour with brittle failure at low strains observed in carbon/epoxy composite structures. Thus, new design concepts are required for crashworthy aircraft structures in composite materials that exploit alternative failure mechanisms such as fibre fragmentation and delamination to absorb impact energy in the structure.

For aircraft structures, the first structural design requirements for better crash protection were established for light fixed-wing aircraft and military helicopters in the Aircraft Crash Survival Design Guide (ACSDG) [1] and the MIL-STD-1290A [2]. The principles established there for crashworthy aircraft structures are now embedded in the Federal Aviation Administration (FAA) 14 CFR Airworthiness Standards [3] and EASA Certification Specifications [4] as civil airworthiness requirements for small transport aircraft CFR 23/CS 23, large transport aircraft CFR 25/CS 25, small rotorcraft CFR 27/CS 27 and large rotorcraft CFR 29/CS 29, as discussed further in Section 10.2. The main requirements for structural crashworthiness are to design the aircraft structure to absorb crash energy for a specified survivable crash scenario so as to protect the occupant space, provide restraint systems to limit occupant loads and accelerations and to provide an escape route through sufficient postcrash structural integrity. As with crashworthy automotive vehicles, active and passive crash safety systems composed of energy absorbing components, primary aircraft structure and seat restraint systems are designed to work together to absorb crash kinetic energy and bring occupants to rest without serious injury. A typical crashworthy aircraft structure requires a stiff safety cage to protect occupants, combined with energy absorbing structural elements that dissipate crash energy and lower occupant crash loads. Composite structures have the ability to absorb impact energy through a controlled failure in progressive crushing as demonstrated in the design of carbon fibre reinforced F1 racing car components, discussed in [5]. By tailoring the fibre type, matrix type, fibre–matrix interface, fibre stacking sequence and fibre orientation, composite crashworthy structures have been shown to have excellent energy absorption (EA) performance characteristics. The challenge for the aircraft designer is to develop composite fuselage structures that meet the appropriate airworthiness structural requirements, without excessive weight or cost penalties being required for the crashworthiness special conditions.

The FAA route to certification adopted by civil aircraft manufacturers is based on the well-known test pyramid for aircraft structures, as set out for composite aircraft in [6]. Hachenberg [7] discusses the corresponding design procedures, which foresee five levels of tests from material characterisation test specimens in Level 1 up to full aircraft structures in Level 5. This is a 'building block' approach as each level strongly relies on the results obtained at the level just below. The complexity of the test specimens and the subsequent costs increase considerably on the next higher level, although the number of specimens is reduced. Such an approach has proven its robustness and efficiency in the last decades and has been applied for most major civil aircraft developments. However, large-scale crash testing of composite aircraft fuselage structures and structural elements is too costly due to the number of crash scenarios, the wide range of composite materials and crash concepts in consideration. Thus, there is considerable interest in design and certification by analysis for composite structures or in practice using analysis to support a reduced structural crash test programme.

This requires the development and validation of computational methods to support the design of a composite aircraft under the full range of flight and service loads defined in the airworthiness specifications. For the structural crash behaviour, improved composites damage and failure models with appropriate finite element (FE) codes are needed and should be validated by materials and structural crash tests at each level of the test pyramid. The successful introduction of polymer composites for the development of commercial transport aircraft and helicopters that meet crashworthy regulations requires an industry wide initiative. This has prompted the FAA to form a Crashworthiness Working Group of the CMH-17 (Composite Materials Handbook, formerly MIL-HDBK-17) [8] with representatives from the aerospace and automotive industries, software companies, academia and national aerospace laboratories. It aims to develop standards for the energy absorbing characteristics of polymer composite material systems with guidelines and practices for experimental testing and design of composite energy absorbers and crashworthy composite structures. The building block approach is being actively promoted, which includes test procedures for composite energy absorber materials and elements, see [9], design analysis methods for elements [10], with future work on aircraft subfloor structures in discussion.

The plan of this chapter is as follows. Section 10.2 gives an overview of crashworthy requirements and design concepts for aircraft structures with focus on composite fixed-wing aircraft and rotorcraft. Section 10.3 reviews failure mechanisms and EA in composite crash absorber elements. After discussion of composites mesoscale ply damage and delamination models used for FE simulation of crush failures, design and analysis methods for composite energy absorbing elements are then presented together with validation by crush tests. This is followed in Section 10.4 by the design of a carbon/epoxy energy absorbing composite subfloor frame structure for a helicopter. This crashworthy concept study was carried out in a collaborative research project between the Cooperative Research Centre for Advanced Composite Structures (CRC-ACS) and the German Aerospace Center (DLR) focusing on development of novel designs and improved methods of design synthesis for crashworthy helicopter structures. This illustrates the building block approach to design and includes a detailed comparison of design predictions with crash tests on three prototype structures. Concluding remarks in Section 10.5 are followed by future trends and references. The chapter concentrates on structural crashworthiness for polymer composite materials, that is, the design of composite crashworthy elements and their application to aircraft subfloor structures. For wider aspects of aircraft safety, the reader is referred to other sources of information on current developments in airframe design, crashworthy seat and restraint systems, postcrash fire protection etc.

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Helicopter Emergency Breathing Systems

Sue Coleshaw, in Handbook of Offshore Helicopter Transport Safety, 2016

8.3 Design

8.3.1 General

The varying development of emergency underwater breathing systems for helicopter occupants has meant that EBS fall into two main design types: compressed-gas systems that provide breathable gas on demand and systems that rely on rebreathing the air in the lungs. Hybrid designs that consist of a rebreather system with additional compressed gas also exist. Each design type has differing advantages and disadvantages which must be evaluated by the end user when selecting a device that will be suitable for particular conditions of use.

8.3.2 Compressed-Air EBS

Most of the compressed-gas systems now on the market use air as the breathable gas. They can be used at depths of 10 m or more and have been designed to be capable of deployment under water. They do not rely on the user having time to take a deep breath of air before submersion to extend the time spent under water (a key disadvantage of rebreather systems). This means that they are particularly suitable for use in accidents when capsize or sinking occurs immediately after contact with the water. (It must be remembered that if deploying under water, the user still has to breath-hold for the time it might take to deploy the device, possibly after overcoming the disorientation caused by a capsize.)

Breathing times of 3–5 min (Brooks & Tipton, 2001) may be achieved, with duration of use dependent upon the size of the cylinder, conditions of use, and ability of the user to control his or her breathing. In the worst case, a gas bottle may be breathed down in less than a minute if, for example, used in very cold water by an individual who is highly stressed and breathing heavily. Because of this limited gas supply, compressed-gas systems should not be deployed too soon in anticipation of submersion. In warmer water and with breathing under control the same gas supply will last for a much longer period. One further factor to consider is that the gas can run out with little or no warning, so the user must know how to deal with this situation.

A disadvantage of compressed-gas systems is the small risk of lung barotrauma (injury caused by rapid or excessive pressure changes) that can occur during rapid ascent. Barotrauma can be caused by expanding air in the lungs that is not exhaled during ascent. The level of risk in shallow water is poorly defined. There are a few isolated case reports of problems caused by lung overpressure during military helicopter underwater escape training (HUET) using compressed-air EBS (Benton, Woodfine, & Westwood, 1996; Risberg, 1997). The first was a case of arterial gas embolism, the second a case of mediastinal emphysema; in both cases the individuals made a full recovery. While the use of compressed air does present a slightly increased risk of lung barotrauma, it should be noted that some similar lung injuries have also been observed during breath-hold dives, possibly due to localized obstruction of the airways or coughing when under water (Henckes, Arvieux, Cochard, Jézéquel, & Arvieux, 2011; Shah, Thomas, & Gibb, 2007). Less serious ear problems have also been reported (Risberg, 1997; cited by Coleshaw, 2003). The effects of pressure change on the ear or sinuses are similar whether using compressed-air or rebreather EBS. Barotrauma is a potential problem only for training as the risk is accepted as being negligible in relation to a real accident.

While most designs of compressed-gas EBS are similar in that they consist of a gas cylinder, first-stage regulator, demand valve (second-stage regulator), and mouthpiece, some have the first- and second-stage regulators very close together, whereas others have a medium pressure hose between the regulators. The products with a hose have the advantage that the relatively heavy gas cylinder can be held securely in a convenient position that will not hinder movement and only the weight of the mouthpiece and hose has to be held by the teeth. If the grip on the mouthpiece is lost the device is still held securely and the user only has to locate the cylinder and hose to find the mouthpiece and redeploy. If a device without a hose is knocked from the mouth there is a danger that the whole unit will be lost unless attached to the body in some way.

Overall, while care must be taken when considering the training requirements for compressed-air EBS, their capability to be quickly deployed under water has meant that they are favored by many user groups and they are known to have saved lives in a number of military helicopter accidents (see Brooks & Tipton, 2001 for examples).

8.3.3 Rebreather EBS

The next EBS design type is the rebreather system, which allows the user to rebreathe air from his or her lungs, exhaling into and inhaling from a counterlung. With a simple rebreather system (rather than a hybrid rebreather, described later), the user must have time to take a deep breath prior to head immersion for the device to perform to its full capability. If there is no time to take a deep breath the user will be dependent upon the volume of air already in the lungs, the worst case being no air to exhale into the counterlung. It goes without saying that in this scenario a rebreather will provide no benefit to the user. Simple rebreather systems may therefore have limited capability in accidents in which capsize or submersion occur immediately after impact with the water.

The period of use of a rebreather EBS is limited by the time it takes for the carbon dioxide concentration to build up in the counterlung and for the oxygen concentration to gradually decrease. This will be influenced by the volume of air in the lungs when use of the rebreather is initiated; with a reasonable initial breath in it is possible to rebreathe from a counterlung for several minutes. Another disadvantage of rebreathers is that the user must overcome hydrostatic pressure when exhaling into the counterlung; the deeper the water the more difficult this will be. Their capability is therefore limited at depths greater than 3–5 m. Hydrostatic imbalance, the difference in pressure between the lungs and the counterlung, depends upon the orientation of the user and the position of the counterlung. With a counterlung on the chest, the greatest breathing resistance will be experienced when the user is in the face-down position. As long as breathing resistance is kept within certain limits (see Coleshaw, 2013), it may cause some discomfort but can be tolerated for the period of time required to escape from the helicopter.

Rebreather systems are generally designed so that the user initially breathes the atmosphere and then switches to breathing into and out of the counterlung. This means that, when time allows, the rebreather can be deployed early, with the switch to breathing from the counterlung made immediately before immersion. This system has the disadvantage that there is an additional action that will add several seconds to deployment time when early deployment is not feasible. Some systems make the switch automatic (by water activation). This removes the additional action on the part of the user but could allow water entry into the counterlung if the user is submerged before the mouthpiece has been deployed.

8.3.4 Hybrid EBS

Hybrid rebreather devices have the basic components of a rebreather system but with the addition of a small cylinder of compressed gas (air) that can be automatically discharged into the counterlung on immersion. Hybrid systems have the advantage over a simple rebreather that they do not rely on the user taking a deep breath of air before submersion; the discharged air is equivalent to a normal lung-full of air. The system will therefore still be functional even if the user does not have time to take a breath. This will be a big advantage in accidents in which there is little warning and rapid capsize. The additional air will also extend the duration of use. One disadvantage of the additional air provided in a hybrid system is that the overall buoyancy of the user will increase compared to other EBS categories. This can make escape a little more difficult, although the user may have sufficient time to overcome this buoyancy effect if the EBS is functioning correctly (see Chapter 9 for a discussion of escape buoyancy).

When used with the additional cylinder of compressed air, a hybrid EBS will present a small risk of lung barotrauma similar to the compressed-air EBS designs. This can be avoided in training by using the EBS as a simple rebreather without the gas cylinder. If this is done, the users must understand the difference between the training and the operational devices.

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The Design of Industrial and Flight Simulators

E. SCOTT BAUDHUIN, in Transfer of Learning: Contemporary Research and Applications, 1987

B SIMULATOR FIDELITY: FLIGHT VISUAL SYSTEMS

The literature on military training device effectiveness is too extensive and varied to examine in full. Therefore, we will focus on the effect of variations in the fidelity of simulator visual systems and displays on transfer in order to illustrate the issues described in previous sections. An issue of major concern with respect to the visual system for a flight simulator is the kind of visual system necessary to satisfy the training requirements. There are basically three types of image simulation which currently exist, each with various advantages and disadvantages relative to this training problem. Two techniques use standard film as the image media, while a third approach utilizes computer-generated images for the visual system.

With standard films it is only possible to have a predetermined number of scenarios which may be filmed in actual flying situations. Given the nature of this general training problem, namely, scenarios where all types of potential hazards can be realistically created, there may be a serious potential for accidents during the filming of a scenario. If the general training requirements indicate that there is not a need for a great number of actual flying scenarios, then this approach may be sufficient.

The second filming approach makes use of model boards which use a miniaturized version of an actual flight scene. The model board approach appears to have technical difficulties since there is no effective way of incorporating moving models or special effects. The model board approach is frequently used in military helicopter trainers to provide a realistic nap-of-the-earth flying scenario where the limited field of view and lens distortions are less critical.

The third possible approach is the use of a computer-generated image (CGI) system where all the necessary visual information can be generated and stored digitally. The obvious advantage of this kind of approach is that it allows for other traffic participants to move in and out of the scene at will and with an independently generated landscape. This approach also allows for all other kinds of factors to be generated, for example, environmental conditions such as day or night, weather, characteristics of the roadway, and so on. New scenarios can be generated quickly and cost effectively. Given the variable number of objects and/or factors which can be developed with such a system and still provide a full 180° angle of vision, the CGI system seems most attractive from an overall cost perspective.

One of the major disadvantages, however, is that CGI systems sacrifice a degree of realism. Individual objects in a CGI system are made up of polygons. The number of polygons that can be processed in real time by an image generator is limited, so tradeoffs are made and the realism of simulated images must also be compromised. One way this problem is minimized is by storing the various objects necessary for any flying scenario at different levels of detail so that those obects that are further away from the pilot's eyes will require less detail and, therefore, fewer polygons. Recent advances in graphics technology have made great strides in enhancing the realism provided by polygon-based image system (cf. Thallman & Thallman, 1985). Whether this tradeoff between the realism of film-derived images and the relative abstractness in the CGI systems approach to image generation is unacceptable remains as a basic human-factor and/or transfer of training research issue.

Rockway and Nullmeyer (1984) studied the effectiveness of the C-130 weapon system trainer (WST) visual system designed to train visually oriented tactical airlift operational tasks. A number of experiments were conducted with personnel from aircrew populations being trained for initial qualification, mission qualification, and continuation training. Most of the experiments compared in-flight performance between those who had received the WST training (simulator training) to the performance of those who had not. Instructors were used to rate performance. The results showed positive transfer as a result of the WST. Thus, compromises in "absolute visual fidelity" were not inconsistent with positive transfer (cf. Semple, Hennessy, Sanders, Cross, & Beith, 1981).

Hughes, Brooks, Graham, Sheen, and Dickens (1982) and Hughes, Graham, Sheen, and Dickens (1983) presented transfer of training findings from an operational exercise designed to assess survival skills under a red-flag combat exercise approximating actual combat conditions. Mission-ready A-10 pilots, who were given simulator training system experience in ground attack skills under simulated threat conditions were found to survive a higher percentage of sorties flown in the subsequent red-flag combat exercise than did a comparable group of pilots who had not received the simulator training. The simulator had a visual system consisting of a wide-field-of-view, 6-arc minute, monochrome computer-generated display system. The authors conclude that, while visual systems continue to be an issue of concern to simulator designers, the ability of this simulator training system to realistically reproduce the critical events in an interactive threat situation such as that found in the typical red-flag combat exercise seems to cast some doubt on previous assertions about the necessity for high physical fidelity.

Holman (1980) examined the overall training effectiveness of the CH-47 helicopter simulators by using a two-part research methodology. The first part utilized a classical two-group transfer of training paradigm with aviators participating in transition training to the CH-47. The second part examined the training benefits attributed to the simulator for aviators who were already operational with the CH-47 helicopter. While the general findings showed that the CH-47 flight simulator is an effective simulator training device, the results did suggest that the simulator may not be adequate for hovering maneuvers, where extensive visual referencing is required at low altitudes.

Wightman, Westra, and Lintern (1985) studied the effect of simulator training on carrier landing. They manipulated the fidelity levels of two visual system factors, field of view and scene detail, in training 72 student pilots on the approach tasks of circling, straight-in, and a segmented combination. Although increased positive transfer was demonstrated for the simulator-trained groups, no effect was found between narrow and wide visual field or high- and low-detail scene conditions. Interestingly, students given 20 trials, as opposed to 40 or 60 trials, on the simulator did no better than controls on transfer performance, indicating that limited simulator practice may produce no real benefit for trainees in some situations. Level of scene detail did effect transfer performance in a study of dive bombing by Lintern, Thomley, Nelson, and Roscoe (1984), however. In short, the requirements for visual fidelity appear to be very much related to the training tasks and the environment surrounding such tasks.

The issue of simulator fidelity can also be viewed in the context of the cost effectiveness of both motion and visual fidelity (cf. Orlansky & String, 1980). In either of these cases, the computer models required to drive these elements are a significant factor in the overall cost of the simulator. Doerfel and Distelmaier (1981) argue that since a simulator is not an "exact" replica of the aircraft, but rather a training or educational device, studies are required to determine approaches needed to achieve educational or training objectives (subjective fidelity). What studies have been done have suggested that subjective fidelity can help to achieve more cost-effective results than if designers only considered the objective fidelity requirements of the simulator.

Nevertheless, there must be limits to the degree to which simulator fidelity can be lowered and still maintain the integrity of the training system. Coward and Rupp (1981) argue that the requirements for flying an aircraft include continuous interpretation of the visual environment from outside and inside the cockpit in order to maintain checks on the status of the aircraft and its location in space. When the flight scenario includes tracking and evaluating the performance and/or status of opposing aircraft such as in a combat situation, the environment is significantly more complex. The successful pilot, in this instance, depends a great deal on the out-of-cockpit visual cues, and such training must therefore take place entirely in the aircraft. Such training provides experience in a high-stress environment which may never be able to be replicated in the simulator training system. Visual cues are critical in certain kinds of flight operations, and the visual cues offered in state-of-the-art computer-generated imagery for simulator training systems may not suffice.

In summary, the transfer studies we have reviewed show that positive transfer can be achieved under varying degrees of fidelity of visual cues in the simulator. It is also clear that much additional multifactor research is needed on transfer of training to sort out effects and to remove some of the artifacts and confounds seen in much of the research. With the increasing emphasis on making simulators a cost-effective alternative to hands-on operational training, the studies of transfer which can isolate the effects of major simulator cost drivers such as visual and motion cues will become more important. In the section which follows, examples of commercial and industrial simulator applications will be reviewed. The focus will be on the development and design process that exists in these areas.

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