Helicopter Pilot's Manual Vol 1 E-Book

Norman Bailey

• Herausgeber: Crowood
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 311

Veröffentlichungsjahr: 2014

Beschreibung

This manual has been produced for students undertaking their basic helicopter training. It concentrates on explaining not only how and why the helicopter flies but also on the correct handling techniques needed to master the flying exercises required to obtain a helicopter pilot's licence. The simpliflied text together with an abundance of diagrams will greatly assist the student to become a better and safer helicopter pilot. This is a revised and updated new edition for 2007.A manual for students undertaking their basic helicopter training, covering principles of flight and helicopter handling. Illustrations throughout.

Leseprobe

THE

HELICOPTER

PILOT’S MANUAL

VOLUME 1 Principles of Flight and Helicopter Handling

Norman Bailey

THE CROWOOD PRESS

First published in 1996 by Airlife Publishing, an imprint of The Crowood Press Ltd Ramsbury, Marlborough Wiltshire SN8 2HR

www.crowood.com

This e-book first published in 2014

New edition 2008

© Norman Bailey 1996 and 2008

All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publishers.

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library.

ISBN 978 1 84797 923 0

CONTENTS

Introduction

1  THE PRINCIPLES OF HELICOPTER FLIGHT

The Lifting Force of the Rotor

Helicopter Systems

Helicopter Controls

Rotor Freedom of Movement

Flapping to Equality

Phase Lag and Advance Angle

Hovering

Power

Limited Power

Forward Flight

Ground Resonance

Vortex Ring

Autorotation

Flying for Range and Endurance

Weight and Balance

Stability

Typical Examination Questions

2  GENERAL HANDLING

Safety Around Helicopters

The Height/Velocity Diagram

Pre-Flight Inspection

Environmental Flying

Icing

Disorientation

Airports and Heliports

Emergency Procedures

Abnormal Vibrations

Weight and Balance

VHF Radio

Transponders

Night Flying

3  AIR EXERCISES

General Notes for Student Helicopter Pilots

Familiarization with the Helicopter and Air Experience

Effects of Controls

Attitude and Power Changes

Straight and Level Flight

Climbing and Descending

Level, Climbing and Descending Turns

Basic Autorotations

Hovering Flight

Vertical Take-Off and Landing to and from the Hover

Basic Transitions

Circuits

First Solo Flight

Sideways and Backward Flight

Spot Turns

Vortex Ring Recovery

Engine-Off Landings

Advanced Autorotations

Forced Landings

Precision Transitions

Quick Stops

Pilot Navigation

Out-of-Wind Manoeuvres

Sloping Ground

Limited Power and Advanced Transitions

Confined-Area Operations

Basic Instrument Flying

Night Flying

Index

INTRODUCTION

The whole process of learning to fly helicopters will be much easier if first you take the time to read about and understand the basic aerodynamic forces that act on a helicopter.

Helicopters lack the aerodynamic control feedback and built-in stability of fixed-wing aircraft. Flying them draws on a pilot’s kinaesthetic senses and ability to extrapolate in four dimensions in real time. This is not something that can be learned overnight, but this book should help you progress more quickly through your initial training.

Few other books offer this combination of helicopter aerodynamic theory and practical hands-on advice in such an easy-to-read style. The first edition proved very popular and now is used by most helicopter training schools because of its simplified approach to learning to fly helicopters.

This new edition provides an update on current training rules and exercises while retaining the easily understood style.

Good luck with your flight training, and I hope you have many safe and enjoyable hours of helicopter flying.

Norman Bailey, DFM

1 THE PRINCIPLES OFHELICOPTER FLIGHT

Helicopters and other related rotary-wing aircraft are widely varied in their concept and configuration. This book concerns primarily the single-rotor helicopter, of the type that employs a compensating tail rotor.

Although the aerodynamics of the helicopter are based on the same laws that govern the flight of a fixed-wing aircraft, the significance of some considerations is somewhat different.

Both rely on lift produced from air flowing around an aerofoil, but whereas the aeroplane must move bodily forward through the air, the helicopter’s rotors (‘wings’) move independently of the fuselage and can produce lift with the aircraft remaining stationary (hovering).

Both autogyros and helicopters have rotating wings (rotor blades), but those of the autogyro are not driven. Instead, they rotate freely in flight under the single influence of the airflow. The helicopter’s rotor blades are engine driven in powered flight, giving it the ability to hover.

Before considering the principles of helicopter flight, it is necessary to explain some terms and definitions.

The principles of helicopter flight.

Aerofoil (Airfoil in USA) An aerofoil is any surface designed to produce lift when air passes over it. On a helicopter, the rotor blades are the aerofoils and normally are classed as symmetrical, because the blade’s upper and lower surfaces have the same curvature.

Aerofoil section.

Chord line This is an imaginary line joining a rotor blade’s leading and trailing edges.

The chord line.

Axis of rotation An actual or imaginary line about which a body rotates.

Plane of rotation This is normal to the axis of rotation and parallel to the rotor tip-path plane. It is at right angles to the axis of rotation.

Tip-path plane The path described by the tips of the rotor blades as they rotate.

The tip-path plane.

The rotor disc The area contained by the tips of the rotor blades.

The rotor disc.

Pitch angle The angle between the chord line and the plane of rotation.

The pitch angle.

Coning angle The angle between the spanwise length of a rotor blade and its tip-path plane.

Coning angle.

Coning Movement of the rotor blades aligning them along the resultant of centrifugal force and lift. An increase in lift would increase the coning angle; conversely, an increase in rotor rpm would decrease the coning angle.

Feathering The angular movement of a rotor blade about its longitudinal axis.

Feathering.

Flapping The angular movement of a rotor blade about a horizontal axis. In fully articulated rotors, the individual blades are free to flap about their flapping hinge.

Flapping.

Dragging The angular movement of a rotor blade about an axis vertical to that blade. The dragging hinge is only incorporated in fully articulated rotor systems.

Dragging.

Angle of attack The angle between the chord line and the relative airflow.

Angle of attack.

Total rotor thrust The sum of lift of all the rotor blades.

Disc loading The ratio of weight to the total main rotor-disc area.

Solidity ratio The ratio of the total blade area to the total disc area.

THE LIFTING FORCE OF THE ROTOR

Lift

To understand how lift is created, first we must review the basic principle of pressure differential. This was discovered by a Swiss physicist, Daniel Bernoulli. Simply put, Bernoulli’s Principle states that as the velocity of a fluid (air) increases, its internal pressure decreases. When a relative wind blows across a rotor blade, the air divides, passing over the top of the blade and underneath it. Essentially, the air blowing across the top moves at a greater speed than that passing below, thereby creating a pressure differential, which results in lift.

The pressure differential.

Lift from a helicopter rotor blade can generally be expressed in the same terms, but because the rotor blade moves independently of the fuselage, the velocity (V2) when hovering in still-air conditions is purely the result of the rotation of the blade (rotor rpm).

Blade Pitch

The wing of an aeroplane is fitted to the fuselage at an angle, the datums being the chord line and a line running longitudinally down the fuselage. The angle between the two is known as the angle of incidence.

Blade pitch.

A rotor blade, when attached to the main rotor head, will also have a basic setting. The datums are the chord line of the rotor blade and the plane in which the rotor blade is free to rotate. This angle between the two datums is the pitch angle.

If the rotor blade had a constant value of pitch throughout its length, problems would arise in relation to blade loading, because each section of the blade would have a different rotational velocity and, therefore, a different value of lift. As lift is proportional to V2, if the speed were doubled, the lift would increase fourfold.

Blade lift.

To avoid this considerable variation of lift, it is necessary to increase lift at the root and decrease it at the tip. This can be achieved by tapering the blade, twisting the blade (washout), or a combination of the two. Even then, lift from the blade will have its greatest value near the tip, but its distribution along the blade will be more uniform.

Relative Airflow

Consider a column of still air through which a rotor blade is moving horizontally. The effect will be to displace some of the air downward. If a number of rotor blades are travelling along the same path in rapid succession (with a three-bladed rotor system operating at 240 rpm, a blade will be passing a given point every twelfth of a second), the column of still air will become a column of descending air.

Induced flow.

This column of descending air is known as the induced flow. Therefore, the direction of the air relative to the rotor blade will be the resultant of the blade’s horizontal travel through the air and the induced flow.

Relative airflow.

Total Reaction

This force acting on an aerofoil can be understood more easily if split into two components: lift and drag. Lift acts at a right angle to the relative airflow, but, as a result, does not provide a force in direct opposition to weight. Therefore, the lifting component of the total reaction must be the part that is acting along the axis of rotation. This component is known as rotor thrust. The other component of total reaction will be in the rotor blade’s plane of rotation and is known as rotor drag.

Total reaction.

Total Rotor Thrust

If the rotor blades are perfectly balanced and each blade is producing the same amount of rotor thrust, the total rotor thrust can be said to be acting through the rotor head at a right angle to the plane of rotation.

Total rotor thrust.

Coning Angle

The effect of rotor thrust will cause the rotor blades to rise until they reach a position where their upward movement is balanced by the outward pull of the centrifugal force generated by the rotation of the blades.

Coning angle.

At high rotor speeds, the blades produce a great deal of centrifugal force, keeping the coning angle low. When rotor speed is decreased, there is less centrifugal force, so the coning angle will increase. As this centrifugal action through rotor rpm gives a measure of control of the coning angle, provided the rotor speed is kept within the specified limits for a particular helicopter, the coning angle will remain within safe operating limits.

There will also be upper limits to the rotor rpm, due to engine and transmission considerations as well as end loading stresses where the blade is attached to the rotor head.

HELICOPTER SYSTEMS

There are many variations in the design of a modern helicopter. Even though helicopters come in all shapes and sizes, however, they share many of the same major components.

Flight Control Systems

Main Rotor Systems

Main rotor systems are classified according to how the rotor blades move relative to the main rotor hub. The main categories are fully articulated, semirigid and rigid.

Fully articulated Each main rotor blade is free to move up and down (flapping), to move back and forth (dragging), and to twist about the spanwise axis (feathering). This type of system normally has three or more blades.

Fully articulated rotor system.

Semi-rigid system Normally, two main rotor blades are rigidly attached to the main rotor hub, which is free to tilt and rock independently of the main rotor mast on what is known as a teetering hinge – as one blade flaps up, the other flaps down. There is no vertical drag hinge.

Semi-rigid rotor system.

Rigid rotor system This system, although mechanically simple, is structurally complex because the operating loads must be absorbed by bending rather than through hinges. The rotor blades cannot flap or drag, but can be feathered. The natural frequency of the rigid rotor is so high that air and ground resonance are less of a problem. What is a problem, though, is that the control loads are high, making stability difficult to achieve.

Anti-torque Systems

Most single-rotor helicopters require a separate rotor to overcome the effect of torque reaction, i.e. the tendency for the helicopter to turn in the opposite direction to that of the main rotor blades.

Torque compensation.

Another form of anti-torque rotor is the Fenestron, often called ‘the fan in the tail’ rotor. This system employs a series of rotating blades shrouded within the vertical tail fin of the helicopter. Because the blades operate inside the ducted area, they are protected from contacting external objects.

Fenestron tail rotor.

Finally, there is the NOTAR (no tail rotor) system, an alternative to the anti-torque rotor. This design uses low-pressure air forced into the tail cone by an internal fan. The pressurized air is fed through horizontal slots and a controllable rotating nozzle to provide anti-torque and directional control.

NOTAR anti-torque system.

Twin-rotor helicopters do not require a separate anti-torque rotor because the torque from one rotor is balanced by the torque from the other, thereby cancelling out the turning tendency.

Landing Gear

Skids The most common type of helicopter undercarriage, skids are suitable for landing on all types of surface. Some are fitted with dampers so that touchdown shocks are not transmitted to the main rotor system. Skids not fitted with dampers absorb such shocks by allowing the cross-tube to flex. Small wheels fitted to the skids can be lowered to facilitate movement of the helicopter on the ground.

Landing skids.

Wheels Usually found on large helicopters, wheels may be fitted in a three-or four-wheel configuration. Normally, the nose wheel is free to swivel as the helicopter is taxied on the ground. To reduce drag in flight, some designs allow the wheels to retract.

Wheel landing gear.

Flotation Many helicopters can be fitted with floatation bags for operations over water. There are two basic types of floatation gear: pontoon floats that replace the skids and are permanently inflated; and pop-out floats that can be inflated in an emergency, either automatically or by the pilot.

Pontoon floats.

Pop-out floats.

Operation of Flight Control Systems

A knowledge of the flight control systems is necessary, as by understanding their operation, you will be able to recognize potential problems when conducting your pre-flight inspection.

Collective Pitch Control (Lever)

Usually operated by the pilot’s left hand, the collective pitch lever controls the lift produced by the rotor. Movement of the lever simultaneously adjusts the pitch of all the blades by the same amount.

Collective pitch control system.

Cyclic Pitch Control (Stick)

Usually operated by the pilot’s right hand the cyclic pitch control changes the pitch angle of the rotor blades in their cyclic rotation. This tilts the main rotor tip-path plane to allow forward, rearward or lateral movement of the helicopter.

Cyclic pitch control system.

Anti-Torque Control (Pedals)

The anti-torque pedals are operated by the pilot’s feet and vary the force produced by the tail rotor to oppose torque reaction. When you apply left pedal, you increase the pitch of the tail rotor blades, which increases the thrust to the right and moves the nose of the helicopter to the left.

Swash Plate Assembly

The purpose of the swash plate is to transmit cyclic and collective control movements to the main rotor blades. In its simplest form, it consists of a stationary plate and a rotating plate. The stationary plate is attached to the main rotor mast and, although restricted from rotating, is allowed to tilt in all directions and move vertically. The rotating plate is attached to the stationary plate by a bearing surface and rotates at the same speed as the main rotor blades. It transmits pitch changes through mechanical linkages. Cyclic pitch changes tilt the rotating plate and alter the main rotor blade pitch through the pitch control arms. Collective pitch changes are made by moving the whole swash plate bodily up and down while maintaining the angle of tilt.

Swash plate assembly.

Trim

Many helicopters are equipped with some form of trim arrangement to relieve the pilot from having to hold the controls against any forces in the system. The neutral position of the cyclic stick changes as the helicopter moves off from the hover into forward flight. The control feel in a helicopter is provided mechanically, and you can adjust this mechanical feel in flight by changing the neutral position of the stick using the trim control.

Frictions

Since the main rotor blades tend to feed back aerodynamic forces to the pilot’s controls, trim springs are used to resist any control motion. Friction controls provide adjustable resistance to control movements.

The Power Train

On a piston-engined helicopter, the power train usually consists of a clutch, main rotor transmission and drive, a tail rotor transmission and drive, and a freewheel unit to allow the rotors to turn freely in the event of an engine failure.

Engine

A typical light helicopter is usually powered by an air-cooled piston engine mounted behind the cabin. A fan is employed to assist engine cooling. This can absorb up to 10 per cent of engine power in the hover.

Clutch

In an aeroplane, the engine and propeller are permanently engaged, but because of the greater weight of the helicopter’s rotor system in relation to engine power, a piston-engined helicopter is usually started with the rotors disconnected from the engine to relieve the load. Even more important, there must be some way to disconnect the engine from the rotors in case of engine failure, since otherwise the rotor would stop with the engine.

Some helicopters use a centrifugal-type clutch, in which contact between the inner and outer parts is made by spring-loaded brake shoes. At low engine speeds, the clutch shoes are held out of contact by springs. As engine speed increases, centrifugal force throws the clutch shoes outward until they contact the clutch drum.

Many helicopters utilize a form of belt drive to transmit engine power to the main rotor transmission. Normally, this consists of a lower pulley attached to the engine crankshaft, an upper pulley attached to the input shaft of the main gearbox, an idler pulley and belt(s). Tension on the belt(s) is gradually increased to regulate the rate of rotor engagement.

Belt drive.

Freewheel

All helicopters are designed so that the main rotors can be disengaged from the engine in the event of a power failure. In helicopters with a belt drive system, this is achieved by means of a freewheel unit (one-way sprag clutch) contained in the upper pulley. When the engine is driving the rotor, inclined surfaces force rollers against the outer drum. If the engine fails, the rollers move inward, allowing the outer drum to continue turning.

Tail Rotor Drive System

A tail rotor driveshaft, powered from the main transmission, is connected to the tail rotor transmission located on the end of the tail cone. The tail rotor transmission provides a right-angle drive and gears to increase the input speed so that the output shaft rotates at optimum tail rotor rpm. The tail gearbox is splash lubricated from its own oil supply. You can check the oil level by means of a sight glass or plug.

HELICOPTER CONTROLS

A helicopter is able to climb and descend vertically, move horizontally in any direction and, while hovering above a spot on the ground, turn on to any selected heading. To achieve this variety of performance, the helicopter is fitted with special controls.

Collective Pitch Control (Lever)

This control gets its name from the fact that when it is raised, it simultaneously increases the pitch angles of all the rotor blades equally. Similarly, when it is lowered, it reduces their pitch angles equally. These changes are called collective pitch movements.

The first requirement is to be able to control the amount of total rotor thrust. We have said already that this force depends on angle of attack, airspeed and size/shape of the aerofoil (rotor blade). The last two can be disregarded, since they are design features. The airspeed of a rotor blade is governed by the speed of rotation, which, in the modern helicopter, is virtually constant, the maximum and minimum limitations being quite close together.

Limits of Rotor Speed

The maximum rotor speed is governed by such factors as maximum engine rpm (piston engine) and transmission limitations (gas-turbine engine). Since the gap between maximum and minimum rotor rpm is so small, there can be no question of varying the speed to control the amount of total rotor thrust. In any case, the response would be too slow because of the considerable inertia of the rotor blades. It follows that the only practical means of control is by varying the angle of attack of the blades, which is done by means of the collective pitch control (lever).

Control of rotor rpm.

Pitch Angle

Variations in blade pitch will cause marked changes in drag, and to maintain constant rotor rpm, changes in power must be made. This is achieved by having a throttle control on the end of the lever.

Cyclic Pitch Control (Stick)

To move the helicopter into horizontal flight, a thrust force is required, which must be produced by the main rotor. This can be achieved by tilting the rotor disc so that the total rotor thrust is angled in the direction of the required movement.

In the case of a two-bladed rotor, if the pitch of one blade is increased while that of the other is decreased by the same amount at the same time, one blade will rise and the other will fall, resulting in the rotor disc being tilted. To keep the rotor disc tilted, the pitch must vary throughout the blades’ 360-degree cycle of travel. This changing pitch is known as cyclic pitch and is achieved by the pilot moving the stick.

Cyclic pitch.

Torque Reaction

Unless balanced in some way, the fuselage will rotate in the opposite direction to the main rotor as a result of torque reaction. As mentioned, the most common method used to overcome this is by fitting a tail rotor.

As torque reaction is not a constant – it varies with power changes – some means must be provided to vary the thrust of the tail rotor. This is achieved by the pilot moving the pedals, which collectively change the pitch, and thereby angle of attack, of the tail rotor blades. The pitch increases or decreases depending on which pedal is moved. When tail rotor thrust equals torque reaction, the helicopter will maintain a constant heading.

Torque reaction.

Additional Functions of the Tail Rotor

Changing heading in the hover By operating the pedals to produce a thrust greater or less than torque reaction, the pilot can alter the heading of the helicopter while hovering over a ground position. The pedals operate in the correct sense, in that a yaw to the right results from pushing on the right pedal, and vice versa.

To maintain a balanced condition in forward flight By using the pedals to keep the balance indicator centralised, the pilot can ensure that the helicopter flies straight.

To prevent the fuselage from rotating in autorotation When the rotors are being turned purely by the reaction to the air and without assistance from the engine, friction will cause the fuselage to rotate in the same direction as the main rotor.

The tail rotor blades are symmetrical in shape and must be capable of being turned to produce plus or minus values in pitch angle.

Tail Rotor Drift

Consider a bar that is being turned under the influence of a couple, YY, about a point X. The rotation will stop if a couple of equal value, ZZ, pulls in the opposite direction.

Tail rotor drift.

The rotation would also stop if a single force were used to produce a moment equal to the couple, YY, but there would now be a side loading on the pivot point, X.

The tail rotor of a helicopter produces a moment to overcome the couple arising from torque reaction, which in turn causes a side loading on the axis of rotation (pivot point) of the main rotor. This side loading is known as tail rotor drift, and unless corrected it would result in the helicopter moving sideways over the ground.

Since the value of a moment is the product of force multiplied by distance, the greater the distance that the tail rotor acts from the main rotor’s axis of rotation, the smaller the force required. In practice, the tail rotor is normally positioned just clear of the main rotor.

Tail rotor drift can be corrected by tilting the rotor disc away from the direction of drift.

This can be achieved by:

•

The pilot moving the cyclic stick.

•

Rigging the controls so that when the cyclic is in the centre, the disc is actually tilted by the right amount.

•

Mounting the engine so that the drive shaft to the rotor is offset.

•

Causing the disc to tilt when the collective lever is raised.

Tail Rotor Roll

If the tail rotor is mounted on the fuselage below the level of the main rotor, the force produced by the main rotor to correct tail rotor drift will create a rolling couple with the tail rotor thrust, causing the helicopter to hover one skid low. This can be overcome if the tail rotor is raised to the level of the main rotor by cranking the fuselage or fitting the tail rotor to a pylon. This condition will only be achieved, however, if the helicopter is loaded with the centre of gravity (CG) in the ideal position. A helicopter is usually designed so that the tail rotor is level with the main rotor at cruising speeds.

Tail rotor roll.

The conventional tail rotor operates in difficult aerodynamic conditions and is susceptible to damage by foreign objects. Also, it is a danger to ground personnel. One solution to these disadvantages is the shrouded, or Fenestron, tail rotor, where the rotor blades are hinged about their feathering axis only and operate within a shroud on the tail fin.

Shrouded Fenestron tail rotor.

ROTOR FREEDOM OF MOVEMENT

Feathering

This describes the movement of the main rotor blade relative to its plane of rotation. Feathering takes place as a result of changes in collective or cyclic pitch.

Feathering.

Flapping

This describes the movement of the rotor blade perpendicular to the main hub. Flapping occurs as a result of collective and cyclic pitch changes, variations in rotor rpm, and changes in speed and direction of airflow relative to the disc, which happen in certain flight conditions.

To alleviate bending stresses that otherwise would occur, the rotor blade is allowed to move about a flapping hinge. In some helicopters, the rotor blades are allowed to see-saw about the rotor hub.

Flapping hinge assembly.

Dragging

This describes the freedom given to each rotor blade to allow it to move in the plane of rotation independently of the other blades. To avoid bending stresses at the root, the blade is permitted to drag about a dragging hinge. Such movement is retarded by some form of drag damper to prevent undesirable oscillations.

Rotor blade dragging.

Dragging occurs because of:

Periodic drag changes When the helicopter moves horizontally, each rotor blade’s angle of attack is continually changing during each revolution to provide asymmetry of rotor thrust. This variation in angle of attack results in a variation of rotor drag; consequently, the rotor blade will lead or lag about the dragging hinge.

Changing position of the blade CG relative to the rotor hub Consider the helicopter stationary on the ground in still-air conditions, with the rotors turning. The radius of the blades’ CG relative to the axis of rotation will be constant.

Changing position of blade CG.

If the cyclic stick is moved, the rotor blade will flap up on one side and down on the other to produce a change in disc attitude. With the helicopter stationary on the ground, the axis about which the blades are turning will not have altered, so the radius of the blades’ CG relative to the axis will be changing continuously through each 360 degrees of travel.

This variation in radius will cause the blade to speed up or slow down about the dragging hinge, depending upon whether the radius is increasing or decreasing. This is known as the Coriolis Effect, and it will also occur when the helicopter first moves into horizontal flight.

Hooke’s Joint Effect This effect is difficult to describe, but basically it is the movement of the rotor blade to reposition itself relative to the other rotor blades when cyclic pitch is applied. The effect is very similar to the movement of the rotor blades’ CG relative to the main rotor hub.

Hooke’s Joint Effect.

Consider the rotor of a helicopter hovering in still air. When viewed from above the shaft axis, rotor blades A, B, C and D appear equally spaced. When a cyclic tilt of the disc occurs, the cone axis tilts, but if still viewed from above the shaft axis, which has not tilted, blade A will appear to increase its radius, and blade C to decrease its radius. Blades B and D must maintain position to achieve their true radial locations on the cone. It follows, therefore, that they must move in the plane of rotation and position themselves accordingly.

FLAPPING TO EQUALITY

Moving the cyclic stick does not alter the magnitude of total rotor thrust, but simply changes the disc attitude. This is achieved by the rotor blades flapping to equality. Consider a rotor blade on a helicopter in the hover where the angle of attack is 6 degrees. A cyclic movement decreases the blade pitch and, assuming initially the direction of the relative airflow remains unchanged, the reduction in pitch will reduce both the blade’s angle of attack and rotor thrust.

Flapping to equality.

The blade will now begin to flap down, causing an automatic increase in the blade’s angle of attack. When the angle of attack reaches 6 degrees again, rotor thrust will return to its original value and the blade will continue to follow a path to maintain a constant angle of attack. Thus, cyclic pitch will alter the plane in which the blade rotates, but the angle of attack remains unchanged.

The reverse occurs when a rotor blade is subject to an increase in cyclic pitch.

Therefore, any change in angle of attack through control action or in-flight conditions causes the rotor blades to flap, and they will do so until they restore the rotor thrust – they have then flapped to equality.

PHASE LAG AND ADVANCE ANGLE

Control Orbit

In its simplest form of operation, movement of the collective pitch lever causes a flat plate mounted centrally on the main rotor mast to rise and fall. Movement of the cyclic stick causes the plate to tilt in the direction in which the cyclic stick is moved.

Rods of equal length, called pitch operating arms, connect the flat plate to the rotor blades. When the plate is tilted, the pitch operating arms move up or down, increasing or decreasing the pitch of the main rotor blades.

Control orbit.

The flat plate can be more accurately described as a control orbit, because it represents the plane in which the pitch operating arms rotate.

Pitch Operating Arm Movement

Consider now the effect of the movement of a pitch operating arm when the control orbit is tilted 2 degrees. (It is assumed that the control orbit tilts in the same direction in which the stick is moved.) If the movement of the pitch operating arm through 360 degrees of travel is plotted on a simple graph, the result will be as shown below.

Pitch operating arm movement.

Travel of pitch operating arm.

Resultant Change in Disc Attitude

The rotor blades will respond to the cyclic pitch change by flapping, and the resultant change in disc attitude can be determined by following the movement of each blade of a two-bladed rotor system.

Change in disc attitude.

Consider the rotor blades to be positioned at A and C when the control orbit is tilted; the pitch operating arms are attached to the control orbit directly beneath the rotor blades. As the blade moves anti-clockwise from A, it will undergo a reduction in pitch, and the blade will flap down. The rate of flapping varies with the amount of pitch change, so the blade will experience its greatest rate of flapping down as it passes position B (maximum pitch change). In the next 90 degrees of travel, the pitch will return from –2 degrees to zero, so the rate of flapping will have died out by position C. The blade that started at position A will flap down for 180 degrees of travel and, therefore, will reach a low position at C.

The reverse will take place with the other blade, which will reach a high position at A. Now the disc will be tilted along the axis BD, 90 degrees removed from the tilt axis of the control orbit.

Phase Lag

When cyclic pitch is applied, the rotor blades will automatically flap to equality. In doing so, the disc attitude will change, the blade reaching its highest and lowest position 90 degrees later than the point where it experiences the maximum increase and decrease of cyclic pitch. The variation between the tilt of the control orbit and the subsequent tilt of the rotor disc is known as phase lag.

Advance Angle

If the control orbit tilts in the same direction as the cyclic stick and, as a result, the disc tilts 90 degrees out of phase with the control orbit, the disc will also tilt 90 degrees out of phase with the cyclic stick. Thus, unless the system is compensated in some way, moving the cyclic stick forward would cause the helicopter to move sideways.

One way to overcome this problem is to arrange for the rotor blade to receive the maximum alteration in cyclic pitch 90 degrees before the blade is over the highest and lowest points on the control orbit. Another way would be to make the control orbit tilt so that it is out of phase with the cyclic stick by the required angle.

Advance angle.

The angular distance that the pitch operating arm is positioned on the control orbit, in advance of the rotor blade to which it relates, is known as the advance angle.

HOVERING

Take-Off to the Hover

To lift the helicopter off the ground, a lifting force must be produced that is equal and opposite to the weight that suspends vertically through the helicopter’s centre of gravity.

Rotor thrust.

When the rotor is turning at flying rpm, with the collective lever fully down, very little rotor thrust will be produced. As the collective lever is raised, the rotor blades will begin to cone up, and eventually the rotor thrust will equal the helicopter’s weight. If the collective lever is raised further, the rotor thrust will increase yet more, and when it becomes greater than the helicopter’s weight, the helicopter will accelerate upward. After a short time, the acceleration will become a steady rate of climb, and the helicopter will continue in this state until the pilot lowers the collective lever.

Take-off to the hover.

Consider the helicopter to be 200 ft above the ground when the collective lever is lowered slightly to stop it from climbing further. The helicopter will now come to the hover. Being well clear of the ground, this condition is known as a free air hover (hover out of ground effect).

Vertical Descent and Climb

If the collective lever is lowered in the free air hover, the angle of attack will reduce, rotor thrust will become less than weight, and the helicopter will begin to accelerate downward. The airflow resulting from the helicopter’s descent will oppose the induced flow and cause the angle of attack to increase. When it reaches its original value, rotor thrust will equal weight and the downward acceleration will become a steady rate of descent.

Vertical descent.

In a vertical climb, the reverse takes place. Increased collective pitch raises the angle of attack, rotor thrust becomes greater than weight, and the helicopter accelerates upward. The airflow from the rate of climb is in the same direction as the induced flow, and the resultant change in airflow direction to the rotor blades will gradually reduce the angle of attack. Again, when it reaches its original value, rotor thrust equals weight and the upward acceleration will become a steady rate of climb.

Ground Cushion Effect

In a free air hover, the resistance to the induced flow is only the resistance of the surrounding air. In a hover close to the ground, the ground itself will resist the induced flow. This will be at a maximum when hovering just above the surface. The ground effect intensifies the pressure differential around the rotor. Accelerated air, having passed through the rotor disc, strikes the ground and is slowed down, which increases the pressure under the rotor. This, in turn, causes a reduction in the induced flow and a consequent increase in angle of attack.

Ground cushion effect.

Therefore, the same angle of attack can be maintained in ground effect (IGE) with less collective pitch and power than would be required out of ground effect (OGE). This reduction in power is possible because of a reduction in rotor drag.

Hovering and ground effect.

Factors that Affect Ground Cushion

Several factors affect the ground cushion:

•

The height the helicopter is hovering above the ground. Ground cushion effect disappears at a height equal to about three-quarters of the diameter of the rotor disc. The lower the hover height, the more intense the ground effect.

•

Nature of the ground. Rough ground dissipates the cushion; long grass absorbs the cushion. Concrete and tarmac surfaces produce the best effects.

•

Sloping ground. This makes for an uneven ground cushion.

•

Wind velocity. The cushion is displaced downwind. Hovering into wind places the cushion nicely underneath the helicopter.

Recirculation