01 - Basic Helicopters

He Wharekura-tini Kaihautu 0 Aotearoa

THE OPE N P0|.YTE(HN|( OF NEW ZEALAND

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CONTENTS

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Basic Operating Principles Controls Structures The Powertrain Safety In and Around Helicopters Appendix:

Table of Definitions

Copyright

{his material is for the sole use of enrolled students and may not be reproduced without the written authority of the Principal, TOPNZ.

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AIRCRAFT ENGINEERING" AIRFRAMES 111

ASSIGNMENT 1 BASIC HELICOPTERS

This assignment is intended to serve as an introduction to the rest of the assignments in the 5S5~3 series. The complete series consists of Assignment

1

Basic Helicopters

Assignment

2

Basic Flying Controls

Assignment

3

Basic Rotors

Assignment Assignment

Q S

Piston Engine Installations Rotating Flying Controls

Assignment

6

Main and Tail Rotor Heads and Blades

Assignment Assignment Assignment

7 8 9

Transmission Systems Helicopter Vibrations Turbine Engine Installations

Assignment 10

Basic Helicopter Flight Aerodynamics

The word Helicopter is derived from the two Greek words: Helicon

=

helix

Pteron

=

wing

and so literally the word helicopter means spiral wing. The history of helicopter flight starts in the mid 1700s when people of many nationalities began making models of helicopters of all shapes and sizes, powered in a variety of ways, such as gunpowder, steam, and electricity. However, vertical flight was known much earlier.

It was first described by the Chinese

alchemist Ko—Hung, who wrote in 320 AD about a toy now known as the "Chinese flying top". In 1907, Paul Carnu, a Frenchman, made the world's first free helicopter flight. His machine reached a height of about 1.7 metres

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_ 2 _ and was airborne for but a'few seconds. In the years that followe d many helicopters were made and flown. In 1936, the successful Focke-Wulf Fw 61 flew for the first time. In 1939, Igor Sikorsky flew his VS 300, which became the RH production model and is the forerunner of the present—day Sikorsky helicopter models. Many of the advances made in the design of the helicopter rotor are due to the work of Juan de La Cierva who, during the development of his "autogyro" re~invented the flapping hinge, invented the drag hinge and its damper, and developed cyclic pitch control of the rotor. So far, several terms associated with helicopters have been used. Before going any further, and to avoid confusion, a list of words and terms as they are used on helicopters and on fixed-wing aircraft is given in the Appendix at the end of this assignment. As part of your work on this assignment you should now read the Appendix and than dg Practice Exercise A that follows here. PRACTICE EXERCISE A

State whether each of the following statements is True or False. 1.

Disc area is the sum of the area of all the blades of a rotor.

2.

The angle between the chord line of a rotor blade and its plane of rotation is called the angle of incidence.

3.

The control that changes the main rotor blade pitch angles all together is the cyclic pitch control

4.

An aircraft pitches about its longitudinal axis.

S.

The propulsion rotor sited at the tail in a more or less vertical plane is the tail rotor-

6.

An aircraft yaws about its normal axis.

7.

The study of the motion of air is called dynamics.

8.

The control that changes the main rotor blade pitch angles differently to each other is the collective pitch lever.

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_ 3 _ 9.

10.

The angle between the chord line of an airfoil and the direction of the airflow (relative airflow) is called the angle of attack. An aircraft rolls about its

lateral axis-

(Answers on page

27)

BASIC OPERATING PRINCIPLES

If two or more airfoils (see Fig. 1) are fioined together, pivoted at the centre, held

horizontal, and then spun around quickly, they will rise straight upwards because of the lift developed by the airfoils as they move through the air. This device FIG. l

is the Chinese flying top mentioned earlier. Should a gust

of wind tilt it to one side while it is flying, then it will move in the direction of the tilt. All the time that the lift generated exceeds its weight, the top climbs, and when the lift is

equal to the weight the top hovers, and when it becomes less, the top descends. The helicopter main rotor operates in a similar way to the flying top, except that it is power driven and its tilt is controlled by the pilot.

Because the main rotor is power driven, a torque reaction equal and opposite to the torque turning the rotor is developed. (Newton's third law of motion.) If this torque reaction were allowed to act unhindered, then the fuselage of the helicopter would turn in the opposite direction to the main rotor. The c component that controls the effect of the torque reaction is usually a tail rotor, which is a vertical, side-mounted propeller ' WhOS@ blade angles can be moved from a positive pitch through 0°

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_ u _ to a negative pitch to vary the side thrust produced.

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Besides

controlling the effect of the torque reaction, the tail rotor is also used to control the helicopter

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about its vertical (yaw) axis when

Dimzciion of rototbn

it is hovering.

The pilot

controls the pitch angle of the tail rotor blades through the tail rotor pedals.

The lift developed by the “h-III T‘o*'cLuQ.'-

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FIG. 2

1

main rotor is altered not by

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Main rotor torque and tail rotor force

speeding up or slowing down the rotor but by increasing/decreasing the pitch angle of all the blades together by the same amount, in

the same direction, and at the same time. This is known as collective pitch. Lifting up the collective pitch lever increases the pitch angles and causes the helicopter to climb. down causes the helicopter to descend.

Pushing it

The reason for changing the pitch angles and not the rev/min is that the inertia of the main rotor would cause a time delay between the opening or closing of the engine throttle and the rev/min of the main rotor increasing or decreasing. By, say, increasing the pitch of the main rotor blades and increasing the engine power output at the same time, the main rotor rev/min stays constant and the power delivered to the main rotor is increased without the time delay due to inertia. The main rotor is tilted through the cyclic pitch control column by progressively increasing and then decreasing the angles of the blades in their orbit. Thus, to move into forward flight, the pitch angle of a rearwardgmoving blade is increased, which causes the blade to develop more lift while a forward—moving blade has its pitch angle decreased to develop less lift. The result is to tilt the total reaction into a forward—leaning position ~— see Fig. H. The main rotor can be tilted in any direction by moving

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_ 5 _ the cyclic pitch control column in the direction desired for the tilt, the helicopter then flies in the direction of the tilt. The actual tilting may be done by 1.

Using a gimbal-mounted main rotor. is called a semi—rigid rotor.

This assembly

See Fig.

3(a).

2.

Aerodynamic forces moving the blades, each pivoted on a horizontal hinge pin, up and down in relation to the centre of the fixed main rotor hub. This assembly is called an articulated rotor. See Pig. 3(b).

3.

Using aerodynamic forces to bend relatively flexible blades and their mount elements up and down in relation to the centre of the fixed main rotor hub. This assembly is called a rigid rotor. See Fig. 3(0)

(a) Semi—rigid rotor

(b) Articulated rotor FIG. 3

(c)

Rigid rotor

Types of main rotor

The main rotor gives energy to a large mass airflow, and because the airflow is accelerated to a low speed only, this offers a most efficient method of hovering.

As a theoretical example, a

helicopter that hovers by passing 500 kg of air each second at a velocity of 20 m/s downwards through its main rotor generates a" lifting force where Force

=

mass per second (%§) X velocity (%> /

=

§§Ll§ (N)*

B

*

This is a variation of the familiar

Force

=

mass (kg) X acceleration

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k

= -2-gin-(N) In both equations the answer is in newtons (N). 555/3/i

it

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F

=

500 (kg) X 20 (m)

1 (s)

=

1 (S)

10 000 (kg m) l (s7) 8

= 10 000 kg m/52 0

= 10 000 N The energy needed to generate this force of 10 000 N is found from Ke

=

i mV

2

=

i X 500 X 202

= 100 000 J where Ke is kinetic energy, m is mass, and V is velocity.

This amount of energy is used each second, so the-power needed is 100 O00 (J) 1 (S)

=

100 O00 W

=

100 kW

If, by some means a smaller mass of air is moved, say, 250 kg, then, to keep momentum the same, the velocity must be increased. r-

Momentum

=

mass X velocity

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- 7 _ _

10 O00 (momentum)

=

40 m/s

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The energy needed has become

K8

=

5 X 250 X 402

=

200 O00 J

Because this amount of energy is used each second Power needed

=

200 O00

(J)

1

(s)

= 200 000 w = £92=§E This is a twofold increase in power just to do the same job as before.

The comparison becomes even more marked if we take a theoretical VTOL jet aircraft of the same weight but with a jet velocity of Q00 m/s. As the momentum of the air lS the same, its mass is now as follows: Mass

=

momentum velocity 10 O00 400

New mass

=

2.§1.=1+oJ r':za Q

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Flying controls in the cockpit

STRUCTURES

The two most common construction methods used at present for helicopters are 1.

Semi-monocoque, and

2.

Girder.

Each method has its advantages, the former leaving a large open box for crew, baggage and payload, but requiring a complex structure and reinforcing for strength.

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