12.01 Theory Of Flight

Question 1

How does a helicopter achieve lift?

Answer 1

Lift is produced by aerofoils (rotors) moving through the air.

Question 2

What is a helicopter’s primary aerofoil?

Answer 2

The main rotor.

Question 3

What is the definition of an aerofoil?

Answer 3

Any surface which gets a useful dynamic reaction from the air.

Question 4

What forces are generated by a helicopter’s blades?

Answer 4

Lift and thrust.

Question 5

What is the definition of an aerofoil’s span?

Answer 5

It is the distance from the root to the tip measured along the centreline.

Question 6

What is an aerofoil’s chord?

Answer 6

It is an imaginary line running from the leading edge to the trailing edge of the blades cross section.

Question 7

Aerofoils used for helicopters are usually the same shape above and below the chord line. This is known as…?

Answer 7

A symmetrical profile.

Question 8

The curvature of the aerofoil is referred to as …?

Answer 8

The camber.

Question 9

If a blade is moving backwards and upwards, what direction is the relative wind?

Answer 9

The relative wind is moving in the opposite direction (forwards and downwards).

Question 10

What is the forward-moving blade known as?

Answer 10

The advancing blade.

Question 11

What is the definition of “pitch angle”?

Answer 11

It is the acute angle between the rotor blade’s chord line and the helicopters reference plane.

Question 12

What will determine a helicopter’s “reference plane”?

Answer 12

The attitude of the main rotor hub.

Question 13

What is the definition of “angle of attack”?

Answer 13

It is the acute angle between the rotor blade’s chord line and the relative wind.

Question 14

How is the angle of attack changed?

Answer 14

By altering the pitch angle of the blades using the collective and/or cyclic levers.

Question 15

According to Bernoulli’s Principle, what is the relationship between velocity and pressure?

Answer 15

As velocity increases, static pressure decreases.

Question 16

What is the relationship between lift and relative wind?

Answer 16

Lift is perpendicular (at right angles) to the relative wind.

Question 17

With respect to a rotor blade, what direction is the relative wind?

Answer 17

Always opposite to the flight path of the blade.

Question 18

What is the relationship between drag, lift and relative wind?

Answer 18

Drag is always parallel to the relative wind and perpendicular to lift.

Question 19

What is the relationship between drag and velocity?

Answer 19

Drag varies as the square of velocity.

Question 20

The imaginary point where the result of all aerodynamic forces of an airfoil is considered to be concentrated is known as the …?

Answer 20

The “Centre of Pressure”.

Question 21

What happens to the centre of pressure as the angle of attack increases?

Answer 21

It will move forward on the blade.

Question 22

Why are Symmetrical Airfoils used on helicopters?

Answer 22

Symmetrical blades reduce any excessive movement of the centre of pressure which can lead to instability.

Question 23

What effect will pitch increase have, besides increasing lift?

Answer 23

Drag will increase.

Question 24

How is premature stalling of a rotor blade prevented when pitch is increased?

Answer 24

Power is increased to maintain the blade’s velocity.

Question 25

Lift will vary dependent on air density, what factors affect air density?

Answer 25

Temperature, altitude and humidity.

Question 26

To achieve thrust what force must be overcome?

Answer 26

Drag.

Question 27

When at rest, a helicopter’s blades, because of their weight, span and flexibility, have what appearance?

Answer 27

Rotor droop.

Question 28

When the rotors start turning, how is rotor droop overcome?

Answer 28

Centrifugal force.

Question 29

A combination of lift and centrifugal force will raise the blades above horizontal, an effect known as…?

Answer 29

Coning.

Question 30

What factors will affect the amount of the answer in Q29?

Answer 30

Lift and weight; a lightly loaded helicopter will have less coning than a heavy one.

Question 31

What will be the effect if blade tips are not all in the same plane?

Answer 31

The blades will be out of track and vibration will occur.

Question 32

To facilitate pitch change, a rotor hub must have provision for a ………. axis?

Answer 32

Feathering.

Question 33

In a conventional layout helicopter (main + tail rotor), how is directional control achieved when increasing power?

Answer 33

As power is increased the pitch of the tail is increased to counteract the torque.

Question 34

What design feature of a conventional layout helicopter assists (unloads) the tail rotor in forward flight?

Answer 34

The vertical fin unloads the tail rotor.

Question 35

Pilot’s inputs arrive initially at what part of the rotor head?

Answer 35

The swash plate.

Question 36

How is the pilot’s signal transmitted from the swash plate to the blades?

Answer 36

Pitch control rods.

Question 37

How is “dissymmetry of lift” accommodated for in rotor head design?

Answer 37

By the use of flapping hinges.

Question 38

What is the definition of “Coriolis Effect”?

Answer 38

When dissymmetry of lift causes a blade to move above or below the rotational plane, the blades centre of gravity (CofG) changes. If the blade goes up the CofG moves inward and the blade accelerates, if the blade goes down the reverse happens.

Question 39

How is the Coriolis Effect accommodated for in rotor head design?

Answer 39

By the use of drag or lead/lag hinges which allow the blades to hunt.

Question 40

How is travel of the blade limited and smoothed, about the drag hinge?

Answer 40

By the use of drag or lead/lag dampers.

Question 41

In which area of the rotor disc is blade tip stall likely?

Answer 41

In the retreating half of the rotor disc.

Question 42

What 3 factors can contribute to retreating blade tip stall?

Answer 42

Insufficient airspeed, too great an angle of attack, heavy wing loading.

Question 43

What is the definition of “Translating Tendency”?

Answer 43

This is a tendency for the whole helicopter to drift in the direction of the tail rotor thrust.

Question 44

How is translating tendency corrected at the design stage?

Answer 44

The situation is normally corrected by either offsetting the mast which changes the tip path plane or by rigging the cyclic system to give the required tilt when the cyclic is level.

Question 45

What is “Ground Effect”?

Answer 45

The helicopter displaces air downwards in the hover, this air forms an air cushion beneath the helicopter. This supports the aircraft whilst hovering and near the ground.

Question 46

When will Ground Effect be lost?

Answer 46

Ground effect will be lost if the aircraft moves faster than 3-5mph.

Question 47

What is the definition of “Vortex Ring State”?

Answer 47

The downward air flow equals the vertical downwards speed of the helicopter and therefore negates the lifting force. VRS occurs at a downward speed of approx. 3 to 4 m/s. The downward airflow created by the rotor deflects the upward airflow sideways, outwards and then down through the top of the rotor. This causes vortexes to form at the blade tips. At the same time, stall occurs at the rotor blades near the rotor head.

Question 48

What is “Autorotation”?

Answer 48

Autorotation is the process of producing lift with the rotor blades as they freely rotate from a flow of air up through the rotor system.

Question 49

How does the pilot land safely when auto-rotating?

Answer 49

The pilot flares the helicopter.

Question 50

What is the tail rotor used for during autorotation?

Answer 50

During autorotation, the tail rotor will be used for directional control only.