From Exercises Sheet

Question 1

What are the two main issues driving the multi-rotor configurations widely used in the early rotorcraft development?

Answer 1

Torque balance.
Rolling moment in forward flight.

Question 2

How does the single rotor helicopter deal with the torque balance?

Answer 2

Tail rotor.

Question 3

How does single rotor helicopter deal with the rolling moment in forward flight?

Answer 3

Flapping motion.

Question 4

What is the difference between the helicopter and autogiros?

Answer 4

1. The rotor powered by the engine in the cruise flight.
2. Autogiros cannot take-off vertically without the help of pre-rotator.
3. Hover capability.
4. The helicopter may be larger than than the autogiros.
5. Autogiros sometime are not equipped with rotor control input.

Question 5

Why does not the autogiro need a tail rotor?

Answer 5

The rotor of the autogiro does not connect with the engine, and it cannot transmit torque to the vehicle.

Question 6

What is the FUNDAMENTAL difference between the helicopter and the autogiro?

Answer 6

The rotor powered by the engine during cruise flight.

Question 7

The function of the swashplate?

Answer 7

Adjust the blade control angle at different azimuth angle.

Question 8

What is the difference between the thrust and lift?

Answer 8

Thrust is in the rotor axial direction.
Lift is normal in the inflow direction.

Question 9

The assumptions of the momentum theory?

Answer 9

1) Existing a steam tube which is an axially symmetric surface passing through the rotor disc perimeter which isolates the flow through the rotor.
2) Incompressible flow.
3) Velocity imparted to the fluid is constant across the disk.
4) Far away from the disk, the air is at rest.

Question 10

What is the definition of induced velocity?

Answer 10

The velocity increment due to the rotor is called Induced Velocity.

Question 11

What is the condition of the momentum theory applicable to the vertical descent flight?

Answer 11

|U_A| > 2* v_i,hover

Question 12

What is a Figure of Merit used for?

Answer 12

It is used to evaluate rotor performance in hover.

Question 13

What is the difference between calculating the rotor power from the momentum theory, compared to the blade element theory?

Answer 13

The power from the momentum theory is the induced power (induced + parasite power in forward flight), and the power from the blade element theory is the all rotor power consumption.

Question 14

Why can’t the rotor provide any thrust in the vortex-ring state?

Answer 14

The vortex ring state sucks the downstream flow back, increasing the normal velocity Vn significantly, which makes most of the blade in the stall condition.

Question 15

What is the most reasonable pilot control strategy to get rid of the vortex ring state?

Answer 15

Push longitudinal cyclic pitch to increase forward speed.

Question 16

From the view of the blade element theory and considering the attack angle and the inflow condition, indicate why the rotor needs the pre-twist angle?

Answer 16

The pre-twist angle allows most of the blade element (especially the blade tip section) to be in the optimal attack angle (maximise the ratio between lift and drag).

Question 17

Demonstrate the function and mechanism of the pre-rotator?

Answer 17

Function: Reduce the take-off distance of the auto-rotator and even allow the autogiro to have vertical take-off capability.

Mechanism: Let the engine link with the rotor on the ground to make the rotor rotate, which will disconnect with the rotor after take-off.

Question 18

Demonstrate the control strategy of the collective pitch during the autorotational landing?

Answer 18

Reduce the collective pitch to maintain the rotational speed, and when the vehicle is close to the ground, increase the collective pitch to add additional vertical acceleration to reduce the vertical speed.

Question 19

In the autorotation process, is the helicopter similar to the autogiro’s rotor. If so, how?

Answer 19

True:
In the autorotation process, the helicopter is similar to the autogiro’s engine, providing no torque to the vehicle.

Question 20

Which azimuth angle will be the highest point if we only consider the dynamic pressure difference in forward flight?

Answer 20

180 degrees

Question 21

Which azimuth angle will be the highest if we only considered the coning angle in forward flight?

Answer 21

270 Degrees.

Question 22

Which azimuth angle will be the highest point if we only consider the longitudinal cyclic pitch (use it to increase forward speed)?

Answer 22

0 Degrees

Question 23

If we want to provide additional right force along from the rotor in an anti-clockwise rotor, which direction should the swashplate change?

Answer 23

Tilt backwards.

Question 24

By optimising the pre-twist angle, which component of the power consumption will be reduced?

Answer 24

Profile Power.

Question 25

By optimising the geometry of the fuselage, which component of the power consumption will be reduced?

Answer 25

Parasite Power.

Question 26

Which component of power will increase due to the tip blade compressibility effect?

Answer 26

Profile Power

Question 27

Why is the induced power reduced with forward speed increases?

Answer 27

The local dynamic pressure is higher.

Question 28

Which components provide the mass, damping and stiffness terms of the rotor flapping dynamic equation?

Answer 28

Inertia force, aerodynamic force, and centrifugal force: respectively.

Question 29

What is the flapping offset influence on the flapping frequency and flapping phase delay angle?

Answer 29

Increase the flapping frequency angle and reduce the flapping phase delay angle.

Question 30

Why the flapping phase angle is close to 90 degrees?

Answer 30

The natural frequency of the flapping motion is close to the rotational speed, and the excitation frequency is equal to the rotational speed. Thus, the system is in the resonance condition, in which the phase delay is 90 degrees.

Question 31

What causes the lead-lag motion and why the rotor must have the lead-lag hinge?

Answer 31

The lag (lead-lag) motion is due to the Coriolis forces provided by the flap and the lead lag hinge is used to reduce this harmonic load damage on the rotor hub.

Question 32

With forward speed increase, how will the reverse flow area change and how will it influence the rotor aerodynamic?

Answer 32

The reverse flow area will increase with forward speed, and it would reduce thrust and increase the rotor drag.

Question 33

How will the flapping offset provide additional control moments and how will it influence the pitching and rolling damping?

Answer 33

The flapping offset gives a moment to let the rotor thrust contribute to the control moment, which is called the hub moment. The flapping offset can make the rotor provide additional pitching and rolling damping.

Question 34

For a rotor system with no flapping offset and its rotational direction is anti-clockwise: When the helicopter is in forward flight: Indicate the direction of increment of the rotor thrust, forward/backward forces, and rotor tilting angle (flapping motion) provided by pushing backwards the longitudinal cyclic pitch controller.

Answer 34

Higher rotor thrust, more backward forces, rotor tilt backwards.

Question 35

For a rotor system with no flapping offset and its rotational direction is anti-clockwise: When the helicopter is in forward flight: Indicating the direction of the additional rotor thrust, forward/backward force and rotor tilting angle (flapping motion) provided by introducing additional lateral cyclic pitch?

Answer 35

Sideward force (direction: act as damping), rotor tilt sideward

Question 36

For a rotor system with no flapping offset and its rotational direction is anti-clockwise: When the helicopter is in forward flight: Collective pitch influence on the rotor forward/backward forces and flapping direction.

Answer 36

Exaggerate the flapping motion, making the rotor tilt backwards and sideward.

Question 37

When the helicopter is in hover, and the rotor is not tilted backward or sideward: Vertical velocity influence on flapping motion.

Answer 37

Not create any sideward tilt, and coning angle will be changed.

Question 38

When the helicopter is in hover, and the rotor is not tilted backward or sideward: If a man is walking from the cockpit to the tail part of the vehicle, how will the trim control input, pitching altitude, rotor flapping, and tail rotor thrust change?

Answer 38

Collective pitching will be higher, additional nose-up pitching altitude, the longitudinal cyclic pitch should be pushed forward, the rotor is flapping forward, and tail rotor thrust will increase.

Question 39

For a single-rotor helicopter configuration, they usually have a pre-setting angle. It means the rotor hub shaft tilts forward: When this helicopter is in hover flight, the trim pitching angle of this type of rotorcraft should be.....

Answer 39

nose up

Question 40

For a sing-rotor helicopter configuration, they usually have a pre-setting angle. It means the rotor hub shaft tilts forward. Compared to the traditional rotor design with no pre-setting angle, how will the pitching altitude and longitudinal cyclic change in the forward flight.

Answer 40

The pitching altitude will be more nose-up, and longitudinal cyclic control input will be more backwards.

Question 41

Here is the x3 rotorcraft and its power consumption figure. This helicopter can fly at a higher forward speed compared to the traditional helicopter Compared to the trimmed collective pitch of this vehicle and the vehicle sharing the same size but without the wing system in hover and high-speed forward flight.

Answer 41

The trim collective pitch of X3 would be higher in the hover but lower in the high-speed flight.

Question 42

Here is the x3 rotorcraft and its power consumption figure. This helicopter can fly at a higher forward speed compared to the traditional helicopter. Indicate the reason why this vehicle ground effect is lower than the single-rotor helicopter configuration.

Answer 42

The rotor wake is affected by the wing system, and therefore the airflow momentum that can hit the ground would be reduced.

Question 43

Here is the x3 rotorcraft and its power consumption figure. This helicopter can fly at a higher forward speed compared to the traditional helicopter What are the two main reasons related to the rotor that limits the helicopter's maximum flight speed?

Answer 43

Advancing tip compressibility effect, stall condition in the retracting side.