Rotorcraft Theory

Review Exercise

Which vehicles can be categorized as rotorcrafts?

Helicopter, Autogiros, Tiltrotor (they must be heavier than air)

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

Balancing the torque of each rotor system and balancing the rolling moment in forward flight

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

By using a tail rotor

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

Use a flapping hinge to balance the rolling moment a provide balance

What are the differences between helicopters and autogiros?

Why does not the autogiro need a tail rotor?

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

What is the fundamental difference between the helicopter and the autogiros?

The rotor powered by the engine in the cruise flight

How do single-rotor rotorcrafts provide control forces and
control moments?

• Vertical force: Main rotor thrust

• Longitudinal force: Main rotor tilt fore/aft

• Lateral force: Main rotor tilt lateral

• Pitch attitude: Main rotor tilt fore/aft

• Roll attitude: Main rotor tilt lateral

• Yaw attitude: Tail rotor thrust/engine torque

How do tandem rotorcrafts provide control forces and control moments?

• Vertical force: Collective main rotor thrusts

• Longitudinal force: Main rotors tilt fore/aft

• Lateral force: Main rotors tilt lateral

• Pitch attitude: Main rotor tilt fore/aft and/or differential main rotor thrust

• Roll attitude: Main rotor tilt lateral

• Yaw: Differential main rotor tilt in lateral directions

How do side-by-side (transverse) rotorcrafts provide control forces and control moments?

• Vertical force: Collective main rotor thrusts

• Longitudinal force: Main rotors tilt fore/aft

• Lateral force: Main rotors tilt lateral

• Pitch attitude: Main rotors tilt fore/aft

• Roll attitude: Main rotors tilt lateral and/or differential main rotor thrusts

• Yaw: Differential main rotor tilt fore/aft

How do coaxial rotorcrafts provide control forces and control moments?

• Vertical force: Collective main rotor thrusts

• Longitudinal force: Main rotors tilt fore/aft

• Lateral force: Main rotors tilt lateral

• Pitch attitude: Main rotors tilt fore/aft

• Roll attitude: Main rotor tilt lateral

• Yaw: Differential main rotor torques

What is the function of a swashplate?

Adjust the blade control angle at different azimuth angles

What is the difference between thrust and lift?

• Thrust is in the rotor axial direction

• Lift is normal to the inflow direction

What are the assumptions of the momentum theory?

• Exiting a steamtube (axially symmetric surface passing through the rotor disc perimeter which isolates the flow through the rotor)
• Incompressible flow
• Velocity imparted to the fluid is constant across the disk
• Far away from the disk, the air is at rest

What is the definition of the induced velocity?

Velocity increment due to the rotor

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

|𝑈𝐴|>2𝑣𝑖, ℎ𝑜𝑣

Right or wrong:

A figure of merit is used to evaluate the rotor performance in the vertical flight.

Wrong: It is used to evaluate rotor performance in hover.

Right or wrong:

Rotor power calculated from the momentum theory and blade element torque is the same.

Wrong: the power from the moment 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

What issues do tandem rotorcrafts experience that results in imbalance?

The differential main rotor thrust unbalances the \n aircraft in yaw.

There is no good alternative to differential \n thrust to change aircraft pitch attitude.

The pilot or the control system must cope with the coupling the results.

Use the following figure to indicate why the rotor cannot provide any thrust in the vortex-ring state

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

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

Push longitudinal cyclic pitch to increase forward speed

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

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)

Demonstrate the function and mechanism of the pre-rotator

• 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

From the view of the blade element theory, how does the rotor maintain the rotational speed?

P75 but im not sure which bit

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

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

Right or wrong:

In the autorotation process, the helicopter is similar to the autogiro’s rotor, providing no torque to the vehicle

Right

Based on this figure, get the rotor rotational direction and the thrust direction of the tail rotor in normal forward flight

Anti-clockwise: the tail rotor direction is to the right if you are seated in the pilot position

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

180°

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

270°

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

0°

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

Tilt backwards

Right or wrong:

The below equations can be both used in estimating the induced velocity in high-speed forward flight?

Right

Right or wrong:

The below equations can be both used in estimating the induced velocity in low-speed forward flight?

Wrong, the second equation cannot be used to calculate the induced velocity in low-speed forward flight

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

Profile power

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

Parasite power

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

Profile power

Why is the induced power reduced with forward speed increases?

The local dynamic pressure is higher

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

Inertia force, aerodynamic force, and centrifugal force, respectively

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

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

Why the flapping phase angle is close to 90°?

• 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°

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

• The lag (lead-lag) motion is due to the Coriolis forces provided by the flap

• The lead-lag hinge is used to reduce this harmonic load damage on the rotor hub

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

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

Using the blade element theory shown in this figure, which parameter will be changed considering the flapping motion?

Vn

Write the comprehensive expression of the blade control angle and indicate which is the longitudinal and lateral cyclic pitch.

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

• 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

i have not done chapter 7 yet

Here is the single-rotor helicopter with the centre of gravity after the rotor hub.

Which components significantly contribute to forward speed damping, vertical speed damping, and pitching damping?

• Forward speed damping: Rotor Fuselage
• Vertical speed damping: Rotor
• Pitching: Horizontal tail

Here is the single-rotor helicopter with the centre of gravity after the rotor hub.

Which components contribute to the velocity stability derivative and incidence stability derivatives?

Please also the direction (+/-) of these stability derivative

Velocity stability (positive means stable)

• Rotor-> positive velocity stability derivative
• Horizontal tail->If the horizontal tail pre-setting angle is positive, negative velocity stability derivative. Otherwise, the positive velocity stability derivative

Incidence stability (negative means stable)

• Rotor-> positive incidence stability derivative
• Horizontal tail->Negative incidence stability

Here is the single-rotor helicopter with the centre of gravity after the rotor hub.

What is the influence of the centre of gravity on the velocity stability derivatives and incidence stability derivative?

• Changing the centre of gravity backwards will increase the amplitude of the velocity stability and incidence stability derivatives

• As the rotor provides unstable incidence stability, it would lead the vehicle more unstable

What are the two most significant coupled control moments provided by the tail rotor?

• Sideward force
• Rolling moment

Here is the x3 rotorcraft and its power consumption figure. This helicopter can fly at a higher forward speed compared to the traditional helicopter.

Find the speed point corresponding to the maximum flight duration and maximum flight range and indicate to it on the figure.

max flight duration - middle of power required line

max flight range - draw a line from 0 to end of power available, where that intersects with power required is the maximum flight range

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.

• The rotor wake is affected by the wing system
• Therefore the airflow momentum that can hit the ground would be reduced

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?

• Advancing tip compressibility effect
• Stall condition in the retreating side

Here is the x3 rotorcraft and its power consumption figure. This helicopter can fly at a higher forward speed compared to the traditional helicopter.

Why this helicopter can fly at a higher speed compared to the traditional single-rotor with tail rotor configuration?

• The wing and propeller will offload the rotor in the high-speed flight
• Thus, the compressibility effect and the stall condition will be delayed to a higher speed range