Chapter 2 Flashcards
What is the difference between the thrust and lift:
Thrust is in the rotor axial direction
Lift is normal in the inflow direction
The assumptions of the momentum theory:
Existing a steamtube which is an 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
The definition of the induced velocity
The velocity increment due to the rotor is called Induced Velocity
The condition of the momentum theory applicable to the vertical descent flight
|UA|>2𝑣 i,hov
directed upwards so slipstream will be above the rotor (windmill brake state)
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.
why a 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, indicate why the rotor needs 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.
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
