Rotors, Hinges And Controls Flashcards
(16 cards)
In blade design, what is the aspect ratio?
Relation of blade width to the chord. (Square Root of the Span / Area).
What is the term given to the imaginary line about which the rotor blades rotate?
Axis of rotation.
What is the name given to the circular path traced by the rotor blades’ tips?
Tip path plane.
What is the relationship between the axis and plane of rotation?
They are perpendicular.
What is the coning angle?
The angle between the tip path plane and the spanwise length of the blade.
When is the area of the rotor disc at its maximum and why?
On the ground with no pitch. Raising the collective increases rotor thrust. The blades rise about the flapping hinge and ‘cone’. The upwards movement is balanced by centrifugal reaction.
What are the implications of high and low NR and why is control important?
NR control: prevents rotor stall (insufficient lift); prevents loss of cyclic control effectiveness; regulates coning by providing centrifugal force.
NR upper limit is due to transmission considerations, blade root loading stress and compressibility due to high tip speed.
What is the designed twist in the blade and what is its purpose?
Washout - reduces blade pitch angle from root to tip to give a uniform distribution of lift.
Rotational velocity of the Kade varies depending on its radius from the rotor head. The tip has a greater rotational velocity than the root.
Lift (rotor thrust) is proportional to V^2 and therefore is greater at the tip. An unequal distribution of lift would create large bending stresses.
Name the three typical hinges found in helicopters.
Flapping, feathering, dragging.
What is the purpose of the flapping hinge?
Reduces stress - allows coning - lift equalisation.
What is the purpose of the feathering hinge?
Allows pitch angle to be changed.
What is the purpose of the dragging hinge?
Reduces stress - allows periodic drag changes through the revolution (conservation of angular momentum - Hooke’s Joint Effect.
What is Hooke’s joint effect?
When cyclic Input is applied (forward in this example), the disc / cone axis is tilted away from the axis of rotation and the shaft.
The front blade has a greater radius than the rear. The middle blades maintain position. Therefore dragging must occur.
What is phase lag?
Phase lag is the difference between the point of control input and the point of the maximum blade response disappearing mechanical problem.
Why does phase lag occur?
In a rotating rotator system, the blade continues to move around the plane of rotation whilst the vertical displacement from the control input takes place.
How is phase lag overcome?
Advance angle: pitch control horn puts the input in advance of the blade
Control rigging: servo Jack ‘rigged’ to put the pilots input in earlier
