Rotorcraft Theory Basics Flashcards
What is rotorcraft?
A heavier than air aircraft with rotary wing generating lift by rotating around a vertical mast.
What are the two main types of rotorcraft?
Helicopters (powered rotors) and Autogyros (unpowered rotor during flight).
How do helicopter differ from autogyros?
Helicopter can hover, take off vertically, and have rotor control input.
Autogyros cannot hover and rely on airflow to power the rotor during flight.
Why do we study rotorcraft?
Rotorcraft principle are used in helicopters, drones, wind turbines, and ship propellers.
What is induced velocity?
The downward velocity imparted to the air by the rotor generating thrust.
What is the relationship between thrust and induced velocity in hover?
T=2ρπ(R^2)(vi^2)
What are the key assumptions of momentum theory?
Flow is incompressible and axially symmetric, velocity is constant across the disc, and far-field flow is at rest.
What is the purpose of a blade element theory (BET)?
To calculate rotor thrust and torque by summing aerodynamic forces along the blade span.
What is the vortex ring state?
A condition where descending airflow reverses upwards through the rotor, causing a loss of lift and control.
How can the vortex ring state be avoided?
By increasing forward speed or reducing the descent rate.
What is the role of the pre-rotator in autogyros?
It spins the rotor during take off to reduce the required speed take off distance.
What is autorotation?
A state where the rotor spins freely, powered by airflow, allowing for emergency landings without engine power.
How does forward flight differ from hover?
In forward flight, blades experience asymmetric loading, with the advancing blade generating more lift than the retracting blade.
How can cyclic pitch control be used in forward flight?
Cyclic pitch adjusts blade angles to tilt the rotor disk forward for propulsion or laterally for balance.
What is the purpose of the blade flapping?
To compensate for asymmetric lift between advancing and retracting blades in forward flight.
What is the retracting blade stall?
A condition where the retracting blade’s angle of attack exceed the stall limit, reducing lift.
What are the main rotor control imputs?
A collective pitch (𝜃_0) for uniform thrust and cyclic pitch (𝜃_1s, 𝜃_1c) for disk tilt.
How is yaw control achieved in a single rotor helicopters?
Through tail rotor thrust, counteracting main rotor torque.
What is purpose of aerodynamic damping in flapping motion?
To reduce oscillations and stabilise the rotor’s response to disturbances.
The flapping motion creates opposing aerodynamic proportional to velocity , reducing oscillation.
What are stability derivatives?
Metric like forward speed damping (M_u), vertical damping (M_w) and pitching damping (M_q) that describe stability contributions.
What is the thrust coefficient (C_T)?
C_T = T/(ρA(ΩR)^2)
What is the power coefficient (C_P)?
C_P = P/(ρA(ΩR)^3)
What is the figure of merit?
M = P _ideal / P_total measuring rotor efficiency.
Higher indicate more efficient rotor.
How do tandem rotors differ from single rotors?
Tandem rotors uses differential thrust and tilt for pitch control, eliminating the need for a tail rotor.
What is unique about coaxial rotors?
They have two counter-rotating rotors on the same shaft to balance torque without a tail rotor.
What are transverse rotors?
Two side by side rotors used for lateral balance via differential thrust.
How is induced velocity on hover calculated?
v_i = sqrt(T/(2ρA))
What is the thrust equation in vertical climb?
𝑇=2𝜌𝜋(𝑅^2)(𝑈_𝐴 + 𝑣_1)*𝑣_1
What is the thrust equation for windmill brake descent?
𝑇 = 2𝜌𝜋(𝑅^2) * (𝑉_𝐷 − 𝑣_𝑖𝐷) * 𝑣_𝑖 𝐷
What are the key factors affecting rotor efficiency?
Rotor speed (Ω), blade chord (c) and pitch angle (θ).
What are the limitation of the momentum theory?
Assume steady, incompressible and axisymmetric floe cannot model unsteady effects like vortex ring state or windmill brake conditions.
What is the role of blade twist in BET?
Reduces inboard stall and improves efficiency and improves efficiency by distributing lift evenly along the blade span.
How does differential collective pitch affect yaw in tandem?
It creates torque to yaw the aircraft but must be balanced to avoid pitch instability.
why does vortex ring state occur more frequently in hover descent?
Hover descent produces a strong recirculation of airflow reducing effective thrust.
