Propeller and Rotor Flashcards
What is the difference between propeller and jet
Propeller:
moves large mass of air at low velocity
Jet:
moves small mass of gas at high velocity
Word equation for Propeller efficiency
(Thrust x axial speed) / (Resistance torque x rotational speed)
At low speeds is propeller of jet more efficient
propeller is more efficient at low speeds
at 0.5M, propellers are noisy and much less efficient
Discuss how number of blades effects the propeller
- Small effect on efficiency
- More blades might be better as distributes thrust more evenly and has a smoother wake
- More blades mean more narrow chords for a given thrust (so practical limits apply)
Chord length can increase to reduce diameter. What effect does this have on efficiency
large diameter often improves efficiency, so would decrease
What dictates the pitch of the propeller
velocity of incoming fluid and rpm
Large pitch does what to efficiency
large pitch increases efficiency, but can stall when axial velocity is low
What is the best choice for diameter-pitch ratio
In the order of 1
Why is large propeller diameter good for efficiency
Large diameter is good for efficiency as it catches larger volume of air to which it can distribute power/thrust
- Restrictions on size is dependant on aircraft
How does fluid density affect a propeller
- does not affect efficiency
- affects its size and shape however
i. e. hydro-propeller requires much smaller parameters as denser fluid
Note: hydro-propeller develop cavitation more easily thus must keep lift coefficient cl<0.5
What is the momentum theory (propellers)
- thrust is uniformly distributed over the disc
- no rotation is imparted to the flow by the actuator disc
- streamtube entering and leaving defines the flow distinctly
- pressure far ahead and behind matches ambient value
What is the actuator-disc theory
- Propeller replaced with infinitely thin actuator-disc
- the disc has same ‘solidity’ ratio as propeller
- the disc increases the velocity of the air (thrust)
What are 4 assumptions for actuator-disc theory (rankine-froude theory)
- Fluid is incompressible
- Properties across any plane normal to flow is uniform
- Disc has no yaw or incidence
- Flow is irrigational and isentropic (has no losses)
Give thrust as a function of a and b (inflow and upstream factors)
T = ρAu^2(1+a)2a = ρAu^2(1+b/2)b
Give propulsive efficiency in terms of T, u and P(power)
η = Tu / P
Give ηf - ideal froude efficiency
ηf = 1 / (1 + a) = 2v/1+v
where v = u1/u3
When is ηf = 0
When there is no velocity change across the disc
What are the losses neglected in actuator-disc theory
- Whirl - rotational flow about rotor axis in the wake of the propeller
- Non-uniform radial loading
- Profile drag (viscous/form drag)
- Interference effects in the hub of propeller
- Compressibility effects i.e. shock formation
Describe blade element theory
- uses assumption that local flow is 2D at any given radius
- Aerodynamic performance of each element determined entirely by geometry of local section
What is pitch
angle between chord line and propeller’s plane of rotation
Equation to calculate pitch element
Pitch = 2 (pi) r tan(β)
Describe pitch
pitch is the geometric value of axial advance of the blade during one revolution of the propeller
For low-speed aircraft what would a fixed pitch propeller be
pitch would be low for max thrust at runway (when no forward airflow speed)
For high-speed aircraft what would a fixed pitch propeller be
pitch would be large
For low speeds however this can create high separation of flow
