Propeller and Rotor

Question 1

What is the difference between propeller and jet

Answer 1

Propeller:
moves large mass of air at low velocity
Jet:
moves small mass of gas at high velocity

Question 2

Word equation for Propeller efficiency

Answer 2

(Thrust x axial speed) / (Resistance torque x rotational speed)

Question 3

At low speeds is propeller of jet more efficient

Answer 3

propeller is more efficient at low speeds

at 0.5M, propellers are noisy and much less efficient

Question 4

Discuss how number of blades effects the propeller

Answer 4

1. Small effect on efficiency
2. More blades might be better as distributes thrust more evenly and has a smoother wake
3. More blades mean more narrow chords for a given thrust (so practical limits apply)

Question 5

Chord length can increase to reduce diameter. What effect does this have on efficiency

Answer 5

large diameter often improves efficiency, so would decrease

Question 6

What dictates the pitch of the propeller

Answer 6

velocity of incoming fluid and rpm

Question 7

Large pitch does what to efficiency

Answer 7

large pitch increases efficiency, but can stall when axial velocity is low

Question 8

What is the best choice for diameter-pitch ratio

Answer 8

In the order of 1

Question 9

Why is large propeller diameter good for efficiency

Answer 9

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

Question 10

How does fluid density affect a propeller

Answer 10

• 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

Question 11

What is the momentum theory (propellers)

Answer 11

• 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

Question 12

What is the actuator-disc theory

Answer 12

• Propeller replaced with infinitely thin actuator-disc
• the disc has same ‘solidity’ ratio as propeller
• the disc increases the velocity of the air (thrust)

Question 13

What are 4 assumptions for actuator-disc theory (rankine-froude theory)

Answer 13

1. Fluid is incompressible
2. Properties across any plane normal to flow is uniform
3. Disc has no yaw or incidence
4. Flow is irrigational and isentropic (has no losses)

Question 14

Give thrust as a function of a and b (inflow and upstream factors)

Answer 14

T = ρAu^2(1+a)2a = ρAu^2(1+b/2)b

Question 15

Give propulsive efficiency in terms of T, u and P(power)

Answer 15

η = Tu / P

Question 16

Give ηf - ideal froude efficiency

Answer 16

ηf = 1 / (1 + a) = 2v/1+v

where v = u1/u3

Question 17

When is ηf = 0

Answer 17

When there is no velocity change across the disc

Question 18

What are the losses neglected in actuator-disc theory

Answer 18

1. Whirl - rotational flow about rotor axis in the wake of the propeller
2. Non-uniform radial loading
3. Profile drag (viscous/form drag)
4. Interference effects in the hub of propeller
5. Compressibility effects i.e. shock formation

Question 19

Describe blade element theory

Answer 19

• uses assumption that local flow is 2D at any given radius

- Aerodynamic performance of each element determined entirely by geometry of local section

Question 20

What is pitch

Answer 20

angle between chord line and propeller’s plane of rotation

Question 21

Equation to calculate pitch element

Answer 21

Pitch = 2 (pi) r tan(β)

Question 22

Describe pitch

Answer 22

pitch is the geometric value of axial advance of the blade during one revolution of the propeller

Question 23

For low-speed aircraft what would a fixed pitch propeller be

Answer 23

pitch would be low for max thrust at runway (when no forward airflow speed)

Question 24

For high-speed aircraft what would a fixed pitch propeller be

Answer 24

pitch would be large

For low speeds however this can create high separation of flow