propeller

Question 1

Explain why the pitch angle of a propeller is a function of its radius.

Answer 1

the pitch angle of a propeller is often designed as a function of its radius to optimize its aerodynamic efficiency, load distribution, tip clearance, and other performance characteristics. The specific relationship between pitch angle and radius depends on the desired operating conditions and performance requirements of the propeller.

Question 2

Explain the principle of operation of a variable pitch propeller.

Answer 2

A variable pitch propeller is one in which the pilot/computer is able to adjust the blade pitch during flight. The blade angle can thus be adjusted to its optimum value for the phase of flight, be it takeoff, climb or cruise.

Question 3

Describe how propeller pitch angle is determined in relation to the angle of attack, α. What is the advantage of variable pitch propellers?

Answer 3

TA is function of β, and TA is basically an aerodynamic phenomenon which is
dependent on angle of attack α.
With variable pitch propeller, the propeller pitch can be continuously varied to
maintain maximum efficiency at all flight velocities. This can be visualised as
riding along the peaks of the propeller efficiency curves shown earlier, as shown
by the dotted line.

Question 4

Describe the principles of operation of a propeller.

Answer 4

• Propellers are made up of aerofoil sections designed to generate an aerodynamic force.
• During operation, the pressure difference of the aerofoil provides thrust to push the airplane through the air.

Question 5

Under what conditions is propeller efficiency equal to zero? Why does this occur?

Answer 5

• Consider a propeller with given n and D, J depends only on 𝑉∞. When 𝑉∞= 0 then J = 0. Thus, propeller efficiency is zero at J = 0 because there is no motion of the airplane, and hence no power available.
• At the other extreme, when 𝑉∞ (and hence J) is made large, the propeller loses lift owing to small angles of attack.
• Also, when the propeller tip speeds are near sonic, η drops dramatically due to shock wave and boundary-layer separation losses.

Question 6

By considering the relative wind speed observed by a rotating propeller, explain why propeller blades are generally twisted, i.e. the pitch angle, , is a function of the radius, r.

Answer 6

• Swept velocity is determined by radius (r) multiply by rotational velocity (ω). In this case, airstream velocity (V∞) and ω are constant from root to tip.
• Since radius is smaller at root, its swept velocity is smaller, where swept velocity is larger at tip.
• Pitch angle (β) must be larger at root [see (a)] and smaller at tip [see (b)] to maintain the same angle of attack (α).