# Aircraft Performance Questions and Answers – Power Producing Power Plant

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This set of Aircraft Performance Multiple Choice Questions & Answers (MCQs) focuses on “Power Producing Power Plant”.

1. What is power producing powerplant?
a) Propulsive force that is directed by increasing momentum of the airflow
b) Propulsive force that is obtained from shaft power by propeller
c) Propulsive force that is directed by decreasing momentum of the airflow
d) Propulsive force that is obtained from engine power by propeller

Explanation: The two forms of powerplant used in aircraft propulsion are thrust-producing and power producing. A power-producing powerplant is the one whose propulsive force is obtained from shaft power by propeller.

2. Which of the following is a power-producing powerplant?
a) Turbojet
b) Turbofan
c) Rocket
d) Piston engine

Explanation: The two forms of powerplant used in aircraft propulsion are thrust-producing and power producing. A thrust-producing powerplant is the one whose propulsive force is obtained by increasing momentum of the airflow through the engine. A power-producing powerplant is the one whose propulsive force is obtained from shaft power by propeller. Examples of thrust-producing powerplant are turbojet, turbofan and rocket. Examples of power-producing powerplant are gas turbine and piston engine.

3. Which of the following statement is correct?
a) Thrust power delivered by an aircraft engine is always less than shaft power delivered
b) Thrust power delivered by an aircraft engine is always more than shaft power delivered
c) Thrust power delivered by an aircraft engine is directly proportional to shaft power delivered
d) Thrust power delivered by an aircraft engine is inversly proportional to shaft power delivered

Explanation: The thrust power delivered by an aircraft engine is always less than shaft power delivered. This is because in a power-producing engine the shaft power have to be converted into thrust during which there occur losses and hence the thrust power delivered by an aircraft engine is always less than shaft power delivered.

4. Which of the following represents the correct formula for propeller efficiency?
a) η=$$\frac{T}{P}$$
b) η=$$\frac{P}{TV}$$
c) η=$$\frac{T}{PV}$$
d) η=$$\frac{TV}{P}$$

Explanation: The correct formula for propeller efficiency is η=$$\frac{TV}{P}$$ where η is propeller efficiency T is thrust and V is true airspeed and P is shaft power. The propeller efficiency is the ratio of thrust-power output by the shaft power input of a powerplant engine.

5. The propeller efficiency depends on the advance ratio and power coefficient.
a) True
b) False

Explanation: The propeller efficiency depends on the propeller advance ratio and power coefficient. Propeller advance ratio is given by J=$$\frac{V}{nD}$$ where J is propeller advance ratio, V is true airspeed, n is rotational speed and D is propeller diameter. Also the pressure coefficient is given by CP=$$\frac{P}{\frac{\rho n^3D^5}{2}}$$ where CP is pressure coefficient, ρ is density, P is shaft power, n is rotational speed and D is propeller diameter.

6. Which of the following relations are correct for pressure coefficient?
a) CP=$$\frac{P}{\frac{\rho n^3D^4}{2}}$$
b) CP=$$\frac{P}{\frac{\rho n^2D^5}{2}}$$
c) CP=$$\frac{TV}{\frac{\rho n^3D^5}{2}}$$
d) CP=$$\frac{P}{\frac{\rho n^3D^5}{2}}$$

Explanation: The propeller efficiency depends on the propeller advance ratio and power coefficient. The pressure coefficient is given by CP=$$\frac{P}{\frac{\rho n^3D^5}{2}}$$ where CP is pressure coefficient, ρ is density, P is shaft power, n is rotational speed and D is propeller diameter. Also propeller advance ratio is given by J= $$\frac{V}{nD}$$ where J is propeller advance ratio, V is true airspeed, n is rotational speed and D is propeller diameter.

7. Which of the following relations are correct for propeller advance ratio?
a) J=$$\frac{TV}{nD}$$
b) J=$$\frac{P}{nD}$$
c) J=$$\frac{V}{nD}$$
d) J=$$\frac{PV}{nD}$$

Explanation: The propeller efficiency depends on the propeller advance ratio and power coefficient. Propeller advance ratio is given by J=$$\frac{V}{nD}$$ where J is propeller advance ratio, V is true airspeed, n is rotational speed and D is propeller diameter. Also the pressure coefficient is given by CP=$$\frac{P}{\frac{\rho n^3D^5}{2}}$$ where CP is pressure coefficient, ρ is density, P is shaft power, n is rotational speed and D is propeller diameter.

8. What is the propeller advance ratio for the power-producing powerplant when the true airspeed is 300m/s and the diameter of propeller is 0.5 m with rotational speed of 117.81rad/sec?
a) 7.01
b) 9.02
c) 5.09
d) 5.89

Explanation: The answer is 5.09 radian. Given advance ratio V=300m/s , n=117.81rad/sec, D=0.5 m. From the equation J=$$\frac{V}{nD}$$ we can find propeller advance ratio. Substitute the given values in the equation J=$$\frac{300}{117.81*0.5}$$

9. What is meant by static thrust?
a) thrust at maximum airspeed
b) power at zero airspeed
c) thrust at zero airspeed
d) power at maximum airspeed

Explanation: Static thrust is the thrust at that is produced at zero airspeed. This static thrust decreases with the increase in airspeed of a power-producing engine. This condition arises when propulsive increases with decrease in airspeed.

10. Which of the following is the correct equation for longitudinal performance equation of motion?
a) $$\frac{\eta P}{V}$$=D-Wsinγ2+mV
b) $$\frac{\eta P}{V}$$=D+Wsinγ2-mV
c) $$\frac{\eta T}{V}$$=D+Wsinγ2+mV
d) $$\frac{\eta P}{V}$$=D+Wsinγ2+mV

Explanation: The correct equation for longitudinal performance equation of motion is $$\frac{\eta P}{V}$$=D+Wsinγ2+mV where η is propeller efficiency, V is true airspeed, P is shaft power, D is propeller diameter, m is mass flow rate, W is weight.

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