# Aircraft Design Questions and Answers – Propulsion – Jet-Engine Thrust Considerations and Turbojet Installed Thrust

This set of Aircraft Design Questions & Answers for Exams focuses on “Propulsion – Jet-Engine Thrust Considerations and Turbojet Installed Thrust”.

1. Propulsive efficiency is defined as _____________
a) ratio of obtained thrust power to energy expanded
b) thrust used by compressed energy
c) aerodynamic efficiency
d) lift required to thrust required at each segment

Explanation: Propulsive efficiency is defined as the ratio of obtained thrust power to the energy expanded. Aerodynamic efficiency is defined as lift to drag ratio. Aerodynamic efficiency is primarily consideration of aerodynamic design.

2. Determine thrust power if aircraft velocity is 150 m/s and thrust force is 1000N.
a) 150KW
b) 1500W
c) 12.056MW
d) 0.0

Explanation: Thrust power = thrust*velocity = 150*1000 = 150KW.

3. Net thrust is given by ________
a) gross thrust minus the ram drag
b) drag into velocity
c) gross thrust divided by weight
d) lift to drag ratio

Explanation: Net thrust is given by gross thrust minus the ram drag. Drag into velocity will give power not the thrust. This power can be calculated by subtracting the excess power from total available power. Lift to drag ratio is called aerodynamic efficiency of the aircraft.

4. Afterburner is used to ________
a) increase thrust of engine
b) increase fuel efficiency
c) increase lift produced by tail
d) reduce fuel consumption

Explanation: Afterburner is used to increase Thrust produced by the jet engine. It is primarily used by fighter aircraft to increase thrust during takeoff and combat. Afterburning will increase fuel consumption and reduces the fuel efficiency.

5. An engine has gross thrust of 150KN. If mentioned designed condition needs to be met is that the ram drag should not exceed more than 18KN then, find the net thrust by engine.
a) 132KN
b) 123N
c) 145 N
d) 345MN

Explanation: Net thrust = gross thrust – ram drag
= 150-18 = 132KN.
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6. Find the acceptable value of ram drag if Turbojet is designed to provide net thrust of 88% of gross thrust. Engine gross thrust is 12.5KN.
a) 1.5KN
b) 2500
c) 23456
d) 4325

Explanation: Ram drag = Gross Thrust – net Thrust = 12.5 – 0.88*12.5 = 1.5KN.

7. Thrust generated by an engine is proportional to mass flow rate.
a) True
b) False

Explanation: Thrust is nothing but the propulsive force which is used to accelerate aircraft. It will be used to provide forward motion. Thrust depends upon number of factors including mass flow rate, exhaust velocity etc. A typical aircraft thrust is nothing but the change in momentum. And hence, it will be affected by mass flow rate.

8. Which type of engine should be selected, if required cruise speed is above 2.1M?
a) Turbojet
b) Piston
c) Reciprocating
d) Piston propeller

Explanation: If required cruise speed is above Mach 2.1 then turbojet engine is suitable among the given options. Piston and reciprocating engine performance is limited to certain speeds. Piston prop engine is limited by tip speed constraints.

9. Which of the following is correct?
a) Typically, higher bypass ratio will give higher efficiency
b) Higher bypass ratio always decreases efficiency
c) Lift is equal to weight always
d) Thrust required is the same as thrust available

Explanation: We can improve engine efficiency by using high bypass ratio. Lift is not always same as weight. Lift is same as weight during cruise segment. Thrust required and thrust available are different from each other. Thrust available is actual thrust produced by engine. Thrust required is the amount of thrust required to oppose the drag.

10. What is the limitation of current engine design?
a) Turbine inlet temperature
b) Inlet flow velocity reduction limitation
c) Flow velocity over wing
d) Expansion fan only

Explanation: Turbine inlet temperature is one of the biggest obstacle of current engine design. For best Thrust and efficiency it is desirable to use stoichiometric air fuel ratio of 15:1. However, this generated tremendous high temperatures. Such high temperatures are beyond the limits of materials used for Turbine.

11. What do you mean by overall pressure ratio?
a) Ratio of pressure at engine exhaust and inlet front face
b) Ratio of dynamic to normal pressure
c) Total pressure divided by stagnation pressure
d) Stagnation pressure divided by dynamic pressure

Explanation: Overall pressure ratio is defined as ratio of pressure at engine exhaust and inlet front face. The overall pressure ratio or opr is used to measure the ability of an engine to accelerate the exhaust. It has direct impact on thrust and propulsive efficiency of the engine.

12. Uninstalled engine data is available from __________
a) manufacturer
b) stability margin
c) thin airfoil theory
d) lifting line theory

Explanation: Uninstalled engine data is available from an engine manufacturer. It can also be found by using preliminary cycle analysis or by using fudge factor approach. Thin airfoil they is used for airfoils with thin profiles. Lifting line theory is an aspect of Aerodynamics.

13. At low subsonic speed, we should select _________
a) low bypass Turbofan engine
b) turboprop always
c) always use ram jet
d) subsonic speed cannot be achieved

Explanation: At low subsonic speed, low bypass Turbofan engine should be adopted among the given options. Turboprop has speed limitation due to propeller. Ram jet engine cannot be used at low subsonic speed as ram jet will require assistance to achieve adequate design condition. Ramjet can be used for speeds above Mach 2.8.

14. Turbofan engine uses a large fan at inlet to ________
a) increase efficiency of the engine
b) increase flow velocity
c) increase wing tip lift
d) increase tailplane lift

Explanation: To Increase efficiency, more specifically propulsive efficiency of the engine a turbofan engine uses a fan. Some air is bypassed around the engine and not being used for combustion. This helps to increase efficiency.

15. For supersonic jet, C – D nozzle is used for the acceleration of exhaust.
a) True
b) False

Explanation: A Typical C – D nozzle consists of convergent section as well as divergent section. Apart from those it also includes a throat section at where flow is in sonic condition. Incoming flow initially has subsonic speed which is increased by convergent section. At throat, flow reaches the sonic speed and as it progresses further in the divergent section it further expands to supersonic speeds.

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