Aircraft Design Questions & Answers for Exams 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
Answer: a
Clarification: 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
Answer: a
Clarification: 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
Answer: a
Clarification: 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
Answer: a
Clarification: 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
Answer: a
Clarification: Net thrust = gross thrust – ram drag
= 150-18 = 132KN.
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
Answer: a
Clarification: 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
Answer: a
Clarification: 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
Answer: a
Clarification: 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
Answer: a
Clarification: 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
Answer: a
Clarification: 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
Answer: a
Clarification: 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
Answer: a
Clarification: 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