# Aircraft Design Questions and Answers – Flight Mechanics – Operating Envelope

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This set of Aircraft Design Multiple Choice Questions & Answers (MCQs) focuses on “Flight Mechanics – Operating Envelope”.

1. Falling diagram represents _________ a) operating envelope
b) lift curve
c) tire sizing
d) empty weight diagram

Explanation: Operating envelope is shown in the above diagram. Above diagram is typically used for fighter aircraft. Fighter operating envelope is one of the complex type of diagram. Lift curve is used to provide relationship between lift and angle of attack.

2. The level flight envelope is determined by ________
a) stall limit lines and from zero specific power
b) wing aileron size
c) spoilers area
d) tail area

Explanation: The level flight operating envelope is determined by using stall limit lines and zero specific power. The zero specific power lit is typically shown for both maximum Thrust and for military thrust. Tail area is based on tail sizing.

3. The highest altitude where Ps is zero is called ________
a) absolute ceiling
b) drag polar
c) geometric height
d) geometric altitude

Explanation: Absolute ceiling is defined as the highest altitude at where Ps is zero. Geometric height or geometric altitude is based on height measured. Drag polar is graphical representation of drag properties.

4. Which of the following is incorrect?
a) Operating envelope will give same values for all the aircraft
b) Load factor at cruise is unity
c) Lift is same as weight during cruise

Explanation: Load factor is defined as ratio of lift to weight. At cruise lift and weight both are equal. Hence, at cruise load factor is unity. Wing loading and Thrust are two different things. Wing loading is ratio of weight by reference area.

5. Operating envelope is also known as ________
a) flight envelope
b) fly by wire
c) stall speed diagram
d) lift curve

Explanation: Operating envelope is also known as flight envelope. Fly by wire is a typical system used for converting pilot’s input to signal. Lift curve is used to illustrate the lift variation with respect to the angle of attack.

6. If static pressure of flow is 1200psf and operating Mach number is 1.19 then, determine the external flow dynamic pressure.
a) 1189.52 psf
b) 12.89 psf
c) 112 psf
d) 2587 psf

Explanation: Dynamic pressure = 0.7*static pressure*Mach number2
= 0.7*1200*1.192 = 1189.52 psf.

7. Determine static pressure if external flow dynamic pressure is 1800psf and flow has Mach number of 2.
a) 642.857psf
b) 1000psf
c) 2400 psf
d) 4673 psf

Explanation: Static pressure = dynamic pressure / (0.7*Mach number2)
= 1800 / (0.7*22) = 642.857 psf.

8. Consider external flow dynamic pressure as 2000psf and static pressure as 800psf. find the approximate value of Mach number.
a) 1.9
b) 5
c) 4.5
d) 2.78

Explanation: Mach number = (dynamic pressure / (0.7*Static pressure))0.5
= (2000 / (0.7*800))0.5
= 3.5710.5 = 1.88 = 1.9.

9. Determine total pressure if static pressure is 1600psf and Mach number is 1.8. Consider flow of air.
a) 9193.2 psf
b) 1000 psf
c) 3450 psf
d) 7892 psf

Explanation: Total pressure = static pressure*[1+0.2*Mach number*Mach number]3.5
= 1600[1+0.2*1.8*1.8]3.5
= 1600*1.6483.5 = 9193.2 psf.

10. If flow has Mach number of 1.2 then, determine the ratio of total and external flow dynamic pressure.
a) 2.4
b) 1
c) 3.8
d) 5.6

Explanation: Given, Mach number M = 1.2
Ratio of total pressure and external flow dynamic pressure = (1.428/M2)*[1+0.2*M2]3.5
= (1.428/1.22)*[1+0.2*1.22]3.5
= (1.428/1.22)*[1+0.288]3.5
= 2.40.

Sanfoundry Global Education & Learning Series – Aircraft Design.

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