# Aircraft Design Questions and Answers – Aerodynamics – Parasite (Zero-Lift) Drag and Drag due to Lift

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This set of Aircraft Design Question Bank focuses on “Aerodynamics – Parasite (Zero-Lift) Drag and Drag due to Lift”.

1. Which of the following is a method for estimating the potato drag?
a) Equivalent skin friction method
b) Mach number
c) Weight only
d) Conceptual design

Explanation: Equivalent skin friction method is one of the typical method used to estimate parasite drag. Mach number is defined as the ratio of the speed of object to the speed of sound. Weight is force due to gravity.

2. Equivalent skin friction coefficient can be used to provide initial estimation of the parasite drag.
a) True
b) False

Explanation: Yes, it is true. Equivalent skin friction coefficient can be used to provide information about the initial estimation of parasite drag. In this method, we estimate the parasite drag based on equivalent skin friction coefficient and wetted area of the aircraft.

3. Equivalent skin friction coefficient may include ______________
a) skin friction and separation drag
b) only wave drag
c) only skin friction drag

Explanation: Equivalent skin friction coefficient includes both skin friction and separation drag. Wave drag is supersonic phenomenon. Wave drag occurs when aircraft is traveling with speeds greater than the speed of sound. Thrust loading is defined as the ratio of the thrust and weight.

4. To get initial estimation of the parasite drag, aircraft wetted area can be multiplied by _____
a) equivalent skin friction coefficient
b) weight
c) lift
d) wing span

Explanation: Initial estimation of parasite drag can be obtained by multiplying equivalent skin friction coefficient with wetted area of aircraft. Lift and weight will be in opposite direction to each other. Wing span is a typically length of the aircraft.

5. Determine the initial estimation of parasite drag coefficient if, equivalent skin friction coefficient is 0.004 and ratio of wetted area to the reference area is 0.8.
a) 0.0032
b) 1.2
c) 4.2
d) 0.04

Explanation: Parasite drag coefficient = equivalent skin friction coefficient*area ratio
= 0.004*0.8 = 0.0032.

6. Consider equivalent skin friction coefficient as 0.0025 and Swet/Sref = 0.6. Evaluate parasite drag.
a) 0.0015
b) 02.34
c) 1.23
d) 0.000025

Explanation: Parasite drag = equivalent skin friction coefficient*(Swet/Sref)
= 0.0025*0.6 = 0.0015.

7. Find the approximate value of wetted area Swet for given reference area of Sref=20 unit and parasite drag coefficient of 0.0028. Consider equivalent skin friction coefficient Cf as 0.003.
a) 18.67 unit
b) 12 unit
c) 20 unit
d) 65 unit

Explanation: Wetted area Swet = Parasite drag coefficient*Sref/Cf
= 0.0028*20/0.003 = 18.67 unit.

8. Determine the value of form factor for fuselage if fuselage fineness ratio f is 1.2.
a) 35.725
b) 12
c) 100
d) 423.2

Explanation: Form factor = 1 + (60/f3) + (f/400)
= 1 + (60/1.23) + (1.2/400)
= 1+34.722+0.003 = 35.725.

9. Flat plate skin friction coefficient depends upon _____________
a) mach number only
b) reynolds number only
c) mach number, reynolds number etc
d) weight of the aircraft

Explanation: Flat plate skin friction coefficient depends upon number of factors including Mach number, Reynolds number, type of flow etc. Mach number is used to provide information about speed of the object with respect to the speed of sound.

10. Determine Flat plate skin friction coefficient if Reynolds number is 123500. Consider flow to be laminar.
a) 0.0038
b) 1.234
c) 0.045
d) 123500

Explanation: Laminar flow flat plate skin friction coefficient = 1.328 / (Reynolds number0.5)
= 1.328 / (1235000.5) = 0.0038.

11. Which of the following is correct for nacelle form factor (FF)?
a) FF = 1+(0.35/f)
b) FF = 1*(0.35/f)
c) FF = 1-(0.35/f)
d) FF = 1/(0.35/f) 