# Aircraft Design Questions and Answers – Airfoil Selection-1

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This set of Aircraft Design Multiple Choice Questions & Answers (MCQs) focuses on “Airfoil Selection-1”.

1. The parameter marked by ’?’ in the diagram is _____ a) trailing edge
c) chord
d) camber

Explanation: The diagram is showing typical airfoil shape. The trailing edge of an airfoil is end point of the airfoil. It is most after portion of the airfoil as shown in the diagram. Leading edge in contrast, is foremost part of an airfoil.

2. The cross-sectional shape of wing is called _____
a) airfoil
b) circle
c) camber
d) chord

Explanation: Wings are designed with airfoil cross-section. Engine inlets are typically circular in cross-sections. Camber and chord are parameters of an airfoil.

3. Aerofoil affects the performance parameters of aircraft.
a) True
b) False

Explanation: Airfoil is cross-section of the wing. Airfoil will affect lift, drag, take-off characteristics etc. In general, crucial performance parameters are affected by airfoil selection.

4. Which of the following is correct?
a) Lift produced by an airfoil is always equal to weight
b) Drag by sphere is more than the drag produced by an airfoil for similar conditions
c) Drag by sphere is less than that of produced by an airfoil for similar conditions
d) Drag by sphere is equal to the drag produced by an airfoil for similar conditions

Explanation: Lift produced is not always same as weight. Sphere is a 3D object. Drag produced by sphere will be more due to increased friction and 3D effects. An airfoil is 2D shape which will have less drag as compare to sphere for all similar conditions.

5. An aerofoil is operating with flow velocity of 100m/s. Aerofoil is symmetric and is at 0° AOA. Determine lift produced by this aerofoil.
a) 10.5 N
b) 0 N
c) 50 N
d) 100N

Explanation: Lift is an upward force produced by pressure difference.
Given, velocity V=100m/s, AOA = 0°.
For symmetric airfoil at 0° AOA pressure will be same as upper and lower surface.
Hence, lift = 0N.

6. Which of the following is not type of an airfoil?
a) Symmetric
b) Camber
c) Whitcomb
d) Vortex

Explanation: Symmetric airfoils have symmetric curvature at both side. Camber are non-symmetric airfoils. Whitcomb is also type of airfoil. Vortex is a phenomena, not the type of an airfoil.

7. Airfoil is a streamline shape.
a) True
b) False

Explanation: Streamline shape is that shape which gives streamlined flow. An airfoil has that characteristic. An airfoil is able to provide much smoother flow than non-streamlined body.

8. A symmetric airfoil is operating with flow velocity of 350m/s. The lift produced by the airfoil is 21 N at 0.008 rad AOA. If chord is c m then, what will be the pressure difference across the airfoil?
a) 21/c Pa
b) 21c/2 N
c) 21 Pa
d) 21c N

Explanation: Given, symmetric airfoil
Flow velocity V=350m/s, Lift L=21N, AOA = 0.008rad
Chord = c m, Span of airfoil = 1 unit
Pressure difference = Lift/Area of airfoil
= 21/c*1 = 21/c Pa.

9. If an airfoil has lift of 200 N at 10° AOA, then what is induced drag produced by airfoil at that AOA?
a) 0 N
b) 100
c) 0
d) 100 N

Explanation: Given, Lift=200N, AOA = 10°
Here, airfoil is mentioned which is 2D shape.
For an airfoil Induced drag Di = 0N.

10. Find location of aerodynamic centre from leading edge if, chord is 10m and airfoil has flow velocity of 0.2 M.
a) 2.5 m
b) 0.2 m
c) 5 m
d) 10 m

Explanation: Given, velocity = 0.2M = 0.2*340 = 68m/s
Chord = 10m
Given velocity is less than the sonic speed.
Hence, location of aerodynamic center is = 25% of chord = 0.25*10 = 2.5m from leading edge.

11. Consider a NACA 2812 airfoil. It is operating with flow velocity of 900m/s with chord of 1m. What should be the maximum thickness of this airfoil?
a) 0.12m
b) 0.21m
c) 0.9m
d) 28.12m

Explanation: NACA-4 digit series is used to define certain airfoil characteristics.
Here, velocity = 900 m/s, chord = 1m
From NACA-4 digit,
Maximum thickness = last two digits in percent of chord=12% of chord = 0.12*1 = 0.12m.

12. An airfoil is subjected to the flow with temperature of 288.16K and flow is ideal. The velocity at the leading edge is ____
a) 0m/s
b) 250m/s
c) 288.16m/s
d) 16.882m/s

Explanation: Given, temperature T=288.16K
For idle flow, at leading edge flow is brought to rest isentropically.
Hence, velocity at leading edge = velocity of flow at the impact to leading edge = 0m/s.

13. Pitching moment co-efficient for an airfoil is given by _____
a) lift*area
b) drag*area
c) section pitching moment/dynamic pitching moment
d) lift/dynamic moment

Explanation: Lift*area will give moment. Drag*area will also provide moment but will be less. The ratio of sectional pitching moment and dynamic pitching moment is called pitching moment co-efficient.

14. Which of these is not part of an airfoil?
b) Chord
c) Camber
d) Fins

Explanation: Fins are extended surfaces which are used to improve heat transfer. Leading edge is foremost part of an airfoil. Camber is curve of an airfoil.

15. We are using the airfoil in the aircraft as _____
a) it is a streamline shape which provides better aerodynamics
b) it is a random shape can be changed to rectangular as well
c) it is easy to make than rectangular
d) it is not a streamline body 