# Aerodynamics Questions and Answers – Downwash and Induced Drag – 2

This set of Aerodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Downwash and Induced Drag – 2”.

1. For a finite wing, the local airfoil section sees the angle of attack which is called as ____
a) Downwash angle
b) Induced angle of attack
c) Relative angle of attack
d) Effective angle of attack

Explanation: The actual angle of attack seen by the local airfoil section is the effective angle of attack. This is the angle between the chord line and the local relative wind. Or in other words,
Effective angle of attack = Geometric angle of attack – induced angle of attack.

2. Induced drag is generated by a component of lift for a finite wing.
a) True
b) False

Explanation: The local lift vector is not aligned perpendicular to the free-stream velocity vector for a wing. Lift is perpendicular to local relative wind. This causes a component of local lift in the direction of drag, called induced drag.

3. Which is the correct statement with regards to the flow over a finite wing?
a) Local lift is vertical
b) Downward velocity increases lift
c) Local lift is inclined to vertical by induced angle of attack
d) Free-stream velocity causes all the drag

Explanation: Wings have the local lift aligned perpendicular to the local relative wind direction. Thus, it is aligned to the vertical by an angle (called induced angle of attack). The downwash causes this alignment and a component of lift generates what we call induced drag. Thus, lift is deceased.

4. Select the incorrect statement for a flow over wings.
a) Induced drag comes from pressure- drag
b) Pressure imbalance makes D’Alembert’s paradox does not occur
c) 3D flow over wings cause induced drag
d) Induced drag is a ‘type’ of pressure- drag

Explanation: For a wing, 3D flow causes a pressure imbalance in the direction of free – stream velocity, which is the drag. Thus, induced drag is a type of pressure-drag. But it is not caused by pressure drag.

5. For an inviscid, incompressible flow over finite wings, D’Alembert’s paradox occurs.
a) True
b) False

Explanation: The induced drag is caused by a component of lift which is not vertical. Therefore, even in the absence of skin-friction or flow separation, drag is not zero. For a wing, D’Alembert’s paradox does not occur.
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6. The trailing vortex increases the requirement of power by the aircraft engine.
a) False
b) True

Explanation: There is higher power required by the aircraft engine to overcome the induced drag, thus leading to wastage of power. This power goes waste as it provides energy to the vortices, serving no useful purpose.

7. Identify the source which does not cause drag on a finite wing.
a) Skin – friction
b) Flow separation
c) Trailing vortices
d) Power provide by aircraft engine

Explanation: The flow over a wing has drag which comes from viscosity (skin – friction drag and pressure drag) plus drag induced by the trailing vortices. The power by the aircraft engine is used to overcome the drag and not cause it.

8. The correct formula for profile drag coefficient over the finite wing is_______
a) cd=$$\frac {D_f+D_P}{Sq_∞}$$
b) cd=$$\frac {D_P}{Sq_∞}$$
c) cd=$$\frac {D_f}{Sq_∞}$$
d) cd=$$\frac {D_f+D_P+D_i}{Sq_∞}$$

Explanation: The profile drag for a wing is the drag caused by viscosity. For moderate angles of attacks, it is same as that for airfoils i.e. a combination of skin- friction drag and pressure drag. The correct formula is cd=$$\frac {D_f+D_P}{Sq_∞}$$.

9. The total drag coefficient for a wing is given by______
a) cd=$$\frac {D_f+D_P+D_i}{Sq_∞}$$
b) Cd=$$\frac {D_f+D_P}{Sq_∞}$$
c) Cd=$$\frac {D_f+D_P+D_i}{Sq_∞}$$
d) Cd=$$\frac {D_f+D_P}{Sq_∞}$$+Di

Explanation: For wing, the total drag comprises of viscous drag plus the induced drag. The coefficient of total drag is given by Cd=$$\frac {D_f+D_P+D_i}{Sq_∞}$$, which is nothing but Cd=$$\frac {D_f+D_P}{Sq_∞}+\frac {D_i}{Sq_∞}$$ i.e. Cd=cd+Cdi.

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