# Aerodynamics Questions and Answers – Downwash and Induced Drag – 1

This set of Aerodynamics Multiple Choice Questions & Answers (MCQs) focuses on “Downwash and Induced Drag – 1”.

1. The airfoil data can also be called as_____
a) Finite wing data
b) Wing data
c) Flight data
d) Infinite wing data

Explanation: An airfoil can be thought of as a wing with infinite wing span. Indeed, the airfoil data is generally called as infinite wing data.

2. The airfoil and the wing behave exactly the same way in terms of aerodynamic properties.
a) True
b) False

Explanation: The airfoil can be thought of as an infinite wing. It is indeed a section of a wing. While on the other hand, wing means a finite wing, until specified otherwise. Clearly, the properties are different for a wing and an airfoil.

3. Which is the incorrect statement with regards to the flow over an airfoil and a wing?
a) Flow over airfoil is 2D
b) Flow over wing is 3D
c) Airfoils have a component of flow along span
d) Wings have a component of flow along span

Explanation: Wings (which by default means finite wings) have a component of flow along span, which gives them 3D characteristics. While airfoils, which are infinite wings, have a 2D flow as the span wise component is absent.

4. The 3D flow characteristic attributed to the wings is a by-product of_____
a) Pressure imbalance over top and bottom surfaces of the wing
b) Pressure imbalance between leading and trailing edge of the airfoil
c) Thickness of the wing
d) Extra lift produced

Explanation: Lift over an airfoil is produced by the pressure difference over top and bottom surfaces of airfoil. But for the finite wings, this creates a by-product in the form of span-wise flow, which gives a 3D flow. Thickness of the wing and extra lift produced are vague options.

5. The span-wise component of flow over the top surface of the wing flows______
a) From wing-tip to root
b) From root to wing-tip
c) Along the free-stream direction
d) Perpendicular to the wing upwards

Explanation: The pressure imbalance is such that there is a low pressure region on top and high pressure region at the bottom. This creates a flow from the bottom surface towards the top surface of the wing. Naturally, the flow moves from the wing-tips towards the wing root.
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6. Which of the following is incorrect for a finite wing?
a) Streamlines on the top surface bend towards the root
b) Span-wise flow at the bottom surface is towards the tip
c) Higher pressure region exists at the top surface
d) Streamlines on the bottom surface bend towards the tip

Explanation: There is higher pressure region at the bottom and lower pressure at the top. Due to which the flow moves from the wing root at the bottom towards the wing-tips, turns upwards and moves towards the wing root at the top surface. Thus, streamlines on the bottom surface bend towards the tip and vice versa happens at the top surface.

7. The trailing vortex is formed at each wing-tip as there is a circulatory motion established by the____
a) Leaking flow at the wing- tips
b) Flowing wind
c) Viscosity

Explanation: The flow has a tendency to ‘leak’ around the wing-tips. This flow makes a circulatory motion that trails downstream of the finite weak, giving rise to trailing vortices. This flow is possible because of viscosity but it doesn’t happen because of viscosity.

8. The downward velocity induced by the trailing vortex in the vicinity of the wing-tips is called as _____
a) Downwash
b) Local relative velocity
c) Effective velocity
d) Wing velocity

Explanation: The trailing vortices, in the vicinity of the wing-tips, drag along with them the surrounding air. This induces a small velocity component in the downward direction. This is called downwash. None of the other terms describe this induced velocity.

9. The angle between the chord line and the direction of free-stream velocity is called the angle of attack.
a) True for airfoil only
b) True for wing only
c) True for airfoil and partially true for wings
d) False

Explanation: For an airfoil, the angle between the chord line and the direction of free-stream velocity is called the angle of attack. For the wings, the precise name is geometric angle of attack. Hence the given statement is only partially true for a wing.

10. For a finite wing, the local relative wind differs by the free-stream velocity by some angle. This angle is called as ____
a) Angle of attack
b) Twist angle
c) Relative angle of attack
d) Induced angle of attack

Explanation: For a wing, induced velocity in the downward direction is present apart from the free-stream velocity. The resultant of both (relative local wind) is inclined to the free-stream velocity by an angle, which is called the induced angle of attack.

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