Aerodynamics - Subsonic Compressible Flow over Airfoils Quiz(MCQ)

1 .

For which of thee flows is Prandtl – Glauert compressibility correction is effective?

A)

Sonic flow

B)

Supersonic flow

C)

Subsonic flow

D)

Hypersonic flow

Correct Answer : Subsonic flow

2 .

Prandtl – Glauert compressibility correction is based on which of the following methods?

A)

Energy equation

B)

Continuity equation

C)

Momentum equation

D)

Velocity potential equation

Correct Answer : Velocity potential equation

3 .

For which of these conditions is Prandtl – Glauert compressibility equation valid?

A)

Thin airfoil, small angle of attack

B)

Thin airfoil, large angle of attack

C)

Thick airfoil, large angle of attack

D)

Thick airfoil, small angle of attack

Correct Answer : Thin airfoil, small angle of attack

Explanation : The Prandtl – Glauert compressibility equation is more or less accurate for small perturbations which occur for thin airfoils kept at small angle of attack. One added condition is that this equation is inaccurate for transonic and hypersonic flows.

4 .

What is a critical Mach number?

A)

Mach number at which wake formation does not exist

B)

Mach number at which sonic flow is achieved on the surface

C)

Mach number at which the coefficient of lift is minimum

D)

Mach number at which transonic flow is achieved on the surface

Correct Answer : Mach number at which sonic flow is achieved on the surface

Explanation : When freestream air flows over an airfoil at suppose Mach number = 0.3, there’s a point on the top surface of the airfoil where the pressure is minimum and the Mach number is maximum such as 0.43. When this freestream velocity is increased, there’s a certain Mach number for which sonic flow is achieved at the top surface. This is known as the critical Mach number.

5 .

What is the reason for a sudden increase in drag beyond drag divergence?

A)

The shape of the airfoil

B)

Prandt- Glauert compressibility effect

C)

Transonic region on top surface of the airfoil

D)

Supersonic region on top and bottom region of the airfoil

Correct Answer : Supersonic region on top and bottom region of the airfoil

Explanation : When the Mach number is increased beyond the point of drag divergence, the drag decreases by a factor of 10 due to the formation of supersonic region in both top and bottom surfaces of the airfoil. This supersonic region is followed by a shock wave which leads to flow separation causing higher drag.

6 .

What is the purpose of supercritical airfoil?

A)

Reduce critical Mach number

B)

Reduce drag divergence Mach number

C)

Increase drag divergence Mach number

D)

Increase lift of the airfoil at supersonic speed

Correct Answer : Increase drag divergence Mach number

Explanation : The purpose of a supercritical airfoil to delay the drag divergence Mach number so that there is a delay in wave drag formation which begins at the transonic speed. It aims to increase the Mach number increment between the critical Mach number and drag divergence Mach number.

7 .

How is supercritical airfoil different from the NACA 64 series airfoil?

A)

Thinner airfoil

B)

Flatter top surface

C)

Less cambered

D)

Flatter lower surface

Correct Answer : Flatter top surface

Explanation : The supercritical airfoils have a flatter top surface compared to the NACA 64 series airfoil. This leads to lesser local Mach values at the top even when it is at supersonic speed. In turn, there is a weaker terminating shock leading to less drag.

8 .

Why does the trailing edge of a supercritical airfoil have a cusp-like shape?

A)

Reduce drag

B)

Increase fuel carrying capacity in the wing

C)

Counter highly positive camber at the front end

D)

Counter highly negative camber at the front end

Correct Answer : Counter highly negative camber at the front end

Explanation : Due to the presence of flatter top surface in supercritical airfoil, the 60% of the front end of airfoil is highly cambered resulting in less lift. To counter the reduced lift, positive camber is provided at the bottom aft section of the airfoil by giving it a cusp-like shape. This increases the lift.

9 .

Why do cargo aircrafts make use of supercritical airfoil?

A)

Better takeoff and landing performance

B)

No vortex formation

C)

Thinner airfoil resulting in less wing weight

D)

More fuel carrying capacity in thicker airfoil

Correct Answer : Better takeoff and landing performance

Explanation : Supercritical airfoils work efficiently even in the transonic regime. It’s unique geometry of large leading edge, flatter top surface and cusp shape at the bottom aft section all result in high lift. This leads to very good take-off and landing performance as during this flight regime, higher lift is required. For this reason, cargo flights make use of supercritical wings.

10 .

What is the value of C_p at the forward surface in a biconvex airfoil?

A)

Zero

B)

Positive

C)

Negative

D)

Infinite

Correct Answer : Positive

Explaination : When there is freestream flow towards a biconvex supersonic thin airfoil, using linearlized theory, the sign conventions states positive value of C_p for the forward surface. The surface is inclined into the free stream velocity.

11 .

What is the lift coefficient over a flat plate at an angle of attack of 3 degrees in a supersonic flow at Mach 4?

A)

0.0512

B)

0.0541

C)

0.0628

D)

0.0714

Correct Answer : 0.0541

Explanation :

12 .

What is the source of wave drag exerted on the supersonic airfoil?

A)

Skin friction

B)

Trailing vortex

C)

Shock waves

D)

Viscous boundary layer

Correct Answer : Shock waves

Explanation : Wave drag is a result of pressure distribution which is exerted over the airfoil. This is a consequence of formation of shock waves and expansion wave pattern due to the presence of supersonic flow.

13 .

What happens to the coefficient of pressure in a supersonic flow for an increase in Mach number?

A)

Increases

B)

Decreases

C)

Remains unaffected

D)

First increases then decreases

Correct Answer : Decreases

Explanation : For linearized supersonic flow, the coefficient of pressure is inversely proportional to

Thus, when the Mach number increases, the coefficient of pressure decreases. This in contrast to the trend followed by the subsonic flow where an increasing Mach number results in decreasing coefficient of pressure.