CH7: Selecting the airfoils and planform

Question 1

Regarding low and high magnitudes, what are the design measures of merit for a wing planform?

Answer 1

High
>CL_alpha
>CL_max
>Wing fuel volume
Low
>CD0
>K’
>Wing Weight

Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 172

Question 2

For subsonic speeds, how does the thickness ratio affect the max lift coefficient? CD0?

Answer 2

At subsonic speeds, increasing t/c…
> Increases CL_max
>Slight increase in CD0

Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 173

Question 3

True or False
By having the wing and fuselage drag rises peak at different Mach numbers, the thrust required to accelerate past M = 1 is increased.

Answer 3

False. The thrust requirement is decreased in this scenario.

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Question 4

How does the thickness ratio change the critical Mach number value? Explain.

Answer 4

As t/c increases, the critical Mach number decreases as supersonic flow occurs earlier on the upper surface, leading to the presence of a normal shock and flow separation.

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Question 5

In supersonic flight, the wave drag increases approximately as the __________ of t/c.

Answer 5

square

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Question 6

If an aircraft is being designed for flight at prolonged supersonic speeds, the thickness ratio (t/c) should be __________ to avoid large wave drag values.

Answer 6

small (on the order of 4% to 6%)

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Question 7

Explain how the location of maximum thickness is related to pressure gradients.

Answer 7

The location of the maximum t/c determines the end of the favorable pressure gradient (decreasing pressure), and the start of the adverse pressure gradient.

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Question 8

In the ideal case, a laminar boundary layer will transition to turbulent when encountering an ________ pressure gradient.

Answer 8

Adverse

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Question 9

Explain why placing the maximum thickness value further away from the leading edge is advantageous from a skin friction drag viewpoint.

Answer 9

By placing the maximum thickness value further back from the leading edge, the boundary layer will remain laminar over a large portion of the wing. This reduces the turbulent portion of the boundary layer and causes a reduction in the total skin friction.

Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 174

Question 10

If the maximum t/c is forward of the airfoil aerodynamic center, the airfoil is termed “_______” and will produce a nose-________ pitching moment.

Answer 10

front-loaded section
nose-up

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Question 11

If the maximum t/c is aft of the airfoil aerodynamic center, the airfoil is termed “_______” and produces a nose-________ pitching moment.

Answer 11

aft-loaded
nose-down

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Question 12

True or False
For a round-nosed, subsonic airfoil, an increase in the leading edge radius will generally result in a large CL_max.

Answer 12

True

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Question 13

True or False
The subsonic CD0 is primarily composed of skin friction and is generally not influenced by the nose shape of the airfoil section.

Answer 13

True

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Question 14

Define “camber.”

Answer 14

The amount (in percent cord) that a line equidistant from the upper and lower surface varies from the chord line.

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Question 15

Positive camber will shift the CL_alpha curve to the ________.

Answer 15

left.

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Question 16

True or False
Chamber at the leading edge has a large effect on changing the lift.

Answer 16

False. It has almost no effect.

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Question 17

What is the primary purpose of leading edge (LE) camber (i.e., deflecting a leading edge flap)?

Answer 17

To delay flow separation of the forward part of the airfoil, producing higher maximum lift coefficients.

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Question 18

Airfoils with positive camber have _______ values of CM_ac, whereas a symmetric section has a CM_ac = ________.

Answer 18

negative
zero

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Question 19

A positive chamber will ______ CL_max.

Answer 19

Increase

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Question 20

In terms of trim, what is a major downside to increasing the chamber?

Answer 20

A positive increase in the chamber will be accompanied by a negative CM_ac. This moment must be trimmed by a down-aft tail load for static stability.

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Question 21

Define the drag-due-to-lift factor K’.

Answer 21

K’ = 1/(piARe)

Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 180

Question 22

True or False
For low-speed (subsonic) flight, AR has little effect on CD0 as the drag is primarily skin friction and independent of platform shape.

Answer 22

True

Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 181

Question 23

True or False
In supersonic flight, the CD0 will decrease with increasing AR.

Answer 23

False. CD0 increases with increasing AR for supersonic flight. This is the reason why supersonic aircraft usually have AR<5.

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Question 24

From a structures standpoint, what is the major problem with a forward-swept wing?

Answer 24

Aeroelastic divergnce.

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Question 25

What are the three primary advantages of a forward-swept wing?

Answer 25

1. Improved area rule distribution
2. Longer lever arm between the wing and tail mean aerodynamic chord (MAC)
3. Reduced tip stall tendencies

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Question 26

The peak wing CD0 occurs in the _____ regime.

Answer 26

Transonic

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Question 27

A disadvantage of wing sweep is the corresponding __________ in the lift curve slope.

Answer 27

decrease

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Question 28

What wing design parameter can be used to control tip stall attributes?

Answer 28

Wing twist

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Question 29

For a high aspect ratio, aft-swept wing configuration, if the wing stalls, the aircraft will _________, further deepening the stall and leading to loss of control.

Answer 29

pitch up

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Question 30

What tapper ratio closely approximates an elliptic lift distribution?

Answer 30

CT/CR = 0.35

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Question 31

What is the main disadvantages to a variable geometry wing platform?

Answer 31

1. Weight and complexity
2. The large shift in the aerodynamic center as the wings are swept back.

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