CH7: Selecting the airfoils and planform Flashcards
Regarding low and high magnitudes, what are the design measures of merit for a wing planform?
High
>CL_alpha
>CL_max
>Wing fuel volume
Low
>CD0
>K’
>Wing Weight
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 172
For subsonic speeds, how does the thickness ratio affect the max lift coefficient? CD0?
At subsonic speeds, increasing t/c…
> Increases CL_max
>Slight increase in CD0
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 173
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.
False. The thrust requirement is decreased in this scenario.
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 173
How does the thickness ratio change the critical Mach number value? Explain.
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.
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 174
In supersonic flight, the wave drag increases approximately as the __________ of t/c.
square
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 174
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.
small (on the order of 4% to 6%)
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 174
Explain how the location of maximum thickness is related to pressure gradients.
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.
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 174
In the ideal case, a laminar boundary layer will transition to turbulent when encountering an ________ pressure gradient.
Adverse
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 174
Explain why placing the maximum thickness value further away from the leading edge is advantageous from a skin friction drag viewpoint.
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
If the maximum t/c is forward of the airfoil aerodynamic center, the airfoil is termed “_______” and will produce a nose-________ pitching moment.
front-loaded section
nose-up
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 174
If the maximum t/c is aft of the airfoil aerodynamic center, the airfoil is termed “_______” and produces a nose-________ pitching moment.
aft-loaded
nose-down
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 174
True or False
For a round-nosed, subsonic airfoil, an increase in the leading edge radius will generally result in a large CL_max.
True
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 176
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.
True
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 176
Define “camber.”
The amount (in percent cord) that a line equidistant from the upper and lower surface varies from the chord line.
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 178
Positive camber will shift the CL_alpha curve to the ________.
left.
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 178
True or False
Chamber at the leading edge has a large effect on changing the lift.
False. It has almost no effect.
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 178
What is the primary purpose of leading edge (LE) camber (i.e., deflecting a leading edge flap)?
To delay flow separation of the forward part of the airfoil, producing higher maximum lift coefficients.
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 178
Airfoils with positive camber have _______ values of CM_ac, whereas a symmetric section has a CM_ac = ________.
negative
zero
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 178
A positive chamber will ______ CL_max.
Increase
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 178
In terms of trim, what is a major downside to increasing the chamber?
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.
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 180
Define the drag-due-to-lift factor K’.
K’ = 1/(piARe)
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 180
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.
True
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 181
True or False
In supersonic flight, the CD0 will decrease with increasing AR.
False. CD0 increases with increasing AR for supersonic flight. This is the reason why supersonic aircraft usually have AR<5.
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 181
From a structures standpoint, what is the major problem with a forward-swept wing?
Aeroelastic divergnce.
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 182
What are the three primary advantages of a forward-swept wing?
- Improved area rule distribution
- Longer lever arm between the wing and tail mean aerodynamic chord (MAC)
- Reduced tip stall tendencies
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 182
The peak wing CD0 occurs in the _____ regime.
Transonic
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 183
A disadvantage of wing sweep is the corresponding __________ in the lift curve slope.
decrease
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 184
What wing design parameter can be used to control tip stall attributes?
Wing twist
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 184
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.
pitch up
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 184
What tapper ratio closely approximates an elliptic lift distribution?
CT/CR = 0.35
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 186
What is the main disadvantages to a variable geometry wing platform?
- Weight and complexity
- The large shift in the aerodynamic center as the wings are swept back.
Fundamentals of Aircraft & Airship Design, Nicolai, Pg. 187
