The first digit of a NACA four-digit aerofoil designation gives
Answer: A
The first digit is the maximum camber in per cent of chord, the second its position in tenths, and the last two the thickness in per cent. A NACA 2412 therefore carries two per cent camber at four tenths chord and is twelve per cent thick.
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NACA six-series aerofoils were developed principally to
Answer: B
Moving the point of maximum thickness aft holds a favourable pressure gradient over much of the chord, delaying transition. The low-drag benefit is confined to a narrow band of lift coefficient and is fragile in service.
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A supercritical aerofoil is selected for a transonic transport because it
Answer: C
The flattened upper surface keeps the local Mach number modest over a long run, so the terminating shock is weak. Aft camber recovers the lift the flattened crest gives away, at the cost of a more nose-down pitching moment.
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The design lift coefficient of an aerofoil is the value at which
Answer: D
Choosing a section whose design lift coefficient matches the cruise value keeps the aircraft in the drag bucket where it spends most of its life. A mismatch means cruising on the shoulder of the polar and paying for it every hour.
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Increasing an aerofoil's thickness ratio generally
Answer: A
A thicker section accelerates the flow more strongly over it, so sonic conditions arrive at a lower free-stream Mach number. The compensation is more room for spar depth and fuel, and usually a higher maximum lift coefficient.
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Aspect ratio is chosen as a compromise between
Answer: B
Correct answer: Induced drag against structural weight
Induced drag falls with aspect ratio,
\[ C_{Di} = \frac{C_L^2}{\pi A e} \]so a slender wing is aerodynamically better. But a slender wing has a longer bending moment arm and a smaller structural depth for its span, so it is heavier for the same strength, and it may also exceed the airport gate box. Aspect ratio is therefore settled where the drag saving and the weight penalty balance, typically between 8 and 11 on a transport.
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Taper is applied to a wing principally to
Answer: C
A straight-tapered planform with a taper ratio around a third approximates elliptical loading closely enough for most purposes, without the compound curvature an elliptical planform would demand. It also moves the loading inboard and relieves root bending.
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Washout, a nose-down twist towards the tip, is applied so that
Answer: D
Reducing the tip incidence keeps those sections further from their stalling angle, so separation starts inboard and spreads outward. Tapered wings need it particularly, since taper alone loads the tip more heavily relative to its chord.
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Dihedral is incorporated in a wing chiefly to
Answer: A
In a sideslip the lower wing meets the flow at a greater effective incidence and lifts more, rolling the aircraft back towards level. Sweep and a high wing position both contribute the same effect, so all three are traded together.
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A high-wing aircraft usually needs less geometric dihedral than a low-wing one because
Answer: B
Flow around the fuselage in a sideslip adds an effective dihedral for a high wing and subtracts it for a low one. Many high-wing transports therefore use little or even anhedral, while low-wing aircraft carry several degrees.
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