A wake survey determines a body's drag by measuring
Answer: C
Integrating the momentum lost across a plane behind the body gives the drag directly, by momentum conservation, without needing to instrument the surface at all. It is particularly useful for sections in a wind tunnel and is the standard way section drag data are obtained.
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The single most important practical reason boundary-layer theory matters to a designer is that it
Answer: D
Separation governs pressure drag, maximum lift, stall behaviour and buffet, and none of it can be obtained from an inviscid calculation. Everything else in this topic -- the thicknesses, the shape factor, transition, control devices -- exists to answer that one question.
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Interference drag arises where
Answer: A
At a junction such as a wing root or a strut fitting, the two boundary layers merge in a corner where the pressure is already recovering, and separation follows readily. Fillets exist to ease that corner, and the drag they save can be a surprisingly large share of the total.
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A boundary layer developing in a corner between two surfaces is particularly prone to separate because
Answer: B
Corner flow accumulates low-momentum fluid from both surfaces at exactly the place where an adverse gradient is usually strongest. The result is an early, three-dimensional separation, which is why wing-body and tail junctions are always filleted and are checked carefully in the tunnel.
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A body has a drag coefficient of \( 1.2 \) referred to a frontal area of \( 0.5 \) m\( ^2 \). At \( 20 \) m/s its drag is nearest to
Answer: C
The dynamic pressure is \( 0.5 \times 1.225 \times 400 = 245 \) Pa, so the drag is \( 1.2 \times 245 \times 0.5 = 147 \) N. A coefficient of 1.2 on frontal area is characteristic of a bluff body such as a cylinder below the drag crisis.
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Assertion (A): Fillets are fitted at wing-body and tail junctions on almost every aircraft.
Reason (R): Corner flows accumulate low-momentum fluid from two surfaces where the pressure is already rising.
Select the correct answer from the options below.
Answer: D
Both are true and the second explains the first. Without a fillet the corner separates early and produces a disproportionate amount of interference drag, and often buffet as well. Easing the corner geometry is a cheap and standard cure.
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Consider the following statements about wake surveys:
1. They determine drag from the momentum deficit behind the body.
2. They require no instrumentation on the body surface.
3. They can distinguish skin friction from pressure drag directly.
Which of the statements given above is or are correct?
Answer: A
The first two are correct and are why the technique is so convenient for section testing. The third is false: the wake carries the total momentum deficit and cannot by itself say how much came from shear at the surface and how much from the pressure distribution.
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Which of the following is NOT a reason a fillet reduces interference drag at a wing root?
Answer: B
A fillet does add a little wetted area, but that is a small penalty rather than a reason it helps. Its benefit comes entirely from easing the corner geometry so the merged boundary layers can negotiate the pressure rise without separating.
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Which statement about drag on a streamlined body is TRUE?
Answer: C
By definition a streamlined body keeps the flow attached, so the wake is narrow and the pressure drag small, leaving skin friction as the principal term. This is precisely the reverse of the bluff-body case and is why reducing wetted area matters so much for a streamlined shape.
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The reason d'Alembert's paradox does not describe real flows is that
Answer: D
The paradox follows correctly from inviscid, attached, steady flow with no wake, and its resolution is precisely that real fluids form boundary layers which leave a wake behind. Prandtl's 1904 hypothesis is what reconciled the mathematics with the observed drag.
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