This set of Aircraft Design Interview Questions and Answers for Experienced people focuses on “Configuration Layout – Wing/Tail Layout and Loft-2”.
1. As a designer, our task is to design wing layout such that the location of MAC from root chord or centre line is at 8ft. Find the appropriate value of the wingspan if wing is rectangular.
Explanation: Given, rectangular wing.
MAC location y = 8ft. As wing is rectangular taper ratio t =1.
Wingspan b = (6*y*(1+ t)) / (1+t+t2) = 6*8*(1+1) / (1+1+12) = 6*8*2/3 = 32ft.
2. A fighter jet is flying with wing span of 90ft. If mean aerodynamic chord MAC is at 30ft from tip chord then, to design such wing which type of planform should I use?
Explanation: Given, wing span b=90ft, location of MAC from tip = 30ft
Location of MAC from root y = b/2 – 30ft = 90/2 – 30 = 45-30 = 15ft.
To find planform let’s find taper ratio first from y.
Location of MAC is given as, y = (b/6)*(1+t+t2) / (1+t). …. (1)
From given options, let’s consider delta wing first.
Delta wing has taper ratio near to none. Let’s consider taper ratio to be zero for idle case.
Hence, by substituting value of taper ratio in eq (1),
Y = (90/6)*1+0+0/1+0 = 90/6 = 15ft.
This value is same as we found earlier from given data. Hence, wing planform is delta.
3. An unstable A/C configuration is a typical wing-aft tail body. IF MAC is 4m then, what would be the approximated location of the CG?
Explanation: Typical location Cg for an unstable wing = 50% of MAC = 50% of 4 = 0.5*4 = 2m.
Explanation: Wings can have different wingtip shapes. Above figure is showing a typical rounded wingtip. A smoothly rounded tip allows air to flow easily around tip. LEX stand for leading edge extension.
Explanation: Above figure is often termed as trailing edge kick. A straightened out trailing edge will increase the flap chord. It also reduces induced drag. Rounded tip will permit air to flow around tip easily.
6. As a designer, our job is to design a wing which has root chord Cr as 1m. What should be the approximate radius of fillet?
Explanation: typically fillet radius can be approximated as 10% of the root chord.
Hence, radius = 10% of Cr = 10% of 1 = 0.1*1 = 0.1m.
Explanation: Wing fillets are arcs as shown in figure. Wing fillets are used to improve smoothness of the root of the airfoil as shown. This will result in improvement of aerodynamic efficiency. A typical fillet is a circular arc.
8. Which of the following is correct?
a) We can generate flat wrap surface by using linear interpolation method
b) Flat surfaces can only be generated by using linear interpolation
c) Linear interpolation will never give flat surface
d) Linear interpolation is only used for flat wrap surfaces
Explanation: A flat wrap surface has same tangent angles for different conic curves. When linear interpolation method is used, it does not always provide constant cross-sections or c/s with same tangent angles. Hence, it may or may not produce flat wrap. We can generate flat wrap by providing some constraints as well.
9. Determine the corrections or otherwise of the following assertion [A] and reason [R]:
Assertion [A]: To generate new airfoil, we use spanwise line instead of chord line in flat wrap interpolation technique.
Reason[R]: Chordwise line will generate flat structure from curves having different values of tangent angle.
a) Both [A] and [R] are true and [R] is the correct reason for [A]
b) Both [A] and [R] are true but [R] is not the correct reason for [A]
c) Both [A] and [R] are false
d) [A] is true but [R] is false
Explanation: In order to provide flat wrapping to wing, it is required to interpolate between airfoil coordinates with same tangent angle or slope. Chordwise lines are affected by wing twist. So, it will not generate flat structure from curves with different tangent angles. Hence, a spanwise line is used for flat wrap interpolation.
10. At maximum thickness point, fillet is in ____________
a) purely vertical plane
b) purely in hp
c) plane at an angle of less than 20° always
d) plane at 45° always
Explanation: Fillet arcs will be normal to wing surface only at the maximum thickness point. At any other section it is not perpendicular to surface. Hence, at maximum thickness point, fillet is in pure vertical plane.
11. Fillet radius is typically more at rear of the aircraft.
Explanation: Fillet arcs can be of constant radius or can be of varying radius. Typically, at rear of an a/c fillet arc radius is more. This will result in reduction of the flow separation.
12. Wing planform is always elliptic.
Explanation: Wing planform is shape generate when viewed from top. Elliptic wings are more efficient in terms of lifting properties. However to manufacture it is also difficult. Hence, wing planform is not always elliptic rather it will be selected according to mission requirements.
Sanfoundry Global Education & Learning Series – Aircraft Design.
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