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As you'd know, certain older transport aircraft in the 20th century, such as Convair 990 or Tu-134 featured very distinctive, large anti-shock bodies situated on the trailing edge of the main wing so that the cross-sectional area transition/distribution of the aircraft would be more subtle and thereby reducing wave drag on the high-subsonic/transonic flight regimes.

What got me wondering though, is when designing a flap track fairing on an aircraft that doesn't feature any dedicated anti-shock bodies, how much of it is the result of consideration regarding the area rule/wave drag, as opposed to considering other drags like profile drag?

I know that the flap track fairings as found on most airliners are not designed to act as an anti-shock body, but thought that it would be valid to assume that it will at least have some effects in terms of the area rule and if so, there might be some consideration put into the design regarding it.

It seems like the more recent design trends like in the case of A380, A350XWB and B777X are such that they are trying to make the flap track fairings as small, slim and streamlined as possible and thereby minimize their aerodynamic effects and instead feature other aerostructures, such as an extended wingbox fairing that trails longer behind the actual wingbox to compensate for decreasing cross-sectional area. Are these observations correct?

If it is correct, how does the question in the title apply for older airliners/transports which didn't feature such designs like an extended wingbox fairing? Would it be safe to assume that flap track fairings had a bigger role in achieving smoother transition of cross-sectional area distribution for those older aircraft?

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To which extent were/are area rule considered when designing the flap-track fairing of transport aircraft?

Fully, at least from the late Fifties on. While first-generation jets like the Caravelle, Tu-104, DC-8 or the Boeing 707 tried to hide their flap tracks as much as possible, with the VC-10 they became common and indeed started life as anti-shock bodies. The Tupolev designs used their main gear bay as a Küchemann body from the Tu-104 on but added flap track fairings only with the Tu-154.

I know that the flap track fairings as found on most airliners are not designed to act as an anti-shock body

You must be mistaken. They started life as anti-shock / Küchemann bodies and were found to be useful as flap track fairings, too.

One picture should say more than 1000 words. The one below (source) is from VC-10 development and shows the lower side of the wing in a windtunnel experiment without (a) and with flap track fairings (b) at Mach 0.9 with oil added to show the streamlines.

Oil picture comparison at Mach 0.9

The bright line in (a) is caused by massive separation. Adding the fairings could avoid this separation. Since the added area of the bodies adds friction drag, they are clearly helping to reduce transsonic drag but would increase subsonic profile drag.

You are quite correct: The extended wing box fairing also helps to smooth the cross section distribution, but is also designed to cover landing gear bays and accessories, thereby helping to allow to have a cargo hold with constant cross section over its full length in the lower fuselage. The extended wing box is needed with wing planforms which have a nearly straight trailing edge at their root. Earlier wing designs were trapezoidal, so that all edges were swept over the full wing span and less cross section tailoring was required.

However, as the picture below (source) shows, the Vickers engineers didn't make full use of those large fairings ...

VC-10 taking off

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  • $\begingroup$ According to the source, those in the picture are anti-shock bodies and not flap track fairings. The layout of the flaps would be also quite "packed" for them to be fairings. $\endgroup$
    – sophit
    May 6, 2023 at 18:29
  • $\begingroup$ @sophit That is why I added the second picture: Yes, they faired the flap tracks, but modern designs use continuous flaps, not the ones with gaps at each track. Their use as anti-shock bodies is a lucky dual-use. The real anti-shock body is the one between vertcal and horizontal tail. $\endgroup$ May 6, 2023 at 19:22
  • $\begingroup$ Thanks for the answer, though may I ask if the above photo of VC-10 wing model under experiment representative of the upper-side of the wingfoil or the lower? I've seen that photo quite a few times and know it's a viscostic flow/boundary layer simulation using an oil, so automatically I'd assume that they are indeed trying to investigate high-subsonic/transonic speeds, but the picture on the right and the boundary layer separation seems like a compression-shock induced separation to me, and to my knowledge, compression shock mainly occurs on the upper-side of the wingfoil. $\endgroup$
    – MK.s
    May 7, 2023 at 1:29
  • $\begingroup$ @MK.s First, it is a windtunnel experiment, not a simulation. Second, the photo shows the lower side of the wing. It was taken at -1° AoA, so a shock on the lower side without Küchemann bodies is quite normal. $\endgroup$ May 7, 2023 at 6:32
  • $\begingroup$ Just to be fair with the oldies 😉: Caravelle, DC-8, 707,... had very simple flaps which didn't need fairings in the first place. $\endgroup$
    – sophit
    May 7, 2023 at 8:35

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