Why Are L/D Ratios Lower at Supersonic Speed?

[Jane-DoH]

I'm curious to an actual reason why L/D ratios tend to be lower at supersonic speed compared to subsonic speed.

I've generally assumed it had to do with the fact that you'd have more drag due to the effects of shockwaves, more turbulence; less lift due to a lower pressure-differential among the upper and lower section of the wing, and the fact that the center of pressure moves aft requiring more nose-up trim to keep the plane level.

[Fitter2]

In steady straight and level flight, lift = weight = a constant.

At speeds around and above Mach 1.0 drag increases for all the reasons you state, in varying proportions depending on the cleverness of the designer, and the aircraft's role.

Why do you need to ask?

[gijoe]

Have a look at the clever way that the designers of Concorde decided to deal with this problem - i.e. by shifting fuel around from the main tanks to trim tanks in the tail to keep things nice and easy. This negated the need for up or down trim of the elevons.

There are some good explanations of this on google.

Nice!

:ok:

[Jane-DoH]

Fitter2

Why do you need to ask?

I've speculated the exact reason for awhile, but it's good to have an actual answer.


gijoe

Have a look at the clever way that the designers of Concorde decided to deal with this problem - i.e. by shifting fuel around from the main tanks to trim tanks in the tail to keep things nice and easy. This negated the need for up or down trim of the elevons.

Yup, still the L/D ratio when supersonic was less than subsonic

What really amazes me is the F-104... despite having a straight wing it still doesn't seem to have serious trim-drag problems and maneuvered real well at even supersonic speeds (though it sucked at low speeds)

My guess is the following:

-> Mass distribution and the length of the wings chord (center of pressure shifts to the 50% chord mark on a straight wing, but if the wing's chord is short -- it's a tapered straight wing, not a delta -- and the mass distribution is right -- I can't really explain it much better than this -- it might not have too much of an effect in pitch change)

-> It's tailplane is relatively large and mounted high (a high mounted surface I assume would have more leverage than if it was mounted on the plane's centerline for the same reason that an aileron mounted far outboard tends to exert more leverage)

-> The contouring of the wing -- it has an curvature that looks like it would form some oblique waves on it's underside which produces a pretty good amount of lift when supersonic

-> The inlet is pretty efficient and well shaped (and if I recall correctly was purposely designed to be oversized as to allow sufficient area for low-speed flight with the excess air routed right around the inlet at higher speeds or something like that) providing good pressure recovery and allowing good amounts of thrust for a given mach number at a given altitude.

[Rhys S. Negative]

What really amazes me is the F-104... despite having a straight wing it still doesn't seem to have serious trim-drag problems and maneuvered real well at even supersonic speeds (though it sucked at low speeds)



Actually, it *blew* at low speeds:

The first production aircraft with BLCS (boundary layer control system) was the Lockheed F-104 Starfighter, where after prolonged development problems, it proved to be enormously useful in compensating for the Starfighter's tiny wing surface.

Back to the thread...

[Jane-DoH]

Rhys S. Negative

That's right. I didn't know there were prolonged development problems; regardless, at what flap setting did the BLCS activate at?