Straight wing and supersonic, Help me please

[chonburi]

http://www.centennialofflight.gov/essay/Theories_of_Flight/supersonic_flow/TH22G5.jpg

Today my aerodynamic class was finished with something unclear... According to the graph shown above tell me optimum efficiency of the straight wing is about just before mach 1 but in the second figure below the straight wing's coefficient drag decreases after mach one, better even swept wing at beyond about mach 1.9 which made me so confused with my humble ealier knowledge that the swept wing is more efficientcy (less drag due to at least shockwave delayed) at supersonic speed. Yes, I see X-1 and F-104 have straight wing even though they flew at supersonic. The question is why the straight wing goes better after mach one? This also led me to doubt how much degree F-14's wings (CADC controlled) or other swing-wing fighter move to approximately at each speed settings above mach 1 (are they fully swept at all time or wing positions depend on CADC)? I don't think any fighter likely to sweep fully forward to try minimizing drag at high speed like figure the below.

http://www.centennialofflight.gov/essay/Theories_of_Flight/supersonic_flow/TH22G4.jpg

[rudderrudderrat]

Hi chonburi,

This link (http://www.centennialofflight.gov/essay/Theories_of_Flight/supersonic_flow/TH22.htm) shows the advantages / disadvantages of various wing lay outs. (Including your diagrams)

Whilst a swept wing will allow a subsonic aircraft to fly faster (delays the onset of supersonic flow over the top of the wing), there is a greater span wise air flow and loss of lift. Once the aircraft is flying supersonically then the delta shape of Concorde with a straight trailing edge is a good compromise.

[Tango51]

The reason the drag rises on the swept wing in the graph as mach number increases is because portions of the swept wing are subjected to the mach bow shock cone thus increasing wave drag on the wings (according to NASA SP-367 where the image comes from - not sure which original NACA report the pic comes from however). The greater the mach number the more the wing is outside the mach cone, the higher the wave drag.

For supersonic fighters the amount of wing sweep is intended to keep the wings inside the mach cone and not outside of it thus you shouldn't see the drag rise you do in your chart compared to an unswept wing. The greater the mach number, the greater the sweep needed to keep the wings inside the bow shock.

http://history.nasa.gov/SP-367/fig99.jpg

Hope that helps!

[bearfoil]

chonburi

It is instructive to consider three types designed by the same man. F-104, U-2, and SR-71.

Three different approaches to three different mission profiles. The F104, an interceptor was built for sheer speed, its weaponry were AIM 7, and two only, (Initially). One engagement and home. The U-2 was meant for high altitude loiter. The F104 and U2 were basically the same aircraft, with very different wings and engine.

Loiter? High aspect ratio, low weight, a razor thin line between Stall and loss of control (high speed). Sometimes the pilot had to balance on a one knot margin. The F104 had potato chips for wings. They were essentially Rocket vanes, with anhedral to ennable a wobble that diminished drag at the transition. Like a fat lady squeezing on pants.

The SR-71. Mach three, ogee wings like Concorde, and two J58 Pratts that guzzled a one off fuel. It could refuel, so economy was not the fence.

The SR-71 has two booms. In the fifties, power was a premium, so aero was critical. The F14 was an old dog, and it had gloves, just like OJ. Power is everything. Make that energy.

The J58 produced more thrust in its intakes than it did in Compressor-Exhaust (except for AB.) Kelly Johnson was the man. He did the P-38 also. I was always taught that a sweep lessened frontal area, though parasitic was about the same. I think once everything gets through the Shock, wing is less important.

[ChristiaanJ]

chonburi,

I would suggest that the first diagram you showed has been much simplified.

Its purpose is only to show that :
- an "optimum straight wing" (with a reasonable aspect ratio, not the one of an F-104 !) is perfectly useless at high speed (near and above Mach 1),
- a swept wing (and a delta too, come to that) is far less efficient in terms of L/D max at low speed,
- the best solution, aerodynamically speaking, is a swing wing, combining the advantages of both, as indicated by the solid line in the diagram.

Of course, swing wings have the disadvantage of greater weight and complexity, one reason why the original design of the Boeing SST never got anywhere....

As to the second diagram in your first post, it does not make sense to me, unless it tries to say, that if you can keep the entire wing inside the Mach cone a straight wing is better than a swept wing.
Possibly true, actually... see again the F-104.
However, you end up with an aircraft that is "hot" at low speeds (the F-104 had blown flaps to mitigate that), and that's not what you want for an airliner....

CJ

[chonburi]

Thank you very much for explanations. They are very helpful to understand that something special of the F-104's straight wing withinin the shock cone must be considered. I know the two diagrams above just for basic study cases so some aircraft may need to be looked individually due to its unique config. based on applicable principles.

[John Farley]

There is another aspect about the swept/straight wing story that I don’t think has been mentioned here.

Structurally the ratio of the thickness of a wing to the chord of the wing is very important. For a given chord the thicker the wing the greater the room for a deep spar and so the wing can be made stiff, strong and light.

In the late 1940s people (even the Germans) did not have the sort of alloys and manufacturing techniques needed to make very thin wings that were still stiff and strong enough.

These relatively thick wings meant more drag and earlier shockwave development. Sweep was just a way of taking a fairly thick wing and making it appear thinner to the airflow on a thickness chord ratio basis without making life much worse for the structural man.

In more recent times it has been possible to make thin stiff wings which do not need to be swept back as in the 104 and missile world.

Never forget that very few aspects of aircraft design can be optimised or explained away in isolation. They all involve compromises. This is especially so if the aircraft role is an extreme one in respect of speed, height or payload range.