advantages of wing sweep

[GM12345]

I have a B.S. degree in A.E. There is an irritating question on the topic of wing sweep that was not answered to my satisfaction. Wing sweep solves the problem of wave drag at transonic speeds and this is the first thing we learn. It acts by reducing the thickness-to-chord ratio the air flow encounters as it passes over the wing. Another way of saying the same thing is the wing is less curved in the plane parallel to the fuselage. But sweep also creates a lot of accompanying problems, some of which are are poor aileron flow-adhesion from additional span-wise flow at low speeds, added difficulty in manufacturing, and a poor lift curve slope affecting landing and takeoff performance as well as lowering the stall speed. The latter leads to more complex slats and flaps. Another complication associated with wing sweep is the shift in the center of lift with speed. So, if the active principle in wing sweep is reducing the curvature the air flow encounters and reduction in the effective thickness-to-chord ratio, why not simply keep the wing straight but use a thinner, lower thickness-to-chord wing? The most convincing answer I find is that you can opt for a straight wing thin enough to avoid wave drag, no problem there, but you will also have a wing so thin that it will have the same problems with slow speed flight but will also be too small inside to contain wing spars, fuel, guns, and landing gear. But is that all? Can you add to the reasoning that wing sweep is always preferable to a high-speed straight wing?

 

[GM12345]

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It's the difference between Figure A and Figure B in that link.

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[btrueblood]

GM,

this link states the reason for wing sweep fairly clearly, along with the associated penalties:

http://adg.stanford.edu/aa241/wingdesign/wingparams.html

"Wing sweep is chosen almost exclusively for its desirable effect on transonic wave drag."

 

[KENAT]

I think you may be right and I was mixing things up a little.

How about, most 'transonic wings' are swept because compressibility effects can be associated with the Mach number normal to the leading edge?

Maybe the below helps, especially section 4.2 helps.

http://www.aoe.vt.edu/~mason/Mason_f/ConfigAeroDesignOpt.pdf

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[KENAT]

By the way GM12345 thanks for your persistence, you've made me dust off a few brain cells. I'm fairly sure I've got it right this time though (will I come to regret saying this ;-)).

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[GM12345]

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Trueblood- Funny, I was reading the very same DeskTop Aero webpage you linked to in your post. Great website. That's why we have these little get-togethers, to stimulate discussion and to remove the rust from being so long away from engineering college. Over the years I have had many such discussions on another website, and everyone who participates wins although not all can claim first, last, or best punch on a given issue. It's easier when you start the thread!

Well, I guess the gist of this issue boils down to the dilemma of why did straight wings lose traction in the annals of transonic aircraft design and allow sweep to take over. The general explanation you get is it pushes the Mach divergence curve up a little bit and I guess I will have to be happy with that explanation. I have no doubt that sweep is the best way to do it given the sheer preponderance of historical evidence to that effect, yet my inquiring mind refuses to let the issue be decided without a satisfying reason for straight wings not being quite as good. I have found a bunch of great links through our discussion, and if I find an answer to this apparent dilemma I will gladly post it here.

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[KENAT]

GM12345 your concern with a very thin straight wing not being as structurally advantageous is I believe one of the reasons.

It's just that once, again, I'm not sure it's the only or even solely prime reason.

I found the link I gave 13 May 10 20:06 quite informative and a bit memory jogging.

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[GregLocock]

There's a couple of other reasons to sweep the wings even of slow aircraft.

me262 was reputedly swept because the CG wasn't where they'd expected it to be.

There's a low speed yaw roll coupling effect that is affected by wing sweep.

Cheers

Greg Locock


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[rb1957]

straight wings are designed into low speed planes, turbo-prop commuter planes like the Dash 8 with a max speed around 300 kts (sort of M.5). Universally faster planes (business jets, Boeings, Airbus', etc) use swept wings. with the structural complexity there has to be some gain (almost certainly aerodynamic), which i think is drag reduction. you mentioned "wave drag" above; remember, shocks start to form at < M1, as the local airflow on the foil goes supersonic. you mentioned F104; look at the plane view of the plane ... it has a very small wing (and a very big engine).

I'd suggest looking into Raymer or Torenbeek (airplane design texts) ... wing sweep is a compromise of a whole bunch of issues.

a thought just occurred to me, i wonder if it reduces the strength of the tip vortex and so induced drag ? maybe the effective semi-span is reduced to b/2*sin(sweep) ??

 

[moon161]

A thicker section (with the same t/c) allows a deeper and stronger spar and wing box. The moment of inertia is proportional to depth^3. You have to make a wing that will hold things up- The F104 wing only has to hold a J-79 and a pilot, low AR = low moment in the spar & wing box.

Higher t/c sections are AFAIK better at low, airspeed.

As for the F111, F14, low sweep and high AR at subsonic cruise speeds lowers the induced drag- longer transit or patrol endurance.

dCl/dalpha is higher for high AR -> good for landing.

References:

http://www.aerospaceweb.org/question/aerodynamics/q0281c.shtml

Raymer is good, so are Jan Roskam's books, at least the 2 or 3 I read. Pardon the 80's typsetting, but they're good.

 

[btrueblood]

Rb, I can't quote a reference, but can point at the fact that high sweep causes higher spanwise flow - which I think would add strength to trailing vortices, not minimize them (all else being equal, i.e. same total lift produced).

 

[mohr]

HI GM12345

Read to Robert T. Jones and his books and NASA papers.....

Cheers

 

[markrkrebs]

I like your question. Lots of high performance (yf23) or transonic (learjet) aircraft don't use sweep. The motivation's I've seen allow an imaginative design approach where you "think" about one thickness ratio & get to use a lower streamwise mach number (or as you say, an effectively thinner airfoil) when considering wave drag. It seems silly.

Consider Clbeta though. Sweep provides that effect without dihedral, which is logistically troublesome. Perhaps there are other reasons in the "lore" of transport design that have resulted in optimizing towards sweep.

 

[gerritgroot]

The advantages of the swept wing concept at high speeds were at first demonstrated by Busemann at some "Volta" Congress.

The publication about that is in german, viz:
Busemann, A., “Aerodynamischer Auftrieb bei Überschallgeschwindigkeit,” Proc. Volta Congr., 1935,
pp. 328-360.

I don't know whether there's a translation. If you find one, let me know on this forum, but I don't think so.

When he published it, nobody took it serious until the germans started applying swept wings in their fast Me262.

Don't expect to find a clear conceptual explication in this publication. Busemann got his point by means of long old-style equations and you sure need previous knowledge about analytical aerodynamics from the 20's to understand it.