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?

 

[rb1957]

another point i think is that sweeping the wing moves the wing tips back and keeps them from interfering with the mach cone (shock) generated by the nose

 

[KENAT]

Wing design (like many other aspects of aircraft) is a compromise of several factors, aerodynamics, structures, manufacturability, ease of operation, safety, radar signature... this was a lesson in my first year at university.

While it varies, so far it's been found that some kind of swept wing (if you count a delta or modified delta as a 'swept wing') usually gives the best compromise.

Be carefully to distinguish between 'swept' and 'tapered' wings, which I'm not sure you do. The taper is largely for structural reasons as I recall/understand it. The type of sweep you're talking about is for aerodynamics in the compressible flow regime. However, some slower aircraft also have slightly swept wings for various reasons such as pilot vision or correcting C of G issues and the like.

F104 was perhaps the most famous proponent of the 'thin wing' approach, though the X1 and other research aircraft also used it.

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[KENAT]

Thanks rb1957, you posted while I was typing. It was my understanding that that was actually the main reason but I was worried my memory was playing up. What the Op talks about is more a benefit of taper.

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[GM12345]

Nah what I am really talking about is, why is wing sweep used for 99% of the transonic aircraft designs we see in the modern age. I am asking is there a single reason that clinches the choice of sweep as opposed to the use of a similarly-capable high-speed straight wing? Per the F-104 fighter and a few designs, I know that it can be done. I am aware that design trades often dictate the final decision favoring one choice over another. But in this case you have a historically supported fact in play, namely that sweeping a wing is the best way to deal with wave drag for any wing that sees the transonic range (>M= 0.83). The most convincing answer I have come across to my question is as stated above, that you get more usable space inside the swept wing than you would if used a straight wing optimized for similar lift and drag characteristics. Thoughts?

 

[GM12345]

Someone mentioned taper, and I am not talking about that. Taper an engineering compromise aimed at achieving the most usable area for a given span. The goal is drag-reduction, optimized by a perfectly elliptical planform shape. Taper gives nice drag reduction with none of the manufacturing difficulty. There are other reason for taper as well, but manufacturing elliptical Spitfire wings during WWII was a huge pain in the butt. This is why P-51 Mustangs and all other airplanes prefer taper.

 

[berkshire]

The eliptical wing of the spitfire was not done for lift distribution, although that was an added bonus. The trailing edge was curved to accomodate machine guns in the wings.
B.E.

 

[KENAT]

GM12345 The spitfire wing was a compromise. While if coincided nicely with the theoretical shape it also made the wing thicker and more chord where they had to house landing gear and guns.

Wing shape is a compromise, trying to deny that or fine one overwhelming main reason in all cases is pointless.

Reading your posts again, I kind of understand what you're saying but It's the explanation of wing sweep that never held up for me. You don't literally take a straight wing and then turn it a few degrees and get a swept wing.

If you are looking for one overwhelming reason then for transonic aircraft I'd go with what rb1957 says. Sweeping the wing far enough keeps it behind the shock wave in compressible flow.

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[GM12345]

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Ok let's limit the scenario to high subsonic speed. This leaves airplanes such as airliners and business jets mostly. A supersonic straight wing can be successful as we know in the F-104, but just to simplify the discussion let us take guns, taper, and mach cone out of the question because none of these things applies to airplanes in this class.

Why not just build a straight wing? Say for a moment I am your company's cost person, you are the chief engineer. We call a preliminary design meeting for our new clean-sheet airliner to discuss some basics of the new airplane. I am complaining that some of the engineers are saying privately while we are having coffee at Starbucks that we can save a ton of money by making our new airplane with a straight wing rather than the usual swept wing everyone else like Boeing and Bombardier makes for theirs. They are telling me they can still get the requisite lift and drag characteristics we want and yet avoid the complexity of a swept design. This straight wing airliner will be the cat's meow on how to save money and enable our company to become the next Airbus.

It is a valid point, or at least I think it is. And I know that swept wings are not simply straight wings turned 25 degrees. So you are dead-set against this straight wing nonsense as chief engineer, and you want a typical swept wing for our new airliner because history dictates it for one thing, and more importantly you know that swept wings are better in some profound way. What way do you have in mind? I am not convinced that the straight wing cannot do all the same aerodynamic things that a swept wing can and a lot cheaper too. Convince me. I would be willing to settle for the "sweep is what you settle on after you work out a huge list of compromises" concept if there were a single high-subsonic airliner in existence with a straight wing, but so far as I know there are none.



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[GM12345]

Correction: taper does apply to subsonic airplanes, my mistake.

 

[KENAT]

For one thing, if you went with a straight wing you'd likely have to design the outer portions of it to sometimes operate in compressible flow. This is what rb1957 brought up and I don't think you've mentioned. From the ever reliable wikipedia:

"One way to avoid the need for a dedicated supersonic wing is to use a highly swept subsonic design. Airflow behind the shock waves of a moving body are reduced to subsonic speeds. This effect is used within the intakes of engines meant to operate in the supersonic, as jet engines are generally incapable of ingesting supersonic air directly. This can also be used to reduce the speed of the air as seen by the wing, using the shocks generated by the nose of the aircraft. As long as the wing lies behind the cone-shaped shock wave, it will "see" subsonic airflow and work as normal. The angle needed to lie behind the cone increases with increasing speed, at Mach 1.3 the angle is about 45 degrees, at Mach 2.0 it is 60 degrees.[1] For instance, at Mach 1.3 the angle of the Mach cone formed off the body of the aircraft will be at about sin? = 1/M (? is the sweep angle of the Mach cone)[2]"

You can't limit the discussion to high subsonic speed except potentially for missiles & the like. Unfortunately aircraft tend to have to land for fuel etc. then take off again so spend some time flying somewhat slower.

Any help?

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[GM12345]

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My proposal that we limit the discussion to the subsonic flight domain was an attempt to avoid the whole mach cone issue. Mach cones are a supersonic kind of consideration, and yet swept wings are used in both flight regimes, supersonic and subsonic. So I thought limiting the question to subsonic speeds and the aircraft made for it would not to preclude anything relevant to the discussion. Rather it should serve to clarify the issue by eliminating mach cone as an explanation. Subsonic airplanes obviously do not use sweep for mach cone reasons because there is no mach cone at subsonic speeds. And yet, you'll never see an airliner without sweep.

Your point ("Unfortunately aircraft tend to have to land for fuel etc. then take off again, so spend some time flying somewhat slower...") is intriguing, one that I think about too. The existence of several variable geometry swept airplanes, such as the F-111 Aardvark and the F-14 Tomcat (again, not trying to go supersonic with my example) plus a long list of mid-subsonic (M=<0.83) business jets illustrates the fact that at slower speeds less sweep is better. This makes a lot of sense, and there are many designs that use non-swept wings for better low-speed performance. Cessna straight-wing jets for example. Of course almost all turboprop and piston aircraft may be added to this list. They all have unswept wings because they do not need the sweep. However, once the airplane is used at around M=0.8 or more it is a natural choice to include sweep. I still wonder why. Perhaps I am hung up on the apparent contradiction that wave drag reduction, which is the huge advantage to sweep, is also easily obtainable by using thin, straight wings.

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[KENAT]

New PostGM12345, I think I see your issue.

Even though the aircraft as a whole may be traveling at less than Mach 1, if it is close to Mach 1 (Transonic) then there may be areas of incompressible flow.

While I'm a bit rusty, in essence leading edges of the aircraft such as the nose or sharply curved surfaces such as cockpit etc. can cause the airflow to become locally incompressible.

Assuming the feature causing this is the nose, or near the nose of the aircraft, and that a Mach Cone is generated by the nose then by sweeping the wing you can stay behind that cone - so in subsonic/compressible flow.

http://en.wikipedia.org/wiki/Swept_wing

may be of more use to you than I thought, though take it with a pinch of salt as Wikipedia is far from infallible.

This is in fact why we have the term 'transonic' otherwise we'd just have subsonic and supersonic.

The further below supersonic you get the less the angle you need to stay behind the Mach Cone as the mach Cone angle decreases. Eventually at some point the leading curved surfaces don't compress the flow enough to turn it supersonic so you don't need any sweep for mach reasons - though there may be other reasons to have it.

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[KirbyWan]

GM12345,

I'm not an expert here, but to clarify discussion I would break down the different flight regiems as follows:

M < .3

This is the slow region where air can be thought of as incompressible. If you stick your hand out your car window with the palm flat to the onrushing air you will feel higher wind pressure on one side of your hand, but your hand is not moving fast enough, even at 100 miles an hour, to appreciably change the air pressure in front of your hand compared with the surronding air.

.3 < M < .8

This is the compressible flow regime. The airplane is moving fast enough that you have to take into account the effects of compressibility, but not so fast that you are approching the transonic region. most turbo prop regional transports fly in this regime such as the dash 8 which has straight wings and can fly up to 414 MPH. Some of the airplanes that fly in this region also have some sweep. Even if you do not need to avoid a shock cone, some sweep may improve aerodynamics. Also do not discount appearance. Airplanes with swept wing look cooler then airplanes with straight wings, even if it's just a slight sweep.

.8 < M < 1.1

This is the transonic region and most commercial jets fly in this region. While the airplane might not fly faster then the speed of sound, some region around the airplane might experience local supersonic airflows. If an airplane flies above the speed of sound it may experience local subsonic flow.

1.1 < M

Supersonic flow region. One point I want to put out there tough it's not really germain to the discussion, The F104 does not have swept wings, but they are so stubby that they are within the Mach cone.

As a non-expert I can't really say this is the reason why all commercial jet wings are swept, but I hope this breakdown of flight regiems helps to clarify the subject and focus the discussion. Feel free to correct my choice of break points. I was thinking of having the compressible/transonic region break at .7.

-Kirby

Kirby Wilkerson

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[GM12345]

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Kirby, I concur with your information. Of course the break points vary per an actual wing in question, although I concur with your suggested break points.

One thing I wonder about here after reading over the other replies is, how we are defining the term "wave drag". My definition of this term is- drag accruing from any type of shock wave on the airplane, and typically for the transonic regime, this would be on the top side of a wing. That's all. Said shock may be as you mention found on a wing flying in the transonic regime which after all is a subsonic flow according to the freestream velocity. You would want to say there are "locally supersonic" flows on such an aircraft, because most of the aircraft is still immersed in subsonic flow. These are the shock waves I am talking about, and the term "wave drag" does not imply that the airplane is flying at or in a supersonic freestream flow. This is supposedly why wings are swept, in order to allay this wave drag effect at say, M=0.82.

Again, what I am worried about is why sweep is always the solution to this problem with wave drag. Why not have a thin, flat wing that is perpendicular to the fuselage. They call it a straight wing. It's a lot cheaper, and there will be no wave drag if you make it thin enough. The only reason I can come up with for not doing this, is such a wing has insufficient internal volume for fuel and landing gear. Otherwise it would be the ideal solution and cheaper to build. The F-104 is an example.

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[KENAT]

GM - you still seem to be ignoring that the nose area can create a 'mach cone' and you can 'hide' the wing behind this with a swept wing.

Certainly for a wing intended to operate in supersonic flow the 'thin straight wing' is an option. However, as I hinted at before, these aircraft usually need to be able to fly slow enough to land & take off - a flight regime where the limitations imposed by thing wings can be a problem as I vaguely recall.

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[KENAT]

By the way, sorry that's I've probably mixed up my terminology in a couple of spots.

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[GM12345]

Mach cone is a supersonic phenomenon. It can't occur in subsonic flight, because sound travels faster than the airplane- by definition. The air molecules have time to adjust to the arrival of the airplane before it gets there. Mach cone however, does form in supersonic flight. By keeping the sweep angle a little bit greater than the angle of the mach cone it is possible to keep the wing inside it subsonic, even though the airplane is flying supersonic.

 

[KENAT]

GM you seem to accept that wave drag can happen due to the curvature on the top of a typical wing.

Do you not believe it can happen due to the curvature of a typical nose or cockpit canopy or the like?

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[GM12345]

--

Yes there may be some shocks, but not those indicated by the term mach cone. More like wave drag. Not the same thing at all and mach cone does not occur in this speed range (M=0.82 ~ 0.95). Wave shocks are local, smallish shocks formed at points of compound curvature on the outside of the aircraft. Contours like the tops of wings, canopies, nose cones and radomes are where you would expect them. Mach cones on the other hand are large, single shock waves extending out from the nose of the airplane. They do not occur at subsonic cruise speeds like these. Sweep may address the issue, but then again I am not talking about supersonic jets, so mach cone does not apply.

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http://en.wikipedia.org/wiki/Mach_number>

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