Why do jets have swept wings?

[GremlinX]

I am curious as to why jets use swept wings.... what is the advantage and why do they use them. This is an honest question that came up last night at a dinner with friends...

[Propstriker]

Here ya go Kieth;

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

In its most basic explaination, less frontal area means Less drag.

[GremlinX]

Thats what I said last night, had to be frontal area... Thanks.

[OverTemp]

Simple answer... increase the Critical Mach number.

Rember Bernouli? The flow over the top is accelerated because it has farther to go around the airfoil. This increases dynamic pressure (the pressure cause by air movement). Since total pressure cannot change, static pressure (pressure caused by air sitting there) must decrease. This decrease in static air pressure on top of the wing is lift, or a major component of the total lift being produced.

Since the air is being accelerated over the top of the wing it's velocity is faster that the static air around it. At some aircraft speed below the speed of sound (usually Mach .75 or above, but I've seen some lower) the speed of the air relative to high points of the wing goes super-sonic. It's called local sonic flow (if I remember correctly). This creates a normal shockwave which creates a number of problems, including excessive drag, loss of lift, movement of the center of lift, etc... This magic speed depends on the airfoil and aircraft shape and it called the Critical Mach number.

By sweeping a wing we send the air over the airfoil at an angle... hard to picture, but some of the air is now moving spanwise along the airfoil instead of directly chordwise along the airfoil. This has the effect of extending the length of the chord line and reducing the harshness of the angles the air must travel over... this lowers the local air velocities. Thus, the airplane can now fly faster before reaching the new, lower, Critical Mach Number.

Swept wings have draw backs... they want to dutch roll (yawing in 1 direction while rolling in another... really bad at high altitudes) so now we have yaw dampers installed in airplanes. The want to stall tip first, and geometry changes and devices are needed to keep aileron effectiveness at low speeds. If the root stalls first the plane wants to tuck over into a nose dive... the balance of lift vs. AOA along the wing must be very carefully balanced. There are numerous other disadvantages but the increase in speed and efficiency is worth it.

Aero is simply a balance of compromises... nothing is free.

I fly heavy full scale... what I've quoted here is all regurgitation from courses etc... over the years and practical experience... I'm sure some of it is a little off though.

[teryn]

No, it's NOT to do with frontal area. In aerodyanmics, you will learn not to jump to intuitive and the conclusions within easy grasp, cause those usually are not right. If it was frontal area, you would just make the wings shorter, right?

	Quote: Originally Posted by GremlinX
	Thats what I said last night, had to be frontal area... Thanks.

[Boulder]

	Quote: Originally Posted by teryn
	No, it's NOT to do with frontal area. In aerodyanmics, you will learn not to jump to intuitive and the conclusions within easy grasp, cause those usually are not right. If it was frontal area, you would just make the wings shorter, right?

Not entirely true, with a swept wing you "could" maintain the same amount of lifting surface while decreasing the frontal area thus decreasing drag, can't do that by shortening the wing. Not saying this is the reason, just sayin'

I didn't read Propstriker's link, but OverTemp's explanation is extremely close to the way I remember it.

BTW: Yes, I realize I opened myself up for a lesson in aerodynamics.

[teryn]

OverTemp's explanation is correct and very well explained.

I don't know all the possible combinations things you could do to achieve a desired result but using sweep is a really stupid way of reducing frontal area. I was just trying to make the point that things that appear to be intuitive is probably not the answer (in GremlinX case). There are a lot more to things than meets the eye...

	Quote: Originally Posted by Boulder
	Not entirely true, with a swept wing you "could" maintain the same amount of lifting surface while decreasing the frontal area thus decreasing drag, can't do that by shortening the wing. Not saying this is the reason, just sayin' I didn't read Propstriker's link, but OverTemp's explanation is extremely close to the way I remember it. BTW: Yes, I realize I opened myself up for a lesson in aerodynamics.

[tommy321]

From the famous Roskam Aircraft Aerodynamics and Performance text book :

Basically, if you take your the critical mach number for a straight wing, and divide by the cosine of your sweep angle, you get a new higher critical mach number. Critical mach number, as has been said is the point at which you start to see a large increase in drag.

Tom

[GremlinX]

Wow... don't me out to be such a bad guy... If you have a 50 foot long wing, and turn it back 45 degree angle, you have a 50 foot wing and X number of frontal area. You have increaced the cord, and hidden some frontal drag. Now... that does not mean I believe that to be the case here.... but please don't be so harsh. Well put OverTemp.

[jwilliams]

You are all wrong, they have swept wings so they can fit more of them in the hangar.
Jeff W.

[teryn]

You are a great guy. Sorry for my bluntness. However, what you claimed in post #3 is misinformation, and people will read that and get the wrong impression of things (we have enough aero misinformation going around). Fortunately, Overtemp explained it accurately.

I presume that you read the Wilkepedia info in the previous post, and I am not sure how you arrived at your "less-than-one-sentence" conclusion from that source.

	Quote: Originally Posted by GremlinX
	Wow... don't me out to be such a bad guy... If you have a 50 foot long wing, and turn it back 45 degree angle, you have a 50 foot wing and X number of frontal area. You have increaced the cord, and hidden some frontal drag. Now... that does not mean I believe that to be the case here.... but please don't be so harsh. Well put OverTemp.

[GremlinX]

Jeff... thats funny.