Degree of Sweepback?
 

On high speed aircraft, how is the angle of sweepback determined? For example, why use 25 degrees as opposed to 30 degrees?

Cheers!

There is quite a lot on wikipedia - search for "swept wing". Design speed has a lot to do with it, whether transonic or supersonic. I suggest you have a good read and then return if you need further explanations!
TP

I have heard that one of the reasons for the degree of sweep on the 747 was hanger size (more sweep equaling shorter wing span), though I am skeptical. If anyone can documentably confirm or refute I would be grateful.

No documentary evidence, but that sounds very unlikely.

Increasing the sweep while maintaining the same wing span and the same root and tip chord achieves absolutely nothing compared to increased span.

Increased span gives more wing area (more lift) and a higher aspect ration (more efficient lifting). But generally does nothing for you at high Mach.

Increasing sweep at fixed span gives no change in area and worse high lift characteristics but gives better high speed (Mach) drag.

I can't conceive of any circumstances where you'd want more span, and decide to settle for more sweep instead; if anything, they are opposing effects, not complementary.

the swept back wing is to make sure the tips stay out of the wake of the shock wave.due to the speed closer to the speed of sound. the faster the plane goes the more swept back it is with relation to the amount of lift needed.

Is this true for all aircraft?

If yes how do you explain the F-104?

After reading "Building the 747", it was surprising to find out that the sweepback of the 747 was a compromise. Designers wanted the plane to go faster than the 707 which had a sweepback of 35 degrees, so 40 degrees was picked. However the engineering for a 40 degree sweep was too complex so they settled at half way 37.5 degrees.

Since then, I imagine there's somewhat more theory behind the degree of sweep.

A misunderstanding of the facts. The requirement is for the wing tips to remain within the shock cone produced by the nose, and the F-104 achieves this because the wing, although straight, is of short enough span to remain within the cone. The angle of the shock cone is related to the Mach number as explained in TyroPicards reference. It should be noted that the shock cone does not form until the aircraft reaches a speed of Mach 1, at that time the angle being 90°, so is not a factor in the design of sub sonic transports. Wing sweep is about controlling the Mcrit of the airfoil and its associated wave drag, in association with numerous other considerations the designers have to ponder to meet the design aims. One of the reasons, among others, the 707 could not be stretched like the DC-8 was because it had too much wing sweep.

That's so stupid...
 

Brian, what happens on all those swept back planes that are never intened to break Mach? Sweeping a wing has to with peformance, more sweep, less drag, higher Vmo speeds for a given max thrust is one perameter...but one get's higher stall speeds in the clean configuration...they get that back with flaps, slats, ect...so they can land with that swept wing. If you sweep a wing too much, take off and landing distances can get unrealisticaly long and also maybe the engines needed can't push such a low drag, low lift wing through the air to carry the load desired. There is a place where the amount of sweep, the lift devices available, the load desired, the engines installed, altitudes flown, fuel amount burned and carried...all add up to a plane that can take off and land at acceptable speeds and distances, fly for acceptable ranges, have decent handling chararistics that Joe Pilot Typed Yesterday can fly it, and carry a few people in back that are paying for all this in a safe, fast enough, efficient enough manner. All while complying with all the FAA type certificated requirements that planes need to go through.

If we're talking about sub-sonic aircarft, would I be right in saying then that a compromise needs to be reached...

In sweeping the wings back to enable Mcrit to be increased, a concession is made with respect to the amount of lift produced, as the swept wing has a lower lift coefficient than a rectangular one. The faster the aircraft goes (in terms of mach number), the more the wings have to be swept back to delay shock waves being formed. A certain amount of lift is required for an aircraft at a certain weight, so the wings can't be swept back too much otherwise this lift won't be produced (keeping all other things constant). With extra sweep, you would therefore need to travel faster, which would then require more sweep to delay the shockwaves ...and the viscious circle continues...

Is it just a question of finding (well, getting the engineers to figure out)the optimum angle of sweep to meet the requirements of cruising at a certain sub-sonic mach number, at a certain weight?

redhouse,
The lift problem isn't at high speed, thats taken care of by V^2. It's the take off and landing bit that's the problem.

Of course you can always go the way of the F111, B1, Tornado and have a swing wing.

How much runway acreage is available? The shortest runway in your route system will usually limit the amount of sweepback. Look at the DC-9/MD80 family - very modest sweepback, well suited to shorter fields.

I would doubt that 747's were designed to fit a hangar; as I recall it was the other way round.

Choosing sweep angle for commercial/business jets is a compromise between critical mach number and maximum lift coefficient. Increasing sweep increases the former (allowing increased cruise speed) but decreases the latter, resulting in worse field performance. So it is not surprising that short range aircraft generally have less sweep than long range ones, because high cruise spped is less important than field performance.
Very high sweep combined with high aspect ratio can produce serious structural problems like wing divergance(bending moment causing increased incidence at tip):ouch:

Keith