Sweepback...?

[Willie Nelson]

O.K. I have done a bit of research on this with the limited books I have with me at present. I have also done a search on the subject and while there are some snippets of useful information and plenty of people who were "asked about it in interviews", I would like to know the aerodynamic functioning of sweepback.

According to A.C. Kermode;

The theory behind this is that it is only the component of the velocity across the chord of the wing (V Cos alpha) which is responsible for the pressure distribution and so for causing the shock wave; the component V Sin alpha along the span of the wing causes only frictional drag. This theory is borne out by the fact that when it does appear the shock wave lies parallel to the span of the wing, and only that part of the velocity perpendicular to the shock wave i.e. across the chord, is reduced by the shock wave to subsonic speeds. As shown (in diagram that I cannot show due to computer illiteracy) the greater the sweepback the smaller will be the component of the velocity which is affected, and so higher will be the mach critical number, and the less will be the drag at all transonic speeds of a wing of the same thickness/chord ratio and at the same angle of attack.

I understand that the point of the sweepback is to simply push back the buffet boundary, and this has something to do with inward airflow over the top and outward airflow over the bottom, however would it not be true to say that if it is "only the component of the velocity across the chord of the wing (V Cos alpha) which is responsible for the pressure distribution and so for causing the shock wave" this simply presents albeit a slower velocity to a higher thickness/chord ratio......and therefore why would that in itself delay the buffet boundary or shock wave?

Only legitimate replies please.....Thank You in advance,

Willie

[Captain Nomad]

I thought sweepback was to INCREASE the shockwave so that when the shockwave hits the control tower the ATC boys spill their coffee...?!!

Sorry - corrupted by Top Gun!

Sorry - not a legitimate reply either...! Have to go and dig up my copy of "Aerodynamics for Naval Aviators" that has been rather neglected since ATPL days. Ah, the memories...!

[Aerodynamisist]

In addition to the info you already have from kermode, Sweepback aids longitudinal stability (rolling plane) IE: unitntenional roll - left wing goes down and presents more wing to the relative airflow - lift increases. It could just be the simple answer that there looking for.

stuffed up sorry it's lateral stability around the longitudinal axis

[Lefthanded_Rock_Thrower]

Sweep Back, it pretty well described in the "Aerodynamics for Naval Aviatiors", Sweep back Sub Sonic or Super Sonic, for high speed sub sonic flights, such as helicopter blades has been designed to reduce drag at the highest velocity part of the blade, the advancing tip, for high speed aeroplanes, read the shock wave bit, especially turning characteristics.

[Willie Nelson]

I might be getting there

So would it be true to say that this works because the air can be accelerated to a slower speed based on having a smaller velocity component over the wing (V Cos alpha) than the free stream velocity. If so that confuses me because why not simply have a longer chord for a given thickness thus also reducing the amount of acceleration applied to the free stream velocity and subsequently achieving the same result, surely I am still missing something here........

Nice to see some people helping out those of us a little challenged in this area.

I was waiting for the standard response it just looks a lot sexier.

[Plas Teek]

And when you come down to it... Looks are everything!