Willie Nelson
23rd Nov 2003, 22:02
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? :uhoh:
Only legitimate replies please.....Thank You in advance,
Willie
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? :uhoh:
Only legitimate replies please.....Thank You in advance,
Willie