Swept Wings/Lat Stability
 

I fully understand the effects of increasing the critical Mach No., however can't find a concise answer to why swept wings have a STABILIZING effect on roll, ie it prevents unwanted role. Anybody can enlighten?

Because of the downwash created by the flow over the wings, a higher streamline hits the tips than hits the roots - the flow being deflected downwards along the chord, which for a swept wing means also along the span. Thus, to the airflow, a flying swept wing, particularly at high AoA appears to have dihedral.

Swept wings have much less lateral stability at low AoA, because there's less downwash being created - at the zero-lift AoA it should have none.

G

The reaction of a swept wing to sideslip is that a restoring rolling motion will dominate the lateral stability quartic. A straight wing will have a dominating spiral motion.

skywaytoheaven,

The question you have posed requires a degree of knowledge of flight dynamics to answer. There is a large degree of coupling between the lateral modes of an aircraft i.e. those motions in roll and yaw. That is, the action of one will induce the other.

With regards to wing sweep, then I agree totally with BEagle. When sideslip is induced, a swept wing will tend to produce a rolling moment reaction as to reduce the angle of sideslip. This occurs as a result of the effective change in flow direction over the wing due to reletive sweep angles being different. The net result is to reduce to dominance of the spiral mode in the lateral responce i.e. the aircraft is more stable in its spiral motion.

Among other reasons, this is why A/C with highly swept wings have particualr problems during crosswind landings (I have been assured that the VC-10 is a good example!)

I am not so sure about the downwash and AoA theory suggested by GtE, but then again, he is probably far more informed than myself!!

Hope this helps,

Cheers,

Cuban_8

Cheers for the information guys, much appreciated!