a.s.elliott(at)cox.net
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 Posted: Thu Apr 10, 2008 7:39 am Post subject: Maneuvering Speed is NOT related to Control Speed |
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Folks:
As a Ph.D. aerospace engineer with >30 years experience in the industry, I am really scared by what people are saying about V speeds. One likely reason is that definitions are not found in the FAA regs, only short descriptions. I am going to *try* to clear it up for maneuvering speed, Va.
[1] There is no way that a designer for a light aircraft can take control deflection rates into account when determining Va. This is only possible in fly-by-wire systems where the maximum rates are controlled by the software, not pilot. As an example, in a modern fighter, the pilot can pull the stick (or handle) full aft and the plane can be made to smoothly go to maximum available G loading without overstressing anything.
[2] Va is a sort of a steady-state speed. It is NOT dynamically defined. It is simply the speed at which the wing, in perfectly coordinated flight, will reach it's stall angle of attack at the same time the aircraft reaches its design maximum load. Since the wing can not produce more lift past stall, the aircraft can not *in a steady-state situation* exceed the G limits at that speed. Note that maximum load factor is not the same as maximum load. Load factors depend on actual aircraft weight. POH definitions of Va are almost always at design gross weight.
[3] I said "sort of steady-state" because most light aircraft can not hold this condition in level flight due to lack of available thrust. That is, the drag associated with the high-G, nearly stalled condition is much larger that the thrust available from the propeller. So the only way to get to that corner of the flight envelope is dynamically, or by falling out of the sky.
[4] It is absolutely, 100% surely possible to exceed the maximum steady-state load in a dynamic condition at Va. (It is possible to significantly exceed, temporarily, the "max" steady lift coefficient of a wing in a phenomenon referred to as dynamic stall. Some airfoils will go to 2X or even more of the steady max CL.)
[5] The definition includes the stricture of coordinated flight. The Va speed does *not* hold for a rolling aircraft, where the loads are not balanced.
SUMMARY: Va does NOT mean you can safely make abrupt control deflections.
FWIW,
Andy Elliott, Mesa, AZ
N601GE (reserved)
601XL/TD, Corvair, building...
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