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Southport, Manitoba, Canada
Steve Pomroy is a professional flight instructor and aviation writer. He has been teaching since 1995 and holds an Airline Transport Pilot License, Class 1 Instructor and Aerobatic Instructor Ratings, military QFI, and an undergraduate degree in Mechanical Engineering. He's written and published three flight training books through his company, SkyWriters Publishing, and has several other books under development. Steve currently teaches RCAF pilot candidates on their Primary Flight Training course.

Monday, September 27, 2010

Va: Not a Bad Speed, Just Misunderstood (Part 2)

Continuing from where we left off last time, here’s some more information regarding maneuvering speed, Va:

Failure "of the control" means that although the control surface itself (and it’s supporting structure) may be able to withstand structural loads resulting from deflection, the rest of the airframe may not. This distinction is most important for the elevator, but may apply to the rudder or ailerons under some conditions. If we apply full elevator at or near Va, it’s possible to cause the structural failure of the wings, or other structurally critical components such as the engine mounts. Read that again—it probably contradicts what you’ve been told previously. Simplified statements such as, "below Va, you can’t overstress the aircraft," are false. Let’s consider why.

The certification standard states that the minimum allowable value for Va is your stall speed (Vs) multiplied by the square root of the limit load factor. The result of this calculation is your load-factor-adjusted stall speed. So, sure enough, we would expect at speeds lower than this to experience stall before overstress—with the stall-induced loss of lift preventing overstress. So far so good. But there are three problems with this reasoning.

First, it ignores negative maneuvers. Full forward deflection of the elevator can still rip your wings off, as your inverted stall speed and limit load factor are normally different from their upright values, and this difference is not accounted for in the certification standard. Bad news, that.

Second, it ignores the effect of "unsteady flow" on the wings. It turns out that wings that are pitching rapidly can momentarily produce more lift than wings at a fixed angle of attack (i.e. – they have a temporarily increased stalling angle). The lift increase varies with the airfoil and the pitch rate, but can be as high as 30%. So if you pitch fast enough, you may be able to overstress your wings by as much as 30% even at or below your load-factor-adjusted stall speed.

Third, and finally, the load-factor-adjusted stall speed is the minimum allowable speed for Va. Manufacturers have the right to increase it. Most don’t, but it’s not unheard of. At maximum gross weight, the Piper Cherokee (PA-28-140) has a stall speed (Vs) of 64 MPH, a limit load factor of 3.8 g’s, and a Va of 129 MPH. Do the math. It’s not a very big difference, but it is there. At this published Va, the control surface can withstand full deflection, but full positive deflection of the elevator, even done slowly, will bend the wings.

The bottom line here is that your maneuvering speed, Va, can be used as a reference speed with regard to control surface strength. But when it comes to combined or multiple maneuvers, it must be taken with a grain of salt. Further, if you want to protect yourself from other types of structural problems (airframe overload or overspeed), these considerations need to be independent of Va.

Hopefully, this helps to clarify a very unclear speed, and will help you to become a safer pilot.
DISCLAIMER: The comments made here (and in Part 1) regarding maneuvering speed assume that the aircraft in question is of conventional configuration with no special or unusual standards applied. Aircraft for which all comments are not accurate may include (but are not limited to) canard and tandem wing aircraft.

Happy Flying!

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