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Thursday, March 10, 2011

Maneuvering Speed - Va

You probably remember the maneuvering speed definition from your Private Pilot study days as something like "that speed where any sudden control movement or vertical gust, i.e., turbulence, will not cause the airplane to experience damage, and the airplane will first stall." Close, but hold off handing out cigars just yet. We need to understand that maneuvering speed is not a static number, like 108 knots. You probably know that Va varies with aircraft weight. However did you know that Va varies with aircraft configuration? For example, Va changes when flaps are lowered (Va decreases). He's a better definition of Va: The stall speed at a particular design limit g-load. And yes, the purpose is to prevent us from bending the airplane. If we look at the v-g diagram below, we see that there's a positive limit and a negative limit.

I don't know about you, but when I'm forced to fly in turbulence, that doggone stuff is both positive and negative. What that means to us is that the POH/AFM Va is stated as an airspeed value in relation to the positive g-load limit, for example, 3.8 g's for your Normal category airplane. There is no published Va for negative g loads. This means that if you fly into turbulence, slow the airplane to a controllable speed BELOW the published maneuvering speed at your airplane's current weight. Whoops! What if your POH/AFM doesn't have a value for your current weight? Use this handy formula: VA-NEW = VA √ (WNEW/WMAX-GROSS) The FAA published a Special Airworthiness Bulletin about this topic. You can find it here.

This is important for all you acro-files, too. We have to understand that if we have a positive design load limit of 6.0 g's, then if we blend control inputs, e.g., elevator and aileron, then the new load limit shrinks to 4.0 g's, and the maneuvering speed decreases to 2 X Vs (if you're interested, I've have the mathematical calculations to support this, drop me a line).

Well, that's enough brain-twisting for a while. It's perfect weather these days if you're a duck or goose, so bone up on the theory, grab some good flying weather when it shows up, and we'll see you at the airport!