Last week’s piece on maneuvering speed inspired me to dig a little deeper into the topic of *V*_{A}. It really is an interesting topic that forces pilots to delve into aerodynamic theory. This week, let’s take a closer look at maneuvering speed.

Perhaps the best source for such understandings is the uber-technical *Aerodynamics for Naval Aviators*. According to this classic text, maneuvering speed can be defined by a simple formula:

**Maneuvering Speed =*** V*_{S}√(n), where *V*_{S} is stall speed and *n* is the limit load factor.

The good book goes on to talk about the V-N diagram and points out that this is the maximum speed at which the wings will stall before exceeding structural limitations. That’s where our common understanding of *V*_{A} comes from.

Let’s run the numbers. According to my Piper Arrow manual, *V*_{S} is 60 KIAS at max gross weight with the flaps and gear up. Our limit load factor is 3.8g, so that’s our value for *n*. So maneuvering speed is calculated to be:

**Maneuvering Speed =*** 60 × √(3.8) = 117*

You might find it interesting to know that the Piper Arrow pilot’s operating handbook defines *V*_{A} to be 118 KIAS at max gross weight. That’s one knot in the less conservative direction. That’s probably due to small errors in our math (you’re supposed to use equivalent airspeed, not indicated airspeed for this stuff).

But there could be another culprit. And that lies in the legal definition of *V*_{A}. More on that next time.

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Va,

what is maneuvering speed
Alas, you have fallen prey to a common and potentially dangerous error.

Va is the indicated airspeed that comes from multiplying the _calibrated_ stall speed by the square root of the load factor (assuming sea level, where CAS=TAS). The same error is very common when people calculate 1.3Vs0 for short field landing Vref. In aircraft where indicated airspeed at stall is much lower than calibrated airspeed, using indicated airspeed means that you are adding 30% of a much smaller number to the approach speed, leading to flying much closer to the stall than anticipated. If I remember correctly, this happens in the Cessna 152, where the published short field speed is close to the indicated airspeed that comes from adding 30% to the _calibrated_ stall speed.

To make this more dramatic, imagine an airplane whose pitot-static system was so poorly designed that it read 0 at stall, even though the calibrated airspeed was, say, 40 knots. Using IAS, you’d add nothing to the stall speed to get approach speed! Using CAS, you’d add 12 knots. A big difference!

Right Jim, and the one knot error in this instance is almost certainly attributed to the back-of-the-napkin use of indicated airspeed (sloppy on my part I know). You actually use Equivalent Airspeed for these sort of things, which for piston pilots is nearly identical to CAS.

Thanks for pointing out the danger with that C152 example — 40 knots is a huge difference, no matter what kind of airplane you’re flying!

Cheers

One Web source, for which I have lost the reference, says that an airplane in turbulence is encountering IMPULSES. Textbooks say that the forces are unknown in impulse situations. Therefore most explanations of maneuvering speed, which depend on Newton’s f=ma law, are inapplicable.

Going back to first principles, then, and thinking this through, one can persuade himself that reduced maneuvering speed is to protect THE PEOPLE AND/OR THE CARGO, not the airframe, a conclusion that would agree with the student pilot’s instincts. Remember, Einstein once said that an hypothesis must be intuitively satisfying before one resorts to the math.

There now, a trial balloon for you to shoot at!

Frank, Burlington, ON