Just read an article regarding Manouvering Speed in the latest Plane and Pilot mag.
I've always wondered if what I thought was Va in my airplane was actually (or close too) Va given my weight, knowing that Va speed goes down as weight goes down. It also goes down as stall speed goes down. Here are a couple formulas. Va was given in the article and Vs I gleaned from "Aviation Safety Journal" as a rule of thumb for calculating stall speed under max gross weight. STOL mods will lower this so know your airplane.

Va=Vs * sqrt(Lmax * W/ Wmg)
Vs=Vsmg - (1 - W/Wmg)*50 (this is 1/2 the percentage of the weight change subtracted from the max gross stall speed)

Where:
Sqrt = Square Root of
Va = Manouvering Speed
Vs = Stall Speed Calibrated for Weight
Lmax = Maximum Positive Load, typically 3.8 for normal category airplanes, 4.4 for Utility, and 6 for Aerobatic
Vsmg=Stall speed at max gross of weight of Aircraft
W=Current Weight of Aircraft
Wmg= Max Gross Weight of Aircraft.

For example in my airplane with half fuel remaining:

Vs=60 - (1 - 1316/1600) * 50 = 51 (ish)
Va=51 * sqrt(3.8 * 1316/1600) = 90 (ish)

Which is a little lower than I have been guessing. If I fly under 90, I should be reasonably safe in strong turbulence or, in reality, light turbulance that I always think is strong. At least, as far as Va is concerned.

Aviation Definition I just pooped out: Manureing Speed. A some speed above Va which is determined by the wings popping off.

Since I am flying experimental I have to be the one to establish the numbers to work with for calculations. It is simpler to just stall the plane at altitude once to determine what that speed will be. It can vary with the weight and the day but will be more accurate than calculations and keep the pilot fresh on the feel of the edge of the flight envelope.

I'm sure this will generate some debate, but here goes:

Va should be Va, regardless of weight. Why? Because while you can generate more G's with full deflection of the control surfaces when lightly loaded, it isn't the multiple of gravity that folds the wings - It is the force applied. Force is mass (we use weight in the earth's gravitational pull) x acceleration (G's). If your mass is down, your G's can go up and equal the same amount of force.

What everyone gets hung up is the max G's the airframe is certified for - This metric is based on the max gross - And is tested with nothing more than a substantial amount of mass (i.e. sandbags) applied across the wing surface, accelerated by 1xG. They don't send some poor dumbass out with a parachute and a G-meter to figure it out.

Ever wonder why there aren't placards everywhere on the newer planes for Va at every weight, only gross? Given our healthy stable of ambulance chasers in this country, if it were a problem, it would be placarded.

In other words, I personally don't get too worked up at trying to find some calculated manuevering speed at partially loaded conditions in order to meet a specific "G".

I've got my asbestos undies on..........

Flynengr

flynengr wrote:I'm sure this will generate some debate, but here goes:

Va should be Va, regardless of weight. Why? Because while you can generate more G's with full deflection of the control surfaces when lightly loaded, it isn't the multiple of gravity that folds the wings - It is the force applied. Force is mass (we use weight in the earth's gravitational pull) x acceleration (G's). If your mass is down, your G's can go up and equal the same amount of force.



I've got my asbestos undies on..........

Flynengr

Here's an explanation as to why Va varies with weight. If you have a math allergy don't click the link.

http://www.nar-associates.com/technical ... screen.pdf

Other articles by the same guy:

http://www.nar-associates.com/technical ... lying.html

dirtstrip wrote:Since I am flying experimental I have to be the one to establish the numbers to work with for calculations. It is simpler to just stall the plane at altitude once to determine what that speed will be. It can vary with the weight and the day but will be more accurate than calculations and keep the pilot fresh on the feel of the edge of the flight envelope.

Very true, there is no subsitute for knowing ones airplane and slow flight practice is something to practice regularly.

I ran through it to see if my estimate of Va was in the ballpark and thought others might also find it useful. I haven't gone through and written down actual stall speeds for various weights and my POH doesn't include anything other than gross. There's also the problem of determining actual stall speed from the indicated airspeed, which I'm sure has been discussed in a thread here somewhere.

by Bonanza Man » Wed Feb 24, 2010 9:00 am

flynengr wrote:
I'm sure this will generate some debate, but here goes:

Va should be Va, regardless of weight. Why? Because while you can generate more G's with full deflection of the control surfaces when lightly loaded, it isn't the multiple of gravity that folds the wings - It is the force applied. Force is mass (we use weight in the earth's gravitational pull) x acceleration (G's). If your mass is down, your G's can go up and equal the same amount of force.



I've got my asbestos undies on..........

Flynengr

Here's an explanation as to why Va varies with weight. If you have a math allergy don't click the link.

http://www.nar-associates.com/technical ... screen.pdf

Other articles by the same guy:

Yep, that's the approach many stick to because that's what the calculation tells them should happen, but it is flawed, in my view as well as others.

If you subscribe to the theory that an airfoil will always stall at the critical angle of attack, regardless of what it is connected to, the extrapolation suggested is pure bunk. The calculation he cites in his article is what we term in the engineering world as "empirical", meaning it isn't derived from theory, it is a "best fit" based on laboratory studies to explain a mathematical relationship. It uses a coefficient to translate relatively similar inputs into similar results. It isn't intended to explain phenomena beyond its intended purpose, which is where they headed with the Va extrapolation. Granted, their argument is the "safer" bet, and that's always the easy argument to make.

The reason stall speed varies with weight is explained by the equation - a wing supporting an increased weight must be flown at an increased speed to maintain a constant angle of attack. When everything is in balance, it works at stall speed, but when the speed drops below, the angle of attack effectively increases due to the relative movement of the airplane to the oncoming air - Thus the stall. Where the equation breaks down lies in the assumption that extrapolating the force at stall speed directly relates to the force on the wing structure and attach points in terms of G, and that lower stall speed MUST mean lower maneuvering speed. What this approach fails to recognize is that the wing doesn't quite see it that way. The wing will stall at exactly the same angle of attack, regardless of the weight inside the wings or cabin.

It might seem that after 100 years we'd have this lift thing completely understood, but believe me we don't. How many pilot oriented books cite Bernoulli's principle (low pressure due to faster air on top of the wing makes it go up) still as the fundamental basis for flight?

Flynengr