Maneuvering speed vs weight
 

FAA says maneuvering speed INCREASES with weight. Why?

This question has been bugging me ever since my PPL days (which is only a few months ago).

I gave the FAA answer on the written test, and passed, but since I'm an engineer, I'm going to figure this one out.

It doesn't make sense to me at all.

Maneuvering speed is the speed at which full and abrupt movement of any ONE control surface can be applied without overstressing the airframe.

Or in other words, the control surface will either stall before a force big enough to overstress the airframe can be generated, or end of travel will be reached.

Why does it INCREASE and not DECREASE with weight?

The stress on an airframe is the sum of 2 forces - gravity pulling the plane down, and lift pulling the plane up.

Lift generated by the wings is proportional to airspeed squared, and the angle of attack (roughly, before stall). That means, at the same speed, and same angle of attack (critical angle of attack), it doesn't actually matter how heavy the plane is. The wings will ALWAYS generate the same amount of lift (the plane could be climbing or descending depending on the weight, but we don't care about that).

Force of gravity, on the other hand, is proportional to mass.

What that means is, at the same airspeed and critical angle of attack, a heavier airplane will stress the airframe more.

So why is it that maneuvering speed increases with weight?

Ah! It makes sense if the stress limit is in G's.

But then the next question would be, why are airplanes rated in G's, and not absolute forces?

A heavy airplane and a light airplane pulling the same G definitely stress the airframe differently?

Think about the load factor. It's lift divided by weight. The same applies to Vs. Some of these things often seem quite confusing but Va is similar to a stall speed (gust + buffer) in that it relates to load factor; if I remember correctly!

Yes, the question is, why is load factor used to rate an airplane, instead of force?

What you say is true if the only criterion was wing bending and shear.

But G-limits are correct for engine mount, seat loads, baggage loads etc. which won't necessarily be lower for reduced gross weight.

Similar arguments apply to consideration of wing fuel weight as that reduces wing root loads but has no effect on the mass-loading effect of the powerplant.

This one has always baffled me also, cant wait for a response in laymans terms that I actually understand .

My understanding is follows:

Your max weight plane has a Va of Xias. Flying at that speed and weight, the difference between your alpha and the alpha to stall the wing is less than can be commanded by full up elevator. As your weight goes down with burned fuel, the same lift requires less alpha, therefore the margin expands. So, unless Va is consciously and correctly reduced by the pilot, he can damage the plane with sudden, full up elevator.

I'm choosing up elevator just as one example of a control input.

I'm sure one can get much more specific and wrapped up in symbols if one likes.

That is the most common explanation I hear, and I don't think it's right.

At the maximum travel, the airplane will always stall (with very few exceptions), regardless of airspeed or weight. Elevator more or less sets the angle of attack, and usually has the authority to go past the critical angle of attack.

That means, the maximum lift produced up full up elevator is the same regardless of airplane weight.

A pokey little Cirrus 22 will easily do 150kias and its Va is 133. I can assure you that even at 120kias, using full elevator in the Cirrus will result in a rapid climb. Sure, in time it will stall, but not very quickly. Now imagine the wing loads at 150 when suddenly commanding full up elevator.

If you're straight and level, there's only 1G on the frame. When in a constant speed climb, there's only 1G on the plane. However, to get from S&L to that CS climb, you have to accelerate (change directions) and that loads the wing. If you are above Va and you do it too quickly or with too much authority, the frame goes ouch.

I think Mark 1 has made the main point.


Imagine you are an engine component, attached to some part of the aircraft by a bracket. Let's say the bracket can support up to 4 times your weight.

Now, how much do you weigh? Your normal weight times the G-factor. If the G-factor goes over 4, the bracket will break or deform.

But for any given aerodynamic force, the G-factor is inversely proportional to the whole aircraft's mass.

Yep, Mark 1 has the correct answer.

Not all limits necessarily pertain to just the wing structure. In fact, in overload conditions, typically the wing is the last item to fail, just after the tailplane/elevator.

A lighter aircraft will react quicker to control inputs as it has less momentum than a heavier aircraft at the same airspeed. I'm sure this ties in somehow but my head's not with it today!

Hi Mathewlai, Its all to do with how the plane reacts to different All Up Weights (AUWs.)

If you throttle back to just 70 knots with just yourself on board, then pull fully back, you will not be able to pull more than say +2G.

Try the same at max AUW (full of passengers and full fuel.) and you will not get +2G but maybe just 1.5G. To get the 2G, you will need to fly faster when fully loaded.

Increase the speed to beyond Va and it is then possible for you to exceed the +4G limit of most planes, with full deflection of the yoke. Again Va will be higher when fully loaded, by a factor that I think is proportional to the squre root of the increase in weight.

You've never done accelerated stalls then. Part of the FAA CPL is to demonstrate accelerated stalls, and what you do is fly along straight and level BELOW Va for your weight, then yank back on the elevator very quickly. The aeroplane doesn't go into a rapid climb as you would expect but stalls. But critically as you are below Va, none of the forces on the aeroplane exceed any design limits so you don't cause any damage.

The heavier the aeroplane is, the easier it is to stall, which is why Va rises with increased weight.

Va is simply the stall speed at the limiting load factor.

Like any other stall speed it is proportional to the square root of the weight.

If the weight increases this causes Vs, Va and all of the other stall speeds to increase.

If we know the value of Va at any given weight we can calculate the new Va at any different weight using the following:

Va at new weight = Va at old weight x Square root of (New weight / Old weight)

The reason is:

So that your aircraft will stall before it can be overstressed due to g loads when you abruptly maneuver the aircraft.

Fly faster than maneuvering speed on a lightly weighted aircraft and abruptly maneuver the aircraft, you will overstress the airframe.
Fly slower than maneuvering speed on a lightly weighted aircraft and abruptly maneuver the aircraft, it will stall first before it can be overstressed.

'Mass' may be more useful than 'weight'
 

I have read that IN FLIGHT the Va airspeed does not vary with wing loading (i.e. G-load). This notion holds that Va varies with aircraft MASS not aircraft WEIGHT. Admittedly, on earth MASS and WEIGHT may be essentially the same value but loading an aircraft in flight with the increased load factor of a level turn (or a pull-up) would increase the apparent WEIGHT of the aircraft and its contents but would not increase the aircraft's MASS. Therefore, this notion holds, Va would not vary.

As an aircraft flies along using its fuel the aircraft's MASS would be reducing and thus so would the aircraft's Va airspeed. Not by a lot I'd guess.

From the relevant flight manuals for Va speeds.

Cessna 207 Basic weight in 2200 pounds region.
3800 pounds 130 KIAS
3050 pounds 117 KIAS
2300 pounds 101 KIAS

Cessna 172N Basic weight 1400 pounds region
2300 pounds 97 KIAS
1950 pounds 89 KIAS
1600 pounds 80 KIAS

I would opine that a 17 knot spread represents a substantial proportion of the 172 speed capability, at 75% power, 2000 PA, standard temp, cruise is 116 KTAS. Ignore Va at your peril.

..in laymans terms... although this may have been posted in the old 2013 thread...

the reason the Va increases with weight, is that it's just to do with stalling speed.... it's the speed you want the aircraft to stall instead of break following a sudden control deflection..
as weight goes down, so does the stalling speed. If the Va stayed the same but weight went down, then there's the chance that a sudden maneuver would bend something instead of stall the aircraft.


Higher weight, means higher stalling speed at various loads... so Va is higher.
Lower weight, lower stalling speed at various loads.... Va is lower.