Explaining Va (maneuvering speed)
 

ever since i started my training, i've had a tough time explaining why the maneuvering speed (at least with a Piper Warrior and a C172) is two different speeds. i know it has to do with the critical angle of attack and load factor, but relating the two is what's tough for me. i tried the search function and didn't see anything about this on this fourm, so thought i'd give it a shot. anyone here have any way to easily be able to explain Va?

thanks in advance,
Colin

The idea behind Va is that you want the wing to stall before it breaks. Consider changing speed while maintaining altitude.

Lift is a function of velocity squared and a direct function of AOA: As speed increases, lift increases a lot so you have to reduce the AOA to keep the lift constant and maintain altitude.

As you slow down, the opposite occurs, you need to increase AOA a lot to maintain altitude.

At a slower AS, you have a higher AOA. This means you are closer to stall than when going fast. If you haul back on the yoke and increase AOA you will get some wing loading but it will stall out quickly which actually protects the wing (no wing loading when fully stalled).

If you haul back on the controls at a high speed, the AOA is very low, so it takes more AOA to reach stall. You might overload the wing before you reach critical AOA and stall.

Va is simply that speed where the AOA is just high enough so that you will just stall before bending/breaking something.

A lighter airplane can develop a high load factor more easily than a heavier, if otherwise similar, one can. They usually share the same "n" figures in order to achieve certification (+3.8 g's/-1.52 g's).

Load factor (n)= Lift / weight; but Lift= q*CL*A; so if CL and A are held the same for two similar airplanes, then only q can be changed. Q is dynamic pressure or speed of the air. So, to get the same ratio of lift to weight in a lighter airplane, you need to go slower. A or alpha would be the same for a given wing design or even different loadings, and is independent of speed, because by the definition of Va, the wing must be at its stalling angle of attack at Va.

Not sure if I am answering the right question or not...

Also remember full abrupt of the controls refers to the Elevator not Ailerons...

Uhh, No...... It applies to all control surfaces, what is does not account for especially with the tail is back and forth movement of the control surfaces, think AA Flight 587.

What do you mean by "two different speeds"? Va decreases with weight but that's a continuous process, so I'm not sure what you are referring to.

But, if you're asking why Va decreases with weight, it's usually a good idea to have a couple of explanations in your toolkit. If you have a student who's mathematically inclined, a too-simple explanation cna make you sound like an idiot. OTOH, if your student is math-phobic, a too-complicated one can leave her with a deer-in-the-headlights stare.

With that in mind, this explanation was derived from a number of online and offline sources.

Let's go back to the definition of maneuvering speed. Euphemistically, it's the speed at which an airplane will stall before it breaks due to a gust or abrupt control movement.

Putting it in slightly other terms, it's the speed at which the wings can suddenly go from their existing angle of attack to their critical angle of attack without increasing the load factor (G-force) beyond the aircraft's design. For normal category aircraft, that design maximum is 3.8 G.

Let's fill this out with some numbers. We are flying an airplane that stalls at 15º AoA. At it's normal 120 KT cruise, it's AoA 3º.

What happens if we suddenly change the AoA from 3º to 15º? Because there is (roughly) a one-to-one relationship between increase in AoA and increase in load, we have just increased the ~1-G cruise load on the wings by a factor of 5 G. Too bad we suffered structural damage at 3.8!!

What we're really trying to do to protect ourselves is increase our AoA so that the gap between our AoA and the critical AoA is smaller. How do we do that? We slow down. When we slow down while maintaining level flight, we reduce power and increase pitch, which increases our AoA. So, let's say that flying our hypothetical airplane level at a 90 KTS takes a 5º AoA. Even that small change means that suddenly bridging the AoA gap only involves a 3-G increase, below the 3.8 G damage point.

Why the slower speed for lower weight? Well, in general, a lighter airplane can maintain level flight at a particular airspeed with a lower angle of attack. So the cruise to critical AoA gap is larger at lighter weights. So we need to slow down more to get our cruise AoA where we need it to be to keep the gap manageable.

That was the great misunderstanding prior to AA587. Yes, you can go full deflection with the controls and then back to neutral (center). What you can NOT do is full deflection and a complete full deflection reversal. A recent presentation showed the forces created on the A300 rudder were about 4-5 times the design load. But aggressive rudder was part of the windshear training and also part of the 737 rudder hard-over training at one time.

Actually, for airplanes certified to Civil Aviation Regulations (CAR) 3 Par. 3.184, the rudder and vertical stabilizer did not have to do anything to pass, save for not fall off or exhibit visible damage. There was no deflection requirement or load rating for the rudder at all. Right off hand I am not sure how many airplanes were made under this particular certification rule, but there are many. All the Cessna piston singles up to a certain point, I would venture a guess somewhere in the late 1960's were CAR 3. This did not necessarily mean they were underbuilt, in fact they were overbuilt because it was thought the tail/ rudder assembly should be stronger than the other flight controls.

The definition of Va from the PHAK "The maximum speed where full, abrupt control movement can be used without overstressing the airframe"

The reason for the range of speeds 88-111 in a Warrior is based on weight
88 = Max Gross Takeoff Weight
111 = Basic Empty Weight

Any flight that you are legally operating (not going over max gross takeoff weight), would fall in that range.

Aircraft loading in flight can be asymmetrical (i.e. with multiple control inputs at the same time) The conventional thinking prior to 587 was that Va was based on wing loading (macro-level), not individual structural loading (micro-level).

Another key here is that maneuvering speed now has a bit to with components that do NOT change in weight even though the aircraft changes in weight. A rough example of this would be engine mounts, trim tabs, etc.

A full-rudder deflection in the A300 was almost the equivalent of doing a snap-roll in the aircraft

On a side note... I kinda like to think of Va as Velocity - acceleration from F=MA