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

"The maximum speed where full, abrupt control movement can be used without overstressing the airframe" - That definition is completely false! Thanks to the FAA and other flight training books, Va has become a big misunderstanding among pilots.

Va is only valid for positive Gs. Va does not protect the aircraft structure under negative Gs and abrupt sideslip under high dynamic pressure (ie AA587).

He is right. It applies only to the elevator up to a determined positive load factor.

I agree, elevator only by definition.

To further confuse matters, some designers may ensure that the other controls surfaces are also protected below Va so for some models it might be all-inclusive...but don't bet on it unless it's documented.

I suspect that the ailerons usually have a greater margin than the elevator anyway in most designs. Aileron inputs don't cause the wing load to change dramatically.

The rudder/vert stab on the other hand may well have lower margins...as was demonstrated dramatically in 2001.

I am unsure of the negative portion of Va, but the VG diagram seems to support this claim if you observe the negative G load would be slower than the positive in most cases.



However there are two rules I was taught in aerodynamics that were beat into my head, specifically because at the time we were doing aerobatic training. Va is predicated on two key points during certification, the loading is:

Symmetrical: Meaning level flight, not in turns where one wing may experience a greater load than the other.

Progressive: This one is a little tricky, notice many sources claim "full abrupt" with regard to controls. Now anyone here who has been in an aerobatic aircraft know of "snap speed" or the speed at which you really can go full and abrupt with the controls. Va is determined based on progressive, non snap, meaning not abrupt, loading. A quick pull back, ok, but don't think you can rip that control to your lap right at maneuvering speed and be completely safe, that is why there are snap speeds for aerobatic aircraft. Can anyone verify this with a source other than my flight dynamics professor, he did not source it for us but used our cap 10 snap speed as an example to prove his point.



There is one last item used for determining load factor, that is the material used. Seems like an obvious one so I usually leave this one out.

Va INCREASES with weight!


This is how I've always thought about it. Va is the highest speed at which you can have maximum deflection without hurting the airplane.

A lighter airplane is more "flickable." It responds faster to control inupts and is more easily overloaded. A lower airspeed is required to prevent the airplane from "flicking" too fast and putting too much stress on the airframe.

A heavier airplane would be more "sluggish," it won't respond as quickly to control inputs. Instead of jerking into a roll like a light airplane would if you went full deflection, the heavier airplane would do more of a slow roll with full deflection, allowing for a faster airspeed to be used with that full deflection.

Here is how I explain it:

Be sure to read this with an open mind, even if math scares you, it is only basic addition which I believe anyone is capable of understanding. Enjoy.

I like to use what I call the practical lift formula, that formula is Lift = Speed + AOA. It is nothing more than a general method for seeing how Speed and AOA work together, increase one the other must decrease to maintain the same lift.

Let us assume we stall at 17 degrees, since Va is related to the amount of deflection we are allowed let's say that we must be at or above 7 degrees for us to be at Va. If we are at 7 degrees we will have a safe amount of deflection, in reality these numbers also don't work but are used here merely for demonstration purposes.

Input some numbers, now these are also unrealistic but will unquestionably demonstrate the relationship with weight and Va.


Aircraft one: Weight = 20 which means Lift must equal 20 to support this weight, remember lift equals weight.

Apply practical lift formula: 20 (Lift) = 7 (AOA) + 13 (Speed)


Aircraft two: Weight = 22, again Lift equals 22 to support that weight.

Apply practical lift formula: 22 (Lift) = 7 (A0A) + 15 (Speed)



Conclusion: At the heavier weight of 22 the speed was able to be 15 instead of 13 at the lighter weight. Obviously these numbers aren't logical, but who cares? They serve the purpose of showing, with an obvious, easily understood, representation that with a heavier weight we can attain a faster speed with the same AOA and thus a faster maneuvering speed.

Disclaimer: I purposely annotated each formula with () to show which each field was. This was done for simplicity with regards to the reader. Since many people don't like math because there are numbers that don't represent meanings this depicts the meaning clearly next to the number.

Wow! Really appreciate the input from everyone! I'm just gonna run this by you guys just to make sure I have it right.

Va is defined as the highest speed that an a/c can be at where abrupt control movements will just cause it to stall, not break. In our C172's, Va is 90 (for the basic empty weight) to 105 (at max gross weight), and if we use abrupt control movements at or higher than whatever speed pertains to our weight, we risk structural damage.

Does it sound like I have it right?

Yea that sounds about right... The easiest way I was able to understand and teach it was take a cessna at max weight, and then at say middle of the road weight.. At the speed that is ref. the pilot goes full back on the elevator. Visually think what the G's would be at the max gross weight at both speeds, and then what would it be like at the lighter weight. At the lighter weight to maintain altitude you must pitch up so as a by product you are closer to your critical angle of attack, vs if u stayed at the faster Va at the lighter weight which would put you at a higher angle to travel to your critical angle of attack and cause higher load factors that could cause structural damage.

Hope that helps

WOW! I guess I screwed the pooch on that one and got it a$$-backwards. Guess I should have posted after my morning coffee.

Va decreases as weight decreases. Va increases as weight increases.

Although fjetter did get those numbers backwards. Higher with higher; lower with lower.

Correct, but you just said "Va decreases with weight." You didn't specify an increase or decrease with weight, but when someone says "X decreases (or increases) with Y" it's assumed that Y is increasing. At least that's how I've always seen in presented. Could be a regional (geographical, not airline) thing. Like "Air temperature decreases 2C with every thousand feet" or "Air pressure decreases with altitude."

Although I guess after re-thinking the way you said it, I picture weight "with" Va, holding hands and strolling down a merry decline together. So yeah, Va would be decreasing with weight :D. So i guess you could say it like that too. I just never heard that kind of statement worded that way.

Just didn't want to confuse the guy. When I read what you wrote, I assumed you meant an increase in weight. Just a matter of semantics, I guess :) .

So what if the aircraft moves into another category. For example, light cessna moves from normal into utility?

No, you're right. My explanation should have been clearer.

Huh?

Maneuvering speed is a structural-related limitation. I'm not sure that it does change with the aircraft's category. The general rule that the max gross weight number is always listed (if there's only one number, it's for max gross). Listing others is pretty much optional, although airplanes that have a wider range of possible weights will tend to list more since Va for 6-seat airplane with lots of fuel and baggage capacity will be significantly lower when someone flies it solo with only a headset and a kneeboard with only 1 hour of fuel left. So I guess that if utility category means less max gross, a manufacturer might well opt to list a Va for max weight when operating in that category.

btw, I don't think anyone mentioned it, but the formula for calculating maneuvering speed (or any load limited V-speed for that matter) at any weight is

V1=V2 * Sq Root (W1/W2)

Where:
V1=Airspeed being calculated
V2=Published airspeed for a given weight
W1=Weight for which airspeed is to be determined
W2=Gross weight for which airspeed is published

Va for light aircraft, the manufacturer bases it on load limit factors. Going from category to category changes the load limit factors (3.8normal to 4.4utility). A 4.4 load limit will allow more airspeed at less weight. Notice how 172s' are placarded with "Va = 105". The safety factor is 1.5 for structural items.

Usually the formula used for (light aircraft manufacturers) Va is Va = stallspeed * sqrt(limit positive load limit factor)

One thing to consider doing, let say aerobatics, is that certain high-g maneuvers require a higher entry speed than Va. Of course these maneuvers are symmetrical, but it really just depends on the amount of residual energy. I think Assymetrical loading has a little more to do with a Vd/Va interpolation.

It's been a few years since I've done one, and we did them ad-nauseum in aersopace college of course but a given manufacturer submits a "VN Diagram" for a new airplane design to the FAA or their designate which must be approved. V stands for velocity in feet per second, and N is load factor. It looks like the ones you see in pilot textbooks, but with a gust envelope added as well, see diamond shaped envelope shown in this one:

http://i284.photobucket.com/albums/l.../VNdiagram.jpg

Just in case we have some european readers, ultimate load factor for them is two times limit load factor for their certifications.

To clear this up since I have had a bunch of questions arise from speaking of limit load and ultimate load factor:

Limit load factor = the published g limitations in your POH and the point where structural damage can occur.

Ultimate load factor = as Ryan said, in the USA 1.5 times limit load or two times it in Europe. This is the point where structural damage is certain and items may begin to come off the aircraft. Ryan: Is it may come off or will come off at ULF? I can't recall for certain.

I have really no idea at all.... my only slightly educated guess would be that it is based off the "weakest link" - that is maybe the wings will hold on but the engine mounts might go or vis-a-vis... or maybe it's when detectable damage occurs.

We just got a Ops Bulletin regarding Va again and thought I would share it.
I re-read this thread before posting and agree that there certainly is some confusion :)

From the Special Airworthiness Information Bulletin (Jan 18, 2011):
USMCFLYR