Question about Va speeds

[white_knuckle_flyer]

So during CPL groundschool, the question came up as to why Va increases as weight increases. I volunteered my answer which was something along the lines of "...because the increased weight under the same forces will accelerate less and therefore undergo less stress." I was a little surprised when the instructor shrugged it off as incorrect, since it had been a satisfactory answer during my PPL flight test.

I now have a better ( I think ) understanding of it and will be the first to admit that my answer is perhaps only a part of the answer and perhaps a very small one at that, but my answer is not totally out-to-lunch is it ? Perhaps, the whole F=ma side of it is secondary to the "real" reason which I "believe" ( sorry I'm being heavy-handed with the quotation marks) is essentially that a heavier plane will have a higher stall speed and therefore the higher speed still allows the plane to stall before structural damage occurs.

I may have missed out mentioning angle-of-attack somewhere in there, but do I basically have it correct ? And if not, could anyone offer me the anwer to end all answers ?

[DanWEC]

You had the right idea initially, but you don't need to confuse the concept with AoA and stall speed... they are immaterial at this point. Might have been lost a bit in translation with your description to the instructor though, but the first part is definitely correct.

There is nothing about AoA that's required. You may mention the control surfaces though. They can only impart a fixed amount of change to the airflow. The heavier plane will react slower to that change, giving less wing loading.

[2R]

FAA-AC-61-21A page 299
Vg load airspeed diagrams are explained.
Bon Appetite

[photofly]

...because the increased weight under the same forces will accelerate less and therefore undergo less stress.

I think that's an excellent single-sentence explanation.

My aide-memoire for explaining the whole Va thing is to consider an example structure of the aircraft: let's say the engine mount. The engine mounts are only strong enough to bear the weight of the engine & propellor multiplied by the maximum g rating of the aircraft (let's say 4.4g for utility category). If you accelerate the airframe by more than 4.4g you will overstress the engine mount. This is a bad thing.

As the aircraft as a whole gets lighter it requires less and less lift (force) to generate a 4.4g acceleration.

From the lift equation, the maximum lifting force the wing will ever generate is proportional to v^2. To limit the lift force to lower values, therefore, the airspeed limit has to decrease.

[photofly]

Incidentally you might be interested to learn that the "real" definition of Va is only tangentially connected to overall loading.

FAR23 is the US regulation under which most aircraft are certified (or else the older CAR3). FAR23 defines a whole list of design speeds, Va, Vb, Vc, Vd etc. which the aircraft manufacturer has to specify as part of the design. Va is a speed at which a full single application of ailerons or rudder is guaranteed not to damage the control. It's also required to be not less than the square root of the g-limit times Vs1. But it may be more.

If it is more, then it's not the limit you think it is: in that case, limiting your speed to Va will not guarantee that the aircraft will stall before it breaks.

A straw poll of the very limited number of aircraft types to whose POH I had access showed me that in all cases the designers had opted to use the lowest value of Va allowed, which is equal to Vs1 * sqrt(g-limit). That means that value of Va is an appropriate stall-before-bend limit. But it doesn't have to be.

I realize that lots of text books and flight instructors aren't aware of the origin of Va, or its "official" definition, and they prefer the one they were taught. Regardless, as far as I can see, Va under the FAA definition is the one that is listed in the POH of an American-certified aircraft.

If anyone is interested, the definition of Va is in FAR23.335(c), and its application to the strengths of control surfaces in 23.441 (rudder) and 23.445 (ailerons). Interestingly, it doesn't appear to mention the elevator strength at all.

FAR23 is at http://www.ecfr.gov/cgi-bin/text-idx?c= ... 10&idno=14

[dr.aero]

Let's say that in the case we're discussing that maneuvering speed equals Vs * sqroot of positive load limit.

Vs increases with weight, therefore, Va will increase with weight.

The reason why is because Va is limited by acceleration and NOT force. You may ask why limit the acceleration when force is what breaks things. The reason is that the internal structures are designed to support a certain mass - Force = mass * acceleration. If you know the mass the structure is supporting and you know the force that's required to compromise the structure, then you can solve for the acceleration required to compromise the structure.

Imagine the floor of the plane where you're allowed to put 150lbs of baggage. For certification they will ensure that the floor can withstand the force created by accelerating 150lbs at the load factor limit plus a 50% safety margin. If your limit load factor is +4.4G then the design load factor is +6.6G - the floor will need to withstand the force of 150lbs accelerated at 6.6G.

If you look at an engineering text you'll see that they say maneuvering speed does not change based on weight and that the lighter you are, the more G you can pull. That's true because they're talking about the wings breaking off. It's force, not acceleration, that breaks things!

A wing will always produce the same force if in the same configuration, speed and angle of attack. It doesn't matter what the fuselage of the airplane weighs. So if you find the speed that produces the force required, at max angle of attack, to break the wings off - that's the maneuvering speed. If you're defining it like that, then you'll notice that the G force required to be pulled before your wings break will increase if you're lighter.

Since Va is based on acceleration and not force - to keep the same acceleration, you need to decrease Va as you get lighter.

If it is more, then it's not the limit you think it is: in that case, limiting your speed to Va will not guarantee that the aircraft will stall before it breaks.

photofly...

Misleading. Look at FAR 23.423(a). If Va was higher, the airplane would need to be able to sustain the loads created at Va with full deflection of the elevator - the only thing that would happen if Va was higher than Vs * sqroot n would be that you wouldn't necessarily stall, but you would need to ensure the airplane doesn't break at that higher load created by the higher Va.

You're mixing up the definition of Va and the maneuvering load requirements. Va is usually never higher than Vs * sqroot n because that would mean the manufacturers would need to potentially add more structural strength (weight) to ensure the airplane didn't break at the Va speed.

Think about it like this - maneuvering speed is determined based on FAR 23.335(c), and then there are other FARs (23.423 being one of them) that ensure that the loads imposed in flight at Va will not compromise the aircraft structure.

[dr.aero]

"...because the increased weight under the same forces will accelerate less and therefore undergo less stress."

I'd also say that that statement isn't correct... but most likely for different reasons than your instructor.

The reason it's not correct is because stress is related to force, not acceleration. Understanding that force is what breaks things and not acceleration is what you need to do first. Acceleration is the result of a force applied - not the other way around. If that's not clear then I'll explain further.

[esp803]

Also a little brain teaser for you (Aerodynamic Buffs please let other respond first) regarding Va and weight...

In a Caravan (like many other airplanes), it is possible to get an upgross kit. On the van it's pretty basic: Wing fences, bigger axles, and a little placard that says "at 9062lbs VA=143" Which is less then the VA of the original gross weighted Caravan (8750lbs)... Do you know why?

E

PS: it's been months since I've flown... I think it's 143, but may be something else, it is less then the original VA at MCTOW though.

[photofly]

Va is usually never higher than Vs * sqroot n because that would mean the manufacturers would need to potentially add more structural strength (weight) to ensure the airplane didn't break at the Va speed.

Usually is not the same as always.

The point is that FAR23 specifies only certain parts of the aircraft that must not break, with full control application, at Va. For example 423 refers to wings and horizontal surfaces, and I see that 23.371 refers to the engine mount. There are myriad other load-bearing elements that aren't required to meet 23.423. Stalling at Va doesn't guarantee you won't break them.

The only general guarantee you have over aircraft strength is the g-load limit. For which a safe stalling speed is sqrt(g-limit) * Vs1. That might be the same as Va, but it might be less. Have you actually checked?

[TopperHarley]

Im no engineer, but another helpful way to look at Va is that, as long as you respect the speed while pulling as hard as you can on the stick, you will stall the airplane before you ever cause structural damage due to the forces. It's better you stall the plane than to break up in the air, at least there's a chance you can recover.

[Colonel Sanders]

I hope you guys never think about torsional loads
on the airframe (e.g. full deflection of elevator and
aileron at the same time. I won't dare mention the
rudder).

[photofly]

Im no engineer, but another helpful way to look at Va is that, as long as you respect the speed while pulling as hard as you can on the stick, you will stall the airplane before you ever cause structural damage due to the forces.

That's what the flight schools teach, but it's not correct.

If you pull hard on the stick at Va you won't break: the wings, the tailplane, or any of the control surfaces. Or the engine mount. But there are plenty of other parts of the aircraft that you are not guaranteed to leave unbroken.

I hope you guys never think about torsional loads
on the airframe (e.g. full deflection of elevator and
aileron at the same time.

I do think about that: what I think is "don't do that."

I won't dare mention the
rudder).

Is this a good time to mention AA587?
http://www.ntsb.gov/doclib/reports/2004/AAR0404.pdf

[dr.aero]

Photofly...

I think what you're missing is FAR 23.391.

[dr.aero]

Colonel...

I definitely have thought about it and it's also quite clear that you should never do that as the airplanes aren't certified/tested for that.

[photofly]

Photofly...

I think what you're missing is FAR 23.391.

§ 23.391 Control surface loads.
The control surface loads specified in §§ 23.397 through 23.459 are assumed to occur in the conditions described in §§ 23.331 through 23.351.

I'm not sure how that helps us ... can you construct a chain of reasoning that links, for example, the strength of the baggage compartment floor, to Va? I can't see one.

I wonder if there are any aircraft for which Va > sqrt(g-limit) x Vs1. Does any know know of one? If there aren't, then it's only an academic distinction.

[Colonel Sanders]

This isn't merely academic.

As an example, Va is 100 mph. At what speed may I:

1) use full aileron and full elevator together?
2) use full elevator and full rudder together?

As I said, this isn't merely academic. Let's
say you're doing a flight to remove the non-aerobatic
restriction from a homebuilt, which has the aforementioned
Va of 100 mph. What speed are you going to recommend
as the maximum speed for a snap roll? (e.g. #2 above)

PS I go with Aerodynamic For Naval Aviators on this.

[Steve Pomroy]

you would need to ensure the airplane doesn't break at that higher load created by the higher Va.

Va is usually never higher than Vs * sqroot n because that would mean the manufacturers would need to potentially add more structural strength (weight) to ensure the airplane didn't break at the Va speed.

as long as you respect the speed while pulling as hard as you can on the stick, you will stall the airplane before you ever cause structural damage due to the forces.

No. Maneuvering speed sets the certification standard for the structural strength of the control surfaces. There is no guarantees that you cannot break other parts of the aircraft at (or below) maneuvering speed. For the structural protection of other aircraft parts, reference the limit load factors and limiting airxspeeds (mostly Vne, Vno, Vb).

I hope you guys never think about torsional loads on the airframe (e.g. full deflection of elevator and aileron at the same time. I won't dare mention the rudder).

Valid point. Maneuvering speed is based on the deflection of a single control surface at a time.

For example 423 refers to wings and horizontal surfaces

The wings are only referenced for canard and tandem wing arrangements, if the wing has a pitch control.

I wonder if there are any aircraft for which Va > sqrt(g-limit) x Vs1. Does any know know of one? If there aren't, then it's only an academic distinction.

PA-28-140. G-120A. Not much of a difference in either case (4 mph in the PA-28, 4 knots in the Grob at max aerobatic weight), but it's there.

As an example, Va is 100 mph. At what speed may I:

1) use full aileron and full elevator together?
2) use full elevator and full rudder together?

Unless that number is published, you really have no way of knowing. In the Grob the elevator + rudder speed is published: 110 knots vs a Va of 165 knots. I've never seen a combination speed published in any other aircraft that I've flown (that doesn't mean it's not done for aircraft I haven't flown, of course).

There's some more clarification here on my blog:
http://www.flightwriter.com/2010/09/va- ... stood.html
http://www.flightwriter.com/2010/09/va- ... od_27.html

Cheers,
Steve
http://www.flightwriter.com
http://www.skywriters.aero

[photofly]

Colonel - you've got access to a range of aircraft data - could I ask you to pull out the POH values of Va and Vs1 for some of them, and post it?

[Colonel Sanders]

In the Grob the elevator + rudder speed is published: 110 knots vs a Va of 165 knots

Excellent. Note the massive decrease when more than one
flight control is fully deflected. Also note that these numbers
line up exactly with what is recommended in Aerodynamics
For Naval Aviators!

2/3 x 165 = 110

you've got access to a range of aircraft data

That's on the bookshelf at home, sorry!

[dr.aero]

Photofly...

I think what you're missing is FAR 23.391.

§ 23.391 Control surface loads.
The control surface loads specified in §§ 23.397 through 23.459 are assumed to occur in the conditions described in §§ 23.331 through 23.351.

I'm not sure how that helps us ... can you construct a chain of reasoning that links, for example, the strength of the baggage compartment floor, to Va? I can't see one.

I wonder if there are any aircraft for which Va > sqrt(g-limit) x Vs1. Does any know know of one? If there aren't, then it's only an academic distinction.

Read 23.391 - note that all the control surface loads that we're discussing are contained within 23.397 through 23.459. Those regulations are assumed to occur in the conditions described in 23.331 through 23.351 - 23.337 falls within that range. Then read what 23.305 says about the structure being able to support loads up to the limit load.