Here is a link that might help with the understanding of Va. There are plenty of other reference online if you want to dig a little further.

[url= http://www.boldmethod.com/learn-to-fly/aerodynamics/va-designed-maneuvering-speed-what-does-it-protect/ ]One explanation of Va[/url]

Hope this helps!

Said it before, say it again, the amount of force the wings can cope with does not explain why Va lowers with lowering airplane weight. When the plane becomes lighter, a given AoA and airspeed will result in greater acceleration, and it is the greater acceleration that will break the airplane. Force = mass times acceleration. F=ma. So, if they force is the same (AoA and airspeed), then the airplane will accelerate more if there is less mass. That is why Va lowers with increased airspeed.

The wings will not be affected by the increased acceleration because the force on the wings is the same. The wings will be no more likely to break off because the force on the wings will be the same. (If there was some delicate structure inside the wing, then it might be affected, like, say the fuel tank mounts, but not the wing itself.)

[color=blue]That is why Va lowers with increased airspeed.[/color]
Very confusing - perhaps the following will help!

The Manoeuver Speed Va can be defined as the max weight stall speed at the aircraft’s designed “limit load”. The limit load for a utility category aircraft (such as the C150) is 4.4g. I will ignore the -ve load limit for simplicity

It is helpful to think of Va as an acceleration limit and not as a force limit - see Vn diagram below.
At speeds Va, aerodynamic forces will over-stressed the aircraft before the wing stalls

[attachment=1141]Vn.jpg[/attachment]

When the manufacturer determines a value for Va, they are not entirely worried about breaking the wing, they are also concerned about breaking other important parts of the airplane, such as the engine mounts, fuel tank mounts and other structural components of the aircraft. These other items don’t directly care how much force (lift) the wing is producing; they just care about the acceleration they are undergoing.

By reducing the mass of the airplane an increase in acceleration will result from any overall force applied (F=ma). That being said, if mass is decreased, a lower Va will be required at the critical angle to reduce the aerodynamic forces to maintain the acceleration at the design “load limit”

The above a very simplistic view of why Va reduces as weight reduces.

When you say that "they are not entirely worried about breaking the wing" it seems like you are agreeing with me, and that I agree with you. The force on the main spar of the wing will be the same at a given AoA and airspeed.

Page 10 of RPL/PPL says "... bending loads on the wings beyond the design limit".

And that is misleading.

(I say "misleading" rather than "wrong", because, with a maximally loaded airplane, well above Va, you could overstress the wings with an abrupt tug on the control column. But that, to take us back to the very start, does not explain how Va decreases with increased aircraft mass.)

Va is a design speed, it doesn't change with weight unless the manufacturer actually provides a range of Va v's weight.

First, I'll offer a few comments on previous posts, and then Va and, finally, we can have a look at why one should reduce Va at weights below the maximum POH weight.

[color=blue][i][b]... Va was limited by the max force the wings could cope with.[/b][/i][/color]

Not quite correct and only looks at one consideration. However, it is a common training definition and well-intentioned.

[color=blue][i][b] If that was true, Va would not decrease with decreased aircraft load.[/b][/i][/color]

We can have a think about why we might reduce Va a little later on.

[color=blue][i][b]Here is a link that might help with the understanding of Va[/b][/i][/color]

Carello's link appears to be to someone who has taken the time to read a bit about the Design Standards, in particular, FAR 23.

[color=blue][i][b]The Manoeuver Speed Va can be defined as the max weight stall speed at the aircraft’s designed “limit load”[/b][/i][/color]

Now, that's often a correct comment, but not encompassing. There are a few things which go into the Va recipe mix.

[color=blue][i][b]if mass is decreased, a lower Va will be required ... to maintain the acceleration at the design “load limit”[/b][/i][/color]

That's getting to the crux of the reduced Va consideration.

[color=blue][i][b]Va is a design speed, it doesn't change with weight unless the manufacturer actually provides a range of Va v's weight.[/b][/i][/color]

Certainly a design speed and this will be at maximum gross weight. Usually, one should expect to see a POH schedule of reduced (effective) Va with reducing gross weight.

Now, we should consider that Va has a number of component considerations. If one has a flick through FAR 23 (and that is a useful exercise) - remember that, if you have a particular aircraft in mind, first you need to look at the Type Certificate Data Sheet to get an idea as to what version of the regulations was relevant to the design of that aircraft. Both of these can be found via the faa.gov website.

A quick run through some relevant bits of FAR 23 (using a superseded version for convenience) indicates that

[color=blue][i][b]23.335[/b][/i][/color] Va must not be less than gross weight clean stall speed x square root (limit load factor). This just says that Va cannot be less than the point identified on the envelope shown in a previous post. If you haven't realised the significance of the curved line at the lower speed end, this is the stall line .. the aircraft is stalled if it finds itself above the line. If Va is selected to be higher than this speed, it need not be higher than Vc.

[color=blue][i][b]23.423[/b][/i][/color] At Va the structure has to be able to withstand a prompt movement of the elevator either to the stop or (and this is important if you have the strength of your friendly neighbourhood gorilla) the maximum presumed pilot loads (found at 23.397)..

[color=blue][i][b]23.441[/b][/i][/color] At Va the structure has to be able to withstand (from an initial steady flight, zero yaw condition) full rudder application or the maximum presumed pilot load.

[color=blue][i][b]23.445[/b][/i][/color] At Va the structure has to be able to withstand (from an initial symmetrical flight condition) full aileron application or the maximum presumed pilot load.

Now, what this says is

(a) from initial, idealised conditions - we are unlikely to be able to achieve this in routine flight activities where we might wish to input significant control deflection

(b) apply a control input for ONE control and ONE input ONLY (and then release the input) - the design case does NOT cover multiple rapid control inputs or inputs for multiple controls simultaneously

(c) the control input is limited by the presumed pilot load capability if that is relevant to the aircraft - it could be possible for a stronger pilot to exceed these presumed forces.

It follows that the pitching consideration may not be the limiting factor for a given aircraft, eg, as I recall, the HS125 Va was predicated on directional stablility considerations with the rudder input.

Be aware that there have been inflight failures of aircraft where operations have been outside the design envelope. You might like to search for reports on the American Airlines Flight 587 mishap. Although the following are not Va related, if you would like to research just how quickly things can get out of hand with flight path control problems, you might also like to look up the losses (at supersonic speeds) of the B58 Hustler (Nov 1959 in Oklahoma) and the SR-71 (Jan 1966 out of Edwards) and, for the latter, TP Bill Weaver's rather amazing survival. In both of these mishaps, the fuselage disintegrated - very rapidly - around the crew, killing three out of four crewmen. I imagine that Weaver didn't bother buying any lottery tickets that week .. he had well and truly used up his quota of good luck !

The AA failure led to some serious rethinking of pilot training and education.

The thing to keep in mind is that many design cases are "line in the sand" considerations and not really intended for the average line pilot to put to the test. Should you go off, fly at Va and play fighter pilot ? .. absolutely NOT .. you would be better advised to treat the aircraft with respect and gentle manoeuvring near Va. At speeds somewhat below Va you should be OK but, at the end of the day, why would you want or need to do this sort of thing in a normal category aircraft ? If you really must, go fly an aerobatic aircraft (while respecting the POH's guidance in respect of speeds for various manoeuvres) which has very much higher structural reserves. Far better if you be gentle with the machine.

Now, Va is a Design Standard thing - the aircraft envelope presumes that the aircraft will NOT be subjected to more than the design load factor (think maximum g-load) which, for many normal category aircraft, is 3.8. For the envelope shown earlier, the intersection of the stall line and the Va speed coincides with the limit load factor for the aircraft at the design (maximum) all up weight .. hence the typical comment (not quite correct) that you will always stall prior to causing damage at or below Va.

If your aircraft is less than gross weight, then the stall line compresses to the left so that, say, at gross weight, book Va you now can pull MORE than the limit load factor prior to the wing's stalling. The only way to preserve the Va pitch philosophy is to reduce the gross weight Va speed to a value which intersects the adjusted stall line at the limit load factor. In this way we don't cause any problems with any other aspect of the aircraft's structural design.

Now, just to clarify. If the aircraft is at max gross and Va, and you pull back and pull 3.8 g, the main spar has a particular amount of force on it. If the aircraft is lighter loaded, and you pull back to, say 4.2 g, how is the force on the main spar increased?

F=ma is the general formula for force, mass and acceleration

F = gross weight x 3.8 is the formula for your airoplane

at a particular AoA and airspeed, F will remain the same. If you decrease the weight of the aircraft by 10%, then this will happen.

F = (0.9 x gross weight) x 1.11 x 3.8
= (0.9 x gross weight) x 4.2

It is the F that is bending the main spar. The acceleration is what the main spar does to the rest of the airplane. Hence, Va decreasing is not to do with protecting the wing.

[color=blue][i][b]how is the force on the main spar increased?
Hence, Va decreasing is not to do with protecting the wing.[/b][/i][/color]

I make no such claims. The considerations are irrelevant to the question of why Va reduces with reducing weight ....

And that is how come it is misleading to say that Va is there to protect the wings.

In case anyone is interested, I will give a different example.

Suppose that an aircraft is flying a stable maneuver accelerating at a constant 3.8 G. Suppose the aircraft suddenly becomes 20% lighter. If the aircraft is 20% lighter (and the AoA and speed remain the same) the acceleration will increase to 4.75 G. The force on the main spar will not increase.

The maths is as follows.

F = ma
= 3.8 m.

F remains constant because the AoA and airspeed remain constant. When the airplane suddenly becomes 20% lighter, with no change in COG, the formula changes.

F = (0.8 m) x (1.25 x 3.8)
= (0.8 m) 4.75

You really are getting to the point of being a little silly in your persistence with things not of much relevance to the discussion ...

[b][color=blue][i]And that is how come it is misleading to say that Va is there to protect the wings[/i][/color].[/b]

Va, most certainly, [i]inter alia[/i], is concerned with protection of wing structure .. how can you possibly say otherwise ? At gross weight, one of the design cases sees the wing stall - without damage - at the limit load factor .... how does this not provide for structural protection ? One needs to note the previous discussion about the design case's not necessarily being directly applicable to what you might be able to do in flight.

[color=blue][i][b]Suppose the aircraft suddenly becomes 20% lighter. If the aircraft is 20% lighter (and the AoA and speed remain the same) the acceleration will increase to 4.75 G. The force on the main spar will not increase.[/b][/i][/color]

Where you are getting a bit lost here is that you are concerning yourself with centroidal loads only. These are certainly of interest, say, for stability and control and performance analyses but don't have a great deal to do with detailed structural analyses. On the other hand, applied and inertial [b]load distributions[/b] are very relevant. The applied loads may well change with deformations due to changed mass distributions now not being within the presumptions of the gross weight analysis - keep in mind that we are dealing with relatively flexible structure here - and it is very likely that the lower weight mass distribution changes things enough to cause some concern with a simplistic approach such as you appear to be pursuing.
[color=blue][i][b]
The force on the main spar will not increase.[/b][/i][/color]

Why are you preoccupied with the spar ? That may not be the critical design case for a given aircraft. In any case, as I suggest above, the changed load distribution may well adversely affect spar (and other structure) loads and margins of safety. That is to say, your view is far too simplistic.

More importantly, your thesis is quite irrelevant. The Type Design is predicated on a nominated limit load factor. You [b]MAY NOT[/b] exceed that load factor, intentionally, under [b]ANY[/b] circumstances. To do so puts the aircraft's operation outside the approved, certificated operating envelope as prescribed in the Type Certificate, Type Certificate Data Sheet, and Approved Flight Manual (or POH, as it is more commonly called for light aircraft). Just what structure within the aircraft might be affected by an exceedance is not published for your information and you would be entering dangerous waters ...

Further, this puts you in the area of illegal operation and the potential for both court-imposed and CASA administrative penalties, not to mention the possibility of action being taken against your licence.

Consider the following from the 1988 Regs ..

[b]138 Pilot to comply with requirements etc of aircraft’s flight manual etc[/b]

(1) If a flight manual has been issued for an Australian aircraft, [b]the pilot in command of the aircraft must comply with a[/b] requirement, instruction, procedure or [b]limitation concerning the operation of the aircraft that is set out in the manual.[/b]

Penalty: 50 penalty units.
......
(3) An offence against subregulation (1) or (2) is an offence of strict liability.

Sub reg (3) means that [i]mens rea[/i] doesn't come into it and, once disclosed as an occurrence, you're done and dusted. A tad silly, I suggest to do what you are proposing - last I noted, a penalty unit is around $210.

Personally, I'd be far more concerned with -

[b]269 Variation, suspension or cancellation of approval, authority, certificate or licence[/b]

(1) Subject to this regulation, CASA may, by notice in writing served on the holder of an approval, authority, certificate or licence (an authorisation), vary, suspend or cancel the authorisation if CASA is satisfied that one or more of the following grounds exists, namely:

(a) that the holder of the authorisation has contravened, a provision of the Act or these Regulations, including these regulations as in force by virtue of a law of a State;

(b) that the holder of the authorisation fails to satisfy, or to continue to satisfy, any requirement prescribed by, or specified under, these Regulations in relation to the obtaining or holding of such an authorisation;

(c) that the holder of the authorisation has failed in his or her duty with respect to any matter affecting the safe navigation or operation of an aircraft;

[b] (d) that the holder of the authorisation is not a fit and proper person[/b] to have the responsibilities and exercise and perform the functions and duties of a holder of such an authorisation;

Sub reg 1(d) is used, I understand, as a catchall provision to slap folk over the wrist where CASA so deems necessary. You can find more than a few tales on the net of pilots who have experienced significant grief due to this provision ...

Thank you for your ongoing interest. You suggested that I was being silly going on about things that were not relevant to the discussion. The original post was that Va is not about preserving the structure of the wing. So that is what the discussion is about. I have posted lots of things that no one has commented about. If you do not think that the original topic is relevant to you, that's fine.

William Kershner's book The Advanced Pilot's Manual, 8th edition, at 52%, or location 5382 of 10379, says the following.

"pounds but not for lower weights. The maneuvering speed must decrease with the square root of the weight decrease, as introduced in Chapter 1.Take the airplane at a near empty weight of 1,500 pounds. Admittedly, it's unusual for an airplane to be able to fly at a weight one-half its gross weight, but there are some models capable of this ratio and it makes for easier figuring. Assume again that the airplane is abruptly stalled at 117 K at the light weight of 1,500 pounds. Since the maximum lift developed depends only on the lift factors just mentioned and has no bearing on the weight of the airplane, 11,400 pounds will be developed as before. The positive load factor will be 11,400/1,500 = 7.6 g's. The first impression is that the occupants will have 7.6 g's working on them and v.iill probably black out (it depends on the length of time they are subjected to the load factor; this abrupt movement would result in only a very short period of 7.6 g's before the stall occurs, so let's forget them). Okay, you say, the wings have the same load as before (11,400), so what's the problem except for a brief discomfort on the part of the pilot and passengers? The wings are all right, but there are "fixed-weight components," such as the engine(s), baggage, retracted landing gear, etc. The airplane's limit load factor here of 3.8 g's is based on an overall analysis of the aircraft components. The engine has gotten no lighter during the flight, and it is, as mentioned, a fixed-weight component. Because of the lighter overall weight of the airplane, the engine and other fixed-weight components are subjected to greater acceleration forces. The engine mounts may not be able to support an engine and accessories that weigh nearly 8 times normal, and the same thing might be said about retracted landing gear, batteries, and baggage. You recall from Chapter 10 that the baggage compartment is placarded for max weight for two reasons: (1) the CG could be moved to a dangerous position and (2) the baggage compartment floor is only stressed to take a certain number of g's with the placarded weight. For instance, the example airplane, which has a limit load factor of 3.8 positive g's, has a limit of 200 pounds weight in the baggage compartment and could have a total force up to 3.8 x 200 pounds, or 760 pounds acting on the floor without structural damage. The pilot who stalled the airplane at the light weight of the airplane at the 117-K speed has caused a force of 7.6 x 200 pounds, or 1,520 pounds, to be exceeded on the floor. Figure 11-5 shows that the lighter weight at the old maneuvering speed results in load factors that can cause problems for the fixed-weight components."

To answer you question, the above is one of the reasons that I can say that Va is not there to protect the wings.

The regs you quote are what is irrelevant to the discussion.

Your thesis is that the change in the load distribution, associated with a lower load, might adversely affect the main spar. Have you got some references for that? I am all ears if anyone can demonstrate that what I say is wrong or simplistic. So, please, show me a reference that credibly says that Va decreasing with decreasing weight is to protect the wing spar because of altered weight distribution. I will be a fan for life.

Just to recap. Kirshner says that if you are at half max weight, in a GA aircraft, and you pull 7.6 g's, "The wings are alright." Bob says that Va is to protect the wings. I've been wrong before. Show me - if you can.

I think you are getting to a straw man argument, here. Kershner was an acknowledged skillful pilot and instructor but he was not, so far as I am aware, a certification engineer .. although I have no doubt he was quite knowledgeable in the discipline. I see naught in your cited reference to his book which is not concordant with what I suggested in earlier posts.

In essence, I can only suggest that you reread what I wrote earlier and, perhaps, visit and have a read of the relevant FARs with your blinkers removed.

First point, Va is a certification definition and only exists, [i]per se[/i], for gross weight, by definition.

Second point, the certification of an aircraft is predicated on a design limit load factor which may not be exceeded intentionally.

Third point, for any weight less than gross for the typical manouevring envelope situation, the maximum speed at which you are able to apply a sudden full elevator input must reduce to maintain the stall-at-limit-load-factor scenario. While you may read references in POHs to reducing Va, that is only a convenient, if somewhat imprecise, terminology .. Va is only defined for gross weight. Now, we all tend to use the term Va for the reduced speed/weight thing. Perhaps that imprecise, but convenient, terminology is what has caused your confusion ?

At gross weight, Va most definitely will provide protection to the wing .. along with the rest of the aircraft's structure. Perhaps you can analyse 23.335 and point to how that view is other than reasonable and correct ? If it helps, the following are the A/L 23-48 words -

[i]Sec. 23.335

Design airspeeds.

(c) Design maneuvering speed VA.

For VA, the following applies:

(1) VA may not be less than VS where--

(i) VS is a computed stalling speed with flaps retracted at the design weight, normally based on the maximum airplane normal force
coefficients, CNA; and

(ii) n is the limit maneuvering load factor used in design.

(2) The value of VA need not exceed the value of VC used in design.[/i]

Looking at your last post, specifically,

[color=blue][i][b]The original post was that Va is not about preserving the structure of the wing.[/b][/i][/color]

Fine, now explain how that is valid for gross weight, which is the weight for which Va is defined ? I note that Va is about ALL the structure, not just the wing ... In your first post, you then go on to suggest that

[color=blue][b][i]If that was true, Va would not decrease with decreased aircraft load.[/i][/b][/color]

Va does not reduce with reducing weight, as Va is only defined for gross weight. The reduced speed at a reduced weight, often referred to erroneously as Va, is so specified not to preserve the wing structure but to preserve the limit load factor basis of the Type certification which, in turn, is relevant to the overall structural design of the aircraft. While I don't have a copy of Bob's texts, I note that the reference you make in your first post does not, at all, show that Bob's text is concerned with reducing weight ?

[color=blue][i][b]11,400 pounds will be developed as before.[/b][/i][/color]

Kershner is being appropriately simplistic for the point of his argument in a pilot forum and I have no concern with that. However, his statement presumes a rigid structure and that the higher pitch rate will have no effect. In respect of the first presumption, we can only speculate without data. For the second, I assure you, pitch rate has a significant effect and it is for this reason that certification stalls (intended to get POH data) are performed at a very low pitch rate. The extent to which these factors may alter the conclusion is more complex that appropriate to discuss here.

In a subsequent post you state

[color=blue][i][b]Said it before, say it again, the amount of force the wings can cope with does not explain why Va lowers with lowering airplane weight[/b][/i][/color]

I have no argument with that. However, your argument's logic is flawed as the reduction in speed with weight has naught to do with the wing structure, specifically. Rather, it is all about maintaining the certification limit load factor.

[color=blue][i][b]The regs you quote are what is irrelevant to the discussion.[/b][/i][/color]

I'll leave that for you to argue with the Bench should your ideas ever be tested in Court.

[color=blue][i][b]Your thesis is that the change in the load distribution, associated with a lower load, might adversely affect the main spar.[/b][/i][/color]

Read what I said rather than what you wish to read ...
[color=blue][i][b]
So, please, show me a reference that credibly says that Va decreasing with decreasing weight is to protect the wing spar[/b][/i][/color]

I suggest that you won't find such a statement in any credible reference because that's not what the story is ... it is a figment of your own imagination drawn, I suggest, from an unfortunate misreading or misinterpretation of what you have read from whatever source.

[color=blue][i][b]Just to recap. Kirshner says that if you are at half max weight, in a GA aircraft, and you pull 7.6 g's, "The wings are alright."[/b][/i][/color]

Subject to the side issues to which I referred earlier, that possibly will be the case. However, the argument puts the aircraft in a situation for which the Design Standards expressly do not account and, simultaneously, put the pilot in a difficult situation if something else lets go .. not to mention the probable argument difficulties the pilot will have in Court should he survive the accident.

Regarding your most recent post, you said the followingt:

"'Said it before, say it again, the amount of force the wings can cope with does not explain why Va lowers with lowering airplane weight'

I have no argument with that. However, your argument's logic is flawed as the reduction in speed with weight has naught to do with the wing structure, specifically. Rather, it is all about maintaining the certification limit load factor."

Does that mean that you *are* saying that Va decreasing with decreasing weight is not about protecting the wing spar? Which is what I said in the very first post.

Regarding your post before last, you said the following.

"'The force on the main spar will not increase.'

... the changed load distribution may well adversely affect spar (and other structure) loads and margins of safety."

Which seems to say that Va decreasing with decreased weight is to protect the main spar of the wing.

I did not intentionally misrepresent what you said. I try and avoid straw man arguments.

As for your comments about court and surviving an accident, I never said, and never implied, that I intended to exceed the design limits of the aircraft. The issue is what the textbook said about what Va was for.

As for the issue of Va only being defined at gross weight and not really changing as the aircraft weight decreases, well, that could have caused some confusion. But only if you had Aspergers. It is quite obvious that, as someone reading the basic text books, that I would use the term Va in the way that basic textbooks use it.

As for William Kershner not being an aircraft engineer, as Forrest Gump might have said, "Aircraft engineer is as aircraft engineer does". So, I invite you to get all crystal clear and clarify your earlier comments. If you can.

What you could have said is as follows.

"'You said that Va was limited by the max force the wings could cope with. If that was true, Va would not decrease with decreased aircraft load.'

Regarding your above comment, Andrew, Va does protect the wings because it defines the maximum force that the wings can cope with. However, your point is correct in that lowering Va with lower weights does not protect the wing, it just protects the rest of the aircraft. (BTW, strictly speaking, Va only applies to the max weight speed. The extrapolated weight that you call Va is not really Va.)"

If you had just written the above, you might have managed to stay a bit calmer. Would the above be a valid response to my first post?