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  • John.Heddles
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John.Heddles replied the topic: Va

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

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

Va, most certainly, inter alia, 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.

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.

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 load distributions 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.

The force on the main spar will not increase.


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 MAY NOT exceed that load factor, intentionally, under ANY 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 ..

138 Pilot to comply with requirements etc of aircraft’s flight manual etc

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

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

Sub reg (3) means that mens rea 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 -

269 Variation, suspension or cancellation of approval, authority, certificate or licence

(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;

(d) that the holder of the authorisation is not a fit and proper person 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 ...

Engineering specialist in aircraft performance and weight control.
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  • Andrewnielsen@me.com
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Andrewnielsen@me.com replied the topic: Va

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.
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  • John.Heddles
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John.Heddles replied the topic: Va

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, per se, 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 -

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.


Looking at your last post, specifically,

The original post was that Va is not about preserving the structure of the wing.

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

If that was true, Va would not decrease with decreased aircraft load.

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 ?

11,400 pounds will be developed as before.

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

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.

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

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

Your thesis is that the change in the load distribution, associated with a lower load, might adversely affect the main spar.

Read what I said rather than what you wish to read ...

So, please, show me a reference that credibly says that Va decreasing with decreasing weight is to protect the wing spar


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.

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."

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.

Engineering specialist in aircraft performance and weight control.
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  • Andrewnielsen@me.com
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Andrewnielsen@me.com replied the topic: Va

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.
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  • Andrewnielsen@me.com
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Andrewnielsen@me.com replied the topic: Va

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?
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  • John.Heddles
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John.Heddles replied the topic: Va

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.

That's precisely what I am saying. The reduction in speed is related to maintaining the limit load factor at stall. However, that's not quite what you stated in your first post .. which was

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

If anything, it appears that the only poster pushing the reducing weight and wing spar concern .. is your good self.

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

Perhaps your techniques for comprehension of the written word differ from mine ?

I never said, and never implied, that I intended to exceed the design limits of the aircraft.

That's a relief .. it appeared to me that you had made a number of suggestions whereby the limit load factor might be exceeded ?

The issue is what the textbook said about what Va was for.


As I don't have the text, I can only go on your first post ... which didn't appear to suggest that Bob's text said anything about reducing speeds ?

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.

Again, I am not able to comment on what Bob's texts may say or not. However, now that you do know the definition, that problem might be resolved ?

So, I invite you to get all crystal clear and clarify your earlier comments. If you can

Perhaps you might assist by putting a series of simple, precise bullet point queries rather than your previous writing style ?. I'm not trying to be difficult .. this topic is rather important for the other training folk who might be reading the thread ... so it is important that it continue to some sort of conclusion.

Va does protect the wings because it defines the maximum force that the wings can cope with.

Va, for the typical situation, will look after the wing structure. However, that is not how it is defined which is what you are endeavouring to suggest ? Again, I draw your attention to what the speed definition is in 23.335.

you might have managed to stay a bit calmer


Those who know me, know that I don't raise a sweat under even extreme provocation .. I am, I assure you, quite calm. That is, I might add, a commonly observed characteristic of most experienced airline pilot folk.

Engineering specialist in aircraft performance and weight control.
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  • Stuart Tait
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Stuart Tait replied the topic: Va

Andrew I have a problem with your demeanour on this forum,, you seem to be trolling for an argument with every post I understand that you wish to contest every point you don't understand, but you are pushing my patience with your tone
Please be a bit more respectful of people who are trying to help you
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  • Andrewnielsen@me.com
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Andrewnielsen@me.com replied the topic: Va

Hi Stuart,
Sorry if you think I am trolling.

Trolls do not admit when they are wrong. I was grateful for being given the opportunity to be wrong about landing configuration and full flaps. The textbook really was an improvement on the standard definitions because "full flap" is more precise than "landing configuration".

As for the thread in question, my original post is valid. Decreasing (the rough definition of) Va with decreasing speed is not to protect the wing spar. If you halve the weight of the airplane and use max elevator at the max-weight Va, you will pull 7 G, but you will not put extra stress on the wing. (The plane will probably break, but the main spar will not.)

John said he agrees with what I said, but objects to the wording. Things John has said about me include the following

• being a little silly
• not of much relevance
• you are getting a bit lost
• Why are you preoccupied
• your thesis is quite irrelevant
• you would be entering dangerous waters
• you in the area of illegal operation
• you are getting to a straw man argument
• your blinkers removed
• caused your confusion
• you to argue with the Bench should your ideas ever be tested in Court.
• Read what I said rather than what you wish to read
• it is a figment of your own imagination
• your techniques for comprehension of the written word differ from mine
• your previous writing style

... all that because I had the temerity to point out something quite true. I even came up with my own solution to the problem, namely:

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.)"
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  • John.Heddles
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John.Heddles replied the topic: Va

It appears that the thread is doomed to a continual going around in circles.

Suffice it to say -

(a) Va is an engineering design limit and not really of direct relevance to the flying folk, albeit that it is declared as a limitation.

(b) the Industry, through the FAA, eventually realised that what the design engineering folk understood by Va had, for a long time, been somewhat modified in practice by the flying folk.

(c) as a consequence, at A/L 23-45 (July 1993), the FAA introduced a new concept, known as the maximum operating manoeuvring speed which better matched the typical flying folk's understanding of what manoeuvring speed might be. The maximum value for Vo (and the minimum value for Va) is the gross weight clean stall intersection with the limit load factor .. ie as defined previously. Operating at, or below, Vo should cause the aircraft to stall for pitching manoeuvres prior to exceeding the limit load factor in most circumstances. The relevant caveat is that the weight/speed reduction consideration is met.

Engineering specialist in aircraft performance and weight control.
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John.Heddles replied the topic: Va

I was having a reread of this thread and it occurred to me that, while

(a) we were talking about the situation for a min Va certification, which possibly is typical for most light aircraft, and

(b) I had noted the introduction of the Vo to differentiate between the not-quite-correct flying community's understanding of Va and what can occur in the certification world ..

I hadn't really made a strong enough point of the fact that Va may be considerable higher than the min Va (where the wing will stall if you pull limit load).

It may be useful for folks to read the FAA's words in the following AC.

AC 23-19A ( www.faa.gov/documentLibrary/.../AC_23-19A.pdf ) has this to say -

48. What is the design maneuvering speed VA ?

a. The design maneuvering speed is a value chosen by the applicant. It may not be less than Vs√n and need not be greater than Vc, but it could be greater if the applicant chose the higher value. The loads resulting from full control surface deflections at VA are used to design the empennage and ailerons in Part 23, §§ 23.423, 23.441, and 23.455.

b. VA should not be interpreted as a speed that would permit the pilot unrestricted flight-control movement without exceeding airplane structural limits, nor should it be interpreted as a gust penetration speed. Only if VA= Vs √n will the airplane stall in a nose-up pitching maneuver at, or near, limit load factor. For airplanes where VA>VS√n, the pilot would have to check the maneuver; otherwise the airplane would exceed the limit load factor.

c. Amendment 23-45 added the operating maneuvering speed, VO, in § 23.1507. VO is established not greater than VS√n, and it is a speed where the airplane will stall in a nose-up pitching maneuver before exceeding the airplane structural limits.


(The "applicant" is the OEM).

My interpretation has always been that, for a light aircraft whose AFM/POH states that one can use (single) control input at Va, then that aircraft's design likely was based on min Va. If the AFM/POH doesn't make the statement then one probably needs to presume that the Va for design was higher than min Va and one shouldn't be pulling other than a checked pitching manoeuvre to keep the loads below limit. For new certifications you may see the Vo definition for which you should see the abrupt control statement.



Do be careful, though, there be dragons out there for the unwary.

Engineering specialist in aircraft performance and weight control.
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