Hi guys,


I can’t find in the Bob Tait RPL PPL books an explanation for why Va reduces with reducing weight.

This is for my son, he failed an RPL flight test today because he couldn’t explain to the testing officer how it worked……he knew it did but didn’t know why…..

Any ideas where it is in Bobs books?

I doubt not knowing the answer to any particular question is a reason for failing an RPL flight test..

However flying above Va on flight test and being hard on controls without knowing what could happen is a different story.

Have a look at this short video, it may help explaining a stall before break concept :
www.boldmethod.com/blog/crash-course/how...ed-va-change-weight/

Appreciate the link.

Nup, he wasn’t able to answer why it decreased with decreased weight. Out of many many pre flight questions he’s was asked this and 2 others he didn’t get exactly correct.
He never left the ground.
Examiner said he must get 100% correct answers to the pre flight part first, so failed him.

Showed my son the video, thanks for that link, it helped my explanation to him.

I’ve been flying professionally for 40 years and couldn’t believe he was failed on the ground questions.

Incredible.

Ah, the dreaded V_a. Probably the most misunderstood concept in the GA training fraternity.

I think that the testing officer was a bit hard - a reasonable fellow probably would have engaged your son in discussion to tease out what he did know, rather than presuming the hesitation was a sign of ignorance ...

The link B suggested is typical of many on the net. Tells a bit of the story, some a bit correct, much a lot wrong, while leaving out the bulk of it. If you lined up 1000 pilots, just about every one of them would parrot off the incomplete usual nonsense story one sees in many flying texts and in many on-line tales of derry-doing.

A more detailed story follows in the next couple of posts (multiple posts to keep away from length limitations).

Manoeuvring Speed Considerations – some background thoughts

Design manoeuvring speed dates back a long way and hasn’t changed to any extent over the years. OK, so the abbreviation was Vp and changed to Va, but we can live with that, I guess.

So, for instance, the requirement in CAR 3 (the regulations preceding the FARs), at 01Nov1949 was

§ 3.184 Design air speeds. The design air speeds shall be chosen by the designer except that they shall not be less than the following values: ......

Vp (design maneuvering speed) = Vs√n where: Vs =a computed stalling speed with flaps fully retracted at the design weight, normally based on the maximum airplane normal force coefficient, Cna. n= limit maneuvering load factor used in design, except that the value of Vp need not exceed the value of Vc used in design

Fast forward to the rules in FAR 23.335, at 11Mar1996,

c) Design maneuvering speed Va. For Va, the following applies:

(1) Va may not be less than Vs√n 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

Each looks to be pretty much the same as the other ? Keep in mind that we are talking about MTOW for Va.

The take away is that we are looking at a minimum speed requirement to be selected by the OEM – ie, the book speed might be somewhat in excess of the usual story you will get from the various net sources, albeit that these are the usual (not quite correct) gospel beliefs amongst the pilot fraternity.

The main consideration about the design manoeuvring speed is that it is the basis for airload calculations used in the structural design of the empennage and the various control surfaces, not the basic wing structure integrity, per se. You can read up on this at FAR 23.

Elevator/tailplane:

Sec. 23.423 Maneuvering loads.
Each horizontal tail surface must be designed for maneuvering loads imposed by the following conditions:
(a) A sudden deflection of the elevator control, at Va, to (1) the maximum upward deflection, and (2) the maximum downward deflection, as limited by the control stops, or pilot effort, whichever is critical. .....
(b) A sudden upward deflection of the elevator, at speeds above Va, followed by a downward deflection of the elevator, resulting in the following combinations of normal and angular acceleration:

(b) is considering a checked manoeuvre where the pilot applies a load and then reduces it, typically to avoid generating excessive g-loads.

Fin/Rudder

Sec. 23.441 Maneuvering loads.
(a) At speeds up to Va, the vertical tail surfaces must be designed to withstand-
(1) A sudden displacement of the rudder control (with the airplane in unaccelerated flight with zero yaw) to the maximum deflection allowed by the control stops or by pilot strength, whichever is critical; .......

Aileron

Sec. 23.455 Ailerons.
(a) The ailerons must be designed for the loads to which they are subjected--
(1) In the neutral position during symmetrical flight conditions; and
(2) By the following deflections (except as limited by pilot effort), during unsymmetrical flight conditions:
(i) Sudden maximum displacement of the aileron control at Va. Suitable allowance may be made for control system deflections.
(ii) Sufficient deflection at Vc, where Vc is more than Va, to produce a rate of roll not less than obtained in paragraph (a)(2)(i) of this section.
(iii) Sufficient deflection at Vd to produce a rate of roll not less than one-third of that obtained in paragraph (a)(2)(i) of this section.

Now, if the OEM selects the minimum permissible design Va, then, providing the g-load ramp up isn’t too rapid (to avoid any significant overswing in CL associated with the necessary time delays for pressure gradients to cause results in airflow) then there is a reasonable chance that the aircraft will stall at or around Va before the basic structure of the wings start to creak and groan excessively and ominously.

If, on the other hand, the OEM selected a Va speed somewhat in excess of the minimum, all bets might just be off ?

So we have a bit of a disjoint between what the pilot folk traditionally thought, and what the engineering design/test and certification folks knew was the story. Aircraft didn’t fall out the sky on a daily basis so the Industry sort of ignored the problem for a long time.

However, the FAA did introduce another limitation at 23.1507 in September 1993, Vo - the operating manoeuvring speed, which was different to Va in the subtlety of “Vo is a selected speed that is not greater than Vs√n established in Sec. 23.335(c). By this stratagem, it was intended that, progressively, Vo would supplant Va in pilot thinking and the traditional pilot approach would be tied to a more appropriate limiting speed. However, due to “certification grandfathering”, there would remain a lot of “old rule” aircraft in the Industry for some very considerable time.

Then there came a massive wakeup call in the aftermath of AA587 in November 2001 – the following accident report is worth a read, I suggest –

www.ntsb.gov/investigations/AccidentReports/Reports/AAR0404.pdf

The basic story was that the A300 took off following a B747-400, encountering moderate wake turbulence. Due to inappropriate rudder inputs by the pilot flying, it experienced a fin and rudder separation (ie it all broke off the aircraft) with subsequent ground impact and a death toll of 265 people in the aircraft and on the ground.

Subsequently, the Industry started to rework training programs with the intent to re-educate the pilot fraternity about Va loads associated with pilot inputs and turbulence events. This, probably, has worked its way down through the airlines satisfactorily (although one still sees internet comments which suggest there may still be some way to go) but will take a long time to achieve the aim in the GA fraternity.

So, where does this leave us as pilots ?

First, min Va is not an unrestricted “yank and bank” speed limit as the old style military would have it - and that erroneous idea killed more than a few military folk in the olden days. As an aside, when next you see an airshow display by a solo F16, you will notice, very clearly, that the pilot, following a high speed run and high-g pull up, will bunt very noticeably prior to rolling into a steep turn. This is to unload the symmetrical g-load in the pull up prior to imposing the rolling wing loads. Certainly the most visually evident example that I can bring to mind.

Second, there is no provision for “rocking and rolling” on a control at min Va. This was a major contributor to the AA587 loss. Basically it is a single input to the stop (or the pilot control design load limit). If the pilot starts cyclical inputs, even somewhat below min Va then things can very easily get out of hand, structure-wise, as happened with AA587.

Third, the basic idea is one control at a time, by itself ONLY, moved to the stop and, subsequently, relaxed to neutral (except for the checked manoeuvre consideration noted above).

Looking at the structural implications, if you are at min Va (=Vs√n), then, if things go right for you, you can apply a strong pitch up elevator input (but no rudder/aileron input simultaneously) and expect the aircraft to stall at or about a g-load equal to the design limit load factor. If you start the manoeuvre at a speed somewhat less than min Va, you will stall prior to reaching the limit load factor. On the other hand, if you start the manoeuvre somewhat above min Va, (or at Va if the limit is above the minimum value permitted) then you can confidently expect to overload the structure well above the limit load factor. Should that, as it very easily could do, put you above the ultimate load factor (1.5 x limit n) then you may well find yourself in a world of real surprise and hurt.

That’s all fine, but where do gross weight variations come into consideration ? For this we need to consider what is called the Vn (or Vg) diagram. So that things don’t get confusing, I have drawn only a part of the normal diagram, as shown below.



The curved line represents the stall characteristic (buzz word term for equation) in positive (pull up) g situations. This comes from the usual lift equation with the maximum lift coefficient value.

Note that stall speed goes to zero at zero g. This was something exploited by NASA’s “vomit comet” www.nasa.gov/missions/research/kc135.html

Now this line will be for ONE gross weight. If the gross weight varies, then the stall characteristic line varies as well. In the case where gross weight is varying, we would have a family of stall curves, a bit like in the following graphic. As the lines move from left to right, this is associated with increasing gross weight. Clearly, the right-most line will be for MTOW while those lines to the left of this particular line represent the stall characteristics as gross weight reduces progressively below MTOW.

Where the limit load factor line intersects the MTOW stall line defines the minimum value of Va. However, please do keep in mind that the OEM may elect to use a speed for Va which is higher than the minimum.



Now, we don’t operate above the maximum gross weight (MTOW) but we can (and routinely do) operate below it and we would then be operating along one of the stall characteristic lines to the left of the MTOW line.

Notice that, should we be operating at a lower weight but at the MTOW min Va, then, due to the different lower weight stall characteristic line, we can pull more (maybe a lot more) than the limit load factor g before we find ourselves stalling. That is to say, we can overload the aircraft. Not a good thing to do. See the following graphic.



There are two ways we can get around this problem. Either we can employ a checked manoeuvre and restrict any pull up to the limit load factor (but that defeats the aim of the exercise) or we can reduce the speed to a point where the reduced speed/limit load factor intersection is at the stall speed for the reduced weight. In effect the speed reduction maintains the idea of min Va at the lower weight.

And that’s all that’s involved in varying Va with weight to maintain the intent of min Va at reduced gross weight.

The reason that this is very important is not for the wings but for all the other stuff bolted to the aircraft. This other stuff is designed on the basis that the applied loads (forces) will not exceed the specified limit and ultimate loads. If we let the pilot pull more g than the design expected, this other stuff might start falling apart and floating around the aircraft – again, not such a good idea.

Now, we can also talk around these ideas in other ways – eg

(a) if we think of Newton’s F=ma, then, for a maximum force to be held constant, if weight varies (strictly, W=mg but we, not quite correctly, think of weight in a similar fashion to mass) then the acceleration varies in the opposite direction ie one goes up, the other goes down. So, for the wing stall, if the weight reduces, we are able to pull more that the limit load factor at min Va before the wing stalls and that is what we want to avoid. So, if the weight reduces, we need to reduce the speed so that the stall still occurs at or near to the limit load factor.

(b) we can talk in rubbery terms of angle of attack as in the typical net presentation. If we are at a lower weight, but at the min Va speed, then we are at a lower angle of attack and we have more available lift (ie the lift coefficient curve) before stalling. Same thing, we need to reduce the speed to bring the available lift increment back to where we stall at or near the limit load factor.

Aside – limit and ultimate load factors

Up to the limit load factor, there can be no significant problems and controls etc have to work normally.

Between the limit and ultimate load factors, minor damage and permanent deformations are acceptable but the aircraft has to hold together for at least three seconds under ultimate loads.

Above the ultimate load factor, you’re out a long way on a very thin limb.

You may be interested in looking at an ultimate load certification test on the net. The following link shows the B777 test program. Notice just how close to ultimate the critical part breaks (1.54 x n) ... there’s not much room for pilot heroics with this stuff.



Hopefully, that all gives you some food for thought ?