Va - what's it all about?

[Gin Slinger]

In my ATPL groundschool notes, I have Va as the design manoeuvre speed.

I can only think Va is nominated during design/certification stages to cover a set of arbitrary manoeuvres at the given speed.

After all, I'm sure it's more than possible for pilot induced structural failure below Va.

Can anyone expand on how Va is established?

Yours, GS – an inquisitive ATPL student.

[john_tullamarine]

Va is a bit more than that.

While it might be feasible to break something with ham-fisted rudder inputs, in particular (and there are other threads running regarding an accident in the US which may have involved this sort of problem), or multiple control inputs in general, a significant protection is offered for simple pitching inputs wherein the wing will stall before you break anything at speeds up to Va. This is the prime intent of the Va design consideration and is achieved by setting the Va speed at the intersection of the positive stall manoeuvring capability of the wing and the design limit load factor. If you care to have a look at FAR 23.333 and .335 you will get the general picture of how it works.

It follows that, if you don't have a declared turbulence speed for a given aircraft, there is something to be said for keeping below the Va speed in severe turbulence.

.. and, unless you are a competition aerobatic pilot, there is something to be said for trying to fly smoothly .. rather than aggressively ... in one's everyday approach to manoeuvring flight.

[Genghis the Engineer]

Va is the "A" point on the V-N diagram, that is, it's the point at which the aircraft should stall exactly at the design structural limit N1. Below that, you should theoretically stall first, above that, you're eating into the safety factors and ultimately can bend the aircraft.

However, it is all a little more complex than that. If you pitch-up aggressively, you can get to a higher AoA before you stall, so you could in theory and practice overstress at a lower speed - albeit through mishandling. (This is called "unsteady aerodynamics" and is a bit of a black art.)

A further issue is that the primary flight controls are designed for full deflection only up to Va, above that the maximum deflection tapers off, but they are also designed for at-least 1/3 deflection at Vne, which is why you're told not to use more than 1/3 deflection above Va.


So, in practice, you are pretty safe from anything except very rapid control inputs, particularly in pitch, below Va. But as JT says, smooth manoeuvring is a good thing regardless !

G

[Dick Whittingham]

I hardly dare write in the company of such distinguished experts, but there is a point here which is ATPL exam referenced, rather than pure engineering.

All agree that Va is a design speed used for calculation of stress effects in certification. In its form of Vs times the square root of the maximum positive load factor (normally 2.5g for JAR25 aircraft)it is a fixed value for design purposes, and Vs is the clean stall speed at maximum certificated takeoff weight.

However, if Vs changes, the calculated speed for Vs x sq rt N will also change. This does not mean that the design Va changes, it means that in practice you will meet the damage/stall point at a different speed. The ATPL examiners, however, have a series of questions asking how Va changes for different weights, using the term Va for the practical damage/stall speed. The answer is, of course, that this speed will change by exactly the same percentage or proportion that Vs changes, the square root of the weight change, or, for small weight changes - up to 20% - half the percentage weight change. Dick W

[john_tullamarine]

Following on from Dick's comments, a related concern is those pilots who try to factor reduced weight as somehow permitting the use of a higher g-loading. While there may be some justification from a wing strength consideration, one ought to keep in mind that the various bits and pieces bolted to the aircraft are designed for a fixed loading ....

So consideration of reducing Va is not a just a matter for ATPL exams .. it still is well in the engineering design and certification arena concerns ....

[Dick Whittingham]

Agree with John, absolutely. The ATPL questions are really about the mathematical manipulation of the formula, not about the real world. Don't get me going on this, I have real problems trying to juggle truth with exams. As Goedel said, to every w-consistent class k of formulae there correspond recursive class signs r, such that neither v Gen r nor negative (v Gen r) belongs to Flg k (where v is the free variable of r).

Or, truth is a more powerful concept than proof.

Dick W

[john_tullamarine]

mmmmm .....

Are you having a bit of a lend of us, here, Dick ?

.. or I am just suffering from that well known problem associated with the observation that "I are just a injunear " ?

[Captain Stable]

As Goedel said, to every w-consistent class k of formulae there correspond recursive class signs r, such that neither v Gen r nor negative (v Gen r) belongs to Flg k (where v is the free variable of r). :eek: :confused: :confused: :eek:

[Gin Slinger]

As Goedel said, to every w-consistent class k of formulae there correspond recursive class signs r, such that neither v Gen r nor negative (v Gen r) belongs to Flg k (where v is the free variable of r).

Yes, this part of the thread made me scratch my head too...

[Dick Whittingham]

Sorry, it was late, and I had the best of a bottle of Barossa valley shiraz inside me. In plain language, Goedel's theorem says that in a formal mathematical system there are theorems that are true, but cannot be proved within the system, and theorems that are false that cannot be disproved within the system. This is really to do with artificial intelligence, or, at least, what is required for artificial intelligence. We can solve problems by stepping outside the system and taking a look at how the system itself works. Computers can't do this unless their software is able to look at the working programme (the formal system) from a higher level.

Only connection with ATPL exams is the running decision whether to teach the truth ( but "What is truth?, said jesting Pilate, and would not stay for an answer") or some simplified sub-truth that gets the right tick in the box.

Us instructors lie awake at night worrying about this.

Recommend "Goedel, Escher and Bach - an eternal golden braid", by Douglas R Hofstadter

Dick W

[J-Heller]

1. Recommend from time to time inserting your head, inverted, into a wash-hand basin full of cool water.

2. Whilst holding your breath, and sealing your nostrils with your hand......

3. Wait until the smoke coming out of your ears begins to abate.

4. Slowly lift head out of the water, and reorientate with the horizon.

5. Walk back into the room, and close all open ATPL Groundschool notes, associated textbooks, and the deranged scrawlings you know as 'notes'.

6. Retreat to the a darkened room and sleep it off.

7. Wake up and come back for more. Then go back to No 1.


(Try and keep the cyclic process down to around every 2 hours)

Cheers,



JH

[Genghis the Engineer]

There's another issue, let me introduce you to what is known to a select few as the "Venton Walters Equation".

For a particularly flexible wing, washout increases dramatically with loading, leading to the relationship:-

Vs = Vs(MTOW,1g).(((W/MTOW)Nz)^CAe)


Where CAe is the aeroelastic constant for the wing, equal to 0.5 for a perfectly rigid wing, but I've seen values as high as 0.82 for some (perfectly safe) microlights, which in some cases moves Va above Vne, creating a mild headache for the certification engineer, and severe migrane for many classical aerodynamicists.

The formula was first proposed by Roy Venton Walters, who designed the Puma and Raven microlight wings about 15 years ago, and despite a rather superficial proof on his part at the time, surprisingly works incredibly well for highly aeroelastic wings that tend to offload lift with loading (generally through increased washout).

G

[john_tullamarine]

Dear me ..... new things to go research and learn a bit about ....... and, following on from Genghis' comments, a recent AeroSoc article on gust responses in very flexible wings makes for interesting reading for those who might like to dig it out .....

Actually, Dick .....

(a) some would say that we engineers have limited intelligence ... artificial or otherwise ..... wives, in particular or so I have been told, are quite adept at such musings ......

(b) in another life as a ground instructor I used to get all bitter and twisted at the tendency of some to teach only that which was directly relevant to the exams ... and the exams, oftentimes, were of little relevance to anything ....... at times I thought I was but a lone voice in the wilderness preaching the need to learn things as well as pass the exam .....

[FlyingForFun]

Ok, I'm in way over my head here, particular with that equation for increasing wash-out on a flexible wing :eek:

But, Genghis, way back near the start of the thread you said:

If you pitch-up aggressively, you can get to a higher AoA before you stall
Is that true? I was under the impression that the wing will always stall at the same AoA whatever. I thought that if you pitch up agressively, you can get to a higher attitude before you stall, but the AoA will be the same as for a gentle pitch-up?

Hides away in the corner wondering how many unbelievably complex formulas Genghis will produce to dis-prove my comments!

Thanks,

FFF
----------

[Genghis the Engineer]

True I'm afraid, the fiction that AoA is always constant at the stall is a rumour put around by aerodynamicists to stop pilots asking awkward questions.

Proof?

At a safe height, in an aeroplane you know well, first stall it decelerating the aircraft at a nice gentle 1 kn/s. Then do it at a dramatic 5 kn/s? You'll doubtless find significantly difference stalling characteristics at the two deceleration rates. This is, at-least partly, due to the difference in AoA, and thus the difference in resultant pitching moment..

But keep this to yourself, or at-least don't put it in an ATPL exam, they'll only mark you down for being a smart-alec.

G

[john_tullamarine]

and, to confuse the issue further, if the pitch rate is high (memory brings an onset figure of, say, 60-70 degrees per second ? .. something in that region) you enter an interesting regime of accelerated stalling where a shed vortex sits spanwise above the wing causing flow reattachment ... and permits a very significant increase both in alpha and generated lift forces.

... but I don't think that this one has filtered down to the exam circuit as yet ..... fortunately most of the examiners aren't engineers ...

[john_tullamarine]

.. like the man said ... not really of relevance to garden variety fixed wing operations .... certainly of interest in high performance aircraft and, more particularly, rotor dynamics .....

... just threw it in for amusement value ...

[john_tullamarine]

..peace be upon you, brother ...... all's well ...

my initial foray into ATPL in the late 60s I did by reading the syllabus .. that was my first mistake

... then I sourced the recommended texts ... wrong ..... again

I found it quite disconcerting that the exams had negligible relevance to either the syllabus or the texts ....

then I discovered the duality of the system ... one ought to learn something useful .. and then learn that which is not necessarily useful other than for passing the exam ......

[Ignition Override]

In the eyes of those who create those exams over there, how do such questions help one to be a safe pilot? Does such knowledge (other than for those at Test Pilot School, i.e. Empire, Edwards AFB, NAS Pax River) help us fly the plane after an engine failure at V1, etc? Or turn the autopilot knob smoother?

After lift-off, maxumum bank at initial climb speeds of at least V2 + 10 knots might be 15 degrees and an extra degree of bank angle (up to 30, with 25 degrees preferred) allowed for every knot above minimum flap/slat retraction speeds etc, should require very little theoretical baloney.

[john_tullamarine]

I guess that it is a balance between minimal knowledge .. which makes for a very minimal and limited monkey-see-monkey-do sort of pilot .. and a very detailed engineering, electronics, avionics, etc., etc., knowledge ... which makes for an unworkable system.

Somewhere in the middle is a sensible, practical, and reasonable middle ground.

In general, knowledge is not a load to have tucked away in the back of the mind .... but, for the average pilot, it probably is better to be practical rather than book work academic ?

[Genghis the Engineer]

Problem is, a certain proportion of all pilots and engineers progress onto other things, whether it's instructors, test pilots / engineers, authors of textbooks, etc. etc.

So, it's an inevitable fact of life that everybody needs educating a bit beyond what they're expected to start out needing, so that they've got the ability to progress beyond.

Another way to look at it, is that you teach more than they'll need, and hope the most important core of knowledge remains.

Yet another way to look at it, is that you never know when that obscure bit of knowledge you never thought you'd need will help you to extract yourself from the mire.

G

[Dick Whittingham]

John and Ghengis and I all agree. There is more to the ATPL than just the specific bits of knowledge that tick the boxes. You are learning to be a pilot, top of the world, pushing the frontiers of understanding, not just of technical matters, but of human behaviour and human failings. Go for it, get all you can.

Dick W

For example, I have just learnt how to spell Genghis.