Va - Manoeuvring speed? ... what manoeuvring speed?

[Head in the clouds]

I don't think that it is due to the rate of change of control input and inertia but due to the fact that at higher weight the aircraft has a higher stall speed and therfore the wing stalls before its structural limits are exceeded. Inertia dosent come into it.

 

Correct - have a look at a V-N or V-G diagram for any aircraft. The 50 fps gust lines give the answer. I'll try to post one of these flight envelope diagrams as a separate subject because it's quite informative.One most important thing about wing loadings - if you deploy flaps in rough air, or with huge pilot induced accelerations - then you are asking for trouble.

 

For a Tecnam P96G, Va is 81 KIAS, and Vfe is 60 KIAS (I've seen 60 given as 'approach' speed in some info.)

 

Flaps retracted + 4.0 g to -2.0 g is the permitted range

 

Flaps 38o (full) + 1.9 g to 0.0g is the permitted range

 

Remember Va is a no flap speed. With flap extended, the wing is much weaker and it's likely the Va for flaps extended is going to be closer to 60. So, the range of IAS seems to be rather limited if you think turbulence and wing loadings.

 

Worth discussing?

 

Absolutely. Let's do it :)

 

In turbulence, particularly severe, you airspeed will be varying and the aircraft will also be subject to fairly large "G" fluctuations.( I've had a good 60 knots variation in a rotor at about 1,000 feet alt., in a c-150). That range would cause a big problem with flaps extended. Control inputs (ailerons mainly) and flap extension can cause loads that are inclined to twist wings. Your control inputs will have a big bearing on whether you survive or not and the throttle IS a control. Large pitch changes are not a good idea and don't apply large control inputs. The situation I was in was made worse by there being quite a few times when there was violent negative "G". The aircraft attitude should be loosely retained at normal for the conditions, which may be climbing, level or descending, with minimum control inputs to achieve the desired attitude, and your airspeed should aim for the safest speed but it will fluctuate about that speed and you can't (and shouldn't) try to stick to an exact speed. Aim for the correct attitude. If you don't pull the stick back it won't stall. You have several tasks Keep the plane from getting out of shape, attitude and not stalling and not imposing severe loads on the airframe..Nev

 

I think that this can best be explained with the diagram below. Take an aircraft with the following characteristics:Stall at MTOW: 50 kts

 

Stall at a light weight: 40 kts

 

VNE: 150 kts

 

Design load factors of +4.5 and -3

 

It would have an operating envelope something like the diagram below. Flight is only possible in the region in the centre of the chart. Attempt to fly outside this region and either the aircraft will stall or the aircraft will be outside its design peramaters.

 

At a speed below Va it is not possible to overstress the wings because the aircraft will stall first. Above Va it is possible to overload the wings.

 

Now stalling speed increases by the square root of load factor. A load factor of 4.5 would mean that the stalling speed will increase by a factor of 2.12. The graphs shows that the range of values for Va from 85kts to 106 knots as the weight is increased.

 

 

Nobody. Yes I understand quite well the principle your using. It doesn't negate the relationship between load factor and acceleration. The reason the wing is being over loaded is due to the load factor being too high yea?How can that load factor be increased by a control input if te aircrafts motion doesn't change? I can assure you, enertia ( acceleration) is required.

I think this is a subject well worth further discussion so I've taken the liberty of starting a new thread with some thoughts previously posted elsewhere.

 

Manoeuvring speed calculation is certainly one of the matters that students seem to have some difficulty getting to grips with, and many old and bold pilots still don't seem to have a clue what it's really about so they just fly below the yellow arc if it gets altogether too bumpy.

 

Some while ago, while researching the crash of American Airlines Flight 587 which showed that the pilot can, in fact, cause a structural overload with use of the controls whilst well within the placarded manoeuvring speed, I came across an excellent explanation of the subject, by a complete layman, which even I could understand easily.

 

Being able to envisage the structural loading has allowed me to treat the airframe in a more kindly manner whether lightly or heavily loaded, because each of those conditions presents separate hazards.

 

I'll write up that explanation in a following post but I'm sure others have good ones too ... we're looking for succinct wording that newbies can relate to rather than graphs and formulae. Structural failure while airborne is rare but does happen from time to time so this is a pretty serious issue worthy of better understanding by all.

 

Here's Mac McClellan's Flying Mag article which discusses the general conditions which caused the vertical stabiliser failure of Flight 587 and Peter Garrison's as always excellent investigative article about the crash, its causes and the Perils of Flying by the Book where Va is concerned.

 

 

[motzartmerv]

Ok nobody. I'm here;)

 

Answer a few question for me.

 

What is the defintion of VA?

 

Is it only to do with the elevator? Or does it state " full deflection of the controls"?

 

It is not simply a matter if increasing or decreasing stalling speed. I can't stress that enough.

 

If you force a change to the aeroplane in ANY axis too quickly the loads can exceed design limits. It's not Just pitch changes.

 

 

[Guest Nobody]

From page 10-17 of "the Pilots handbook of Aeronautical Knowledge" by the FAA

 

VA

—

 

the calibrated design maneuvering airspeed. This is

 

the maximum speed at which the limit load can be imposed

 

(either by gusts or full deflection of the control surfaces)

 

without causing structural damage. Operating at or below

 

manuevering speed does not provide structural protection

 

against multiple full control inputs in one axis or full control

 

inputs in more than one axis at the same time.

[motzartmerv]

:) .... Thank you. So does it mention the stall? Does it single out the Pitch attitude and Axis?

 

Either by Gusts or full defection of controls.. What do you think that statement means? It means a gust OR a full deflection will cause a change TOO rapidly and the deisgn limits can be exceeded.

 

Definition of inertia" a property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force."

 

A heavier aeroplane will RESIST change better then a light aeroplane will. It CAN NOT be overloaded without a rapid change.

 

 

[Guest Nobody]

From FAR 23.441 about the vertical surfaces,

 

(a) At speeds up to VA, the vertical surfaces must be designed to withstand the following conditions. In computing the loads, the yawing velocity may be assumed to be zero:

(1) With the airplane in unaccelerated flight at zero yaw, it is assumed that the rudder control is suddenly displaced to the maximum deflection, as limited by the control stops or by limit pilot forces.

 

(2) With the rudder deflected as specified in paragraph (a)(1) of this section, it is assumed that the airplane yaws to the overswing sideslip angle. In lieu of a rational analysis, an overswing angle equal to 1.5 times the static sideslip angle of paragraph (a)(3) of this section may be assumed.

 

(3) A yaw angle of 15 degrees with the rudder control maintained in the neutral position (except as limited by pilot strength).

[facthunter]

The situation is far more complex than just saying.... I'm below Va, I can do anything with the controls and it won't break. You have to think hard to show a place where you use the full extent of any control. A couple of examples are ( elevator )flare with a Tailwheel plane. ( Rudder) Sideslip at limit. These are done at quite slow speeds nowhere near normal cruise.

 

A Cessna 210 is often cruised at above the Va speed and if you want to twist a wing off one just go into the circuit fast and crank on some aileron entering a turn. The Airbus rudder quoted, the F/O applied opposing rudder inputs consecutively and the plane yawed excessively due to inertia and above design loads resulted in the entire vertical stabiliser coming off.

 

The pilot must have a sense of what his/her control input has on the aircraft, more so the Effect of controls and FURTHER effect of controls teaches you. ( This is only roll produces YAW and yaw produces ROLL). There is no secondary effect of pitching))

 

The Effect on the aircraft structure from aerodynamic loads resulting from applied control forces. at varying airspeeds , and attitudes and relative airflows, as encountered in gusts, is far more complex, especially as you have to take into account inertia effect of the aircraft moving on 3 axis .This might be more difficult for people who haven't done a lot of science. Nev

 

 

[poteroo]

 

 

 

[Head in the clouds]

..... so the Effect of controls and FURTHER effect of controls teaches you. ( This is only roll produces YAW and yaw produces ROLL)

I don't think yaw is a secondary effect of roll, but it certainly is an effect of increased induced drag from the downward deflected aileron, and reduced induced drag from the upward deflected aileron, in most aircraft.

 

There are also some aircraft with ailerons that are designed to increase their drag when deflected up, by having the nose of the aileron protrude into the airstream below the wing. On a few types this is sufficient to equal the drag induced by the down deflected aileron, and hence cancel any adverse yaw effects. This means that the aircraft will roll around the point without any rudder input. I seem to recall that the Cassutt II is one such example, though it may have been another of the Formula One types, the designer decided that the intentional aileron drag was less than the rudder drag while rolling hard to turn around the pylons, so the aileron drag was the lesser of two evils for a race plane.

 

On the Va subject - I think there is some confusion between inertia and momentum. Inertia is the resistance of a body at rest to move, and relates to its mass only. Momentum is the resistance of a moving body to change direction and relates to its mass and velocity (Mass x Velocity). But - are we sure that momentum actually has anything to do with it?

 

 

[poteroo]

Note the difference between structural damage and structural failure on the RV9A envelope.

 

Note the 50fps lines - this is equal to 3000fpm and is certainly going to break an aircraft with flaps extended.

 

The Va in the RV9A is way below the yellow arc,Vno and you can see that if you hit a 50fps gust at Vno - your RV is likely to feel pain!

 

Have to go. We should all discuss these numbers and what they mean because too many of our pilots think that even flying in the yellow is OK.

 

happy days,

 

 

[ben87r]

I don't think yaw is a secondary effect of roll, but it certainly is an effect of increased induced drag from the downward deflected aileron, and reduced induced drag from the upward deflected aileron, in most aircraft.There are also some aircraft with ailerons that are designed to increase their drag when deflected up, by having the nose of the aileron protrude into the airstream below the wing. On a few types this is sufficient to equal the drag induced by the down deflected aileron, and hence cancel any adverse yaw effects. This means that the aircraft will roll around the point without any rudder input. I seem to recall that the Cassutt II is one such example, though it may have been another of the Formula One types, the designer decided that the intentional aileron drag was less than the rudder drag while rolling hard to turn around the pylons, so the aileron drag was the lesser of two evils for a race plane.

 

On the Va subject - I think there is some confusion between inertia and momentum. Inertia is the resistance of a body at rest to move, and relates to its mass only. Momentum is the resistance of a moving body to change direction and relates to its mass and velocity (Mass x Velocity). But - are we sure that momentum actually has anything to do with it?

Yaw does produce roll, and roll does produce roll, they use differential, and , frise ailerons to reduce the drag differential but this will have varying effects throughout the fligh envelope. First thing they teach with a multi endo is engine out, yaw causes roll, roll creates more yaw, pitch down and spiral dive if uncorrected

 

 

[Head in the clouds]

Yaw does produce roll, and roll does produce roll, they use differential, and , frise ailerons to reduce the drag differential but this will have varying effects throughout the fligh envelope. First thing they teach with a multi endo is engine out, yaw causes roll, roll creates more yaw, pitch down and spiral dive if uncorrected

Yes, yaw certainly does have roll as a secondary effect but I think you miss the point about roll not causing yaw. Roll (or rolling) doesn't cause anything except a bank angle. The adverse yaw is an aileron effect, not a roll effect. I.e. ailerons cause roll, they also cause adverse yaw - but roll, itself, doesn't cause yaw.

 

 

[facthunter]

Sorry I couldn't agree less. I never mentioned application of controls so aileron drag is not an issue. If the plane rolls (for whatever reason, Could be a gust) the aircraft banks and the fin and rudder will weathercock the plane causing it to YAW to the lower wing . Nev

 

 

[Head in the clouds]

If the plane rolls (for whatever reason, Could be a gust) the aircraft banks and the fin and rudder will weathercock the plane causing it to YAW to the lower wing . Nev

Weathercock the plane? That's a new one on me, I'd better go back to theory school ... I know what you're getting at but ... anyway let's stick to Va eh?

 

 

[Bruce]

Australia Aerobatic Club ‘CASA Guidelines'

 

3.13 MANOEUVRING SPEED

 

3.13.1 Manoeuvring speed (VA) is the speed above which full deflection of the elevator control will exceed aircraft structural limitations. Below VA the aircraft will stall before structural limits can be exceeded. VA will be specified in the aircraft's flight manual and placarded on the instrument panel. Full control deflection of any flight control should be avoided above this speed.

 

3.13.2 It is important to note that VA is established at the aircraft's maximum all-up weight or maximum aerobatic weight, and that at lighter weights it is possible to exceed G limitations at speeds less than the specified VA.

 

3.13.3 It could be argued that exceeding G limitations at a lighter weight may not necessarily overstress the wing structure because the lift forces imposed at the lighter weight for the same G are proportionately less, and therefore the wing structure should be strong enough to withstand the load. However, other airframe components such as engine mountings, attachments and other equipment still experience the full G loading and these structural components could fail even though the wing does not.

 

3.13.4 Thus, to stay within a safe operating envelope, the pilot should manoeuvre near VA with caution, monitor the accelerometer rather than rely solely on airspeed limitations, respect rolling G and flick roll limits, and be cautious in the use of abrupt control inputs.

 

• Never exceed the G or VNE limits of the aircraft
   
   
  

 

 

 

 

 

• Do not pull significant G above VA in turbulent conditions (a gust could overstress the aircraft)
   
   
  

 

 

 

 

[facthunter]

Some good points there, particularly things like engine mounts and SEATS If the seat collapses on the controls it's not your day. When you do a steep turn, you can cut your slipstream and sometimes it nearly throws you on your back. Likewise manoeuvring and hitting a gust can take your "G" loadings up quite high. Relaxing the back stick under those conditions might be appropriate if your flight path allows. Nev

 

 

[motzartmerv]

Weathercock the plane? That's a new one on me,

Really? Quite an accurate description of the effect. Ive always argued with the theory books, secondary effects of aileron being 'yaw' but in the opposite direction, followed by yaw in the same direction of the turn due to nev's weathercocking effect. All the text books will say 'yaw in the same direction' is the secondary effect of aileron. In most light aircraft this is simply not true. Yes, it does yaw in the same direction, but only AFTER a yaw away from the turn. Any Jab jockey with lazy feet will agree.

 

 

[poteroo]

Va comprehension!

 

 

 

[Head in the clouds]

All the text books will say 'yaw in the same direction' is the secondary effect of aileron. In most light aircraft this is simply not true. Yes, it does yaw in the same direction, but only AFTER a yaw away from the turn. Any Jab jockey with lazy feet will agree.

Hmmm - I've never seen a text that describes it other than adverse yaw i.e. yaw in the opposite direction from the roll, but then I haven't read every text on the subject ...

 

Anyway, secondary effects of controls, weathercocking and whatever really has nothing to do with Va (Design Manoeuvring Speed). I started this thread because a solid understanding of the reasons for the Va limitations at varying weights is critical to not overloading the airframe and failure to do so can be catastrophic. Therefore I consider this to be a critical subject for flight safety.

 

Adverse yaw, weathercocking and the like I would have described as relatively trivial by comparison but might be interesting to some, so perhaps someone could start another thread to discuss that if they want to?

 

djpacro must have missed this discussion and has kindly also started a thread on the Va subject with some very good input, his post is also embedded here -

 

The definition of Maneuver Speed is:The design maneuvering speed (VA) is the speed below which you can move a single flight control, one time, to its full deflection, for one axis of airplane rotation only (pitch, roll or yaw), in smooth air, without risk of damage to the airplane.

 

..... more.

It's well worth taking the time to read the contents of the link as they are the first info we have had posted here which describes the formula for determining the Va at weights lower than MTOW. In effect the formula is that the speed should be reduced from Va to a new Va which is the old Va multiplied by the square root of the lesser (new) weight divided by the MTOW.

 

That article also provides a passable description of the reason for the reduced Va at lower weight but might still prove a little hazy for some of us.

 

At the beginning of the thread I asked what succinct descriptions people used for the reason for varying Va in layman's terms without graphs and formula but so far we've only had graphs, charts and mention of inertia and accelerations. Is this all that students are provided with to reach an understanding of the subject? I certainly wasn't given anything at all better when I did my GA exams and it was a long while before I came across a more understandable explanation. The charts do tell a student about airspeeds to avoid at varying weights but they don't actually provide any understanding of the reasons or the physics behind it, do they?

 

A couple of typical questions that students used to ask -

 

How does the inertia (momentum?) affect the airframe and help it to cope with the 'acceleration' at higher weight?

 

Wouldn't one suppose that if the airframe is more heavily loaded and at, say 100kts, then when a gust (or control input) produces a force on the wing, and the wing tries to pull the airframe upward, surely the heavier the airframe, the more it will resist the wing, so the load must be higher? If not, what prevents it from being higher than the lighter airframe at the same speed, surely the lighter airframe will 'go with the wing' more easily and so unload/reduce the force? Like a yacht and a catamaran perhaps, the yacht gets a gust and heels over and unloads the sail, the catamaran with the same size rig gets dismasted because it can't unload the sail by heeling to the applied force ...

 

It's been stated above that the stall of the wing doesn't come into it. Is that actually true? The stall certainly isn't mentioned as any part of any formula regarding Va, so does that actually mean that the stall isn't a consideration in determining the Va at varying weights?

 

 

[motzartmerv]

Hmmm - I've never seen a text that describes it other than adverse yaw i.e. yaw in the opposite direction from the roll, but then I haven't read every text on the subject ...

HIC. Thank you for the thread, as you say its a very misunderstood, and under taught subject.

Not to put to fine a point on it, but I wonder how long it has been for you?

 

The two most highly used and published texts on BAK over the last 20 years for use in GA are The Trvor thom (ATC) and the Bob tait BAK.

 

To quote Trevors Thom: Chapter 1-6 The primary effect of moving the ailerons is to roll the aeroplane. A secondary or further effect is to cause yaw, since when the aeroplane is banked, the nose will have a tendency to slip sideways INTO the turn.

 

To Quote Bob Taits BAK: page 1.10 Note, The primary response to the application of aileron was roll. However the sidelsip which resulted from the roll produced yaw. In the absence of any other control inputs, the secondary effect of aileron is to produce yaw in the direction of bank.

 

 

The CPL aerodynamics exam also had " Yaw into the direction of turn" as required knowledge.

 

I also note in Bobs text on the same page " The aircrafts response to the offset airflow is similar to that of a weather cock- infact its sometimes called weathercocking."

 

 

 

 

 

 

I just thought I would give the above references for you, so you can see what people who have learned to fly in the last 2 decades have been taught.

 

 

[Head in the clouds]

HIC. Thank you for the thread, as you say its a very misunderstood, and under taught subject.Not to put to fine a point on it, but I wonder how long it has been for you?

The two most highly used and published texts on BAK over the last 20 years for use in GA are The Trvor thom (ATC) and the Bob tait BAK.

 

To quote Trevors Thom: Chapter 1-6 The primary effect of moving the ailerons is to roll the aeroplane. A secondary or further effect is to cause yaw, since when the aeroplane is banked, the nose will have a tendency to slip sideways INTO the turn.

 

To Quote Bob Taits BAK: page 1.10 Note, The primary response to the application of aileron was roll. However the sidelsip which resulted from the roll produced yaw. In the absence of any other control inputs, the secondary effect of aileron is to produce yaw in the direction of bank.

 

 

The CPL aerodynamics exam also had " Yaw into the direction of turn" as required knowledge.

 

I also note in Bobs text on the same page " The aircrafts response to the offset airflow is similar to that of a weather ****- infact its sometimes called weathercocking."

 

 

 

 

 

 

I just thought I would give the above references for you, so you can see what people who have learned to fly in the last 2 decades have been taught.

Hi MM. Thanks for that, not very helpful to a thread on Va but obviously important to you ... yes it's been a long while but nothing has changed much and as I said some posts ago in response to FH - I see what you're getting at. However, weathercocking, secondary effects of controls etc are still a trivial matter and more advantageously discussed elsewhere. I don't know of any cases of either which have caused inflight structural failure but I could be wrong about that too!

 

On a matter that is far more critical to flight safety, and relevant to this thread, do you perhaps have anything that might be helpful to people wanting a better understanding about Va at varying weights?

 

 

[motzartmerv]

Hic, I was replying to your comments on the matter ;)You contended comments made by myself and other instructors, so I was replying to that. If you dont wish to discuss it......

 

I think it was me who highlighted the knowledge gap and offered some ways to approach the problem in the other thread.

 

The idea that its only related to stalling speed changes with weight is a common 'miss conception' and my point the whole time has been that inertia is the biggest factor when talking about load limits and manourvering. The accident referenced in Dj's article above highlights this precisely where an airliner was misshandled to the point of structural failure well below the quoted VA of the aircraft. And it had nothing to do with the stall, or the stall speed.

 

To explain it, imagine an aeroplane that is massively massively heavy. Hundreds of times over its MTOW. Ignore the obvious problems with this and just think about whats going to happen when the stick is suddenly pulled back. If the aeroplane is super super heavy, its inertia will cause it to continue on a straight line ( as defined by newtons first law) until its acted upon by an unbalanced force. So to cause a change in its direction (acceleration) a force proportional to its massive weight needs to be applied to it. Now, the elevator is designed to cause a change to the aeroplane AT A SPECIFIC AND DEFINED MASS. If this aeroplane is 100 times heavier then this defined mass, the elevator will NOT apply enough force to cause a change in its direction (acceleration) and it will continue along the straight line with NO INCREASE TO THE LOAD FACTOR. Its the CHANGE in direction, acceleration, that causes the load increase.

 

While its silly to imagine this overweight aeroplane, the physics are still the same when you add or remove normal weights too and from and aeroplane.

 

Its the rate of change in direction that is the issue.

 

The dangerous part about all this is that while we define a VA, it by no means insures full and rapid control deflections wont cause the inertia to reach dangerous levels.

 

 

[Head in the clouds]

OK, I like your good example - allow me to be the Devil's Advocate and play the part of the student just learning the subject, if you will?

 

So - we have this massively heavy plane. And you said "the elevator will NOT apply enough force to cause a change in its direction", but the elevator never actually applies the force to change the plane's direction, the elevator applies a force behind the CG to change the attitude of the plane, and, if the elevator is deflected upward, the underside of the wing is then presented to the slipstream i.e. it has increased its AoA (angle of attack) and the wing then tries to cause a change in the direction of the plane.

 

Regardless of how heavy the plane might be, the elevator will always be able to swing the plane's longitudinal axis around its lateral axis and thereby increase the AoA since it only has to cause a slight change in the longitudinal balance to achieve that.

 

So - since the plane (load in the fuselage?) is massively heavy and won't respond to the wings' pulling, why doesn't the wing just get torn off the fuselage?

 

And - if the plane was really light instead, and flying at the same speed, why does the "reduce Va speed if the plane is lighter rule" suggest that the wing would actually get torn off or fail, even though the plane is really light and could respond to the wing pulling it upward, when logic would then suggest that the upward movement would unload the wing, because the plane would then be travelling in the direction the wing is pointing and so the AoA (and hence the load) would be reduced again?

 

 

[facthunter]

The disposition of the mass along the fore and aft axis has an inertia effect too. In this instance it would be rotational inertia and would resist pitch changes and cause the plane to flatten in a spin. The same mass concentrated near the Cof G would not affect things the same way. Engines, wing and tip tanks work the same way with roll. Hard to start and stop roll with a lot of mass away from the longitudinal axis . Nev

 

 

[Head in the clouds]

The disposition of the mass along the fore and aft axis has an inertia effect too. In this instance it would be rotational inertia and would resist pitch changes and cause the plane to flatten in a spin. The same mass concentrated near the Cof G would not affect things the same way. Engines, wing and tip tanks work the same way with roll. Hard to start and stop roll with a lot of mass away from the longitudinal axis . Nev

Granted - but I hope we can agree that in the above very heavy plane example the heavy cargo would normally be loaded close to, or on the CG, about which, or very close to which, the rotation caused by the elevator would occur, and so the elevator can still effect pitch/attitude changes.

 

 

[motzartmerv]

A bit circular there HIC. Does the elevator apply a force or not? In one sentence you stated it doesnt and then it does. (not a great instructional technique) I would simply say that yes it certainly does apply a force.

 

I was attempting to keep the problem simple to clarify it. If we are looking for students to understand the issue then the KISS principle should be applied.

 

How quickly the aeroplane ACTUALLY responds to AOA increase is the exact issue we are trying to understand.

 

The load factor will not increase until the aeroplane describes a new direction, so there for no, the wings will not tear off UNTIL the new direction is attained. Which is the whole point behind understanding inertia..

 

The lighter plane WILL respond to change much faster due to its lack of inertia and there for the LOAD factor WILL increase to critical levels. ..