Design speed VA
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Design speed VA
Somebody can explain the diagram JAR 25.333 (b) Why the point A2 is not coincident with the point A? Is it due to the fact the manoeuver which must be done at VA is done a little before?
Thanks
Thanks
A very interesting question, and I have to confess that despite having working in this area for about 10 years, I haven't the foggiest. For the benefit of others, the diagram in question is <A HREF="http://www.jaa.nl/jar/ihs_data/frameset.html?../av_vault/46000/46480/46480.htm" TARGET=_blank>here[/url.
And the JAA definition of airspeeds is [url=http://www.jaa.nl/jar/ihs_data/frameset.html?../av_vault/46000/46481/46481.htm]here</A>
I'm used to Va not being Vs*N1^½ - that is fairly common in more aeroelastic airframes. But in that case it is still at point A.
25.335 clearly says that: -
Va may not be less than VS1 n , which I think is a typo on the JAA website and means Vs1*n^½, since it certainly isn't a linear relationship. n in this case is the limit load factor at Vc, known to most of us as N1. This makes sense to me, since in my experience, aeroelastic effects leading to a non-square-root stall curve, are generally in a sense that gives a higher Va, never lower.
However it also says that:-
Va need not be more than VC or the speed at which the positive CNmax curve intersects the positive manoeuvre load factor line, whichever is less
Which indicates that Va may at a speed below point A.
So, in summary. I haven't the faintest idea. Perhaps the diagram is a hangover from some previous requirement?
Anybody else got a clue?
G
And the JAA definition of airspeeds is [url=http://www.jaa.nl/jar/ihs_data/frameset.html?../av_vault/46000/46481/46481.htm]here</A>
I'm used to Va not being Vs*N1^½ - that is fairly common in more aeroelastic airframes. But in that case it is still at point A.
25.335 clearly says that: -
Va may not be less than VS1 n , which I think is a typo on the JAA website and means Vs1*n^½, since it certainly isn't a linear relationship. n in this case is the limit load factor at Vc, known to most of us as N1. This makes sense to me, since in my experience, aeroelastic effects leading to a non-square-root stall curve, are generally in a sense that gives a higher Va, never lower.However it also says that:-
Va need not be more than VC or the speed at which the positive CNmax curve intersects the positive manoeuvre load factor line, whichever is less Which indicates that Va may at a speed below point A.
So, in summary. I haven't the faintest idea. Perhaps the diagram is a hangover from some previous requirement?
Anybody else got a clue?
G
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Perhaps the diagram is meant to be no more than a general indication of the things involved. Exactly where Va finishes up on the axis will surely depend on the detail of any specific design. But it must meet the various criteria specified - like those mentioned by Genghis
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Thanks for the attempt of explanation. I am ground instructor and when I have to define VA and to present the diagram it is not consistent. I am try to find the reason.
If somebody has got a clue?
If somebody has got a clue?
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Perhaps the Va and Va2 limits are weight related:
At hight mass, the wing will stall first: higher Va. At high mass and speed at or below Va the wing is "self protected" to any sudden elevator displacement.
At low mass, the wing will exceed G limitation before stalling: lower Va.
it's my understanding when I fly aerobatics...a full tank is better....maybe wrong.....
At hight mass, the wing will stall first: higher Va. At high mass and speed at or below Va the wing is "self protected" to any sudden elevator displacement.
At low mass, the wing will exceed G limitation before stalling: lower Va.
it's my understanding when I fly aerobatics...a full tank is better....maybe wrong.....
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stardust,
That is essentially correct, as the wing lift at stall is a relative constant amount of force, so the load factor drops as the aircraft's mass increases.
Of course, the ultimate load factor is not a fixed quantity, becasue it changes based on the distribution of loads. If the fuel is in a belly tank, the wing stresses on the roots are the same, even at the lower load factor due to a higher mass (the wing lift is the same at stall, so the loads on the fuselage joint are the same). If the fuel is in the wings, then the wing root loads are truly lower at stall.
For fixed masses distributed around the fuselage, such as the engine, your rule of thumb is quite right (except that generally these items are strengthened to withstand crash loads that are much higher than flight loads).
That is essentially correct, as the wing lift at stall is a relative constant amount of force, so the load factor drops as the aircraft's mass increases.
Of course, the ultimate load factor is not a fixed quantity, becasue it changes based on the distribution of loads. If the fuel is in a belly tank, the wing stresses on the roots are the same, even at the lower load factor due to a higher mass (the wing lift is the same at stall, so the loads on the fuselage joint are the same). If the fuel is in the wings, then the wing root loads are truly lower at stall.
For fixed masses distributed around the fuselage, such as the engine, your rule of thumb is quite right (except that generally these items are strengthened to withstand crash loads that are much higher than flight loads).
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Genghis, can you tell me where to find your diagram?
I would guess that the .333 diagram is just a copy from the FAA book, and does not relate exactly to the JAA text which specifies Va in words.
Geddifroggy, according to all the feedback we have the JAA examiners think that Va is Vs1g times the square root of the clean config limiting positive g, and that's it. No question that I have seen goes beyond that.I have seen nothing on application of the standard gust speeds,as in Genghis' posted diagram, but there is a specific JAA "standard gust" that varies with height. I have forgotten what it is, but again, this has never been questioned.
Let's not go too deeply into this, the JAR certification rules are a minefield, and pilots do not need to know 99% of them.
Dick W
I would guess that the .333 diagram is just a copy from the FAA book, and does not relate exactly to the JAA text which specifies Va in words.
Geddifroggy, according to all the feedback we have the JAA examiners think that Va is Vs1g times the square root of the clean config limiting positive g, and that's it. No question that I have seen goes beyond that.I have seen nothing on application of the standard gust speeds,as in Genghis' posted diagram, but there is a specific JAA "standard gust" that varies with height. I have forgotten what it is, but again, this has never been questioned.
Let's not go too deeply into this, the JAR certification rules are a minefield, and pilots do not need to know 99% of them.
Dick W
Dick,
The JAA and FAA codes (which are nearly, but not quite the same) are downloadable at...
http://www.jaa.nl/jar/jar.html
For JAA Codes
and...
http://www.faa.gov/avr/AFS/FARS/far_idx.htm
For FAA codes.
The JAA codes site is a bit more user-friendly, but the FAA site contains all the interpretative information. The JAA site is generally better for the diagrams.
Frankly, for teaching ATPL students, if I were you, I'd stick to part 23 - part 25 is full of additional engineering detail which, apart perhaps from a few elements in the performance section, won't really add anything useful to a pilot's knowledge.
Incidentally I've been thinking deeper thoughts about this. I
think that the A2 point on the part 25 diagram is just there so that design teams, should they wish, can reduce Va with smaller flight weights. This is in one light valid, since by doing so, you'll ensure that the permitted g-loadings are not exceeded. However, in practice, the g-loadings are determined at MTOW, and reaching a bit more g, at Va, at a lower weight, actually will still impose the same overall loads upon the airframe - so that would really just be a case of extra operating requirements for no particularly good reason.
Then again, much of the JAA seems to be run on that premise, or am I getting cynical in my old age ?
G
N.B. If you're down at Gloucester and want the odd groundschool lecture for your studes, I'm reasonably local and open to offers.
[ 11 December 2001: Message edited by: Genghis the Engineer ]
The JAA and FAA codes (which are nearly, but not quite the same) are downloadable at...
http://www.jaa.nl/jar/jar.html
For JAA Codes
and...
http://www.faa.gov/avr/AFS/FARS/far_idx.htm
For FAA codes.
The JAA codes site is a bit more user-friendly, but the FAA site contains all the interpretative information. The JAA site is generally better for the diagrams.
Frankly, for teaching ATPL students, if I were you, I'd stick to part 23 - part 25 is full of additional engineering detail which, apart perhaps from a few elements in the performance section, won't really add anything useful to a pilot's knowledge.
Incidentally I've been thinking deeper thoughts about this. I
think that the A2 point on the part 25 diagram is just there so that design teams, should they wish, can reduce Va with smaller flight weights. This is in one light valid, since by doing so, you'll ensure that the permitted g-loadings are not exceeded. However, in practice, the g-loadings are determined at MTOW, and reaching a bit more g, at Va, at a lower weight, actually will still impose the same overall loads upon the airframe - so that would really just be a case of extra operating requirements for no particularly good reason.Then again, much of the JAA seems to be run on that premise, or am I getting cynical in my old age ?
G
N.B. If you're down at Gloucester and want the odd groundschool lecture for your studes, I'm reasonably local and open to offers.
[ 11 December 2001: Message edited by: Genghis the Engineer ]
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Genghis, Gedifroggy,
We have two threads running here. One is, what do we teach for the JAR exams?, and the other is, what do we need to know to feel confident in our teaching?
The first is easy. As I said, the JAR ATPL question bank has a very simple definition of Va, and supplementary questions seem to be limited to how Va changes when Vs1g changes, usually because of a change of mass.
The second problem is more difficult. First, I would hold, in the real world, that Va does not change with change of mass, as it is a design speed, a standard to use in stress calculations. It is a worst case, and fixed. You could say that the speed at which you could deploy full control and remain inside the structural limits would change with mass, but Va is allways Va.
Anyway, to get 2.5g in a level turn you would need over 60º bank, and the aircraft would have been shouting "bank angle, bank angle" at you long before that. It pure pitch manoeuvres it is difficult to see where you would be pulling to the stall above Va, except in a complete upset at high speed, and in any case the control laws will be protecting you.
So the exact definition of Va is academic as far as the JAR exams are concerned. Equally so with Vb. Vb is a design speed, and is not Vra. Vc, likewise, is not your cruising speed, and is probably going to be more or less Vmo/Mmo. Vd can entirely theoretical. Design speeds are for designers. Pilots use Flap and gear placard speeds, Vra and Vmo/Mmo. Designers use stress diagrams, pilots use buffet boundary charts.
So, I lke to know exactly what the JAR rules say, but I don't teach them. I teach how to read and use the charts and information that will come out in the operating manual when th designers have finished designing and the certification guys have certified the aircraft.
Gedifroggy, e-mail me at [email protected] if you want to carry on with this. Genghis, interesting proposal. I should warn you that Alex doesn't pay me much, and I'm his father! E-mail us with your contact address please.
Dick W
We have two threads running here. One is, what do we teach for the JAR exams?, and the other is, what do we need to know to feel confident in our teaching?
The first is easy. As I said, the JAR ATPL question bank has a very simple definition of Va, and supplementary questions seem to be limited to how Va changes when Vs1g changes, usually because of a change of mass.
The second problem is more difficult. First, I would hold, in the real world, that Va does not change with change of mass, as it is a design speed, a standard to use in stress calculations. It is a worst case, and fixed. You could say that the speed at which you could deploy full control and remain inside the structural limits would change with mass, but Va is allways Va.
Anyway, to get 2.5g in a level turn you would need over 60º bank, and the aircraft would have been shouting "bank angle, bank angle" at you long before that. It pure pitch manoeuvres it is difficult to see where you would be pulling to the stall above Va, except in a complete upset at high speed, and in any case the control laws will be protecting you.
So the exact definition of Va is academic as far as the JAR exams are concerned. Equally so with Vb. Vb is a design speed, and is not Vra. Vc, likewise, is not your cruising speed, and is probably going to be more or less Vmo/Mmo. Vd can entirely theoretical. Design speeds are for designers. Pilots use Flap and gear placard speeds, Vra and Vmo/Mmo. Designers use stress diagrams, pilots use buffet boundary charts.
So, I lke to know exactly what the JAR rules say, but I don't teach them. I teach how to read and use the charts and information that will come out in the operating manual when th designers have finished designing and the certification guys have certified the aircraft.
Gedifroggy, e-mail me at [email protected] if you want to carry on with this. Genghis, interesting proposal. I should warn you that Alex doesn't pay me much, and I'm his father! E-mail us with your contact address please.
Dick W
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To understand Va you need to remember there is the design Va speed and the Va as placarded in the flight manual etc. The latter must be less than or equal to the design Va. The pilot only knows the latter. Va1 and Va2 are only relevant to the designer.
Va on the 25.333 is the design limit just referred to for which the aircraft structure must be designed to withstand loads associated with certian manoeuvres eg rolling as per 25.349(a)(2). (Roll rates are faster at slow speed)
Now Va1 and Va2 in 25.333 are related to the manouevre pitching accelerations (ref 25.331(c)(2)(i) &(ii)). tailplane loads etc must be etsblished for points on the line A2 to D2 in 25.333 - note this includes point A as well. Remember that Va in the FM is an upper limit (usually placarded as "do not make abrupt manoeuvres above this speed").
The designer however has to design for pitch accelerations throughout the flight speed range: The lower the speed - the higher the required pitching acceleration for design purposes (ref the equation of 25.331(c)(2)(i) &(ii)). Hence speeds less than Va may result in the highest tailplane loads etc.
Z
Va on the 25.333 is the design limit just referred to for which the aircraft structure must be designed to withstand loads associated with certian manoeuvres eg rolling as per 25.349(a)(2). (Roll rates are faster at slow speed)
Now Va1 and Va2 in 25.333 are related to the manouevre pitching accelerations (ref 25.331(c)(2)(i) &(ii)). tailplane loads etc must be etsblished for points on the line A2 to D2 in 25.333 - note this includes point A as well. Remember that Va in the FM is an upper limit (usually placarded as "do not make abrupt manoeuvres above this speed").
The designer however has to design for pitch accelerations throughout the flight speed range: The lower the speed - the higher the required pitching acceleration for design purposes (ref the equation of 25.331(c)(2)(i) &(ii)). Hence speeds less than Va may result in the highest tailplane loads etc.
Z
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Just to point out that as JAR 25.335(b)(2) (c)(i) says that Va may not be less than Vs1 times the square root of CnMax, the diagram at 333 has to be wrong, as it shows a lower Va. A1 and A2, agree, are just points that indicate certain flight conditions, but if 335 is right, A2 is the same as A. Note that the current Internet 335 has left out the necessary square root sign in the reference given above.
Note also that in the current Internet JARs there is no 331(c)(2)(i) or (ii), (2) having been reduced to a single sub-para.
I note these discrepancies only to illuminate what a b.....s muddle the JARs are. Gedifroggy raised the point in relation to teaching for the ATPL exams, and I repeat my advice to all in that game - stay out of detailed explanations of the JARs. The questions are basic and limited.
Dick W
Note also that in the current Internet JARs there is no 331(c)(2)(i) or (ii), (2) having been reduced to a single sub-para.
I note these discrepancies only to illuminate what a b.....s muddle the JARs are. Gedifroggy raised the point in relation to teaching for the ATPL exams, and I repeat my advice to all in that game - stay out of detailed explanations of the JARs. The questions are basic and limited.
Dick W
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First thanks to everybody for your contributions. I have asked the question not because I don't know the JAR 25-333 or the FAR but because of this strange position of points A1 and A2, sure I am teaching the basic(Vs and n) but it is my job of instructor to answer as deeply as possible to any question from my students when they request an explanation. Sorry, it is not my philosophy to give the minimum: I have been a civil crew member during 25 years and I respect my ex-colleagues and my junior colleagues: the ground school is not only helping youngs to pass an exam, it is also providing foundations for a whole long professional career and the following phase of instruction (type rating/simulator) reveals quite often a too weak instruction. Coming back to the question, I understand that - until another better explanation-points A1 and A2 correspond to a lighter weight at which the described manoeuver is done.
Thanks to everybody
Thanks to everybody
