Design manoeuvre speed, Va
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Design manoeuvre speed, Va
Design manoeuvre speed, Va, is usually taken to be the EAS at which you can apply full control deflection without exceeding the manoeuvre envelope limits. In the positive pitch sense this is the speed at which you can just pull max g at the stall, mathematically Vs times the square root of the limiting load factor. I have always thought this was a maximum for Va, but there might be lower speed limits in roll or yaw.
However, JAR25.335 on design airspeeds says that Va must not be LESS than this figure. It then goes on to give a drawing of the flight envelope in 25.333 which shows a Va below this figure, and adds a statement that Va NEED NOT BE higher than the Vs x root N figure.
Does anyone have a definitive view on how Va should be defined or calculated? This is important for JAR exam candidates, who get asked questions about it.
Dick W
However, JAR25.335 on design airspeeds says that Va must not be LESS than this figure. It then goes on to give a drawing of the flight envelope in 25.333 which shows a Va below this figure, and adds a statement that Va NEED NOT BE higher than the Vs x root N figure.
Does anyone have a definitive view on how Va should be defined or calculated? This is important for JAR exam candidates, who get asked questions about it.
Dick W
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I know nothing about JAA ATPL exams, so what I say may only be correct, and not necessarily JAR-FCL approved.
Va may not be less than Vs1*SQRT(n1), which in practice usually means that it's exactly that figure. But, a few aircraft by virtue of clever FBW systems, or good old fashioned aeroelasticity, will offload lift allowing a higher value of Va. The only class of aircraft for which this is often true is weightshift microlights (which are very very aeroelastic), I've not come across any FAR/JAR-25 airliner for which it is true.
Primary flight controls are required by every standard that I know of (which includes all FARs, JARs, BCARs and Def-Stans) to be stressed to the greater loads of full deflection at Va or 1/3 deflection at Vd. This is where the requirement comes from that you mustn't apply more than 1/3 control deflection above Va.
So, whilst technically Va could be different in each of pitch, roll, and yaw, good practice and all the usual standards require that it's the same for each. (I once got away with making it the same as Vf, thus giving the pilot even less numbers to remember, but you don't pull a stunt off like that often).
G
Va may not be less than Vs1*SQRT(n1), which in practice usually means that it's exactly that figure. But, a few aircraft by virtue of clever FBW systems, or good old fashioned aeroelasticity, will offload lift allowing a higher value of Va. The only class of aircraft for which this is often true is weightshift microlights (which are very very aeroelastic), I've not come across any FAR/JAR-25 airliner for which it is true.
Primary flight controls are required by every standard that I know of (which includes all FARs, JARs, BCARs and Def-Stans) to be stressed to the greater loads of full deflection at Va or 1/3 deflection at Vd. This is where the requirement comes from that you mustn't apply more than 1/3 control deflection above Va.
So, whilst technically Va could be different in each of pitch, roll, and yaw, good practice and all the usual standards require that it's the same for each. (I once got away with making it the same as Vf, thus giving the pilot even less numbers to remember, but you don't pull a stunt off like that often).
G
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Thanx Gengis for this and the reply on the Testers forum. JAA do not disclose their references, but it is clear from feedback that they think Va is at all times the basic figure of Vs times root n. My worry about the max/min definition arose from the fact that at higher speeds you can overstress the aircraft by pulling, and that seems to negate the whole idea of Va. Then there is the example diagram in JARs that clearly shows an example Va lower than the stall/n limit corner.
Big problem for us is that we want to teach the real life truth, but also need to get the students past the examiners, which sometimes means teaching cr**.
But hey, who said it was easy
Dick W
Big problem for us is that we want to teach the real life truth, but also need to get the students past the examiners, which sometimes means teaching cr**.
But hey, who said it was easy
Dick W
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FWIW - it is my understanding that for most wings the flaps up setting allows biggest increase in Cl for a given gust.
Additionally, of course, such encounters tend to occur at higher EAS in the clean config.
The above needs to be balanced against the reduced max LF with flaps deployed...
Additionally, of course, such encounters tend to occur at higher EAS in the clean config.
The above needs to be balanced against the reduced max LF with flaps deployed...
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Normal gust response is a function of lift-curve-slope * EAS / Wing-loading.
Whilst flaps will increase CL.max, I'm not sure that they always have the same effect upon dCL/d.Alpha, you probably want to check the data sheet for the wing. On most "conventional" wings I don't think that the flaps (as you say) have much effect on the lift slope.
G
Whilst flaps will increase CL.max, I'm not sure that they always have the same effect upon dCL/d.Alpha, you probably want to check the data sheet for the wing. On most "conventional" wings I don't think that the flaps (as you say) have much effect on the lift slope.
G
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Evidently. This cropped up during some mod programs years ago on a couple of early 125s. Apparently the full rudder test point is more effective at raising an eyebrow than is the concern for pitching loads. As the matter didn't impact on the program, I didn't bother pursuing it at the time so I have no further details with which to amuse.
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I was told Va was the speed to fly near and in turbulance and it gave the best low and high speed stall protection, this was on the L1011 and the 767-300er.
Hope this is not total rubbish, however I never did stall.
Your considered comments please?
Hope this is not total rubbish, however I never did stall.
Your considered comments please?
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There's another term "Vra" which is the recommended rough air speed.
In practice this is usually the same as, or slightly less than Va. Often they co-incide, leading to the confusion, although the definitions aren't identical.
The rational is that at Va you are at the highest speed at which a stall will occur rather than an overstress, so it should give as you say the best protection. If you do stall an L1011 in turbulence, do let us all know, it should be worth a few beers!
G
In practice this is usually the same as, or slightly less than Va. Often they co-incide, leading to the confusion, although the definitions aren't identical.
The rational is that at Va you are at the highest speed at which a stall will occur rather than an overstress, so it should give as you say the best protection. If you do stall an L1011 in turbulence, do let us all know, it should be worth a few beers!
G
