Va compared to Rough Air Speed
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Va compared to Rough Air Speed
I fly turbo-props for a living, not been able to get on any jet aircraft yet, though hopefully should be there soon!
My question concerns the difference between Va and the rough air speed. Va is almost always lower than rough air speed, why is this? Va is supposed to allow safety margins above stall at the limit of the airplanes g-loading envelope, correct? If so why wouldn't this also cover the speed to be flown in rough, turbulent air when the gust conditions encountered can replicate the same conditions affecting Va.
I've run across a few chaps who would like the speed flown in turbulence to be far below the rough air speed in the FCOM so I am curious as to what should be flown.
Any input would be greatly appreciated.
Thanks!
My question concerns the difference between Va and the rough air speed. Va is almost always lower than rough air speed, why is this? Va is supposed to allow safety margins above stall at the limit of the airplanes g-loading envelope, correct? If so why wouldn't this also cover the speed to be flown in rough, turbulent air when the gust conditions encountered can replicate the same conditions affecting Va.
I've run across a few chaps who would like the speed flown in turbulence to be far below the rough air speed in the FCOM so I am curious as to what should be flown.
Any input would be greatly appreciated.
Thanks!
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BHS239:
I’m quite sure there are differences in certification requirements (don’t have the regs handy right now). But the overall stress for the aircraft (turbulence vs. manoeuvering) is NOT the same. Manoeuvering means pilot-induced loads via the control surfaces, which is not considered in turbulence, at least for certification. So if you have a published turbulence speed, stick to it rather than getting close to stalling.
happy landings
I’m quite sure there are differences in certification requirements (don’t have the regs handy right now). But the overall stress for the aircraft (turbulence vs. manoeuvering) is NOT the same. Manoeuvering means pilot-induced loads via the control surfaces, which is not considered in turbulence, at least for certification. So if you have a published turbulence speed, stick to it rather than getting close to stalling.
happy landings
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Thanks for the reply. being new to the airline system alot of guys tend to give you thier own made up thoery of flight so to speak, so a forum of this nature is superb to combat fiction with fact.
Thanks alot for your input.
Thanks alot for your input.
Va is all about protection by stalling before a maximum load factor is attained.
Turbulence penetration is concerned with protection from stalling AND the likelyhood of exceeding maximum load factor from a gust.
Turbulence penetration is concerned with protection from stalling AND the likelyhood of exceeding maximum load factor from a gust.
Va is the maximum speed at which full control (rudder, aileron/spoiler or elevator/tailplane) may be applied without exceeding the structural limits of the aircraft. The previous post describes the fundamentals of turbulence penetration speed well. As has been said above, there is no direct relationship between the two.
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>LOMCEVAK Va is the maximum speed at which full control (rudder, aileron/spoiler or elevator/tailplane) may be applied without exceeding the structural limits
It is interesting that you included rudder. In fact, I believe the definition of maneuvering speed in many books ought to be less simplistic. However, for the moment, consider that aerodynamic equations assocoated with maneuvering deal primarily with lift (G's) and airspeed. It might be found that today many pilots have re-thought how rudder use fits into these equations.
It is interesting that you included rudder. In fact, I believe the definition of maneuvering speed in many books ought to be less simplistic. However, for the moment, consider that aerodynamic equations assocoated with maneuvering deal primarily with lift (G's) and airspeed. It might be found that today many pilots have re-thought how rudder use fits into these equations.
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(a) recent events implicate rudder reversals which are not addressed in the Standards.
(b) some aircraft have Va limited by rudder considerations in that directional qualities become a problem .. if my memory serves me correctly, this was the case with the early 125s.
(b) some aircraft have Va limited by rudder considerations in that directional qualities become a problem .. if my memory serves me correctly, this was the case with the early 125s.
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If we are talking JARs, then all the data is on the JAA site. It is a copy of FARs, so I suppose the same information is on the FAA site.
JARs give a fairly basic definition of Va, which is a design speed, assessed as basic stall speed times the square root of limiting positive g.
Vb is the design speed for application of gust criteria for certification. The definition is complex, and is in JARs in full. Vb is higher than Va. Rough air speed is not a design speed. It is a recommended speed, chosen to give you the best chance in turbulence of not hitting either the low speed or high speed buffet boundaries. Again, JARs explain how it is chosen, but a simple explanation is that it has to be higher than the minimum calculated Vb, and can be about half way between Vstall and Vmo/Mmo.
Although Va, calculated at max. certificated TOW, will not change, published limiting speeds for manoeuvre or for full control deflection will change with aircraft weight, as basic stall speed changes.
But I only quote what was wrote in JARs. John T and others are your real expert sources.
Dick W
JARs give a fairly basic definition of Va, which is a design speed, assessed as basic stall speed times the square root of limiting positive g.
Vb is the design speed for application of gust criteria for certification. The definition is complex, and is in JARs in full. Vb is higher than Va. Rough air speed is not a design speed. It is a recommended speed, chosen to give you the best chance in turbulence of not hitting either the low speed or high speed buffet boundaries. Again, JARs explain how it is chosen, but a simple explanation is that it has to be higher than the minimum calculated Vb, and can be about half way between Vstall and Vmo/Mmo.
Although Va, calculated at max. certificated TOW, will not change, published limiting speeds for manoeuvre or for full control deflection will change with aircraft weight, as basic stall speed changes.
But I only quote what was wrote in JARs. John T and others are your real expert sources.
Dick W
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The current FAR 23 words may be of use (you need to hyperlink to 23.335 as this link doesn't want to work as I had hoped) and, just above 23.335. is a downloadable picture of the envelope which might assist in understanding. If your penchant is for heavy metal, then the equivalent current FAR 25 words will increase the level of mental distress once you hyperlink to 25.335 ....
The standard caveat applies .. the Standard is an evolving beast and the present words may be quite different to those which applied in the past to any particular aircraft model in question. To resolve this, it is necessary to look to the Type Certificate Data Sheet for the particular machine and then find the archive version for the Design Standard.
I think Tinny's description is pretty close to the mark and generally adequate for pilot purposes ... and certainly a lot easier than wading through the rule swamp ...
The standard caveat applies .. the Standard is an evolving beast and the present words may be quite different to those which applied in the past to any particular aircraft model in question. To resolve this, it is necessary to look to the Type Certificate Data Sheet for the particular machine and then find the archive version for the Design Standard.
I think Tinny's description is pretty close to the mark and generally adequate for pilot purposes ... and certainly a lot easier than wading through the rule swamp ...
Last edited by john_tullamarine; 13th Jan 2003 at 11:55.
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Va ist the stationary, clean stallspeed for the positive maximum g-load at MTOW. If your plane is slower than Va, it will stall before your g-loads reach the limit value, if your plane is faster than Va, (or lighter than MTOW) you can exceed the g-limits by applying full up elevator.
Va is calculated directly from the 1g clean stall speed at MTOW by multiplying with the square root of the limit g´s.
Because Va is the natural limit speed for applying full up elevator, it is defined also the limit speed for any other full control surface deflection. (so you design all controls for the given Va value)
Vra is a performance value selected by the manufacturer, without any direct physical relation to loads or aerodynamics. It must be choosen smaller or equal to Vc. Vc can also be coosen by the manufacturer, but it must be greater or equal to an emperical calculated speed, depending just on the wing loading (JAR/FAR 23, small aeroplanes and commuters) or must provide a safety margin to Vb (JAR/FAR 25, large aeroplanes). Once you have choosen Vc, you must use it to calculate gust loads.
Gust loads are not static aerodynamics, so the Cl max value for the airfoil (and hence the wing) is greater than the static one. Therefor at Va you can experience gust loads, that are greater than the limit g-loads, depending on various parameters.
Quite similar to Va, Vb ist the speed at which the maximum positive gust leads to limit g-loads.
For most transport airplane the (normally assumed) positive 50 ft/s (JAR/FAR 25 : 56 ft/s) gust does NOT exced the g-limits at Va, so Vb is greater thav Va, but that does not naturally apply to all aircraft. You calculate your g-loads at Vc for the 50/56 ft/s gust, which are greater than the Vb limit loads (because Vc is greater than Vb but the gust is the same), and design your structure to carry this load.
Afterwards you choose an additional safety margin and define Vra as the speed, up to which the plane can be flown in strong gusts.
So to answer your original question :
If you want to sell a plane, it must be fast. Therefor you want to define Vra as large as possible.
On the other hand, you don´t want to have to high limit g-loads, because this makes your plane heavy, so mostly only the mimimum required value is choosen, leading to a relatively small Va.
On almost any large plane Vb ist higher than Va, and you choose Vc even larger. Same applies for the minimum required Vc for small aeroplanes. And you dont want to choose Vra with too much nargin to Vc, because you can´t sell a slow plane.
So Vc is larger than Va for any conventional plane, and safety margins don´t sell, therefor Vra is also normally higher than Va.
Va is calculated directly from the 1g clean stall speed at MTOW by multiplying with the square root of the limit g´s.
Because Va is the natural limit speed for applying full up elevator, it is defined also the limit speed for any other full control surface deflection. (so you design all controls for the given Va value)
Vra is a performance value selected by the manufacturer, without any direct physical relation to loads or aerodynamics. It must be choosen smaller or equal to Vc. Vc can also be coosen by the manufacturer, but it must be greater or equal to an emperical calculated speed, depending just on the wing loading (JAR/FAR 23, small aeroplanes and commuters) or must provide a safety margin to Vb (JAR/FAR 25, large aeroplanes). Once you have choosen Vc, you must use it to calculate gust loads.
Gust loads are not static aerodynamics, so the Cl max value for the airfoil (and hence the wing) is greater than the static one. Therefor at Va you can experience gust loads, that are greater than the limit g-loads, depending on various parameters.
Quite similar to Va, Vb ist the speed at which the maximum positive gust leads to limit g-loads.
For most transport airplane the (normally assumed) positive 50 ft/s (JAR/FAR 25 : 56 ft/s) gust does NOT exced the g-limits at Va, so Vb is greater thav Va, but that does not naturally apply to all aircraft. You calculate your g-loads at Vc for the 50/56 ft/s gust, which are greater than the Vb limit loads (because Vc is greater than Vb but the gust is the same), and design your structure to carry this load.
Afterwards you choose an additional safety margin and define Vra as the speed, up to which the plane can be flown in strong gusts.
So to answer your original question :
If you want to sell a plane, it must be fast. Therefor you want to define Vra as large as possible.
On the other hand, you don´t want to have to high limit g-loads, because this makes your plane heavy, so mostly only the mimimum required value is choosen, leading to a relatively small Va.
On almost any large plane Vb ist higher than Va, and you choose Vc even larger. Same applies for the minimum required Vc for small aeroplanes. And you dont want to choose Vra with too much nargin to Vc, because you can´t sell a slow plane.
So Vc is larger than Va for any conventional plane, and safety margins don´t sell, therefor Vra is also normally higher than Va.
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Great replies. I appreciate your taking the time to find the information and the links. Thank you.
Additionally, here is an article that is essentially spot-on.
http://www.aopa.org/asf/asfarticles/2003/sp0301.html
Additionally, here is an article that is essentially spot-on.
http://www.aopa.org/asf/asfarticles/2003/sp0301.html
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Another thought on the subject. If you look at JAR 25.331 on Flight manoeuvre and gust conditions, only manoeuvre in the pitching plane is considered. Again in JAR 25.333 the Flight manoeuvering envelope considers the “V-n diagram” the emphasis is on the pitching plane. Although a rolling limit is imposed it is not the rolling forces that are being considered but the asymmetric loading of the wings during a pitch manoeuvre with bank on.
Considering the accident with American Airlines Airbus A300-600R Flight 587 (facts available on the NTSB web site) perhaps we are flawed in our conventional thinking that VA is the design manoeuvering speed for all the controls.
Reading JAR 25.335 on Design Airspeeds carefully, the wording appears to imply the full application of elevator only.
Perhaps this accident has highlighted the folly of assuming that VA includes all the aerodynamic controls. If so, a radical change is needed in our training ,to “re-program” generations of pilots, to ensure the next accident is not because some poor mis-informed pilot has applied full aileron well within VA , only to find the last round of applause was his wings clapping hand behind his back!
Considering the accident with American Airlines Airbus A300-600R Flight 587 (facts available on the NTSB web site) perhaps we are flawed in our conventional thinking that VA is the design manoeuvering speed for all the controls.
Reading JAR 25.335 on Design Airspeeds carefully, the wording appears to imply the full application of elevator only.
Perhaps this accident has highlighted the folly of assuming that VA includes all the aerodynamic controls. If so, a radical change is needed in our training ,to “re-program” generations of pilots, to ensure the next accident is not because some poor mis-informed pilot has applied full aileron well within VA , only to find the last round of applause was his wings clapping hand behind his back!
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Flamgat,
If you read further on the accident ... you will note that the concern is not so much with a single, steady application of a control ... for which the design standards cater .. but with control input reversals .. for this situation the design process does not consider structural implications.
If you read further on the accident ... you will note that the concern is not so much with a single, steady application of a control ... for which the design standards cater .. but with control input reversals .. for this situation the design process does not consider structural implications.
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The effects of rapid rudder inputs were well understood in the 1950's and lessons learned well.
THEN enter the jonny-come-lateys at AA who taught maximum rapid rudder use.
SOME never learn...or forget.
THEN enter the jonny-come-lateys at AA who taught maximum rapid rudder use.
SOME never learn...or forget.
Per Ardua ad Astraeus
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BHS:
<I've run across a few chaps who would like the speed flown in turbulence to be far below the rough air speed in the FCOM so I am curious as to what should be flown.>
There is immense confusion about speeds to fly in 'turbulence'.
There is NOTHING wrong with flying below Vra in 'turbulence' as long as the 'turbulence' is not life-threatening. When it is, fly Vra. Reducing speed will (?always? - standing by to be corrected!) improve the ride for the salary payers in light or even moderate chop/turbulence without threatening the aircraft's structural or aerodynamic integrity. When it looks as if the bumps are getting so bad you think you will either stall or break up - fly Vra - and avoid.
<I've run across a few chaps who would like the speed flown in turbulence to be far below the rough air speed in the FCOM so I am curious as to what should be flown.>
There is immense confusion about speeds to fly in 'turbulence'.
There is NOTHING wrong with flying below Vra in 'turbulence' as long as the 'turbulence' is not life-threatening. When it is, fly Vra. Reducing speed will (?always? - standing by to be corrected!) improve the ride for the salary payers in light or even moderate chop/turbulence without threatening the aircraft's structural or aerodynamic integrity. When it looks as if the bumps are getting so bad you think you will either stall or break up - fly Vra - and avoid.
