manoevering speed
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manoevering speed
Could someone please explain manoevering speed - what it means and how to determine it. Sorry if this has been discussed before, I can't find it.
Consider the flight envelope / V-N diagram...
(this is just an example I have "borrowed" from a University website)
Va is the speed at which the stall speed curve (known as the 0-A) curve meets the positive g limit. The point at which this happens is called A (next significant point going clockwise is B, etc.) The main significance of this is that at or slower than Va, the aircraft will stall before you reach the positive g limit - so the main structure can't normally be overstressed.
A secondary significance of Va is that the structures people are then required to ensure that the structure of the flying controls are stressed for full deflection at any speed up to Va, and 1/3 of full deflection at Vne. Hence the fact that pilots are advised not to use more than 1/3 deflection above Va.
Normally, the stall speed curve 0-A follows the shape Vs=Vs1.N^½, so va will be Vs1 times the square root of the positive g limit. This isn't necessarily true for some more exotic flying machines such as flexwing microlights which show quite non-standard stall speed versus loading characteristics.
G
(this is just an example I have "borrowed" from a University website)
Va is the speed at which the stall speed curve (known as the 0-A) curve meets the positive g limit. The point at which this happens is called A (next significant point going clockwise is B, etc.) The main significance of this is that at or slower than Va, the aircraft will stall before you reach the positive g limit - so the main structure can't normally be overstressed.
A secondary significance of Va is that the structures people are then required to ensure that the structure of the flying controls are stressed for full deflection at any speed up to Va, and 1/3 of full deflection at Vne. Hence the fact that pilots are advised not to use more than 1/3 deflection above Va.
Normally, the stall speed curve 0-A follows the shape Vs=Vs1.N^½, so va will be Vs1 times the square root of the positive g limit. This isn't necessarily true for some more exotic flying machines such as flexwing microlights which show quite non-standard stall speed versus loading characteristics.
G
A fair point. Va is calculated at MTOW.
At a lower weight, the aircraft will stall at a lower speed, and thus the normal acceleration limit will be reached at a lower speed. But, the normal acceleration limit is also calculated at MTOW and if you work through the theory, you could increase the positive g limit as you reduce weight. Please forgive my pathetic attempts to do equations in PPruneScriptTM but...
Normally Va = VsMTOW x N1MTOW ^½
But, working with total loading on the wing, you can assume that N1actual=N1MTOW (MTOW/W)
And also, scaling stall speed with weight, Vs1actual = Vs1MTOW x (W/MTOW)^½
Thus...
Va = Vs1MTOW x (W/MTOW)^½ x (MTOW/W)^½ x N1^½
Which cancels out to Va = VsMTOW x N1MTOW ^½
Or in other words, Va can be considered constant regardless of aircraft weight, since the structural strength of the aircraft is actually based upon N1 at MTOW, and not just N1 in isolation.
Clearly there are going to be exceptions to this, but only in more complex types, particularly military aircraft with variable wingborne stores, since in those cases the load distribution over the wing is not constant. But even in those cases, unless Va is actually reduced by this, you'd have to leave it alone because of the effect upon stressing of the primary flying controls - this I am pretty certain is universal to all military and civil airworthiness codes.
G
At a lower weight, the aircraft will stall at a lower speed, and thus the normal acceleration limit will be reached at a lower speed. But, the normal acceleration limit is also calculated at MTOW and if you work through the theory, you could increase the positive g limit as you reduce weight. Please forgive my pathetic attempts to do equations in PPruneScriptTM but...
Normally Va = VsMTOW x N1MTOW ^½
But, working with total loading on the wing, you can assume that N1actual=N1MTOW (MTOW/W)
And also, scaling stall speed with weight, Vs1actual = Vs1MTOW x (W/MTOW)^½
Thus...
Va = Vs1MTOW x (W/MTOW)^½ x (MTOW/W)^½ x N1^½
Which cancels out to Va = VsMTOW x N1MTOW ^½
Or in other words, Va can be considered constant regardless of aircraft weight, since the structural strength of the aircraft is actually based upon N1 at MTOW, and not just N1 in isolation.
Clearly there are going to be exceptions to this, but only in more complex types, particularly military aircraft with variable wingborne stores, since in those cases the load distribution over the wing is not constant. But even in those cases, unless Va is actually reduced by this, you'd have to leave it alone because of the effect upon stressing of the primary flying controls - this I am pretty certain is universal to all military and civil airworthiness codes.
G
Last edited by Genghis the Engineer; 17th Apr 2003 at 22:19.
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Thanks, G & L, I'll try to work through and understand this.
But what prompted the question is:
"...Va can be considered to be constant regardless of aircraft weight..."
The Cessnna 206 has different Va's
3 600 lbs 120kias
2 950 lbs 108kias
2 300 lbs 96kias
Now, after a paradrop (very light +-2 300lbs), the descent is at about 140kias with turns at 30 degrees and sometimes light to moderate turbulence.
Should I be concerned?
All the best,
p
But what prompted the question is:
"...Va can be considered to be constant regardless of aircraft weight..."
The Cessnna 206 has different Va's
3 600 lbs 120kias
2 950 lbs 108kias
2 300 lbs 96kias
Now, after a paradrop (very light +-2 300lbs), the descent is at about 140kias with turns at 30 degrees and sometimes light to moderate turbulence.
Should I be concerned?
All the best,
p
Ah, subtly different question.
The C206 is I believe a part 23 aeroplane, in which case a gust analysis would have been done as part of the flight envelope determination. This should have led to determination of a maximum speed for flight in rough air.
This should be in the POH somewhere and designated either VB or VRA . So long as you are staying below that in moderate or worse turbulence, I don't think you've anything to worry about.
I've never worked on any of Cessna's products (flying a Cessna single occasionally is bad enough) but I'd guess the progressive reduction in Va is down to certification office concern about turbulence causing g-loadings to detach ancilliary items like batteries, crew and so-on - which is fair enough, but wouldn't jeapordise the whole aircraft.
Also, do you have a g-meter? If you have, and are not seeing anything worse than about 3g in those descents (limit is 3.8 I believe, but pitch rates can increase the value at extremes of the aircraft) then my uninformed instinct is that you're probably okay.
G
The C206 is I believe a part 23 aeroplane, in which case a gust analysis would have been done as part of the flight envelope determination. This should have led to determination of a maximum speed for flight in rough air.
This should be in the POH somewhere and designated either VB or VRA . So long as you are staying below that in moderate or worse turbulence, I don't think you've anything to worry about.
I've never worked on any of Cessna's products (flying a Cessna single occasionally is bad enough) but I'd guess the progressive reduction in Va is down to certification office concern about turbulence causing g-loadings to detach ancilliary items like batteries, crew and so-on - which is fair enough, but wouldn't jeapordise the whole aircraft.
Also, do you have a g-meter? If you have, and are not seeing anything worse than about 3g in those descents (limit is 3.8 I believe, but pitch rates can increase the value at extremes of the aircraft) then my uninformed instinct is that you're probably okay.
G
G,
I disagree with your statement: "Va can be considered constant regardless of aircraft weight, since the structural strength of the aircraft is actually based upon N1 at MTOW, and not just N1 in isolation". It is true that structural strength is a function of N1 x W, but there is inevitably an overriding maximum value of N1 which must be respected irrespective of AUW.
P,
I suspect that the different values of Va at different AUWs are based on a constant N1 limit. I agree with G that it is Vb that is important for the flight phase that you describe. Remember that Va is relevant to full deflection control inputs and the stall, not g limit exceedances in turbulence.
I disagree with your statement: "Va can be considered constant regardless of aircraft weight, since the structural strength of the aircraft is actually based upon N1 at MTOW, and not just N1 in isolation". It is true that structural strength is a function of N1 x W, but there is inevitably an overriding maximum value of N1 which must be respected irrespective of AUW.
P,
I suspect that the different values of Va at different AUWs are based on a constant N1 limit. I agree with G that it is Vb that is important for the flight phase that you describe. Remember that Va is relevant to full deflection control inputs and the stall, not g limit exceedances in turbulence.
Of course you are right LOMCEVAC. I was referring to the primary airframe strength, but there are all sorts of other bits like engine mounts, seats, batteries, baggage tie-downs which are stressed to N1, and at best it would smart a bit if those went, even if the wings and fuselage do hold together.
Pulling two POH off the shelf, I note that the PA28 and C152 both show variable values of Va, which a quick back-of-envelope sum indicates that are a function of constant N1.
Which raises an interesting philosophical point. A great many aircraft have Va determined at MTOW and placarded, but actually it should vary with weight in a way that, frankly, very few pilots will bother to work out (and I suspect just as few are taught to, my PA-28-161 POH for example refers to linear interpolation - now I know what that means but suspect strongly that I'm in the minority). An argument could readily be made that this is dangerous and we should perhaps in such aircraft standardise on the lower value of Va, say calculated at minimum permitted flight weight?
G
Pulling two POH off the shelf, I note that the PA28 and C152 both show variable values of Va, which a quick back-of-envelope sum indicates that are a function of constant N1.
Which raises an interesting philosophical point. A great many aircraft have Va determined at MTOW and placarded, but actually it should vary with weight in a way that, frankly, very few pilots will bother to work out (and I suspect just as few are taught to, my PA-28-161 POH for example refers to linear interpolation - now I know what that means but suspect strongly that I'm in the minority). An argument could readily be made that this is dangerous and we should perhaps in such aircraft standardise on the lower value of Va, say calculated at minimum permitted flight weight?
G
G,
There are 2 aspects to this. One relates to Vs, the other to the design max speed for the application of full aileron and rudder (which really is the main implication of Va for a pilot). I would want aircraft designed such that I could apply full aileron and rudder at the highest speed possible, requiring Va to be based on max TOW. Irrespective of what Va is quoted as, the N1 exceedance potential remains the same at speeds above "manoeuvre speed" or corner speed as it is referred to in combat aircraft.
L
There are 2 aspects to this. One relates to Vs, the other to the design max speed for the application of full aileron and rudder (which really is the main implication of Va for a pilot). I would want aircraft designed such that I could apply full aileron and rudder at the highest speed possible, requiring Va to be based on max TOW. Irrespective of what Va is quoted as, the N1 exceedance potential remains the same at speeds above "manoeuvre speed" or corner speed as it is referred to in combat aircraft.
L
By your logic (which I accept), you have have a choice of...
- A single value of Va, combined with means of determining whether normal acceleration limits have been exceeded, and a defined value of Vb, or...
- Multiple values of Va at different weights.
Clearly combat aircraft prefer the former, whilst part 23 aircraft (at-least the non-aerobatic ones) prefer the latter (although they do normally define Vb - although see my note below). I can see that either makes since in the operating environments for which the respective aircraft are designed.
Incidentally, I've just been looking at part 23 and a couple of POH. Vb is not required by part 23 to exceed Vc - which effectively means that they become co-incident. Then the Piper and Cessna POH that I've got on the shelf don't declare either, but give Vno, defined as "Maximum structural cruising speed - do not exceed this speed except in smooth air and then with caution". This seems permitted by part 23 which requires Vb to be defined, but not to be declared in operating data. A subtly different definition of Vno to that used in Def-Stan 00-970 which uses it as a speed with a safety margin below Vne.
Speaking for myself I'd rather have more instruments and single non-variable limits than less instruments and variable limits that you can get wrong.
G
- A single value of Va, combined with means of determining whether normal acceleration limits have been exceeded, and a defined value of Vb, or...
- Multiple values of Va at different weights.
Clearly combat aircraft prefer the former, whilst part 23 aircraft (at-least the non-aerobatic ones) prefer the latter (although they do normally define Vb - although see my note below). I can see that either makes since in the operating environments for which the respective aircraft are designed.
Incidentally, I've just been looking at part 23 and a couple of POH. Vb is not required by part 23 to exceed Vc - which effectively means that they become co-incident. Then the Piper and Cessna POH that I've got on the shelf don't declare either, but give Vno, defined as "Maximum structural cruising speed - do not exceed this speed except in smooth air and then with caution". This seems permitted by part 23 which requires Vb to be defined, but not to be declared in operating data. A subtly different definition of Vno to that used in Def-Stan 00-970 which uses it as a speed with a safety margin below Vne.
Speaking for myself I'd rather have more instruments and single non-variable limits than less instruments and variable limits that you can get wrong.
G
I know that various codes have had different definitions over the years but, just taking the current FAR's for this discussion:
"VA means design maneuvering speed ....
The maximum operating maneuvering speed, VO, must be established as an operating limitation. VO is a selected speed that is not greater than Vs x sqrt(n) .....
The value of VA need not exceed the value of VC used in design .... VC need not be more than 0.9 VH at sea level."
Maneuver speed for the Pitts S-2A is 154 mph CAS with its max aerobatic weight of 1500 lb. Maneuver speed for the Pitts S-2B is 154 mph CAS with its max aerobatic weight of 1625 lb i.e. less than Vs x sqrt(6) as permitted by the note above regarding 0.9 VH. The Pitts does not reduce VA with reduction in weight, even for those models where VA equals the stall speed at max weight times sqrt (n). From the S-2A Flight Manual:
"... at indicated airspeeds of 154 mph or less, you may apply full aileron, rudder, or nose-up elevator deflection without exceeding the airframe minimum design loads ......" From flight test measurements of an S-2B - the limit load factor of 6 g can be achieved from a sharp pull-up at 140 mph - a function of the dynamic and power effects on maximum lift.
The Decathlon Flight Manual takes a different view.
"Maneuvering speed (VA) is the maximum speed (for an established operating weight) at which full and/or abrupt use of the elevator control will not cause load factors in excess of the +6 G's in Normal Operations or -5 G's in Inverted or Outside Operations.
Full and/or abrupt movement of ailerons may be used at speed up to VA provided that the load factor does not exceed a +4 G's or a -3.2 G's. Use of ailerons above VA or above +4 G's or -3.2 G's should be smooth and limited to deflections which will cause a roll rate not exceeding that roll rate achieved with full aileron at VA.
CAUTION: Full abrupt use of the aileron with simultaneous use of full abrupt elevator at VA may produce loads in excess of design limits."
VA reduces by 3 mph for each 100 lb below maximum weight.
No mention of rudder here.
To give some general guidance for aerobatic pilots, some of us have been developing this Advisory Circular for (too many) years.
AC 91-075(0) - Guidelines for aerobatics (21K Adobe Acrobat file) published September 2001
see Section 10.9
For info on measured flight loads relevant to this subject take a look at
SAE 70002
Pilots are not told enough about flight load limitations due to use of controls - the two examples above are about as good as it gets. The CAP 10B AFM excels in this respect - a short extract:
"Full deflection of any one of the flight controls is authorized up to 146 mph, no matter if you are in negative or positive and with reservation that you are staying in the flight envelope."
"VA means design maneuvering speed ....
The maximum operating maneuvering speed, VO, must be established as an operating limitation. VO is a selected speed that is not greater than Vs x sqrt(n) .....
The value of VA need not exceed the value of VC used in design .... VC need not be more than 0.9 VH at sea level."
Maneuver speed for the Pitts S-2A is 154 mph CAS with its max aerobatic weight of 1500 lb. Maneuver speed for the Pitts S-2B is 154 mph CAS with its max aerobatic weight of 1625 lb i.e. less than Vs x sqrt(6) as permitted by the note above regarding 0.9 VH. The Pitts does not reduce VA with reduction in weight, even for those models where VA equals the stall speed at max weight times sqrt (n). From the S-2A Flight Manual:
"... at indicated airspeeds of 154 mph or less, you may apply full aileron, rudder, or nose-up elevator deflection without exceeding the airframe minimum design loads ......" From flight test measurements of an S-2B - the limit load factor of 6 g can be achieved from a sharp pull-up at 140 mph - a function of the dynamic and power effects on maximum lift.
The Decathlon Flight Manual takes a different view.
"Maneuvering speed (VA) is the maximum speed (for an established operating weight) at which full and/or abrupt use of the elevator control will not cause load factors in excess of the +6 G's in Normal Operations or -5 G's in Inverted or Outside Operations.
Full and/or abrupt movement of ailerons may be used at speed up to VA provided that the load factor does not exceed a +4 G's or a -3.2 G's. Use of ailerons above VA or above +4 G's or -3.2 G's should be smooth and limited to deflections which will cause a roll rate not exceeding that roll rate achieved with full aileron at VA.
CAUTION: Full abrupt use of the aileron with simultaneous use of full abrupt elevator at VA may produce loads in excess of design limits."
VA reduces by 3 mph for each 100 lb below maximum weight.
No mention of rudder here.
To give some general guidance for aerobatic pilots, some of us have been developing this Advisory Circular for (too many) years.
AC 91-075(0) - Guidelines for aerobatics (21K Adobe Acrobat file) published September 2001
see Section 10.9
For info on measured flight loads relevant to this subject take a look at
SAE 70002
Pilots are not told enough about flight load limitations due to use of controls - the two examples above are about as good as it gets. The CAP 10B AFM excels in this respect - a short extract:
"Full deflection of any one of the flight controls is authorized up to 146 mph, no matter if you are in negative or positive and with reservation that you are staying in the flight envelope."
Given a particular load factor limit I've always thought Va has to be proportional weight (unless it's fast enough at a lower weight that a higher weight limit isn't worth bothering with).
My logic is that at a lighter weight a force applied to the airframe by control surface deflection will produce a greater acceleration than the same force applied to a heavier fuselage. Greater acceleration equates to a greater LF. Ergo, reduce Va at lower weights to limit the maximum acceleration ability.
My logic is that at a lighter weight a force applied to the airframe by control surface deflection will produce a greater acceleration than the same force applied to a heavier fuselage. Greater acceleration equates to a greater LF. Ergo, reduce Va at lower weights to limit the maximum acceleration ability.
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Tinstaaflhas it right.
The easy definition of maneuvering speed is that speed where a stall will be experienced prior to reaching design limit load factor. In other words, you will reach aerodynamic limits before you overstress the aircraft, since you can recover a stall more easily than you can a bent wing.
The real structural load points of the aircraft, like the engine mounts and the wing roots (assuming primarily wing fuel tanks), see a constant mass (the engine for example), so LIGHT gross weight is more critical, since the wing at stall will produce the same lift regardless of mass, but at low mass the aircraft will achieve higher load factor.
Va varies with mass, and is slower at low mass as a result. In fact, it plots nicely along a V squared line, where the mass varies inversely with Va squared.
The easy definition of maneuvering speed is that speed where a stall will be experienced prior to reaching design limit load factor. In other words, you will reach aerodynamic limits before you overstress the aircraft, since you can recover a stall more easily than you can a bent wing.
The real structural load points of the aircraft, like the engine mounts and the wing roots (assuming primarily wing fuel tanks), see a constant mass (the engine for example), so LIGHT gross weight is more critical, since the wing at stall will produce the same lift regardless of mass, but at low mass the aircraft will achieve higher load factor.
Va varies with mass, and is slower at low mass as a result. In fact, it plots nicely along a V squared line, where the mass varies inversely with Va squared.
Yes, light weight is critical for many load cases. Its just a matter of defining the flight envelope and working thru them. The important thing is to make it clear to the pilot what the limits are. There are few flight manuals which achieve that objective.
VA is largely up to the designer. VA min is specified (by FAR 23 for example). There are provisions for having VA less than the corner of the flight envelope. On the other hand, the designer may choose a higher VA.
Consider that last point. VAmin is specified as being nominally at the corner of the flight envelope (I say nominally as it ignores dynamic effects on CLmax, cg position etc) yet I can choose a higher VA - and I can choose it to be constant at all weights. I can determine all the appropriate load cases per FAR 23. Clearly, that VA has nothing to do with the stall line - its only related to limitations on control application. Of course, full elevator can take it beyond the limit load factor - hence the term "checked maneuver" in those load cases.
VA is largely up to the designer. VA min is specified (by FAR 23 for example). There are provisions for having VA less than the corner of the flight envelope. On the other hand, the designer may choose a higher VA.
Consider that last point. VAmin is specified as being nominally at the corner of the flight envelope (I say nominally as it ignores dynamic effects on CLmax, cg position etc) yet I can choose a higher VA - and I can choose it to be constant at all weights. I can determine all the appropriate load cases per FAR 23. Clearly, that VA has nothing to do with the stall line - its only related to limitations on control application. Of course, full elevator can take it beyond the limit load factor - hence the term "checked maneuver" in those load cases.
