High Altitude Stall
Guest
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are we forgetting that the ASI is not a speed indicator at all .... only a differential pressure gauge ?... some assumptions and fancy designing labels the dial in speed instead of pressure ... if the conditions right here and now replicate the calibration assumptions then the gauge gives a correct speed .... at all other times it is wrong to a greater or lesser extent ... hence the various engineering tricks of different airspeeds to make the thing tractable for analysis .... worth reading the relevant sections of an engineering undergrad text on aerodynamics and the thing becomes a little easier to accept ..
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BOAC - agreed, it a localised acceleration of the airflow to M 1.0, but at 0.61 surely that is unlikely. Standard B757 cruise is0.80 with MMo at 0.84, and we still experience no high speed buffet. However, I do not think this about pulling G at altitude.
JF. Excuse my apparent ignorance, but if you are pulling harder and harder in you r 0.80 spiral dive, are you not increasing your wing loading. Stall speed, being directly related to effective weight, will increase with increased weight. How far adrift am I?
Unfortunately, I am still baffled by the HTBJ reference I quoted earlier which states that the EAS stall speed increases with altitude. EAS means nothing to do with intrument corrections, position error, or compressibility error; this is the stuff that is important to the aerodynamicists - or at least that's what me Mum told me! So...why does it increase with altitude?
Guys, if you have already explained it and I have not cottoned on please excuse me and send me to back of the class , but I really am curious about this one.
JF. Excuse my apparent ignorance, but if you are pulling harder and harder in you r 0.80 spiral dive, are you not increasing your wing loading. Stall speed, being directly related to effective weight, will increase with increased weight. How far adrift am I?
Unfortunately, I am still baffled by the HTBJ reference I quoted earlier which states that the EAS stall speed increases with altitude. EAS means nothing to do with intrument corrections, position error, or compressibility error; this is the stuff that is important to the aerodynamicists - or at least that's what me Mum told me! So...why does it increase with altitude?
Guys, if you have already explained it and I have not cottoned on please excuse me and send me to back of the class , but I really am curious about this one.
Guest
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Wilfred has been confusing EAS and TAS ... EAS removes density as a variable by definition and, on a simplistic view would suggest that stall EAS is independent of altitude. However, CL also is influenced by Re with the result that CLmax will be reduced at higher altitudes. Solving for EAS stall speed at CLmax then yields a value in excess of the SL figure. Shock separation is not relevant to the discussion.
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Considering Wilfred's concern about his first assumption, it is quite correct to relate density's decreasing with height to an increase in stall TAS. Recall, though, that EAS relates to SL density to remove the height variation in density from the calculations .. ie
local rho TAS squared= SL rho EAS squared
local rho TAS squared= SL rho EAS squared
Guest
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Gentlemen, I believe we're over-complicating the original question with talk of compressibility and EAS (although I am finding it quite interesting).
Wilfred - if we consider only TAS and IAS, then TAS Vs will increase with altitude, and IAS will ramain the same, for the reasons you outlined. Remember that, taken from the lift formula, '1/2rhoV* = IAS'
And I do believe EAS Vs will increase slightly with altitude. I might have to verify with the 17 or 18 text books I've got up in the shelves though.
Great thread and answers folks...
Wilfred - if we consider only TAS and IAS, then TAS Vs will increase with altitude, and IAS will ramain the same, for the reasons you outlined. Remember that, taken from the lift formula, '1/2rhoV* = IAS'
And I do believe EAS Vs will increase slightly with altitude. I might have to verify with the 17 or 18 text books I've got up in the shelves though.
Great thread and answers folks...
Guest
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Could the increase in stall speed be linked to Reynold's number. Which states
R=(Rho*V*L) / viscosity
Where V=TAS, and L=chord length
Thus;
A reduction in density at higher altitude (all other factors the same as at low alt) will reduce R and hence reduce Cl max.
Therefore if you maintain the same weight (hypertheticaly) you would need a higher AoA at the same speed, thus less margin over the stall AoA, therfore you would stall at a higher speed????
I might be on the right track, but then again, it is 2:30 in the morning........
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Eat My Vapour Trail
R=(Rho*V*L) / viscosity
Where V=TAS, and L=chord length
Thus;
A reduction in density at higher altitude (all other factors the same as at low alt) will reduce R and hence reduce Cl max.
Therefore if you maintain the same weight (hypertheticaly) you would need a higher AoA at the same speed, thus less margin over the stall AoA, therfore you would stall at a higher speed????
I might be on the right track, but then again, it is 2:30 in the morning........
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Eat My Vapour Trail
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John Tullamarine, Noooo. I am not knowingly confusing TAS with EAS. As I said above, EAS corrects for instrument, position and compressibility, and for a given TAS at altitude, this is the equivalent flow of air over the airframe, ie what your hand would feel out of the window, allowing for compression.
It is EAS that is important in 1/2RhoV2SCl questions, as that is what the wing feels.
Keep it coming folks!
It is EAS that is important in 1/2RhoV2SCl questions, as that is what the wing feels.
Keep it coming folks!
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Wilfred.. you are getting there .. as I suggested earlier and vapour trail reiterated .. the problem relates to how you simplify things .. CL depends on a number of things .. including Re (Reynolds Number).. at altitude CLmax is reduced ... solving for stall speed then gives an increased value for Vs .. don't get too hung up on EAS .. it is basically something we engineer chappies need to make the maths a little easier to process ... basically EAS is a "standardised" TAS .. all the other "speeds" are the system's way of getting around the fact that the ASI's calibration lets us down anywhere other than at standard sea level
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Wilfred,
I'll make my pitch & jump in here. Probably a little off but, here goes.
As the boundary layer travels over the top aft surface of the wing, it begins to decelerate and reaches the aft stagnation point at the trailing edge. Normal flow separation in a stall occurs when the lower levels of the boundary layer don't have insufficient energy to overcome the adverst aft pressure gradient on the wing. They prematurely stagnate, ie, before the trailing edge, and separate.
Perhaps the "mach no." effect is the beginning of the onset of the shock wave forming on the wing as a result of higher speed or altitude flight. Depending on wing laminar flow designs, this may happen a quite low mach number but since the drag ( read fuel flow ) effect was minimal, it was disregarded. However, it may have a noticeable effect on boundary layer energy levels. Hence the reason for the stall speed increase.
Is that what you were asking? Phew, I need another red!
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Goddamit! Burnt another one
I'll make my pitch & jump in here. Probably a little off but, here goes.
As the boundary layer travels over the top aft surface of the wing, it begins to decelerate and reaches the aft stagnation point at the trailing edge. Normal flow separation in a stall occurs when the lower levels of the boundary layer don't have insufficient energy to overcome the adverst aft pressure gradient on the wing. They prematurely stagnate, ie, before the trailing edge, and separate.
Perhaps the "mach no." effect is the beginning of the onset of the shock wave forming on the wing as a result of higher speed or altitude flight. Depending on wing laminar flow designs, this may happen a quite low mach number but since the drag ( read fuel flow ) effect was minimal, it was disregarded. However, it may have a noticeable effect on boundary layer energy levels. Hence the reason for the stall speed increase.
Is that what you were asking? Phew, I need another red!
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Goddamit! Burnt another one
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This thread has meandered somewhat .. the answer to the original question is found in any basic engineering text on fluid flow .. either aerodynamics or hydrodynamics ... fluid forces depend on
(a) velocity
(b) mass density
(c) a measure of size
(d) viscosity
(e) compressibility
and no other variables ....
Running through the usual dimensional analysis tricks ... these come down to the following terms ...
(a) mass density
(b) a characteristic area
(c) velocity squared
(d) Reynolds Number (Re)
(e) Mach No (M)
and no others ...
Re is important at low speeds, M is important at high speeds .. and so we tend to ignore one or the other as appropriate, and, in general, ignore Re for most CL considerations.
Re depends on density and viscosity and viscosity is roughly proportional to the square root of the temperature.
At height the end result is that the CLmax reduces .... solving for speed at the (1g) stall gives
Vs squared is proportional to 1/(CLmax x rho)... end result is that the stall speed goes up at height ..... measured as TAS if you are dealing with actual rho .. or measured as EAS if you are using standard SL rho ...... and that is the story ....
(a) velocity
(b) mass density
(c) a measure of size
(d) viscosity
(e) compressibility
and no other variables ....
Running through the usual dimensional analysis tricks ... these come down to the following terms ...
(a) mass density
(b) a characteristic area
(c) velocity squared
(d) Reynolds Number (Re)
(e) Mach No (M)
and no others ...
Re is important at low speeds, M is important at high speeds .. and so we tend to ignore one or the other as appropriate, and, in general, ignore Re for most CL considerations.
Re depends on density and viscosity and viscosity is roughly proportional to the square root of the temperature.
At height the end result is that the CLmax reduces .... solving for speed at the (1g) stall gives
Vs squared is proportional to 1/(CLmax x rho)... end result is that the stall speed goes up at height ..... measured as TAS if you are dealing with actual rho .. or measured as EAS if you are using standard SL rho ...... and that is the story ....
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John Tullamarine
Its really nice to go away for a few days and on return find somebody has sorted out your problems. Hope you have now convinced Wilfred!
I had hoped that my attempt at convincing W that the Cl max would reduce with height (thanks to the stalling alpha reducing with increasing speed and or height, due to compressibility) but I only seemed to confuse him further – judging by his comments on wind up turns.
JF
Its really nice to go away for a few days and on return find somebody has sorted out your problems. Hope you have now convinced Wilfred!
I had hoped that my attempt at convincing W that the Cl max would reduce with height (thanks to the stalling alpha reducing with increasing speed and or height, due to compressibility) but I only seemed to confuse him further – judging by his comments on wind up turns.
JF
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you have me a little perplexed .. the stall speed increases a little due to Re changes .... compressibility is irrelevant at stall speed values ... I think you are thinking of the compressibility increase in EAS as Hp increases for a normal cruise value airspeed ..... the typical Vmo envelope shows this sort of effect ...
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JM
No, what I was thinking of was the effect I quoted in an early post repeated here:
What I am talking about is the breakdown in the simple relationship between stalling speed and applied G. Simple (incompressible) consideration of the lift equation (L = Cl x ½ x density x wing area x speed squared) tells us that if we have a wings level stalling speed of say 100 kts, then at 141.4 kts (or whatever the square root of 2 is) we should be able to pull 2g, ie just manage a 60 deg bank steady level turn at the stall. But somehow one can never quite achieve this and the shortfall at 173.2 kts is even greater (root 3 when we would expect 3g from simple theory).
Every aircraft I have flown demonstrates this point even at circuit height. The designers I have whinged to always blame compressibility. But you feel it is more Re?
JF
No, what I was thinking of was the effect I quoted in an early post repeated here:
What I am talking about is the breakdown in the simple relationship between stalling speed and applied G. Simple (incompressible) consideration of the lift equation (L = Cl x ½ x density x wing area x speed squared) tells us that if we have a wings level stalling speed of say 100 kts, then at 141.4 kts (or whatever the square root of 2 is) we should be able to pull 2g, ie just manage a 60 deg bank steady level turn at the stall. But somehow one can never quite achieve this and the shortfall at 173.2 kts is even greater (root 3 when we would expect 3g from simple theory).
Every aircraft I have flown demonstrates this point even at circuit height. The designers I have whinged to always blame compressibility. But you feel it is more Re?
JF
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This is not something I have come across before .... when you have been looking at a banked stall, what were the conditions ie ..smooth air ?.. rate of change of bank ? ..rate of change of airspeed ? etc ... might give a clue to the answer ... I would have expected that, in a steady banked turn .. with the airspeed constrained to reduce slowly, the stall speed would be predictable. PEC ought not to be the problem .. Re is not varying ..and the mach number is too low to have any significance .... interesting ...
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JT
You asked about conditions at the stall. The manoeuvre I am thinking of is the wind up turn (WUT). It is a very common and bog standard flight test technique used in all development and certification programmes to establish the Manoeuvre Boundary of any aircraft (how much g can it pull at what speed)
The technique is to start at the test height and slowly increase alpha at the required speed until the stall. Once full power is not enough to maintain speed (ie at the thrust boundary) you overbank and lose height to maintain the speed steady.
The aim is to have quasi steady state alpha and speed (albeit with changing height) at the stall (a bit like the must be less than 2kts per sec speed reduction used for 1g certification stalls) Once the height loss is significant you start the manoeuvre above the test height and (if you are lucky) have it on condition as you pass the height.
As I say this is bread and butter of flight test but always produces points that sit on a curve that is a tad less than V squared. And that despite the beneficial effect of high power which has to have a small component assisting lift (indeed not so small at high alpha)
It is this gradual reduction in Cl max as speed increases that I have always been told is down to compressibility effects.
JF
You asked about conditions at the stall. The manoeuvre I am thinking of is the wind up turn (WUT). It is a very common and bog standard flight test technique used in all development and certification programmes to establish the Manoeuvre Boundary of any aircraft (how much g can it pull at what speed)
The technique is to start at the test height and slowly increase alpha at the required speed until the stall. Once full power is not enough to maintain speed (ie at the thrust boundary) you overbank and lose height to maintain the speed steady.
The aim is to have quasi steady state alpha and speed (albeit with changing height) at the stall (a bit like the must be less than 2kts per sec speed reduction used for 1g certification stalls) Once the height loss is significant you start the manoeuvre above the test height and (if you are lucky) have it on condition as you pass the height.
As I say this is bread and butter of flight test but always produces points that sit on a curve that is a tad less than V squared. And that despite the beneficial effect of high power which has to have a small component assisting lift (indeed not so small at high alpha)
It is this gradual reduction in Cl max as speed increases that I have always been told is down to compressibility effects.
JF
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sorry .... misinterpreted the thrust of your previous post ... the majority of my FT (not overly extensive) experience has been on performance not handling (other than basic work). The phenomenon to which you refer, I suggest, is either going to be a result of one or more of
(a) unsteady flow affecting the PEC depending on the manner in which the stall is approached
(b) compressibility if the aircraft concerned are of sufficiently high performance with attendent high stall speeds
(c) Re if the data be not reduced to SL
(a) unsteady flow affecting the PEC depending on the manner in which the stall is approached
(b) compressibility if the aircraft concerned are of sufficiently high performance with attendent high stall speeds
(c) Re if the data be not reduced to SL