high altitude stalls

As many of you know, the common passenger plane stalls at higher IAS at high altitude, due to higher mach number that leads to air compressibility issues or something. Since our super sonic fighter jets stall at pretty high speeds too and fly even higher, i'd presume they would also stall at considerably higher IAS at very high altitudes than at sea level.

I have three supersonic planes for DCS, the F15, Su27 and Mig-21. F15 and Su27 stall at same speed at high altitude (40000ft+) than at 6500ft, but the Mig-21 might show somewhat slower stall speeds at low altitude, 20-30kmh or so? I know the FC3 models are not study level sims but they use PFM so... for comparison, according to charts, the DC10 will stall almost 40kts higher IAS at 40000ft compared to 15000ft at clean config (150kts -> 190kts or so).

So how do the real world fighters perform at high altitude? Is the DCS modules realistic on this? If not, i'd hope it would be fixed since this would greatly limit high altitude maneuvers more realistic.

Thanks for comments!

IAS is a measure of pressure, not actual speed. So as long as you have enough pressure, you won't stall. Compressability alters this somewhat, but you have to keep in mind that these are supersonic aircraft, not subsonic ones. If you look at something like the F-86 and MiG-15, which can both have too little IAS and too much mach at the same time, they can run into those problems in theory. In practice I've never found it a problem as they simply don't fly high enough. But they can have some very narrow operating speed brackets at higher altitudes.

You are comparing a MiG-21 wing to a SU-27 wing...

Here's a link to a thread that may apply here...

http://forums.eagle.ru/showthread.php?t=142026

Hope that helps.

Mike

You are comparing a MiG-21 wing to a SU-27 wing...

No, i am comparing Mig-21 to Mig-21, and Su27 to Su27, and F-15 to F-15 at different altitudes.

Here's a link to a thread that may apply here...

 

http://forums.eagle.ru/showthread.php?t=142026

 

Hope that helps.

 

Mike

Yeah, that thread was started by me actually :-). I have studied this issue a bit now and it still is a bit odd, i totally understand the subsonic wing's high speed buffet margin, but not totally a low speed margin.

Wikipedia generally says that IAS stall speed stays generally unchanged, but the charts clearly states otherwise. It certainly stays unchanged at low speed aircrafts, because the air compressibility comes into play at mach 0.3 and above. That probably is the reason why U2 has almost constant IAS stall speed as seen here: https://upload.wikimedia.org/wikipedia/en/7/75/CoffinCornerU2.png

If you watch that closely, you will notice that the IAS stall speed actually rises, but very little.

Here we have the stall margins for a Dc10: https://dl.dropboxusercontent.com/u/10225958/cleanConfig_stallBoundary.png

Now we have a big difference.

Now, i don't know how supersonic wing behaves differently from transsonic/subsonic, but that's what i want to know. I'd like to see similar graphs about any supersonic fighter, but haven't found any yet :\ ...

IAS is a measure of pressure, not actual speed. So as long as you have enough pressure, you won't stall. Compressability alters this somewhat, but you have to keep in mind that these are supersonic aircraft, not subsonic ones. If you look at something like the F-86 and MiG-15, which can both have too little IAS and too much mach at the same time, they can run into those problems in theory. In practice I've never found it a problem as they simply don't fly high enough. But they can have some very narrow operating speed brackets at higher altitudes.

At first i didn't get this but now i think i got it.

The low speed buffet is actually low speed mach buffet, while we are flying slower than cruise speed with our SUBsonic aircraft, with high alpha, the air over the wing starts to go closer to mach 1, causing mach tuck, as in high speed mach buffet.

So, supersonic wing can handle all kinds of mach-issues, so the mach buffets won't happen at any stage of course (probably).

At first it was hard me to realize that mach would be again a problem, even when the speed gets lower. I hope i am right this time, because now i actually understand q corner! :smartass:

thanks

Compressability should not be a factor, as a good IAS system will be calibrated to take this into account.

The following assumes straight and level flying:

The amount of lift generated by a wing is influenced by mass flow of the passing air. As you fly higher the air gets less dense so you need to fly at a higher TAS (true air speed) to get the same mass flow and the same lift. However, air speed indicators measure IAS (indicated air speed) which also takes account of air density, so this is all taken care of. The amount of lift generated at a given IAS at sea level will be the same IAS at high altitude.

The difference with high altitude flight however, is the reduced density of the air makes it less sticky and more likely to separate (stall); this leads to a reduction in critical AoA with altitide.

Flying high with reduced critical AoA results in a higher stall speed which gives the impression of less lift, but if you remain below critical AoA then lift generated at a given IAS is the same at any altitude.

A reduced critical AoA does however result in less lift available; this is why turns etc require more care high up, and why it seems less lift is available.

The low speed buffet is actually low speed mach buffet, while we are flying slower than cruise speed with our SUBsonic aircraft, with high alpha, the air over the wing starts to go closer to mach 1, causing mach tuck, as in high speed mach buffet.

 

So, supersonic wing can handle all kinds of mach-issues, so the mach buffets won't happen at any stage of course (probably).

 

At first it was hard me to realize that mach would be again a problem, even when the speed gets lower. I hope i am right this time, because now i actually understand q corner! :smartass:

 

thanks

Not quite.

Mach number is the ratio of an aircraft's TAS to the local speed of sound (local as in the air an aircraft is currently flying through).

The local speed of sound is a function of air temperature; as temperature falls, the local speed of sound reduces.

The produces a double whamy: As you climb the air gets less dense so your TAS has to increase to maintain IAS (this means you're generally accelerating towards higher mach numbers in a climb). But it also gets colder as you climb, so the local speed of sound reduces too. Hopefully that makes sense.

Now for buffeting.

Low speed buffet is caused by the wing stalling. As per my previous post, at higher altitudes, this occurs at higher IAS (and as per this post, also higher mach numbers). There is no low speed mach buffet, it just seems that way for the reasons listed.

High speed buffet IS mach related. I'm less well informed about this, but I believe you're on the right track about wings being designed for supersonic flight. You're certainly right that higher AoA will cause air to accelerate even more over the top of the wing. Buffet is also caused by shock waves; IIRC, in particular, shock waves generated at the front of the aircraft hitting wingtips etc further aft (this is why supersonic aircraft are pointy, so stop this from happening).

Low speed buffet is caused by the wing stalling. As per my previous post, at higher altitudes, this occurs at higher IAS (and as per this post, also higher mach numbers). There is no low speed mach buffet, it just seems that way for the reasons listed.

Thanks for your answer again!

I read the FAA's Airplane Flying Handbook's Transition to Jets -section about these issues, and i will now quote:

"There are also occasions when the buffet can be experienced at much slower speeds know as "low speed Mach buffet."."

Then it says that too slow speed + altitude is causing high angle of attack.

I quote some more: "This very high angle of attack would have the same effect of increasing airflow over the upper surface of the wing to the point that all of the same effects of the shock waves and buffet would occur as in the high speed buffet situation

"The higher the airplane flies, the thinner the air and the greater the angle of attack required to produce the lift needed".

Well this clearly says it actually IS mach buffetting. And is exactly the same effect, but initiated by a different reason (high aoa @ low airspeed vs just high airplane's mach)(?).

So, i still think that both buffets are caused by mach-issues. This would still explain why some airplanes (slow ones) do not suffer these issues, like u2, which stall speed is raised only by a few knots at 75000(!) ft for example.

But let's forget mach-issues for a while, because our beautiful supersonic aircrafts do not suffer any mach issues.

If thinner air requires more aoa as you and the handbook stated, then ALL planes should stall at lower IAS speed at altitude, because critical aoa is reached at earlier point (or speed.)? UNLESS this is infact only true due to some kind of "mach-induced flow separation due to wing that can't handle these issues (subsonic wing)." Meaning, this would after all be true only in high speed subsonic jets, not in supersonic jets, not in WW2 spitfires, or turbo charged cessnas.

But if all planes suffer this aoa issue, then our fighers in DCS too, should stall at higher IAS if there are limited angle of attack -range compared to low altitudes?

Now, maybe i am wrong again or seeing things in a twisted angle, feel free to correct :)

Thanks for your answer again!

I read the FAA's Airplane Flying Handbook's Transition to Jets -section about these issues, and i will now quote:

 

"There are also occasions when the buffet can be experienced at much slower speeds know as "low speed Mach buffet."."

 

Then it says that too slow speed + altitude is causing high angle of attack.

 

I quote some more: "This very high angle of attack would have the same effect of increasing airflow over the upper surface of the wing to the point that all of the same effects of the shock waves and buffet would occur as in the high speed buffet situation

Interesting thanks. I personally think that the authour is insane, because if they state that the "low speed mach buffet" is caused by the same effect as high speed buffet, why not just call it high speed buffet and avoid the confusion, but that's just my opinion!

"The higher the airplane flies, the thinner the air and the greater the angle of attack required to produce the lift needed"

That statement is missleading, borderline wrong. The following is the equation for lift:

L = 1/2 ρ V2 x S x CL

They're assuming that as you climb at a constant speed (V), density (ρ) reduces so you have to increase AoA (CL) to compensate. But V represents TAS, not IAS. As mentioned previously, in the climb, TAS increases while IAS remains the same so there's no need for AoA (CL) to increase at all.

So, i still think that both buffets are caused by mach-issues. This would still explain why some airplanes (slow ones) do not suffer these issues, like u2, which stall speed is raised only by a few knots at 75000(!) ft for example.

"High speed buffet" is caused by an aircraft flying at high mach numbers it wasn't designed to fly at.

"Low speed buffet" is caused by air separating from the wing (stall).

There may be other terms like "low speed mach buffet", but I've never heard of them before. As above, I just think that it's a poor choice of words as it doesn't really describe anything different and sounds similar to something else!

If thinner air requires more aoa as you and the handbook stated, then ALL planes should stall at lower IAS speed at altitude, because critical aoa is reached at earlier point (or speed.)? UNLESS this is infact only true due to some kind of "mach-induced flow separation due to wing that can't handle these issues (subsonic wing)." Meaning, this would after all be true only in high speed subsonic jets, not in supersonic jets, not in WW2 spitfires, or turbo charged cessnas.

 

But if all planes suffer this aoa issue, then our fighers in DCS too, should stall at higher IAS if there are limited angle of attack -range compared to low altitudes?

 

Now, maybe i am wrong again or seeing things in a twisted angle, feel free to correct :)

As above, thiner air only requires greater AoA if you maintain TAS, but pilots don't fly by TAS so it's a moot point. Thiner air only reduces critical AoA, but while you remain below critical AoA, you'll get the same lift at a given IAS at any altitude.

DCS planes should indeed stall at a lower AoA at high altitudes. I'll give that a test this evening!

As above, thiner air only requires greater AoA if you maintain TAS, but pilots don't fly by TAS so it's a moot point. Thiner air only reduces critical AoA, but while you remain below critical AoA, you'll get the same lift at a given IAS at any altitude.

 

DCS planes should indeed stall at a lower AoA at high altitudes. I'll give that a test this evening!

Let's first make sure we are talking about same things. Critical AoA = the point where stall occurs, or, it's the point just before stall. Right?

Now, suppose our plane stalls at 17 degrees at sealevel, and is reduced in thinner air to, just throwing numbers here, 15 degrees, wouldn't that give you higher IAS stall speed. (which was the whole point of this thread)?

Let's exaggerate: If the stall speed would be reduced so much i could only add 1 degree of AoA at cruising speed, then i would stall almost immediately below cruise speed? but if, on the another hand at SL i could add +20 degree of AoA, that would give me more margin speedwise ?

EDIT: About the low speed buffet: "The low-speed buffet boundary is defined by the occurrence of buffet due to high-lift-induced boundary layer flow separation. As such, it can be associated with Mach Number effects, but the principal effector is Reynolds Number"

I am not even TRYING to understand "reynolds number" at this point, but i think that also hints that while scientifically it is not mach-buffet, this issue is something that would not happen with lower mach-speeds, and is, in a way, mach related? That would explain the simplified "low speed mach buffet" explanation for pilots (not for fluid dynamics -students) for simple understanding.

Let's first make sure we are talking about same things. Critical AoA = the point where stall occurs, or, it's the point just before stall. Right?

 

Now, suppose our plane stalls at 17 degrees at sealevel, and is reduced in thinner air to, just throwing numbers here, 15 degrees, wouldn't that give you higher IAS stall speed. (which was the whole point of this thread)?

Yes all the above is correct. I got slightly side tracked by that strange "low speed mach buffet' statement. :music_whistling:

What I was trying to emphasise is that it's the reduction in critical AoA at high altitude that results in higher stall speeds, not that the wing gets less efficient at generating lift (while below critical AoA).

Let's exaggerate: If the stall speed would be reduced so much i could only add 1 degree of AoA at cruising speed, then i would stall almost immediately below cruise speed? but if, on the another hand at SL i could add +20 degree of AoA, that would give me more margin speedwise ?

Yeah that's right. This most often comes into play during turns at high altitude. Turns require additional lift, so if you're too close to stall speed / critical AoA when you make the turn, you can end up in trouble.

EDIT: About the low speed buffet: "The low-speed buffet boundary is defined by the occurrence of buffet due to high-lift-induced boundary layer flow separation. As such, it can be associated with Mach Number effects, but the principal effector is Reynolds Number"

 

I am not even TRYING to understand "reynolds number" at this point, but i think that also hints that while scientifically it is not mach-buffet, this issue is something that would not happen with lower mach-speeds, and is, in a way, mach related? That would explain the simplified "low speed mach buffet" explanation for pilots (not for fluid dynamics -students) for simple understanding.

Reynolds number is basically a way of defining how viscous or sticky a fluid is. The lower the reynolds number the more sticky it is and the less likely it is to become turbulent. Reynolds number increases with altitude because the air gets less dense and therefore less sticky. This is why critical AoA reduces at altitude; air no longer sticks to the wing as easily, separates, and creates turbulence (a stall).

Reynolds number is basically a way of defining how viscous or sticky a fluid is. The lower the reynolds number the more sticky it is and the less likely it is to become turbulent. Reynolds number increases with altitude because the air gets less dense and therefore less sticky. This is why critical AoA reduces at altitude; air no longer sticks to the wing as easily, separates, and creates turbulence (a stall).

Yes, alright, thanks for that =)

BTW i have tried maneuverability at high altitudes (30000ft+) in DCS modules, with F86F sabre most recently. In my tests, all that makes maneuvering more difficult at an altitude seems to be the engine power. At high altitude and in 10000ft i can do similar 3-4G turn, if the initial speed is same (little above 200 knots), but the fact that i can add air speed more easily lower down increases my over all maneuverability. Not sure how is this right or wrong, but to me it seems that aerodynamically the plane is as maneuverable in 32000ft as in 10000ft. (quick test).