correct as is Su-27: Wing buffet at high altitude at medium ange of attack

[Pavlin_33]

For some reason the Flanker buffets at 15° AoA at high altitude (around 12km), but does not when the same AoA is attained near sea level.
As if some different physic are at play at higher altitudes, even though the angle of attack as well as the indicated air speed are (roughly) the same.Tracks in the attachment. Please ignore the part where the AoA goes over 15 degrees, this is not relevant to the issue reported here.

Thanks in advance.

Su-27-high-buffet.trk Su-27-low-no-buffet.trk

[Ironhand]

I won’t be able to take a look at the tracks until tonight or tomorrow at the soonest. So I don’t know what speeds are involved but stall onset at high altitude occurs at a lower AoA than at low or medium altitude. So what’s happening doesn’t seem incorrect on its face. The IAS numbers may be similar but the TAS/Mach numbers most definitely aren’t.

[Pavlin_33]

8 hours ago, Ironhand said:

...So I don’t know what speeds are involved but stall onset at high altitude occurs at a lower AoA than at low or medium altitude. So what’s happening doesn’t seem incorrect on its face. The IAS numbers may be similar but the TAS/Mach numbers most definitely aren’t.

Hi @Ironhand,
could you please provide some more details as why would this be happening? What I mean is: why would the lower AoA cause this at higher altitude?
The only thing I can think of is the Mach number, but I am not 100% sure how this would cause the buffet. A part from that, IAS should not matter at all as the stall, ie. flow separation is only a function of AoA and not the speed.

[Yo-Yo]

42 minutes ago, Pavlin_33 said:

Hi @Ironhand,
could you please provide some more details as why would this be happening? What I mean is: why would the lower AoA cause this at higher altitude?
The only thing I can think of is the Mach number, but I am not 100% sure how this would cause the buffet. A part from that, IAS should not matter at all as the stall, ie. flow separation is only a function of AoA and not the speed.

Flow separation IS a function of Mach.

[Pavlin_33]

35 minutes ago, Yo-Yo said:

Flow separation IS a function of Mach.

Thanks for pitching in @Yo-Yo.
I just checked my references and indeed there is a AoA limit based on the Mach number:

I guess this is what's being simulated with the buffet. I will double check latter if the tracks coincide with the table.

[draconus]

What I read is "the angle of attack necessary to maintain level flight in thin air is greater than down low" and "at high altitudes where even slight maneuvering may require high angles of attack due to the low density of air in the upper atmosphere" - so yes, there are "some different physic at play at higher altitudes".

[Ironhand]

4 hours ago, Pavlin_33 said:

… IAS should not matter at all as the stall, ie. flow separation is only a function of AoA and not the speed…

Speed has everything to do with it. The air flowing over the wing at 12,000 m altitude is moving roughly twice as fast as at low altitude. So the conditions are not the same. But I am not the one to explain it because I don’t fully understand all the physics involved.

Your high altitude flight was somewhere in the M 1.1-1.2 region, wasn’t it? That puts you somewhere in that steep critical AoA drop off from 18 to 8 degrees on your chart.

 

…..

[Pavlin_33]

2 hours ago, Ironhand said:

...
Your high altitude flight was somewhere in the M 1.1-1.2 region, wasn’t it? That puts you somewhere in that steep critical AoA drop off from 18 to 8 degrees on your chart.

Yup, assuming the value is linear, the limit for 1.2M should be around 15 degrees.

5 hours ago, draconus said:

What I read is "the angle of attack necessary to maintain level flight in thin air is greater than down low" and "at high altitudes where even slight maneuvering may require high angles of attack due to the low density of air in the upper atmosphere" - so yes, there are "some different physic at play at higher altitudes".

Actually it was just about the max AoA before the buffet and subsequent (accelerated) stall happens - nothing related to level flight. I did not, however, take into account supersonic flow