Stall speed changes with altitude.

[bubbers44]

Also the book said the low speed buffet caused by supersonic airflow over the wing reduced the efficiency of the wing meaning you would have to increase the AOA to compensate therefore reaching stall AOA at a higher IAS.

 

[Bjammin]

I can buy that, but again we have to be talking when there is only a few knots between stall and mach buffet. I'm sure this only happpens ABOVE the max altitude for the airplane. Except for the U-2.

 

[MauleSkinner]

I can buy that, but again we have to be talking when there is only a few knots between stall and mach buffet. I'm sure this only happpens ABOVE the max altitude for the airplane. Except for the U-2.

But we (or at least I) am NOT talking about "stall buffet"...There is a point where, due to increasing angle of attack, airflow over the wing goes supersonic, creates a shock wave (more accurately probably an expansion wave) that causes buffet...it's not a stall...depending up on the airplane, you may or may not be able to hold altitude, and you may or may not be able to increase power to fly out of it.

As an example, the Falcon 10 will do this at FL390 and Mach .69 (about 210KIAS) if you encounter turbulence or increase the g-load slightly above 1 g. The airplane will buffet, the buffet will continue after you reduce the load or leave the turbulence, and at least in the ones I flew, you will have to descend 3-400 feet to get past the drag divergence. Again, no stall, just low-speed mach buffet. Well below the 45,000-ft max altitude for the airplane.

Note also that this is the low-speed end of "coffin corner", and with an Mmo of .87, the upper limit is over 270KIAS, for about a 60-knot spread.

Fly safe!

David

 

[Bjammin]

Mach .69 seems like way to low of an airspeed.

At FL400 (Max is FL410) I don't get any buffet above the correct AOA for stall buffet and this correlates to a specifc speed at a certain weight. I am usually paying attention to AOA here and not trying to get a specific speed or mach number. On my next functional check flight I will get these numbers for you.

As I said, if your theory were correct for all cases I would expect buffet (from the low speed condition supersonic flow) at an angle of attack less then that for actual low speed stall buffet and I don't experience that.

Wing shape will also have alot to do with these conditions as well. A high speed, swept back wing will have less speed difference between the top and bottom of the wing and will thus not experence this condition until way higher.

The T-45 has a MMO above mach 1, but you still get mach buffet at about .86 and tuck at .88

 

[bubbers44]

Last time I was in low speed mach buffet was in an MD80 flying a one hour flight to LAX from SFO with a new captain on the airplane. We were light but he got slow trying to get to 370 to get on top of the clouds. At 500 to go I said we are getting slow so he selected altitude hold to accelerate but unfortunately that pathetic airplane doesn't fly well at high altitude with out close attention so it slowed down some more and as the buffeting got worse and the flight attendants wanting to know what was wrong we had to descend to keep from going deeper into buffet. After that experience I never let an airplane get slow at altitude again.

 

[MauleSkinner]

Mach .69 seems like way to low of an airspeed.

Depends on the airplane...for a Falcon 10 it is...for a Citation it's not...for a Beechjet it's about right for the climb.

At FL400 (Max is FL410) I don't get any buffet above the correct AOA for stall buffet and this correlates to a specifc speed at a certain weight. I am usually paying attention to AOA here and not trying to get a specific speed or mach number. On my next functional check flight I will get these numbers for you.

As I said, if your theory were correct for all cases I would expect buffet (from the low speed condition supersonic flow) at an angle of attack less then that for actual low speed stall buffet and I don't experience that.

Again, it depends on the airplane...I've never seen it in the Hawker, although we do have charts for it. The airplane just becomes a serious slug long before you get to that speed/AOA (still well above "stall") that I've never been tempted to get close.

Fly safe!

David