Absolute ceiling question

[Matthew]

I got the latest AOPA magazine the other day and was looking at their monthly quiz.

There was a T/F question about absolute ceiling. I don't have it with me right now, so I'm going from memory.

The question was something like: At absolute ceiling, an airplane is near its stall speed.

The answer was False - absolute ceiling is where Vx=Vy, both above stall speed, any other speed will result in a descent.

I always figured that at absolute ceiling, the airplane had finally reached its zero climb rate, and that it would have a high enough angle of attack at that point it WOULD be at the edge of a stall even though it was at Vx/Vy.

What really is the angle of attack at that ceiling? Is it near critical?

edit: OK, I'm seeing it now. In the Warrior I'll run out of power before I run out of wing. What about in an airliner or bidness jet?

 

[DGlaeser]

The AOA at absolute ceiling is where drag is at it's minimum, and that is above stall.
Review the drag curves for an airplane. Parasite drag increases with speed, induced drag decreases with speed. Total drag is "U" shaped, and the bottom of the U is where the Vx and Vy speeds are.

You are definitely flying slow to get there (Vx/Vy) so you are 'near' stall (just like you are any time you are flying at Vx/Vy). The closer you get to absolute ceiling the slower your climb rate. So you will be tempted to increase AOA to climb, but you will sink instead of course (or stall if you are too aggressive) - so this is the scenario you are likely envisioning.
Theoretically you will never get to absolute ceiling exactly. That is why Service Ceiling was created, it is the altitude where your climb rate is 100 ft/min.

The AOA is 'high' (we don't know the value without something to measure it), but definitely lower than stall AOA.

 

[Matthew]

The AOA at absolute ceiling is where drag is at it's minimum, and that is above stall.
Review the drag curves for an airplane. Parasite drag increases with speed, induced drag decreases with speed. Total drag is "U" shaped, and the bottom of the U is where the Vx and Vy speeds are.

You are definitely flying slow to get there (Vx/Vy) so you are 'near' stall (just like you are any time you are flying at Vx/Vy). The closer you get to absolute ceiling the slower your climb rate. So you will be tempted to increase AOA to climb, but you will sink instead of course (or stall if you are too aggressive) - so this is the scenario you are likely envisioning.
Theoretically you will never get to absolute ceiling exactly. That is why Service Ceiling was created, it is the altitude where your climb rate is 100 ft/min.

The AOA is 'high' (we don't know the value without something to measure it), but definitely lower than stall AOA.

I was picturing a case where Vx, Vy, and Vs all met.

 

[poadeleted20]

I was picturing a case where Vx, Vy, and Vs all met.

Nope -- there is a bit of room on the back side of the power curve before the stall occurs, but if you're at absolute ceiling, you're while you're in sinking in that section.

 

[Sac Arrow]

Nope -- there is a bit of room on the back side of the power curve before the stall occurs, but if you're at absolute ceiling, you're while you're in sinking in that section.

I seem to recall U2 pilots referring to that as the "coffin corner", where a stall at 70,000+ feet can be disastrous.

 

[poadeleted20]

I seem to recall U2 pilots referring to that as the "coffin corner", where a stall at 70,000+ feet can be disastrous.

That was not a Vx/Vy issue, it had to do with maximum Mach limits. Up at like 70,000 feet, equivalent airspeed is still pretty high with a very low indicated airspeed, and you're right on the edge of a stall one way but Mach buffet the other. Remember that the U-2 has a very long straight wing, and thus has a relatively low Mach limit. See this article for more on that.

 

[flyingron]

I seem to recall U2 pilots referring to that as the "coffin corner", where a stall at 70,000+ feet can be disastrous.

Many panes have coffin corners. It's when the stall speed and the critical mach come together. The U2 is perhaps different in that it regularly operates in the an area where those numbers are very close together. My CFI who was a U2 pilot says he frequently had one wing in stall buffet and the other in mach buffet in turns.

 

[MAKG1]

I seem to recall U2 pilots referring to that as the "coffin corner", where a stall at 70,000+ feet can be disastrous.

Yes, but that's because of a different curve intersection. U2s at maximum altitude operate in a few-knot window between stall and Mach buffet. The latter is not an issue for our spam cans….

If the speed of sound were higher (or the U2 had swept wings), Vy and Vx would be at a higher speed. But, you don't minimize drag when you're throwing shock waves around.

 

[Sac Arrow]

Makes sense.