Coffin Corner

desertdog71

desertdog71

Girthy Member
Anybody have a good description of this? I read a little on this subject but I must admit I did not comprehend at all. :(
 
It's a region (at high altitudes) where the TAS for low-speed buffet (stall) encroaches upon Vmo, where mach tuck (high speed buffet) occurs. The aircraft that I think this is a huge issue with is the U2 (I *think*), but it happens in bizjets and possibly airliners too.

The issue is any change in aircraft pitch will usually be enough to induce one or the other (or both)
 
desertdog71 said:
Anybody have a good description of this? I read a little on this subject but I must admit I did not comprehend at all. :(
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As you go up to higher altitudes, your airplane stalls at a relatively constant IAS, but the higher you go, the higher the TAS you have to maintain to stay above stalling speed. This defines how slow you can go.

On the high speed side, we have the speed of sound to contend with. Civil jets all have a Mmo speed that cannot be exceeded. In my airplane it is M.80. The speed of sound is dependent on temperature. The colder the air, the lower the speed of sound. As altitude increases temperature drops from 15C at sea level to a relatively constant -56C in the stratosphere.

So, picture a graph, with Altitude on the left and TAS along the bottom. The stall line shows how stalling speed expressed as TAS increases with altitude. The line on the right is the speed of sound. It decreases with temperature, until you get to the stratosphere, where temperature stays pretty constant. The top of the graph would be the service ceiling of the aircraft.
In almost anything you will fly, there is a pretty wide margin at the top of the graph.
 
:confused: :confused: :confused: :confused:

So it is similar to where Vx and Vy converge at higher altitudes and where they cross is the Absolute Ceiling?

I need to dig that up and read it again. I can't remember where I was reading it though. Its among my 20 or so books I am sure.
 
desertdog71 said:
:confused: :confused: :confused: :confused:

So it is similar to where Vx and Vy converge at higher altitudes and where they cross is the Absolute Ceiling?

I need to dig that up and read it again. I can't remember where I was reading it though. Its among my 20 or so books I am sure.
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For civil aircraft, they should never actually cross, no aircraft would be certified to an altitude where they get that close together. Although I think I read somewhere that some of the early lears had something like a 40 kt window to operate in at their max ceiling.

The best example is the U2, since it has a straight wing, and is not really optimized for high mach flight. That and the fact that it can fly at something like 70,000', means that there is only a few knots between stall and high speed buffet.

Your best bet is to get a book, like Flying Jets by Pendleton, that will explain everything and have a diagram. I searched and could not find a diagram on yahoo or google.

If it helps any, you could pick a stall speed for your imaginary airplane, say 100 KIAS, then use your e6b to find the TAS at various altitudes and plot them on a graph. Then draw in the speed of sound (661 KTAS at sea level, decreasing to 573 KTAS in the stratosphere). Then draw in the service ceiling of your imaginary airplane at 45,000 MSL. The narrow area at the top of your graph is coffin corner.
 
ananoman said:
For civil aircraft, they should never actually cross, no aircraft would be certified to an altitude where they get that close together. Although I think I read somewhere that some of the early lears had something like a 40 kt window to operate in at their max ceiling.

The best example is the U2, since it has a straight wing, and is not really optimized for high mach flight. That and the fact that it can fly at something like 70,000', means that there is only a few knots between stall and high speed buffet.

Your best bet is to get a book, like Flying Jets by Pendleton, that will explain everything and have a diagram. I searched and could not find a diagram on yahoo or google.

If it helps any, you could pick a stall speed for your imaginary airplane, say 100 KIAS, then use your e6b to find the TAS at various altitudes and plot them on a graph. Then draw in the speed of sound (661 KTAS at sea level, decreasing to 573 KTAS in the stratosphere). Then draw in the service ceiling of your imaginary airplane at 45,000 MSL. The narrow area at the top of your graph is coffin corner.
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Thanks for the excellent explanation. I understand this now, and I kind of feel dumb for not getting it right away. :confused: :)
 
Isn't that where you punt directly into the corner and try and get he ball to go out of bounds within the 5 yard line and without letting the ball go into the endzone. Don't want that touchback but at the same time you want to give the other team the worst field position as possible :)
 
Timbuff10 said:
Isn't that where you punt directly into the corner and try and get he ball to go out of bounds within the 5 yard line and without letting the ball go into the endzone. Don't want that touchback but at the same time you want to give the other team the worst field position as possible :)
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You, my friend, play Madden. Do you have '07 for the PS2? If so, my Dolphins will tear you up online sometime if you wish :)
 
haha, I haven't bought a madden game since Madden 98. The best one was Madden93 though, no one could beat me on that one! I don't think I have played the game since I was in college back in 02... Nothing like a good Madden tourny in the dorms.
 
Besides the U2, I think the Early Learjets were prone to uncontrollable oscillation at higher altitudes also...L20 or L35 if I remember correctly.
 
nosehair said:
In helicopters, it's high altitude (over 30 feet) and low airspeed (under 30 kts)
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You a helo guy, nosehair?

And are you refering to the shaded area in a height/ velocity diagram... heard it called a deadman's curve, but not a coffin corner. Learn something new every day.
 
Buffet Margin? Pfffshaw. Just put a lower ZFW into the FMC to make that level.*


(*do that and I will find you and beat you with a pointy stick.)
 
Somebody who has DC8 time might be able to comment better but I think somebody told me that at max altitude a DC8 has a 10 knot margin.
 
I was reading somwhere in the CRJ tech notes that the overspeed protection is disabled above 390 due to the fact that if it pitched up to slow down it will probably result in a stall and the stick pusher pushing the aircraft down into an overspeed situation begining an sort of vertical dutch roll. I didn't get a chance to try it out but it sounds no fun what so ever.
 
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