Coffins Corner

[Hawker rider]

Indeed, if you have a halfway decent book on aviation aerodynamics it will show an example of the so-called coffin corner envelope.

the parabolic figure it describes will also move inwards when you start putting a certain load on the airframe. the coffin corner itself might not be such a big deal in normal cruise. It will make both margins, high speed and lower speed quite a bit smaller when you encounter some turbulence, for example if you have a load factor of 1.3g's on the airframe your 15 knots margin might now be a 2 knot margin, or leave you outside the coffincorner.

I believe this effect was firstly really "developed" when the spitfires has enough power to reach high altitudes and when they started pulling combatmanoeuvres they ended outside this envelope.

 

[Snoopy58]

http://www.airliners.net/open.file?...C0zMA==&ODJ9dvCE=&O89Dcjdg=&static=yes&size=L

Scroll to the right of the picture & look at the left side of the PFD, you can see both the hash marks at the high end of the speed tape, and at the low end. The caption says the picture was taken at FL 400, and since there is about 40 knots split between the top & bottom, they can't have been so heavy as to be at max weight for the altitude; the range can be somewhat narower than that. Of course, as was pointed out, in jets like the U-2, the range gets really small; let's not go there!

Snoopy

 

[TheRaven]

...which for something like the U2 at high altitude was about an 8 knot difference between stall and overspeed / mach tuck.

I don't think the U2 had a problem with Mach tuck, as this phenomenon occurs primarily on swept wings as the center of pressure moves aft rapidly. The U2 doesn't have a swept wing, and while the CP moves aft as speed increases, its not nearly as violent as in a swept wing aircraft....Most airplanes need to be trimmed as they accelerate because the CP is moving aft, but Mach tuck is what happens when you run out of trim.

As an aside, I just saw a NASA TR-1 (U-2 without the cameras) here in BGR for a couple weeks in DEC...it was doing some atmospheric research.

 

[enigma]

Actually, the Lear series may not have as tight of an envelope as some military fighters; it can still get pretty darn close. In fact, I first heard the phrase when referring to the Lear 23 and 24. Then again, how close is close? If I recall correctly there isn't an actual convergence of the two speeds in a Lear, but you darn sure had to respect that little baby when you were going fast up high. It’s all a matter of perspective.
Respectfully,

JayDub

JayDub, I think that Avbug is correct. Even for the Lears, coffin corner is a long way away as long as you stay in the certified operating envelope.

You are correct about the need for pilots to respect the airplane. The problem with early Lears had to do with stick force required to hold above/below trim speed. Simply put, they don't take nearly as much force at high speed to pull the wings/tail off as they do at low speed. Sorry, that wasn't simple, this should be; control feel was not linear and they are easy to overcontrol at high speed (very light in pitch). Take an untrained pilot who thinks that it is cool to go faster than barber pole, put him in an early lear and he may find himself falling from the sky minus his tail. But that scenario hasn't happened in about thirty years.

regards,
enigma

 

[avbug]

The reduction in force required to hold the aircraft off trimmed speed, below that required for certification, is why the Lear 35 has a limitation of .74 without the autopilot; it performs the mach trim function.

The lear, even at it's highest certified altitudes, doesn't come close to "coffin corner."

 

[TZRav8r]

The U2 s surely did not have a problem with the coffin's corner! I saw '13 Days'. The way The U2 was able to crank and bank when under fire, Kevin Costern would never mislead me! Holywood should always be trusted! Why would they steer me wrong?

 

[JayDub]

Enigma,

Thanks for clearing that up for me. Ahhh yes, having to pulse trim... the memories. I was told if you held the trim down in any one direction more than 2 seconds in a Lear, you were way behind. Take care and have a great weekend.

Respectfully,

JayDub

 

[willbav8r]

There are a coupla Singapore Airlines heavy drivers out there that could give you first hand accounts of their experiences climbing to max ceiling with max weight aircraft.

They have reduced "G" factors (so cleared to climb higher?), and have come unstuck on several occasions........

 

[TurboS7]

Most coffins have 4 of them. If you comply with the limitations of the aircraft that we fly today it does not exist.

 

[TurboS7]

It you want to talk 737 here is something interesting to do. The yellow buffets lines at say 410 represent buffet at 46 degrees of bank 1.3 G. If you want to see where the real buffet margins are just punch in a high ZFW in the FMC. You will see the yellow magenta lines come together but the red majenta lines that represent where you will fall out of the sky they will not move, they are determined by the angle of attack indicator plus all the other stuff going into the ADIRU's then to the FMC. The yellow stall wiskers come up also.