Coffins Corner

[Eagleflip]

"...and the Speed of sound decreases with altitude..."

Not exactly, but close enough. Acutally, Mach is dependent on a formula more complex than needed here, but (in our normal atmosphere) the variables are actually more or less constant--except for temperature.

If you enjoy math, take a gander at a summary of the equation
here.

In short, the only thing that affects mach for any altitude is temperature. Since the temp at 36K (that crazy tropopause) is quite low, Mach will occur at a lower indicated airspeed.

What was mentioned earlier about indicated airspeed creeping ever closer to an stall speed with increased altitude is correct.

 

[avbug]

It should also be noted that "coffin corner" exists much higher than civil aircraft can go. The envelope for any civil aircaft at maximum certificated altitude is still quite large.

 

[JayDub]

It should also be noted that "coffin corner" exists much higher than civil aircraft can go. The envelope for any civil aircaft at maximum certificated altitude is still quite large.

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

 

[TZRav8r]

The real coffin corner

Actually the coffin corner is the one corner in the hangar where they put a plane that some poor turd bought and didnt realize how much it would run him to reannual the thing! Our coffin coner has been occupied by a 182 for the past two years!

 

[Snoopy58]

Actually, the Lear series may not have as tight of an envelope as some military fighters;

If an aircraft can safely exceed Mach 1 (as most modern military fighters that can reach such altitudes can), then the upper end of the "coffin" isn't really there. It's a problem for airframes that have an upper limit defined by some fraction of the speed of sound.

Civil aircraft don't have anything like the narrow range of the U-2, but it absolutely does become a factor for a heavy aircraft climbing to its max ceiling. The example that comes obviously to mind is the 737-700, which displays both upper (Mach buffet) and lower (minimum margin above stall buffet -- I think it assumes some G load, either for bank or turbulence or a combination of both) limits on the Mach/AirSpeed Indicator. At high altitudes & heavy weights, the arc that is NOT yellow or red can get pretty small, and strong mountain wave or turbulence becomes severe no-fun. Descend a couple thousand feet & the available variance in speed opens way up.

If I can find a good picture of the MASI I'll post the link, just for grins.

Cheers!

 

[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."