From: StarAerospace@aol.com
Message-ID: <50.7bc1785.29b99b37@aol.com>
Date: Thu, 7 Mar 2002 23:42:31 EST
Subject: Anyone not know about coffin corner?
To: lancair.list@olsusa.com
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<< While on the subject of Lear 20/30, the story about "coffin corner" might 
be interesting to those potentially-high-flyers.
 
 Jack Kane >>

Jack raises an important point here that we in the industry often take for 
granted that everyone wanting to fly high should know:  coffin corner.

It's the combination of increasing altitude at a given Mmo leading to lower 
and lower IAS until the difference between stall and Mach overspeed becomes 
too small to stay in during normal flight and turbulence.  The critical 
factor most often overlooked is that as we go to lower IAS, the lift 
coefficient increases and with it, the critical Mach of the wing decreases.  
So Mmo goes down with IAS.

Many aft cambered airfoils (including NACA 63, 64, 65) with a design lift 
coefficient of .4 or greater or significant aft camber demonstrate a Mach 
overspeed phenomenon called "Mach tuck".  This is extremely dangerous and 
occurs when the negative moment coefficient change with Mach # is non linear 
and negative.  In simpler terms, if you go to the wing's Mach limit and the 
aircraft noses down causing an acceleration that blows you right through your 
overspeed limit without enough time to react, that's Mach tuck.

The Lancair IV wing may or may not exhibit Mach tuck at certain combinations 
of airspeed and altitude (CL vs. Mach), we don't know until it's tested.  We 
do know that any aircraft taken to high enough altitude will eventually reach 
a point where it's wing will no longer support it below adjusted Mmo.  For 
the TSIO-550 and turbine Lancairs, we'll probably run out of power long 
before that altitude due to the low wing loading.  It is likely that we will 
hit a stability limit first.

There are secondary "coffin corners" with respect to stability limits and 
engine power.  As you climb and lose engine power, you can find yourself 
literally behind the power curve at very high TAS!  IAS is the indicator, all 
you high flyers beware and take care.

Thanks also to Jack for pointing out the difference between using the mass of 
a metal prop (actually rotational inertia) for "damping" (actually lowering 
the excitation frequency) and using an inherently energy absorbing material 
as a rotational damping medium.  I think my explanation was oversimplified 
for this audience.

Eric
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