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<< Lancair Builders' Mail List >>
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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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