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<< Lancair Builders' Mail List >>
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When Brent Regan and I prepared for the Denver to Oshkosh race several
years ago, we did extensive flight testing and research on
compressibility, aerodynamic heating, and instrumentation errors as well
as Mach limits. Our research indicated that the Lancair IV was dive
tested at up to Mach 0.6, but you don't want to go this fast. Neither
did we. We set a hard limit at Mach 0.58, and targeted a maximum
descent speed of Mach 0.56. We raced in summer when the outside air
temperature was well above standard, increasing the speed of sound. At
race cruise conditions (this from memory) we were indicating about 210
knots at 27,000 feet (about 90% power) which was about Mach 0.54 as I
recall. So cruise is unlikely to exceed the Mach limit unless you are
making something approaching 400 horsepower or thereabouts.
The hooker is the descent. At those flight conditions, we could only
descend at about 500 feet per minute as this put us right up against our
Mach limit. As you descend, temperatures rise, Mach number decreases
(for a constant IAS), and thus you can slowly increase the rate of
descent (all the while holding 90% power, or a bit more if the cooling
improves enough) while staying within the Mach limit.
I calculated a descent profile based on Mach number and flight testing,
and created a table we used during the descent since we did not have a
Mach meter in the airplane. I would read the maximum allowable rate of
descent for each 100 feet of altitude, and Brent would fly that descent
rate. This continues until you reach Vne on the air speed indicator
which occurred around 12,000 feet, if I recall correctly. The vertical
speed indicator pegged at 4000 feet per minute long before this maximum
descent rate was reached. So the descent started very gentle and ended
up quite rapid. However, due to the speeds involved, the deck angles
were never steep.
So the message is: cruise with Mach impunity, but descend with care.
Especially in the winter time. Unless you want to be a test pilot.
Additional note: The E6B conversion from IAS to TAS (or equivalently the
use of the little sliding ring on the outside of the indicated air speed
meter) will be in substantial error giving you a TAS about 15-20 knots
high. The reasons are two fold. First compressibility becomes
significant at these speeds, giving rise to about 1/3 of the error.
More importantly, at Mach 0.5 the aerodynamic heating is about 20F and
this gives you an erroneous indication of the outside air temperature.
It is actually colder outside than you think. The aerodynamic heating
(unavoidable anywhere on the airframe) contributes about 2/3's of the
error. So when you go to 25,000 feet and use your whiz wheel to compute
a TAS of 310 knots, subtract about 20 knots and you will be getting
something close to the truth.
Fred Moreno
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