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<< Lancair Builders' Mail List >>
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Regarding the earlier inquiry about Lancair IV speeds, generally it is
true that pilots quote speeds like fishermen quote the size of their
fish. In the case of the Lancair IV, they may be doing so unwittingly.
The Lancair IV is traveling fast enough that air speed errors that can
be negligible at low speeds become appreciable. These are in addition
to the normal errors concerning instruments, static port locations,
leaks in the pitot-static system and such, all of which contribute to
additional errors. For this discussion, we will assume that the builder
has eliminated all these errors so that the Indicated Air Speed =
Calibrated Air Speed. (This is a BIG assumption, and the suspicious
among you should always question its validity.)
Additional high speed errors arise from two sources. The first is
compressibility effects that become appreciable as the Mach number
exceeds about 0.3. The Lancair IV cruises about Mach 0.5, so
compressibility effects are non-negligible. Most pilots are aware of
this effect.
The larger error arises from errors in reading the outside air
temperature. At Lancair IV speeds, aerodynamic heating can be in the
vicinity of 20F or so. It occurs on the leading edges from compression
of the air, but it also occurs over the skin due to frictional heating.
The frictional heating temperature rise depends on a number of details,
but is generally about 81% of the temperature rise at the leading edge
(stagnation point) compression heating. So effectively the whole
airplane is "hot:" on the leading edges from compression, and where the
velocity is higher (and pressure lower), from frictional heating.
The CAFE guys remembered the compressibility effects, but neglected the
frictional heating effects in their report about Derek Hine's Lancair IV
published in Sport Aviation. They suggested a 305 knot cruise speed
which corrects to about 290 knots, just as one would expect.
The net result is that you can not accurately measure the outside air
temperature from inside the airplane without making a correction to the
number provided by the thermometer. So when you grab your E6B computer
(or use that little ring around the outside of the airspeed indicator)
you are basing your TAS calculation on the wrong temperature. And
neglecting the effects of compressibility.
I wrote a little computer program to analyze these effects when Brent
Regan and I raced Brent's airplane several years ago. I plotted curves
for various air speeds, altitudes, and outside air temperatures. Later
I found that Jeppesen makes a "Graduate Level" E6B used by the jet guys
to calculate TAS that takes all these effects into account. I bought
one, and then had to buy the book to learn how to use it.
Here is the simplified bottom line, useful for correcting the figures
you calculate with your E6B:
1) If you are cruising at 10,000 feet, and your E6B suggests your true
air speed is 250 knots, SUBTRACT 7 knots to get the true figure.
2) If you are cruising at 20,000 feet, and your E6B suggests your true
air speed it 290 knots, SUBTRACT 13 knots to get the true figure.
3) If you are cruising at 30,000 feet, and your E6B suggests your true
air speed is 320 knots, SUBTRACT 18 knots to get the true figure.
For the Legacy and two place guys, some of whom are going nearly as fast
as the Lancair IV, here are some corrections for lower speeds:
10,000 feet, 200 knots, subtract 3 knots
20,000 feet, 250 knots, subtract 8 knots
30,000 feet, 270 knots, subtract 12 knots.
Fred Moreno
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LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
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