Paul:
Real DATA! Many thanks.
Let’s see: if I interpret correctly
the 110 HP 0-235 with higher compression pistons puts out 125 HP (plus 8=-9%) at
the same RPM? Is that right, a guess, or a reasonable certainty?
Comment on measuring OAT which is only an
issue beginning above maybe 200-250 knots. If the OAT measurement is made
outside of the air stream (like the skin temperature, for example), it is still
subject to aerodynamic frictional heating. One can calculate this via
something called the Recovery Temperature which varies a bit from stagnation
temperature rise. But for our purposes, the recovery temperature rise (which
is effectively the skin temperature on unheated parts of the airframe) is about
80% of the stagnation temperature rise. In short, there is no way to
avoid the aerodynamic heating error. It goes as the square of the
TAS. You have to allow for it. It is always there, and it will
always create an error when you calculate corrections with your E6B.
That is why a lot of Lancair IV pilots
think they cruise at 300 knots plus. They don’t. The combined
compressibility error (about 1/3) and aerodynamic heating error (about 2/3’s)
total something around 20 knots making true cruise speeds about 285 knots….
Just like the book.
Sorry.
Fred
-----Original Message-----
From: Paul Lipps
[mailto:elippse@sbcglobal.net]
Sent: Tuesday,
25 April 2006 12:43
PM
Subject: Drag Data
Frank! I have never flown around with someone with
another 235, so I have nothing for comparison. I have flown alongside Klaus
several times and he and I had the same IAS/TAS on our systems, with him, of
course, throttled back to stay with me. I have a well calibrated pitot and
static. I've done many two-way runs using my autopilot and altitude hold
to calibrate them vs GPS groundspeed; TAS always comes out within 1 MPH of GPS
GS avg. My two OATs are also mounted out of the airstream, so they show no
stagnation rise and I get correct dalt and TAS. My O-235 has 9.7:1 pistons, and
I get about 125 hp. I get 214 mph TAS at 1000' dalt, 2950 rpm, 206 mph TAS
at 8000' dalt, and 198 mph TAS at 2750 rpm, 14,500 dalt, 5.7 gph, all
when leaned for best power. I designed the prop to give 200 mph
TAS at 2800 rpm, 10,000' dalt; I actually get about 201 mph TAS. ROC at
1000' dalt, 2430 rpm, 110 mph IAS, 1350 lb is 1500-1550 ft/min. My
induction system gives me the correct pressure recovery, which is total pressure
minus carb drop. Lycoming charts for the MA-3A shows the carb drop to be
1.1" at 12,500 pressure altitude. I have 43 gallons total, so I
can get 1250-1300 miles with reserves, no wind, at altitude. I adjust my flap
reflex angle to give me the best speed. Varying it can change speed 3 mph or
more from max (8 deg) reflex when at higher altitude and higher
weight. My wing-tip additions increase my span to 25.2', area to 77.5 sq
ft, and AR to 8.17:1.