Message-ID: <000001c19195$27cd38c0$376c8a90@direcpc.com>
From: "Fred Moreno" <FredMoreno@bigpond.com>
To: "Lancair list" <Lancair.list@olsusa.com>
Subject: Questions on Lancair IV performance
Date: Sun, 30 Dec 2001 19:59:01 +0800
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Many thanks to Carl Cadwell for publishing some actual data for his
plane.  He posted the following:

"My TAS
speeds at various altitudes are:
9000 209 kts
18000 234 kts
21000 246 kts
25000 264 kts"

These figures were at 75F lean of peak at 15.5 gallons per hour, as
reported by Carl.  Later in the same Lancair List, George Braly
suggested that the power output at these settings would be 212
horsepower or 61% of 350 horsepower.  (This corresponds to a specific
fuel consumption of about 0.438 pounds per horsepower.)

I plugged the numbers into a little program I wrote, and also plotted
the results.  The speeds at lower altitudes seem way low.  So my
question is: Why?  What is going on here?  What have others experienced?

Here is the basis for my question.  If I use standard day atmospheric
properties, a prop efficiency of 85%, 212 horsepower, and a flat plate
area of 2.1 square feet (in the ball park for the Lancair IV, although
2.2 may be closer to the truth) I get Carl's numbers on the head for
25,000 feet.  If I use 262 horsepower (75%) holding all else constant, I
get 283 knots, close enough (within experimental error of say 2-3%) to
trustworthy numbers from other sources for these conditions.

Now if I put in lower altitude data, I get speeds substantially higher
that Carl's data: about 220 knots for 9,000 feet, and about 245 knots at
18,000 feet.  (And 205 knots at sea level.)

Since Carl used GPS data and made three speed runs in three directions
at the same altitude, I assume that the readings he obtained represent
pretty good data.

So I am left wondering: what is going on here?  Is horsepower going down
with altitude, or is drag coefficient going up, or what?   Drag is going
up as the air gets thicker, but the flat plate area which is the frontal
area times the drag coefficient should remain more or less the same.
Yet at 9000 feet, a speed of 209 knots corresponds to a flat plate area
almost 11% greater (or power 14% less) that that calculated at 25,000
feet.  These are BIG differences with altitude.

Somebody please help me to understand what is going on here.  Have other
Lancair IV drivers seen similar performance at low altitudes?  (Please,
let's confine our sharing to data obtained using GPS readouts taken at
steady conditions going several conditions to take out the effects of
wind and eliminate the errors of pitot static systems and instruments,
compressibility, aerodynamic heating etc.).

"With all thy getting, get thee understanding."  (Malcolm Forbes).

Fred Moreno

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