Hi all,
I would like to add a few comments on the subject of aircraft cruise
performances:
1. At a given power, with any aircraft type, higher is faster. That is because
higher mean less density, therefore less drag. For a given power, the altitude
at which you will get the best speed is when you need full throttle to get this
power.
2. Law of physics: for a given true airspeed (TAS), indicated airspeed (IAS)
decreases
by the square root of density. Density decreases with altitude and temperature.
To measure and compare aircraft performances, forget IAS and measure
TAS using the 3 vector GPS method. This should be done in smooth air, at the
same power, at the same density altitude and same load.
3. With my aircraft, XS Monowheel 912ULS, Airmaster prop, whenever I want to
evaluate
the effect of a drag reduction mod, I climb to 8500ft density altitude.
This can be FL85 and minus 2 deg.C or FL75 and 10 deg.C or FL65 and 20 deg.C
or any other combination giving a 8500ft density altitude, depending on the
prevailing
conditions. Going full throttle eliminate the effect of any error of
the manifold pressure instrument. The actual MP will be about 22 In.Hg. Prop RPM
set at 5000. According to the Rotax Owner Manual, this setting gives 59 percent
of max continuous power. This setting give me a consistent 135 kts TAS. I
do not care what IAS is, as this is not relevant. TAS does not change
significantly
with the aircraft weight and/or balance. Average fuel burn measured on
many long cross country at this setting is around 19 liter per hour.
Regards
Remi
XS mono F-PGKL
1160 hours
.
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