>I thought I would take my 914 up to 10,000 feet and run it at 100%
>power to see if it would do the mythical 200 mph. Here are the data
>I recorded.
>
>OAT 16 c
>10,000' @ 29.92
>
>North GPS 129 knots
>South GPS 175 knots
>East GPS 167 knots
>West GPS 140 knots
>
>
>I plugged these numbers into the 4 Axis IAS spread sheet that I got
>from the Europa club and it tells me that the TAS is 178 knots with
>a density altitude of 12,310', but..... this is where I get
>confused. None of my ground speeds are 178 knots, so am I
>misunderstanding something here, or are the calculations in the
>spread sheet incorrect.
Paul - as there are several TAS-from-GPS spreadsheets to choose from
on the Europa Club CD, I'm assuming from your description that you
are using the one called "gps-tas by trk.xls", that I derived from
Popular Flying articles published 1998 (since all the others ask for
only 3 observations).
It requires that the aircraft is flown on TRACKS (NOT headings) of N,
E, S & W, and the GPS groundspeed recorded for each. It is also of
interest to record the indicated airspeed (IAS), which should of
course be THE SAME for all 4 tracks, and the headings steered to
achieve the required tracks. For true airspeed, the pressure altitude
and outside air temperature should be recorded as you have done.
It's important for most of these calculations that speeds are all in
the same units (kt, mph, kph) and that bearings are all referred to
the same datum (True or Magnetic).
I plugged your numbers into the "PF" spreadsheet (NB important to
enter them in the right order for this file!) and got a mean airspeed
of 153.95 with a range of 3.34 between the highest and lowest
estimates. The wind was 26.85 at 330.
According to my (mechanical) E-6B computer, a TAS of 154kt would give
an IAS of about 127.5kt at your observed height & temperature. Was
that the IAS you observed? The E-6B also offers a density altitude of
about 12300. Note that there is no reason for any of the groundspeeds
to be the same as your TAS, unless you are flying in absolutely nil
wind.
And, to get back to the question first asked, 154kt is about 177mph
so, yes, the 200mph seems to remain in the realm of fable!
I just looked again at the files available on the Club CD and
observed that the ones contributed by Dave Bosomworth do include
stuff for RAS, TAS & density altitude, so I now realise you have
probably been using that version. I have not personally checked those
files (although no-one else has complained so far!) so will have to
refer you to Dave for an authoritative opinion. As far as I know, he
does not at present inhabit this list, so I will copy this message to
him.
regards
Rowland
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