Alan Stewart wrote:
> Meanwhile, back on Earth...
>
> I think most ordinary mortals with 'attitude' will fly at speeds similar to
> the ones I've quoted.
>
> ..and of course, it's not for me to stand in judgement over the performances
> which other claim.
>
> One thought though. TAS based on GPS readings is easily over-estimated.
> Leaving aside the density/altitude calculations which are frequently aired,
> the most common reason is failure to accurately compensate for wind.
>
> It is essential to either:
>
> 1/ Fly on a day with zero wind.
> 2/ Fly DIRECTLY into and away from the wind vector. (pretty difficult to
> achieve).
> 3/ Use a recognised formula to produce TAS from trigonometry.
>
> I suspect that most GPS/TAS calculations which are not performed with
> rigourous attention to detail will be over-estimated by a few knots. Arent
> simple 'two-way averages' and 'rectangular tracks' are likely to be wrong
> unless the weather is calm ?
>
> I have an Excel spreadsheet (4Kb) which relies on averaged GPS speeds from
> three, accurately flown tracks. The resulting TAS should be fairly close to
> the true airspeed provided:
>
> 1/ The aircraft is at constant altitude throughout the test.
> 2/ The track and GPS speed are fairly constant. (requires a day without
> gusts and thermals)
> 3/ The engine RPM is constant.
> 4/ The wind direction and speed are constant throughout the test period.
>
> I found that I needed to fly for about 5 minutes per sector, in order for
> the GPS reading to average out. I also found it quite hard to hold a track
> to within +- 1 degree.
>
> If anyone would like to examine and add constructive criticism of the
> method, I'll gladly refine it. I'd like to derive a method that is as fair
> and as accurate as possible.
>
> What is the most simple and ACCURATE way to determine TAS ?
>
> Alan
>
Alan,
What I didn't mention in my earlier e-mail posting
is that I first fly a number of headings to determine
the wind direction within 10 deg. or better.
I then fly my two reciprical tracks upwind and
downwind. With a worst case wind speed
of 10 kts and a 10 deg error in flying directly
with/against the wind, I figure there will be
an error of 1.7 kts, certainly non-trivial.
Being off 15 deg with a 15 knot wind would
lead to an error of nearly 4 knots, a significant
error.
With a constant speed prop my rpm's are
constant within 10 rpm. I use the autopilot
to maintain course within a couple of degrees
(allowing most of my attention to be on holding
IAS and altitude), and perform the tests in the
morning with typically < 10 knots wind.
Could I get a copy of your Excel spread sheet?
I have seen pointers to such spread sheets in the
past but forgot which website they were located in.
Another issue is how to determine % power.
The Rotax info does not give much in the way
of rpm, manifold pressure vs power data.
We have assumed that the fuel flow is a
resonable measure of % power, based on
100% = 7.1 gph as the reference point for the
912S. Of course this is only approriate at low
altitudes on the non-turbo engines, and at fairly
high power settings (say, 55% and higher). Any
comments on this?
regards,
Terry Seaver
A135 / N135TD
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