Three leg GS to TAS calculation

john smith wrote:

If each leg is the same distance, use time not airspeed. Then convert
the average for one leg length. Headwinds, tailwinds, crosswinds
cancel.

OK, I'll agree with that, but it's a LOT easier to fly for a fixed TIME
than for a fixed distance, unless you fly at about 10' AGL......

--
Marc J. Zeitlin
http://marc.zeitlin.home.comcast.net/
http://www.cozybuilders.org/
Copyright (c) 2004

And it is the easiest of all to simply forget about time and distance
altogether and use the website.



Marc J. Zeitlin wrote:
john smith wrote:


If each leg is the same distance, use time not airspeed. Then convert
the average for one leg length. Headwinds, tailwinds, crosswinds

cancel.

OK, I'll agree with that, but it's a LOT easier to fly for a fixed TIME
than for a fixed distance, unless you fly at about 10' AGL......

And it is the easiest of all to simply forget about time and distance
altogether and use the website.



Or if you don't have internet access while in the air, use the gps ground speed
for tracks of 000, 120, 240. Stay on each track for about one minute for the
acceleration/deceleration to equalize.

Root mean square of the three different ground speeds will give you the true
air speed with a 0.236% error even with winds 30% of your true air speed.

Example:

Wind 045 @ 30 knots
True air speed: 100 knots

GPS track 1: 000
Ground speed 1: 76.5 knots
Heading: 12.2 degrees

GPS track 2: 120
Ground speed 2: 87.9 knots
Heading: 103.2 degrees

GPS track 3: 240
Ground speed 3: 128.7 knots
Heading: 244.5 degrees

GS1(76.5)^2 = 5852.25
GS2(87.9)^2 = 7726.41
GS3(128.7)^2 = 16563.69

sum of square GS1, GS2, GS3 = 30142.35

average of sum (30142.35/3) = 10047.45

square root of average (10047.45) = 100.23696922792508295043396893834 knots

Thus a 0.2369692279250829504339689383% error.

Or in practical terms for every one hour of flight, your estimated leg time
would be 8.5 seconds in error. Quite acceptable I'd say.

Advantage of this method is you can do it inflight with a log sheet to record
each ground speed for tracks of 000, 120 and 240. Use a calculator that has a
square root function.

You can do this quite quickly and for many different altitudes to gather data
for your own speed/density altitude/power charts.

Give it a try.


paul k. sanchez, cfii-mei
on eagles’ wings
2011 south perimeter road, suite g
fort lauderdale, florida 33309-7135
305-389-1742 wireless
954-776-0527 fax
954-345-4276 home/fax

www.iflyqpa.com/cas

paul k. sanchez wrote:
And it is the easiest of all to simply forget about time and distance
altogether and use the website.




Or if you don't have internet access while in the air,

You don't need internet access in the air. Just write down the three
airspeeds on a piece of paper and do it later.