Three leg GS to TAS calculation

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

Marc J. Zeitlin wrote:
john smith wrote:


With four 90 deg legs of the same length, you just average the sum.


Only if you want the wrong answer.

Think about it.

Headwinds and tailwinds do not cancel



"john smith" wrote in message
...
If each leg is the same distance, use time not airspeed. Then convert the
average for one leg length. Headwinds, tailwinds, crosswinds cancel.

Marc J. Zeitlin wrote:
john smith wrote:


With four 90 deg legs of the same length, you just average the sum.


Only if you want the wrong answer.

Think about it.

10 knots of headwind one way isn't 10 knots of tailwind on a reciprical
course?

CB wrote:
Headwinds and tailwinds do not cancel

Mike Rapoport wrote:
: A little better actually. On Saturday I measured 140kts at 5000' and 50F at
: the recommended cruise setting.

Excellent! I guess 0.35 MU-2 is around 110kts?

: The airspeed indicator seems to read correctly at cruise but it reads at
: least 8kts high around 55kts.

I can't speak for the Helio, but many Cessna airplanes have a conversion
table in the POH that describes the CAS - IAS differentials. It's quite
large at lower airspeeds, as I recall. I presume that its due to pitot/static
errors that are not cancelled out (as they could be with an air data
computer). Do you suppose this may be the reason for the discrepancy?
The Cessna tables also noted that despite the error between CAS and IAS,
the range markings on the airspeed indicator were accurate. In other words,
Vs0 was at, say, 44kts IAS and that was marked on the airspeed indicator,
while the calibrated Vs0 was a different number. I can't remember the exact
wording, and I gave away my Cessna stuff years ago.

Anyway, have fun with the new toy.
--
Aaron Coolidge

"Aaron Coolidge" wrote in message
...
Mike Rapoport wrote:

: The airspeed indicator seems to read correctly at cruise but it reads at
: least 8kts high around 55kts.

I can't speak for the Helio, but many Cessna airplanes have a conversion
table in the POH that describes the CAS - IAS differentials. It's quite
large at lower airspeeds, as I recall. I presume that its due to
pitot/static
errors that are not cancelled out (as they could be with an air data
computer). Do you suppose this may be the reason for the discrepancy?
The Cessna tables also noted that despite the error between CAS and IAS,
the range markings on the airspeed indicator were accurate. In other
words,
Vs0 was at, say, 44kts IAS and that was marked on the airspeed indicator,
while the calibrated Vs0 was a different number. I can't remember the
exact
wording, and I gave away my Cessna stuff years ago.

I'm not sure of the cause of the descrepancy. It could be water in the
pitot line or an error in the instrument. I am aware that most airplanes
have increasing IAS errors at lower speeds. My issue is that the IAS points
for my airplane seem to be quite a bit higher than in the skimpy flight
manual and higher than other people are getting. The Helio was certified in
the bronze age and only has minimal documentation. There are no detailed
charts for anything.

Anyway, have fun with the new toy.

I am!


Aaron Coolidge

Plane flies at 100 TAS, 50 knot head wind outbound on a 100 mile leg
and 50 knot tail wind on the return. The outbound leg takes 2 hours
100/(100-50). The return takes .66 hours 100/(100+50). Total for the
200 miles is 2.66 hours giving an average speed of 75 knots.

On Fri, 10 Sep 2004 23:54:04 GMT, john smith wrote:

10 knots of headwind one way isn't 10 knots of tailwind on a reciprical
course?

CB wrote:
Headwinds and tailwinds do not cancel

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.