Sender: marc
Message-ID: <3AFE2F91.C7FE79CD@mclemente.net>
Date: Sun, 13 May 2001 01:54:09 -0500
From: "Marc F. Clemente" <marc@mclemente.net>
MIME-Version: 1.0
To: robsmiley@home.com, lancair.list@olsusa.com
Subject: Re: Groundspeed as a Flight Reference
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Reply-To: lancair.list@olsusa.com

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"Robert Smiley" <robsmiley@home.com> once worte:
> An addition to the information provided by Toni Durizzi would by to fly
> three equal length legs in an equalateral triangle. Average the three times
> and calculate the airspeed.  This will compensate for head and crosswind
SNIP

Even if there was no crosswind component, flying two legs and averaging
the speed would not work.  Consider a plane going 100 kts (true
airspeed) and wind out of the north at 50 kts.  Go north for 100 miles
(2 hours).  Then go south for 100 miles (0.67 hours).  Total time is
2.67 hours for 200 miles (of ground covered).  You would have calculated
your true airspeed to be 75 kts.
   Actually, doing it with a triangle would not work anyhow. Consider a
triangular course.  The first leg is track 360.  Then a left turn to
track 240.  Finally another left turn to track 120.  You will end up at
your starting point.  Each leg is 100 miles.  Your plane goes 100 kts
(true airspeed).

If the wind is coming from the north at 50 kts:
  Leg 1 will take you 2 hours
  Leg 2 will take you 0.86 hours, with a 115 kt
        ground speed and a 25 degree crab angle.
  Leg 3 will take you 0.86 hours, with a 115 kt
        ground speed and a 25 degree crab angle.
  The total is 300 miles (on the ground) in 3.74 hours.
  In this scenario you would have calculated your true airspeed to be
about 80.3 kts.  The reason this method won't work is because you have
actually gone 374 miles in the air.

A square won't work either.  And neither will a perfect 360 degree DME
arc.

The way to do it is fly two legs in opposite directions, using the gps
to keep good track.  Maintain constant indicated airspeed (obviously). 
For example, fly on a track of 360 then fly on a track of 180.  Record
your ground speed and crab angle for each leg.  The crab angle should be
the same for both legs.
Then,
TAS = ( GS1 + GS2 ) / ( 2 * cos(theta) )
where:
TAS = true airspeed
GS1 = leg 1 ground speed
GS2 = leg 2 ground speed
theta = crab angle
I am not a mathematician.  And I am sure somebody will correct me if I
am wrong.....Marc


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