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TAS measurement
Can somebody please explain to me the technique to measure your plane's true
airspeed by flying in 3 or 4 different directions for a while and recording GPS ground speed ? Do you just fly the same amount of time each leg and average the groundspeed readings ? |
#2
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"Bravo Delta" wrote in message ... Can somebody please explain to me the technique to measure your plane's true airspeed by flying in 3 or 4 different directions for a while and recording GPS ground speed ? Do you just fly the same amount of time each leg and average the groundspeed readings ? Go out and fly three headings that are 90 different from each other. It doesn't matter what they are but it is easier to just use cardinal headings like N, S, E or W. Let the ground speed stabilize and write down the ground speed. Then plug it into one of these websites. The website will also tell you the wind direction and speed. To verify the results use four cardinal headings and use any three of them. You will see the TAS and wind come out the same. www.reacomp.com/true_airspeed/index.html www.csgnetwork.com/tasgpscalc.html |
#3
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"Bravo Delta" wrote in message ... Can somebody please explain to me the technique to measure your plane's true airspeed by flying in 3 or 4 different directions for a while and recording GPS ground speed ? Do you just fly the same amount of time each leg and average the groundspeed readings ? Assume you want to know TAS for a given IAS. You'll have to repeat the following recipe for each IAS you're interested in. ALL speeds must be in the same units (knots, mph, ft/sec, or whatever). You will fly N and get GPS ground speed g1, then E to get g2, then S to get g3. Now it's time to calculate. There are three preliminary calculations needed, for P, A, and Q, as follows: P = (g1^2 + g3^2)/2 A = arctan(g3^2 - g1^2, 2*g2^2 - g1^2 - g3^2) [you have to use the arc tangent function which gives the angle counterclockwise from the positive x axis, usually called something like ATAN2(x,y) in spreadsheets] Q = (g3^2 - g1^2)/(4*cosA) Then, at last, TAS = sqrt((P + sqrt(P^2 - 4*Q^2))/2) Also, wind speed (assumed constant!!) = absolute value of Q/TAS and wind angle = 360 deg - A (mod 360). Hope this helps. If still uncertain, see Performance of Light Aircraft p. 31. John T. Lowry, PhD Flight Physics |
#4
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Can somebody please explain to me the technique to measure your plane's
true airspeed by flying in 3 or 4 different directions for a while and recording GPS ground speed ? Do you just fly the same amount of time each leg and average the groundspeed readings ? One more source: http://www.cockpitgps.com/other_arti..._compass_and_c alcul.htm John Bell www.cockpitgps.com |
#5
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When doing acceptance flights for purchased aircraft one of the issues is
always how much is the true air speed. Since you are working with an airspeed indicator that you don't know, an outside air temperature probe that you do not know temperature rise factor, etc, it is best to use you GPS ground speed. Technique usually requires a pilot-flying and pilot-non-flying crew. During the test use the same power (rpm/manifold pressure), fuel flow, and holding altitude. Fly a track of 000 for about 30 seconds so that the acceleration/deceleration forces can equalize, note down the ground speed. Call it groundspeed A Fly a track of 120 for about 30 seconds so that the acceleration/deceleration forces can equalize, note down the ground speed. Call it groundspeed B Fly a track of 240 for about 30 seconds so that the acceleration/deceleration forces can equalize, note down the ground speed. Call it groundspeed C A^2 + B^2 + C^2 = sum of (ground speeds^2) sum of (ground speeds^2) / number of tracks (3) = average of (ground speeds^2) square root of average of (ground speeds^2) = true air speed at that altitude with that outside air temperature. Just a $1.99 calculator from Office Depot can do the trick. Now of course the question comes in about the validity of the formula. Lets try 100 knots true air speed, wind 045 at 30 knots (realistic winds at less than 12k feet). Using our handy dandy Sporty's E6B-F lets input the values in the HDG/GS function; WDIR 045 WSPD 30 CRS 000 TAS 100 GS A 76.5 HDG 12.2 degrees WDIR 045 WSPD 30 CRS 120 TAS 100 GS B 87.9 HDG 103.2 degrees WDIR 045 WSPD 30 CRS 000 TAS 100 GS C 128.7 HDG 244.5 degrees Ground speed A^2 (5852.25) Ground speed B^2 (7726.41) Ground speed C^2 (16563.69) sum of A^2, B^2, C^2 = 30142.35 sum of the squares / 3 = 10047.45 square root of average = 100.23696922792508295043396893834 knots Please note that even with a wind that is 30% of your true air speed, the percent error using this formula 0.236%. This would cumulate to about 8.5308922053029862156228817788 seconds of leg time error for every 1 hour of flight. Close enough I'd say. 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 |
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