Thorp Air Command - T18.net

I flew a 4 leg "box" course the other day to check my TAS against what mathematically my TAS should read. I used a spreadsheet to calculate my numbers when I got back home. Using this spreadsheet, you input data from 3 legs. Here's the deal though....when I put in the data from 3 randomly chosen legs, the calculated TAS would match what I read in the airplane. But, if I chose another 3 random legs to enter into the spreadsheet, the calculated TAS was 10 knots lower than what I read in the plane. What am I missing here? Does it matter what 3 legs I choose?

It sounds like you are using GPS speed instead of airspeed. I can’t think of any reason you would need to fly a box to find true airspeed.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Ryan,

I use an online site to my calculations. It is: http://indoavis.co.id/main/index.php

It works for me and seems to match any calculations I have done.

Regards,

Jim

Did you use 3 randomly chosen legs or 3 legs that touch each other consecutively? The 3 legs should be 90 degree turns that follow one another. Direction of turns doesn’t matter. Let it stabilize fully on each leg before taking gps speed measurement. No power or altitude changes obviously. Let us know speed, power setting, fuel flow, altitude and engine when you get it nailed

I just flew a box so i could have the data for the 4 cardinal headings. I only entered 3 of the legs into the spreadsheet. The spreadsheet did ask for GPS speed, along with some other variables. I cant recall now which of the three legs I in placed into the spreadsheet. I'll just go fly it again. I need something to do I guess. I'm going to use the link Jim provided to compare to this spreadsheet.

What are you trying to find out? If I remember correctly, “True airspeed” is calculated from indicated airspeed, pressure and temperature. None of those values change flying a box. It’s beginning to sound like you are trying to verify the airspeed indicator but you would use all four legs (or just 2 perpendicular) for that otherwise the winds will have a major effect.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

I just decided to check and see how accurate my TAS indication was. The online calculator matched what my plane is reading.

To verify airspeed indicator, do a gps ground speed check using the following calculator. It takes wind out of the equation.
https://www.eaa62.org/technotes/speed.htm
Then take indicated airspeed and other data from the same run and correct to true airspeed using this calculator.
http://www.indoavis.co.id/main/tas.html
The two numbers should be very close together or your indicated airspeed system is in error. Be ready to be disappointed tho....they are never as fast as in our stories