Speed Testing Math and Accuracy

[mansboat]

Originally Posted by Kevin Horton

It is very hard to know exactly how accurate our magnetic compasses are. .........

When I installed my second D10 to use as an HSI, I calibrated its compass with my GPS. Since the 496 has a 200ms update rate, this is very easy on the ground. Roll a few feet in one direction and you know what the mag heading of the airplane is. This is way more easy than using a compass rose!

I also used it for the in flight portion of the cal in a virtual no-wind situation.

Anyway, for this discussion, make a compass correction table using the GPS on the ground. In the air, you'll know how to correct the compass to be sure you are pointed direct into or away form the wind.

The errors are different when banked into a turn, so you must be going straight for this to work.

[Kevin Horton]

Originally Posted by mansboat

When I installed my second D10 to use as an HSI, I calibrated its compass with my GPS. Since the 496 has a 200ms update rate, this is very easy on the ground. Roll a few feet in one direction and you know what the mag heading of the airplane is. This is way more easy than using a compass rose!

This probably works OK, as long as there isn't anything in the area that affects the local magnetic field.

There are quite a few airports that have all kinds of magnetized stuff underneath taxiways and runways. In my day job, I am privy to numerous reports of problems with compasses on a certain Canadian-built regional jet not agreeing with runway heading at numerous airports. It turns out that the wing is low enough to put the flux valve too close to all the junk, rebar, etc that is underneath the aircraft. This AAIB report provides examples of several airports with local magnetic anomalies.

In theory, whoever sets up a compass rose should be confirming that the local magnetic field is such that it agrees with the lines painted on the rose (see the Australian compass rose guidelines for example). So, if you want to use the GPS to do your compass swing, it still might make sense to do it at the compass rose.

[hevansrv7a]

I still believe the 2-way method is good enough and Ellipse seems to agree.

That said, I checked my GRT's TAS against the NTPS method using 120 degrees between the three legs and got a much different result. The NTPS number was 6 or 7 kts lower. I then re-calibrated the GRT and it brought the disagreement down to near nothing. I think this means that the method is good but somehow I did not get it right the first time when applying it to the instrument. A possible reason might be that I only had one IAS entry in the GRT's table. Now there are two. I don't know, but perhaps that allows a calculation of a slope that could not be done with only one. The error correction was only 1 kt different but it made a much bigger difference in the TAS window immediately.

I also had a thought about magnetic issues as raised by Kevin. A double check would be to slowly vary the heading back and forth to find the highest and lowest speeds by the GPS. If the headings are right they will already be at the highest and lowest groundspeeds.

[hevansrv7a]

I confirmed today with GRT that the function may not work right unless there are a minimum of three entries and it works best if they are at high, medium and low speeds.

For me this means: be more careful in using these fancy new instruments. But it also means that the 2-way speed test is still valid within reasonable limits.

[grantcarruthers]

I have done a rough 2 way as you describe and "finding the wind" is VERY easy. this is a little different as I don't have the GRT to compare heading and track but the principle is the same. I just let the plane turn in a very shallow bank and watch the ground speed vary. Using the DG, if I'm turning NE to SE and hit my peak at 80 degrees, and that peak holds until 110 degress then I know the wind is coming from 270 degress. Fly the speed run heading 90 degress, turn and fly at 270 degress. Average them. Done.

Finding the wind is FAR more accurate than using the forcast wind and takes literally less than a minute if you have GPS ground speed available.

If the 3 and 4 leg runs are designed to eliminate wind errors then I fail to see how they can make much of a difference in the world of GPS and a good 2 way with KNOWN wind direction.

Magnetic variation doesn't come into play in this and compass error is irrelevant. Unless your DG precesses 90 degress in 10 minutes or less!!

So IMHO your 2 way with GRT ought to do as well as 3/4 leg runs especially for "close enough". I'll have to go try comparing them now, whats 10 gallons for trivias sake.

[Doug]

To each his own but two observations:

1. I think you guys are missing the point of the *method*. The very elegance of it is that you need ANY three GPS track and G/S readings to give you a solution. The accuracy comes for free. No other instrument or its accuracy enters the equation.

When you think you might want to take a data set - you can use whatever heading you currently have, stabilize - record the data + altitude and OAT for good measure, turn right or left to another convenient heading somewhere around 80 to 130 degrees , stabilize and record T & G/S, another turn in the same direction approx same amount but any heading will do - record the last data point.

Probably done in the time you have sniffed around for the wind direction.

2. In calibrating the GRT ASI using the GPS method you have closed the loop on the instrument. You have positively determined the instrument+pitot+other minor errors and dialled in the corrections and can now use the TAS readout as an accurate measure.

Careful initial calibration will define the error tolerance - or expressed another way the confidence you can apply to that TAS figure.

Doug Gray

[hevansrv7a]

Originally Posted by Doug

To each his own but two observations:

1. I think you guys are missing the point of the *method*. The very elegance of it is that you need ANY three GPS track and G/S readings to give you a solution. The accuracy comes for free. No other instrument or its accuracy enters the equation.

When you think you might want to take a data set - you can use whatever heading you currently have, stabilize - record the data + altitude and OAT for good measure, turn right or left to another convenient heading somewhere around 80 to 130 degrees , stabilize and record T & G/S, another turn in the same direction approx same amount but any heading will do - record the last data point.

Probably done in the time you have sniffed around for the wind direction.

2. In calibrating the GRT ASI using the GPS method you have closed the loop on the instrument. You have positively determined the instrument+pitot+other minor errors and dialled in the corrections and can now use the TAS readout as an accurate measure.

Careful initial calibration will define the error tolerance - or expressed another way the confidence you can apply to that TAS figure.

Doug Gray

Doug, I completely agree with you. If the GRT would use 3-way, I'd do it. The beauty of the GRT calibration is that you only have to do it once and then you can get highly accurate readings of changes with much less time and fuel used.

[dtaylor]

Well, i disagree with Doug.

The easiest way by far to determine TAS is to do 4 90 degree runs at constant altitude, and plug the numbers into one of the calculators on the web:

http://www.reacomp.com/true_airspeed/index.html
http://www.csgnetwork.com/tasgpscalc.html

(I have a GRT and i can tell you that callibrating TAS on it is secondary. Do a proper 90 degree 4 run test to verify any figures you are seeing.)

David T
Lancair Legacy RG

[Davepar]

Well, I disagree with David. Doug's method is much easier than the 4-way 90 degree method and the spreadsheet produces very accurate results. I used it to fix a 3-4 knot error with my Safair static ports.

[Davepar]

To make things even easier, I made a web site calculator version of Doug Gray's method of computing TAS from GPS. Hopefully he doesn't mind. I actually improved on the spreadsheet slightly by avoiding division by small numbers.

Calculator is at: www.dualrudder.com/tas.php
Code is at: www.dualrudder.com/tas.phps in case somebody wants to check my work.

Dave