Wind Direction and Velocity on the PFD

[thibault]

I performed the National Test Pilot School (NTPS) procedure to verify TAS. This is the three leg test where the aircraft is flown at constant power, airspeed, altitude over three GPS tracks while recording the GPS track and the GPS groundspeed values. These six values can be used to solve for TAS and Wind Direction and Wind Speed. The NTPS website has a spread sheet that does all the calculations for you at http://www.ntps.edu/information/downloads

What I found was that the Dynon calculated TAS (which is IAS corrected for pressure and temperature) was within two Knots of the NTPS calculation, so that is good.

The wind direction was way off and wildly different during each of the three legs. So my question is, since the Dynon TAS is pretty good, does the problem probably point at the compass calibration?

My rationale is that significant errors in OAT, static pressure or pitot pressure measurement would have made the TAS off by more than 2 Knots

 

[Edwardoc]

I'm just repeating what Dynon posted but they reported an incorrect mag heading will significantly affect wind vector calculations. I don't recall what post it was in but Mag heading must be accurate to receive correct wind vector calculations and it will affect GS calculations also.

 

[thibault]

Thanks.

I agree on the wind calculations, but I thought groundspeed is purely the difference between GPS position measurements divided by a time interval. Most GPS receivers have no heading measurement input, so they can only deal with ground track, not heading.

 

[Edwardoc]

You may be correct there. I'll bet Dynon knows.

 

[dynonsupport]

Groundspeed displayed on your GPS or on the Dynon is purely from the GPS. Technically modern receivers calculate this from Doppler shifts, not from two position solutions over X time, but the effect is the same.

The wind arrow and velocity are dependent on:

1) Mag heading
2) OAT
3) Airspeed
4) Altitude
5) GPS track
6) GPS groundspeed

5&6 are basically never wrong assuming you have a GPS lock

2,3,4 all make up TAS, which the original poster said is accurate

Thus, #1 is the issue, and it makes a huge difference. If you do the vector math, just a few degrees error means the winds need to be large. So you need to get your mag as accurate as possible if you want the winds to be useful.

 

[thibault]

Support,

I thought that was the case, so I started looking again at the ADAHRS physical installation and then I will repeat the compass calibration.

I leveled the aircraft and then checked the level of the ADAHRS to the A/C. The roll is about .4 degrees off. The pitch is off .1 degree or less. I have noticed that when taxiing on level ground straight ahead that the edge of the ball is touching the left index line. To date I have flown with that same indication by adjusting rudder trim. Do you think that the ball indication on the ground would be caused by that small roll error or is it maybe misalignment of the ADAHRS along the longitudinal axis of the A/C? It is pretty difficult to do the longitudinal alignment in a Lancair because the reference lines are "fuzzy" on a composite airframe. If it is likely the longitudinal alignment of the ADAHRS, which direction do you think I should twist the ADAHRS to bring the ball to the center when moving on the ground? Also, any idea how much twist it would take to move the ball from just touching the left index line to center it between the lines? The answer would tell me how much of a slot I might need to cut in the mounting shelf to permit the twist. Such a pain to work back there that some idea of how much change is needed would be nice to have before starting.

If instead it is the roll error causing the ball to be off center, then I can easily shim the low side.

Thanks for any ideas!!

 

[Dynon]

It's the roll that would cause the ball to be non-centered, not the yaw. If the airplane is flying a little uncoordinated, that could cause it too (by virtue of one wing being slightly low to compensate for it not flying perfectly straight). In short, a little bit of twist in yaw won't cause what you're seeing, but roll will.