Wind Vector Variations between D10 and Skyview

madb1rd

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Dec 8, 2007
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I regularly fly formation with another like airplane RV-3 and RV-8, mine has a Skyview, the other has a D-10. Both have the latest upgrades although the problem has persisted since we first noticed it. Both have been swung per Dynon compass swing procedures.

The problem is with both planes in tight formation compass heading will be plus or minus one or two degrees. GPS track will be exactly the same and true airspeed will be within one or two kts, but wind vector will vary widely. Usually the heading will be within 10-20 degrees and the variance can be either way. However velocity can be greatly different, with the D-10 usually reading lower.

I have not been able to correlate the magnitude of error to anything, such as crosswinds. As an aside, I have noticed the inertia ball on the Skyview seems to wander off screen at about 40kts crosswind.

So, I guess my question is do these two units differ in how they derive wind vector values? One would think that with compass heading, GPS track and true airspeed very close the two units would be equal.
 

Dynon

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They don't differ, but even a few of degrees of compass error gets really magnified on the winds calculation.

It only takes a few degrees. If you're off by 3 degrees at 100 knots in still air, for example, you'll get a 5 knot wind indicated. 10 degrees at 150 knots would be a 26 knot crosswind. Every degree matters.

Play with http://www.csgnetwork.com/e6bcalc.html to see the effects (first to calculators combined)

Another factor at play that can change magnitudes between seemingly-similar aircraft are pitot/static position errors. One way to check that is to ask what your formation partners' IASes are when you're going the same speed. If they're different, that would explain some of the differences you're seeing too.
 

madb1rd

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Dec 8, 2007
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Does IAS matter if TAS is identical? It seems to me that TAS is the better factor, and in our case they will often be one kt apart. We've done this a few times in really smooth air where TAS was one kt different, both were on the same GPS track due to the formation and heading was within one to two degrees and the units would show 50-80% difference in wind speed and 30 degrees different in direction. What I was wondering is they both must having some averaging lag for readability, could these be different enough to where any one snapshot would not lead to equal results because the two averaging processes are not in sync. The fact that we are talking to each other sharing values takes time and introduces some variation so it may just be we can't sync a comparable snapshot with any accuracy. thx for the insights...
 

Dynon

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Yes. Fundamentally the differential pressure between pitot and static drives IAS, and then TAS is a computation that's derived from IAS (at least as it is able to be computed by an indicator or piece of avionics). So if IAS is wrong, TAS ends up with some error too. Winds do have some smoothing for legibility, but over longer periods of time (a bunch of seconds, not minutes) and in straight and level flight, things should match up if all other things are equal. Incidentally, a perfect pitot/static system is not trivial to produce across all flight regimes. Even production airplanes often have some error. The tables that describe calibrated airspeed (the "actual" airspeed experienced by the airplane, which matters aerodynamically and for things like performance calculations) and indicated airspeed are basically tables that tell you how bad the pitot/static position error is in the aircraft at various speeds.
 
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