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Wind direction and Outside air temperature (Read 2450 times)
Jan 27th, 2013 at 2:26pm

jurnal   Offline
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Mundelein, IL

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I have had my Dynons since 2007 which came with the Flight Design CTLS I bought. I have been very happy with the information they display with 2 interesting exceptions. Wind direction; Yesterday I flew a 320 heading for 45 minutes. The Dynon D100 showed winds at my altitude to be 310 @ 12kts. Spent 30 minutes on the ground and reversed course to 140 at the very same altitude and the winds were 100 @ 16kts. This was very typical where I always seem to be flying with a headwind. Is this a common experience?
Second; The outside temperature reading mostly settles in at 44 to 47F regardless of altitude. Yesterday it was 19F on the ground and the Dynon showed 38F at 3500 feet. Does this mean I have to change the location of the temp sensor? Are these common problems? Undecided
 

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Reply #1 - Jan 28th, 2013 at 8:05am

Skysailor   Offline
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The easiest way to tell is a simple check. My D10A does the same thing but it is because the previous owner (who installed it) mounted the OAT probe in the NACA vent for the cabin air. Air leaking out of the engine cowling during flight goes right to the probe. The probe is very sensative. It does not take much. On my plane the OAT is absolutely correct on the ground. In the air, I always show a headwind component. The reason is the OAT reads higher than it really is (just like your plane). The D10A looks at indicated airspeed and converts it to true. When it compares true airspeed to the GPS groundspeed, it accounts for the difference as wind. My plane typically indicates 140 knots but a quick peek at a chart will show that 140 indicated with a temp of 27 degrees C is much higher than the same indication when the actual temperature is 12 degrees C.

On the plane I am building, the probe is halfway out the wing on the underside. My DAR agrees with the location and says he always recommends a location under the wing and outside the prop arc. While other locations may be OK he finds he has never had a problem with heat contamination on the planes he inspects when they are able to do this.
« Last Edit: Jan 28th, 2013 at 8:08am by Skysailor »  
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Reply #2 - Jan 28th, 2013 at 7:14pm

Dynon Avionics   Offline
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Dynon Technical Support

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If OAT right on the ground at ambient but wrong in the air, that might be engine exhaust (assuming the OAT can possibly catch some), or , sunlight, in installations where it's not shaded.

Beyond that, also, non-optimal static port placement can cause IAS to be erroneously high or low, which then causes TAS to follow suit. Winds is basically your TAS/heading vector compared to your GPS speed/ground track vector. GPS is basically never very wrong, so it's the inputs to TAS/heading that mess things up. Those inputs include heading, OAT, pitot, and static.
 

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