Airspeed lag on takeoff and GPS mismatch

[Indaloamman]

Is there a "normal" lag before my D180 should start reporting airspeed? It doesn't register airspeed until 25-30 knots, and during climb reports on the order of 20 knots less than the GPS.

In level flight into an 8-10 knot wind D180 and GPS are within 3-5 knots, and with flying the wind the airspeed and GPS diverge by 10-15 knots.

 

[Dynon]

If your D180 is on the latest software, it should come alive at 20 knots. So that part isn't unusual.

A discrepancy between IAS and GPS speed could be due to the GPS set to read in mph and the EFIS in knots, or it could be wind. Or, it could be a pitot or static issue: An obstruction in either the pitot or its line could would cause issue with just airspeed. An obstruction in the static line would cause issues with both airspeed and altitude. One quick way to demonstrate that it's properly responsiveness on the ground would be to apply some pressure to the pitot (you can literally blow into it with someone watching). Finally, if either the pitot or static port was aerodynamically not reading what it's supposed to be, due to some odd airflow issue, that could cause fundamental issue, especially at different angles of attack. Oh, and if you're using our AOA/Pitot, having the Pitot and AOA lines mixed up can cause IAS to look artificially low at times.

 

[Terke]

We have the same problem with a SkyView system. We have the latest software load and are flying at sea level. We have an analog airspeed indicator in the cockpit and calibrated it. The whole while the SkyView system was behaving strangely, but according to the folks on the phone that was all normal. After the first three flights I downloaded the data and did some graphing. I subtracted the IAS from the GPS GND speed during various take-off runs. The error between the two gets as high as 42 knots. During the flying I did notice that the SkyView system behaved abnormally, but since I had an analog gauge I wasn't too worried.
We are make triple sure that the pitot and AOA lines are not switched. The lines themselves are all clean and clear. The static ports are in aerodynamically clean locations. So now were are scratching our heads.

Any suggestions would be very welcome

 

[dynonsupport]

You mentioned you calibrated the analog gauge. When you did this, presumably with a pitot/static test set, what did SkyView read?

Did you have a transponder encoder test run before first flight? If so, what did SkyView's altimeter do?

As a note, 42 knots is an error you could easily get if SkyView's static was plumbed to the cockpit or AoA instead of your static ports.

 

[Terke]

While calibrating the analog gauge the SkyView's readings were extremely erratic until around 150 mph then it settled down and agreed well with the test equipment.
The transponder encoder was tested before the first flight by a shop who was familiar with Dynon systems. I did not look at the altimeter while he was running his test, but at the conclusion only a 20 or 40 from recollection.

Since then we did another calibration of the SkyView per the procedure published in the installation manual. The SkyView's values were accurate to within 1mph of the test equipment. However, the value are still erratic in the lower ranges while calibrating. While in flight the values jitter as well. I have attached a graph showing the GPS and IAS values while on approach.
The variations on the graph make the speed tape impossible to read.

Thank you

 

[Dynon]

So buffeting of either the pitot or static pressures could cause erratic readings, but if you were seeing erratic movement while applying pressure in a very controlled manner (with a test set or similar), that could indicate something else going on. Can you take a video of the PFD in flight? One other thought: Does that graph encompass a pattern on approach? IE, should there have been a time where airspeed was less than groundspeed. If this was a long straight-in approach, your graph could be normal if you had a constant headwind. If you've got a pattern in there though, the fact that airspeed is always higher than groundspeed might point to a static issue. What does VSI and altitude look like in-flight? Where is static being read from, and what kind of aircraft is this?

 

[Terke]

I have graphed the hour long flight. The majority of the flight was at an almost constant 1500 ft msl, with occasional climbs to 2000 msl. It was flown at almost constant power setting to check speed and fuel flows. The conditions were somewhat turbulent, and the aircraft is a GP4. I purposely flew a long triangular pattern holding the headings and altitudes pretty constant. I had noted before that at worst the GPS ground speed dropped to the indicated, but always showed higher. I have not made a video in the air yet, but will do.
The VSI looks pretty normal. Not jittering there. There are two static ports on either side of the fuselage, on a long flat section, and about halfway between the wing and the tail. Currently the ports are flush with the fuselage, but we are going to test them by raising their profile a little bit with washers.

 

[Terke]

Here is a video of today's flight. Auto pilot - ON, on heading and altitude. Mild turbulence. On this heading we were showing about 8mph tailwind. On the opposite heading we showed no wind.
https://youtu.be/OYbEJ516Fqk

 

[Dynon]

Yeah, something doesn't look right here. IAS always being above ground speed, no matter the direction, is clearly wrong). There are some aerodynamic effects that could possibly explain it, but it could also be the ADAHRS. Give our tech support team a call at 425-402-0433 or an email at support at dynonavionics dot com so that we can offer some more individual support on these issues.