D180 IAS Reading Way Too High...

[Piper_J3]

Synopsis of my problem….   I bought a flying RV-12 with Dynon D180.  I have units set for statute (fpm, mph).  The airspeed indicator consistently shows IAS as 60 mph too high.  In level cruise flight I show 180mph and I’m indicating about 125 mph on final approach.  In addition, the White Arc for Vfe is set at 150 mph.  I did some troubleshooting and found the original builder forgot to open the static port rivets on each side of the fuselage.  I drilled this out and verified the ports were open and connected to the static tubing.  This didn’t make any difference and IAS reading sere still in the range of 60 mph too high.  I have set zero pressure value and this too didn’t make any difference.

Recent information…  Three weeks ago I sent the D180 unit to Dynon telling them IAS was reading ~ 60 mph too high.  Dynon replaced the airspeed sensor and returned the unit to me.  I installed the unit today and still have IAS reading 60 mph too high.

So here’s my thoughts…

1) Perhaps Dynon didn’t repair and completely check out the unit and returned it to me with same problem, or maybe a scaling factor is wrong in the software.   

2) Maybe somehow I’m getting over-pressurization in the pitot line.  Maybe a hole in the tubing where it runs inside the engine cowl.  The cowling most likely is at higher pressure.

3) Maybe too much vacuum created in the static line.  I could disconnect the static line from the back of the D-180 for a test.   

4) Maybe having Vfe white arc set at 150 mph is causing some interaction.  Tomorrow I will configure airspeed color thresholds per D180 Installation Guide and RV-12 POH.  It's my understanding that D180 limits are preprogrammed by installing Van's RV-12 software, so I don't understand how white arc Vfe is set at 150 mph by default.

If anyone has any ideas on how to troubleshoot this problem I’d appreciate hearing.

Piper J3

 

[jakej]

Here's some ideas & IMO -

A. disconnect the static - for test purposes with a fast taxi or circuit it won't matter much & may give good info for troubleshooting.
B. If you can borrow a ASI then plumb it into the system, in parallel, then blow into the pitot tube carefully to see if the reading is the same on both.

In reference to your 1) - I've seen the Dynon test equipment & they would have checked it out properly.

Where exactly on the wing is your pitot mounted ?

Jake J

 

[Piper_J3]

The Van's RV-12 uses a geared Rotax 912 ULS engine.  The pitot tube is sticking out the front of the centerline of the prop spinner.  The gearbox shaft is hallow and the pitot tubing runs thru the hallow shaft.  Just like a 20mm canon on a Messerschmitt Bf 109...

 

[dynonsupport]

The inside of a plane is generally at lower pressure than the outside of the plane. This will cause over-reading of the airspeed if your static is ported inside the plane. I have a guess that the builder ran static to the AoA input, and the static is just connected to nothing and thus ported to the cabin. If the static ports really were blocked originally, your altimeter would have been totally broken, so if they were, and that made on difference, then they weren't plumbed to static.

First things first is always to find out if you have a pitot or static issue. A quick in-flight check of this is to overfly a runway about 100' above at cruise speed and then at a slower speed. If your altimeter reads different on the two passes, you have a static issue.

You can also play with things like vents in the cabin and if opening and closing them causes an altitude change, then you are reading in the cabin.

If you have a leak in the cowl causing pitot pressure, you'll get airspeed even when you aren't moving on the ground when you rev up the engine.

 

[Piper_J3]

Fixed...  Problem turned out to be human error - mine.  I had IAS units set to kilometers/hour instead of mph.  Works fine now.  Time to eat some humble pie...  :-[