AOA calibration - RV-4 with GAP-26

[flyboy62000]

Has anyone had issues getting AOA to behave correctly in an RV with Garmin GAP-26 probe? I replaced the original pitot tube with the GAP-26 and airspeed calibration is spot-on. However, the AOA side displays some odd behavior. When I view the recorded AOA data in Flight Data Recorder, I see that the AOA increases linearly from 0 deg AOA at 130kts IAS to 5.5 deg AOA at 100kts IAS, then resets to 0 deg AOA at 96kts IAS and then continues linearly to stall at 16 deg AOA with flaps up. The behavior is the same in landing configuration with full flap. I tested this probe on another aircraft and did not have the same behavior. Probe is installed forward of spar in outboard end of bay forward of aileron bellcrank inspection panel. Works fine for stall warning and approach AOA as long as I inhibit AOA above 96 KIAS, but I'd like to for it to work at any airspeed so the warning tone works during higher speed maneuvering. I wonder if the probe is getting some weird airflow change around the wing at 95 KIAS and a longer mast might improve it. It wouldn't appear to be propwash effects as the airspeed is accurate throughout the speed range from stall to VH. Any experiences or advice is greatly appreciated.

Adam
RV-4 O-320

 

[Ironflight]

What do you have the probe hooked to at the “pilot’s end”….G3X, glareshield-mounted display (only), some other AoA indicator?

 

[flyboy62000]

What do you have the probe hooked to at the “pilot’s end”….G3X, glareshield-mounted display (only), some other AoA indicator?

MGL iEFIS, I had the same setup in the other plane and it worked perfectly without this little "step" in the AOA pressures. I thought flaps might be the issue so I ran the calibrations clean and still found the same little step in the data where it increases then goes back to zero at 95kts then increases again. The only thing I can think of is that there is something happening in the airflow at the probe.

 

[Ironflight]

Hmmmm…OK - so I have an MGL EFIS in my little jet, with a Dynon AoA/Pitot tube on the nose, and it has alwasy worked well….but I have only used it in approach mode, so can’t tell you what it does at higher speeds. But your report with actual pressures does make it sound like something finny is going on at the probe end, or in the plumbing… or at the pressure sensor (which is part of the iEFIS, right?). Honestly, I have never seen problems like your describing due to flow at the pitot - you can TRY a longer mast, and see if that works (I won’t discount the possibility), but I would be more suspicious of the pressure transducer in the MGL.

 

[flyboy62000]

Hmmmm…OK - so I have an MGL EFIS in my little jet, with a Dynon AoA/Pitot tube on the nose, and it has alwasy worked well….but I have only used it in approach mode, so can’t tell you what it does at higher speeds. But your report with actual pressures does make it sound like something finny is going on at the probe end, or in the plumbing… or at the pressure sensor (which is part of the iEFIS, right?). Honestly, I have never seen problems like your describing due to flow at the pitot - you can TRY a longer mast, and see if that works (I won’t discount the possibility), but I would be more suspicious of the pressure transducer in the MGL.

Thanks, Paul. I appreciate your input. The little jet looks fun, maybe that'll be my next build. If I figure it out, I'll be sure to post what I find.