What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

AOA rivet, OAT, Pitot location

FinnFlyer

Well Known Member
Using Van's standard 1/4" bent tube for pitot.

The Dynon OAT probe is fat: 5/16", round and 1" long.

I want to add a simple rivet in the leading edge skin for AOA input. I assume the rivet hole should see the oncoming wind straight in at 30 degrees. Does the RV-4 wing really stall at around 30 degrees?

Does this location look about right? (about 18" forward of LE bottom skin edge, longerons level)

view


In order to minimize drag I'd like to put the AOA rivet, OAT probe and pitot in a line.

Should the OAT probe be located before or after the pitot tube? Would the oncoming air be heated by collision with the pitot tube?

Thinking about 3D printing a streamline for the OAT probe, in which case the OAT probe probably is best located after the pitot, if not significantly heated by encounter with pitot tube.

Finn
 

Attachments

  • AOA rivet.jpg
    AOA rivet.jpg
    63.8 KB · Views: 319
I know nothing about where one would put an AOA sensing port, but I can say that most straight wings stall at 16-18 degrees, not 30.
 
30 degrees?

I got the 30 degrees from this thread:
https://vansairforce.com/community/showthread.php?t=34040&page=6

But perhaps the RV-12 wing is very different from the RV-4 wing?

On the other hand, the Dynon Pitot/AOA probe supposedly would work on both an RV-12 and an RV-4. I think that's where the 30 degrees from horizontal came from.

I wonder how critical the location really is. Some variation is obviously taken care of with the Dynon AOA calibration.

Finn
 
I used a rivet on the lower wing skin a few inches back from the leading edge for AOA on my 10. Scott gave me some good advice about turbulence based upon their experimentation with the 12 AOA port placement. In the end, I was more concerned about this turbulence than the specific angle from centerline.

The location that I settled on for my 10 has worked out really well and the usage seems just as reliable as the pitot based AOA signals that I get on my 6A. My kit had a gretz heated pitot and didn't want to buy a more modern one just for the AOA function.

I will try to find my notes about placement location, though your airfoil shape is different and may require something a bit different than mine did.

Larry
 
Last edited:
Thank you Larry.

Can you elaborate on the turbulence issue?

Is that related to where on the wing (outboard near tip, further in or next to tank) it's located?

Sure would appreciate your notes.

Finn
 
Thank you Larry.

Can you elaborate on the turbulence issue?

Is that related to where on the wing (outboard near tip, further in or next to tank) it's located?

Sure would appreciate your notes.

Finn

The turbulence is really flow separation and it happens on the lower airflow surface, just back from the leading edge during high angle's of attack. It is not related to a span-wise position on the wing and doesn't apply to the pitot stalk units, as they are far away from the wing airflow.

I will find the details from Scott later today. Basically they found that if you get the rivet too close to the LE, the flow separation present can foul that AOA reading. It was better to go back a bit further (beyond the 30* tangent line) to have the rivet in cleaner air that is better attached to the surface. It was this process that was used for the Van's approved location of that rivet on the 12 wing. They picked a spot that gave good variability of data and without turbulence instead of an angle that was theoretically optimal.

Going back from the 30* point doesn't inhibit getting increasing pessure at full stall AOA's. It just reduces the variability a bit. My G3X shows the inches of pressure at each calibration point and there appears to be plenty of delta between the capture points.

Larry
 
Last edited:
Thanks Larry,

Looking forward to the notes.

Pondering why 30 degrees is considered ideal if wing stalls around 20 degrees.

Looking in an earlier thread, Joe Gores used a small fitting which brought the opening maybe 0.5" off the skin.

Has anyone actually use a rivet on the RV-4 wing? Sure would like to hear where and the result.

Finn
 
I assume the rivet hole should see the oncoming wind straight in at 30 degrees. Does the RV-4 wing really stall at around 30 degrees?

Bad assumption (Not at all uncommon). The AOA indicator is looking at the differential pressure between the Pitot and the AOA port.

Most AOA indicators have some method of calibration thereby reducing the accuracy necessary for the AOA tube angle.
 
Thanks Larry,


Pondering why 30 degrees is considered ideal if wing stalls around 20 degrees.

Finn

If the wing stalls at 22*, you want some headroom, so the rivet should not be straight on at 22*. Also, don't want the angle changes so small that the pressure differences between differrent angles of attack become too small for the instrument to capture. Not sure what Mel was trying to say, but the pressure in the AOA tube will rise as the angle of the tube (inside rivet) increases from 90* to 180* from the "relative wind" hitting the wing at any constant airsped. He is correct that this pressure in he AOA tube, must also be compared to the pressure in the pitot tube (essentially the constant 180* reference) in order to compute the AOA of the wing.

Not an Areo Eng, so don't really know what optimal is.

Larry
 
Last edited:
AOA rivet location

Decided to drill out a foremost rib rivet and put an AOA pop rivet there. That makes the location more like 70/20 degrees rather than 60/30. Puts it a bit further back -- about 2.25" measuring down around the skin from LE tip to the rivet. Hopefully that will be a working compromise between turbulence and sufficient delta for the Dynon D-100 to calibrate.

If it doesn't work I can simply drill out the pop rivet and replace it with an oops rivet (1/8" shank, 3/32" head) and no harm done.

Finn
 
Decided to drill out a foremost rib rivet and put an AOA pop rivet there. That makes the location more like 70/20 degrees rather than 60/30. Puts it a bit further back -- about 2.25" measuring down around the skin from LE tip to the rivet. Hopefully that will be a working compromise between turbulence and sufficient delta for the Dynon D-100 to calibrate.

If it doesn't work I can simply drill out the pop rivet and replace it with an oops rivet (1/8" shank, 3/32" head) and no harm done.

Finn

Using a rib rivet is a good idea. Wished I thought of that. Let us know how it calibrates.

Larry
 
Please keep us posted on this. I would love to install it on my 4.

Bob. BuRns rv4 n82rb
 
Works great

Finally got around to try a Dynon D-100 AOA calibration with my AOA pop-rivet.

No problems at all. Located 17 1/8" forward of the bottom rear edge of the leading edge skin in the foremost rib hole. My reasoning was that if it didn't work I could drill out the pop rivet and replace it with an Oops rivet and no one would know.

I'm convinced that the location is not critical. In other words, the Dynon can handle any variation resulting from location differences.

Finn
 

Attachments

  • AOA-rivet.jpg
    AOA-rivet.jpg
    57.7 KB · Views: 142
Back
Top