What's new
Van's Air Force

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

Question for Aerodynamics Expert

lr172

Well Known Member
I will be home brewing an AOA probe for my GRT system. I have a good handle on the angle to use, but am trying to get creative on the mounting and avoiding 6" of 1/4" tube. I have a Gretz pitot and stalk. I am thinking of attaching a 3/16" tube from the stalk about 1.5" about the pitot itself and pointing forward, possibly forward and angled to the side to avoid turbulence at the stalk. I know that the input needs to be far enough from the wing surface to stay out of the flow around the wing, but don't know how far I have to be.

I am guessing that the standard stalk used on the RVs is far enough away and probably has some margin as well. Does anyone know if a tube 1.5" above the standard pitot location would still be outside the airflow layer on a 10 wing? If it matters, I am not overly concerned with accuracy at cruise speeds, just the mid-range and slow speeds.

Thanks for any guidance you can provide.

Larry
 
It likely won't be out of the aerodynamic influence of the wing, but I don't think that matters. What you want is for the AOA indicator to be repeatable. I presume this is a probe with differential pressure ports? So as long as that differential pressure is always the same at the stall then you are good to go provided you calibrate it properly. You might not be measuring the free stream AOA, but that's not the objective.

Big jets have AOA vanes which show the local flow. In test the actual AOA is measured by vanes on a nose boom (and those are wrong too) and the 2 are compared. AOA is also measured by putting an inclinometer on a reference surface in the airplane and measuring that when the climb rate is as close to zero as possible. All 3 will be different, but again, that's ok. Stall vanes on a round fuselage tend to read higher than free stream by about 150% plus a constant offset, which varies with flap setting.

Your main concern should be to ensure that at the high AOAs you aren't getting any disturbed flow off the existing pitot probe. Accelerated flow is one thing, but separated flow is another - bad. That will mess things up. So it has to be a good deal above that probe. Make sure it is above a line running something like 20 deg above and behind the tip of the pitot.

When you test it with stalls you will see if you get consistent results. The measured differential pressure at stall will be different for flaps up vs flaps down. That is normal. Otherwise it should be pretty consistent if you do a gradual stall entry (1 kt/sec). If you do dynamic entries the results will change.

And yes I do this for my work.
 
Thanks Scott. I don't think it is a differential measurement, at least not at the pickup. This is for the GRT setup (same as Dynon, I believe). I believe they take the aoa pressure, just like pitot pressure and compare to the static at the instrument. The input is angled down about 60* and uses a flat front. Therefore, the pressure induced into the circuit increases as the wing angles up into the relative wind.

I was a bit worried about the flow separation around the pitot. Maybe I will extend the tube out of the stalk forward, down and to the side, maintaining 2" separation from the pitot and stalk. I can't imagine 2" of 3/16 tube hanging in the breeze would cause too much drag. I realize the easiest answer is another pitot tube, like the one van's recommends (simple bent 1/4" tube) but I don't really like the look of that.

EDIT: Ok, I am think of extending a tube straight forward from the stalk, maybe 2-3" forward. It will be 2-3" above the pitot. Will this be forward enough from the stalk to avoid the air that separates in front of it? The tube/structure will have a rectangular end with a face that is 60* from the air flow, with the entrance hole bored perpendicular to that face. This design is similar to what Dynon is doing on the pitot with AOA.

______________________
___________________ . /
___________________\\/
____________________/




Larry
 
Last edited:
I don't understand your "sketch". Will it be ahead or behind the stock pitot? You need to draw a line up and back around 20 deg from the tip of the pitot and keep the aoa above that (that's a guess).
 
On the RV-12, we just drill a strategically located hole under the leading edge of the wing, put a pop-rivet in the hole, and plumb an AOA air line to the ADHRS.
 
On the RV-12, we just drill a strategically located hole under the leading edge of the wing, put a pop-rivet in the hole, and plumb an AOA air line to the ADHRS.

Cool!! Does it give you reliable results? Can you give me an idea where on the leading edge you put the rivet? I am guessing the rivet face pointing about 30 degrees down from the level line of the wing.

Larry
 
I don't understand your "sketch". Will it be ahead or behind the stock pitot? You need to draw a line up and back around 20 deg from the tip of the pitot and keep the aoa above that (that's a guess).

sorry, hard to draw pictures with text. It will stick straight forward from the pitot stalk/mount. I envisioned the end of the tube to be about 1-2" behind the pitot entry point and 2-3" above it. Well above the 20* line from the pitot entry hole.

Larry
 
Larry -- If you are interested in the "pop-rivet" AOA sensing port used in the RV-12 I suggest that you SEARCH the VAF / RV-12 website -- there is a lot of discussion as to its location, plumbing, etc. As far as I know, the AOA readings from this port are accurate and useful. -- David
 
Thanks Scott. I don't think it is a differential measurement, at least not at the pickup. This is for the GRT setup (same as Dynon, I believe). I believe they take the aoa pressure, just like pitot pressure and compare to the static at the instrument. The input is angled down about 60* and uses a flat front. Therefore, the pressure induced into the circuit increases as the wing angles up into the relative wind.

Larry

I don't have a dog in this hunt, but think about what you describe. Comparing *any* pitot reading to static can't give you useful info for aoa. And even then, you've described a differential measurement. Even simple airspeed is a differential measurement; that's why there are two ports on the airspeed indicator.


Charlie
 
I don't have a dog in this hunt, but think about what you describe. Comparing *any* pitot reading to static can't give you useful info for aoa. And even then, you've described a differential measurement. Even simple airspeed is a differential measurement; that's why there are two ports on the airspeed indicator.


Charlie

Sorry Charlie, but I don't know how the software computes AOA. I know that the primary input is a non-differential pressure input at the source. There is not a second port, so it is not differential at the source. Clearly airspeed is computed on the concept of the difference between pitot pressure and static pressure (i.e. differential). The CPU/SW also has access to static data, but I have no idea if it uses it for the AOA computation, though I speculate it has to in order to provide repeatability across varying conditions. The AOA physical circuit is not differential (only one port).


Larry
 
Larry -- If you are interested in the "pop-rivet" AOA sensing port used in the RV-12 I suggest that you SEARCH the VAF / RV-12 website -- there is a lot of discussion as to its location, plumbing, etc. As far as I know, the AOA readings from this port are accurate and useful. -- David

Thanks David. I will go search that sub-forum.

Larry
 
I know that the primary input is a non-differential pressure input at the source. There is not a second port, so it is not differential at the source.
The Dynon AOA Pitot Tube has two ports... One for Pitot, and the other for AOA. I understand the AOA is calculated as a differential between the two ports. Internally in the electronics, of course. So yes, there's only "one input" for AOA, but that's not the only thing the AOA circuit uses to determine your AOA.
 
Back
Top