Replies to This Discussion

Permalink Reply by Brian Wolfe on May 18, 2009 at 7:12pm

How about a differential pressure sensor just like the ones for measuring airspeed, but used to measure the difference in pressure between the top and bottom of the wing or body. Seems like as one surface angles into the on coming airstream it's pressure should increase with respect to the other surface.

Brian

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Permalink Reply by Roger Gerrard on May 19, 2009 at 2:40am

The pressure differential top to bottom of the wing is a function of AoA and wind speed. Airspeed is used as an approximation for AoA but changes with wing loading. Where this becomes most pronounced in our aircraft is in rapid pull ups or high bank angles in level turns. The beauty of the AoA is to get best performance of the wing independent of the airspeed.

I'll take a look again at the relationship of the cord-wise airspeed and AoA relative to the pressure differential above and below the wing.

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Permalink Reply by Sarel P. Wagner on May 19, 2009 at 3:05am

I have done extensive research on AoA sensors. None available that I could find to the hobbyist :( The problem with the pressure sensors is that it needs calibration, basically a wind tunnel procedure..... The vane idea with optical sensor seems to be the better option, low weight and easy to build/integrate and measure. Calibration should be easy as well.

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Permalink Reply by Kim Skatun on June 24, 2009 at 3:24am

Any progress on this topic, sounds like an interestin project. AoA is way better than airspeed...

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Permalink Reply by Roger Gerrard on January 10, 2010 at 7:20am

Check out the stuff Helmut Lelke did

http://www.charlesriverrc.org/articles/asfwpp/helmutlelke_asfwpp.htm

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Permalink Reply by Roger Gerrard on January 10, 2010 at 7:23am

I worked on F-16 and X-29 flight control systems. I love this stuff. I wonder if feeding back pitch rate instead of (or in addition to) AOA would work. It might be easier to get a more reliable sensor for angular rates. Also, feeding back "G" (also called Nz or vertical acceleration) could be useful for the same reason. If AOA is really needed, a sensor made with pressure taps could avoid having moving parts. It could be fun to have all three sensors and experiment with different gains for each and combinations.

Dave Rajczewski

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Permalink Reply by nerd on December 13, 2009 at 12:53am

Calibrating the AoA would be simple if you were to do it while flying. Measure the lowest angle of attack and then measuring the highest angle of attack before a stall.

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Permalink Reply by Stephen Carlson on January 9, 2010 at 2:50pm

Hey, this thread topic looks familiar. Check out my post in my blog. I figured I could use the P-effect using little pager motors with uni-directional micro props as tiny potential difference generators. I've made some progress since my blog post last month, see pics attached...

http://diydrones.com/profiles/blogs/alphabeta-sensor-probe-wo

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Permalink Reply by Bhargav Gajjar on January 17, 2010 at 5:51am

Can you use an accelerometer ? Then you can integrate the output of the chip to find angular displacement of your vehicle.

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Permalink Reply by Ken on January 17, 2010 at 6:35am

That won't work.
Angle of attack is not the relationship of the aircraft axis with the earth, it is the relationship of the aircraft axis with the relative wind. An aircraft's flight path may be 20 degrees above the horizon, and the aircraft axis may be, for example, 24 degrees above the horizon.
Use of this information can be helpful in flying the aircraft at the most efficient speed without having to do a bunch of computation to compensate for weight, and it is useful in determining if the wing is nearing a stall, again without doing a bunch of computation to adjust stall speed for weight.

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Permalink Reply by Guy McCaldin on January 17, 2010 at 6:14pm

I think a rotary position sensor with magnetic encoders would be the simplest way to do it. I'm not aware of any commercial products suitable for small UAVs, but you could probably base the design off those used for small to mid sized (manned) aircraft: http://www.spaceagecontrol.com/Main/AngleOfAttackSensor

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Permalink Reply by Gagarien on January 18, 2010 at 4:29am

A rotary position sensor and a vane would be the simplest I recon, something like a cannard mounted to the front out in clean air if you have a pusher, pivoted at the centre of pressure or there someware of this cannard(symetrical profile) and a potentiometer to give analog values out. Calibration by data logging straight and level and stalls (as far as I know most airfoils stall at +/- 16 deg.).

I have done some simple test with a vane and a pot. but found the pots. I have even the small one's from a servo had a high internal friction from the cantacts to be sensitive enough for the vane to move at low airflow, so the idea is a larger vane like a symetrical airfoil that would alway be relativly true to airflow or somthing like that.

AOA can be more usefull than airspeed eg. given AOA well calibrated would result in a certian airspeed irrespective of wind direction etc. can be set up for maximum endurance, best climb rate, auto land were you can be assured the lowest airspeed without stalling etc.etc. ---- my 2 cents.

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