AOA Pitot probe

[Marfel]

Hi,

I'm a student in engineering and i have to write a report about the existing angle of attack measuring systems available on the market.
I have seen many systems which measure 4 pressures to determine the AOA.
I wonder how the EFIS can compute AOA with only the 2 pressures available with the Dynon's AOA pitot probe.

The first port gives the total pressure (static pressure + dynamic pressure). The second port gives a pressure that depends on the AOA. But I suppose it also depends on V² and rho. How does the EFIS compute the AOA ? What is the use of the total pressure ?

Thx.

 

[Etienne]

I'm also an engineer, and wondered about the same thing...

My guess is that the second port measures a vertical component of the airflow. Bear in mind that the Dynon does not give an actual AoA readout, only a proportion of a maximum AoA recorded when calibrating, so whether it can internally calculate true AoA or not, it doesn't matter.

At a given ratio between the two measured dynamic pressures, the AoA will always be the same. If it were measuring a pure horizontal and pure vertical component of dynamic pressure (wrt the aeroplane's front-to-back axis) then the true AoA would be arctan(DPv/DPh)...

That's my understanding of the way it works looking at the manual and playing with the two pitot lines...

 

[Marfel]

Thank you Etienne.
Can anyone confirm Etienne's guess ?

 

[dynonsupport]

Think of it this way conceptually - the pitot port on our AOA/Pitot probe is specifically designed to be as insensitive to AOA changes (within reason) as it can be. If you look down the snout of our pitot, you'll see an interesting shape that is highly engineered and in fact wind tunnel-tested. The AOA port is designed with the exact opposite goal. Both are referenced to static, and so both measure dynamic air pressures. So when you run the math, you end up not having to deal with any of the exponential stuff - it all cancels out. In the end, you basically compare the ratios (that's a bit of an oversimplification, but will do for this discussion) of the pressures on those ports.

Also, note that we're not measuring absolute AOA. The low end of your calibration isn't necessarily zero AOA or the AOA of zero lift. Neither is the other end necessarily perfectly critical AOA. Neither truly matter if what you're looking for is some AOA value in the middle that gives you a good indication as you get close to critical (for stall prevention), as well as enough resolution in the meat of the display to be able to manage your energy state on approach (Navy aircraft AOA indexers work similarly, and have much less resolution than our displays).