I have a Rutan Long EZ, (pusher). The pitot tube is a simple tube, cut flush with the pointed nose of the aircraft so that the cross sectional is normal (perpendicularly aligned) with the aircraft longitudinal reference line. As I understand it the AOA subsystem in the AHARS determines AOA by comparing the dynamic pressure in the normal pitot tube line with the pressure in a separate pitot line that measures pressure at a 45 degree angle. Would it be possible to install another pitot tube in the nose which is cut to also measure the dynamic pressure at a 45 degree angle, as the Dynon AOA-capable pitot tube is doing, and connect this to the Dynon AHARS AOA pressure sensor input? Would this work. The angle of the opening in this tube could be adjusted so as to generate pressure readings that would be consistent with whatever the AHARS needs to produce useable AOA values.
AOA subsystem
- Thread starter cbretana
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thibault
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- Oct 25, 2009
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In theory, yes. The difficulty is "How do you calibrate yours?" Whatever you install, you have no way to compare the output of your installation to what the Dynon AOA tube would have produced if it had been installed. The Dynon pitot and AOA openings are located relatively far below where the flow field interacts with the A/C structure. So, it seems quite a leap to expect the same outcome without a way to measure/prove it.
I'm thinking that all I want to use this for is ro give me an aural indication of what would be optimum AOA for turning, just a safe margin below where the canard stalls on my Long EZ. I can test for that and then just file the end of the tube sticking out of the nose appropriately, to "tune" the system so that it accurately indicates that optimum AOA. The optimum AOA is just before the canard stalls. In the Long EZ, the canard is not capable of generating enough nose up to get the main wing to stall. The canard stalls first, drops and immediately begins flying again. So if you hold full aft stick in any flight regime what you get is a continuous pitch oscillation, at about one to two cycles per second. and the aircraft drops five or ten feet with each oscillation. But as the main wing never stalls, you maintain aileron (roll) authority throughput the oscillations.
So, if on a test flight, the cockpit aural indication is occurring too far below the target AOA, (where the canard stalls), I would file down the tube to make the end opening steeper (with respect to FRL). If it occurs at too high an AOA, (too close to canard stall), I would file it shallower, and keep adjusting until the aural tone is occurring at the point I find most comfortable/useful.
So, if on a test flight, the cockpit aural indication is occurring too far below the target AOA, (where the canard stalls), I would file down the tube to make the end opening steeper (with respect to FRL). If it occurs at too high an AOA, (too close to canard stall), I would file it shallower, and keep adjusting until the aural tone is occurring at the point I find most comfortable/useful.
People have done this, usually with success. The angle is shallower than 45 degrees, but it's also more of a complex shape that's designed (and wind tunnel tested) to be less sensitive to yaw. The pitot port on our AOA tube is also designed to be insensitive to AOA changes.