How Does The AOA System Work?

[Dynon101]

derstand that the AOA system watches the two pitot tube ports and when the AOA tube port becomes more and more exposed to the airstream it will corolate that indicated pressure to the pressure that was seen during the calibration flights and indicate a stall.

The question I have is how does the ADAHRS input correlate with this second pitot port pressure?

I am installing this AOA system in a Velocity (canard) airplane and the advantage of a canard airplane is that the "stall" is so gentle that it is more like a mush because the canard (front wing) will only nibble at the stall and the nose does not really lower it just sort of viberates as opposed to the traditional stall with the nose lowering sharply. Should I then knowing this feature of the canard airplane fly the calibration flights and when I know that front wing is going to mush stall should I push the stick forward to trick the system and make it think that is the stall point?

If the calibration flights are all made with wings level what happens during a steep turn? In other words if the calibration flights were all done at wings level with only one G then what happens let's say on a turn from base to final when the airplane is in a 30-degree bank?

Thanks

 

[swatson999]

In other words if the calibration flights were all done at wings level with only one G then what happens let's say on a turn from base to final when the airplane is in a 30-degree bank?

Thanks

It's an Angle of Attack indicator, so it will indicate a stall when the wing exceeds its critical angle of attack.

 

[Dynon]

AOA does not depend on G-load. When you are turning base to final at 30 degrees, though, you're necessarily flying at a higher AOA in the turn to maintain flight.

Basically, the AOA indication doesn't care whether or not it sees a stall. The highest AOA you demonstrate during calibration is going to be the red zone. So even if you have an aircraft that doesn't conventionally stall, getting the aircraft into the highest AOA that you ever want to experience is basically what you want to do. Similarly, for aircraft that have stalls that have bad behaviors, you might not want to actually experience one. even in calibration. So there again, just get it a close as you you'd ever want to be.

 

[Dynon101]

Thanks for your help. I still am confused as to the operation of the AOA system of Dynon.

A traditional AOA vane is mounted to the fuselage and therefore airspeed is irrelevant because it just sees the angle of the relative wind to the fuselage and zaps out a signal for the display.

The Dynon system uses what is essentially airspeed... the relative wind strikes the AOA port of the pitot tube and as the AOA increases the AOA port becomes more exposed and more pressure is forced down to the AOA sensor... this is still just air pressure being forced down the tube.

So in a GENTLE pull up from a high speed descent the airspeed could be 120 knots so a slight increase in AOA would force more air pressure (increases at the square of airspeed) down the AOA port and even though only a small portion of the AOA port is exposed there is way more air being jammed in there.

So if X pressure at 57 KIAS is being forced down the AOA port during a stall and since that pressure triggers the stall indication then I would think that the same X pressure being sensed during a slight pull up at 110 knots would trigger the same stall indicating... unless there is some other factors than pressure being sensed by the AOA port of the pitot tube.

 

[dynonsupport]

The AoA system senses the difference between the pitot and the AoA. Not just the AoA. The pressure due to aircraft velocity is removed because it's common to both ports, and only the AoA pressure remains which is independent of speed.

The Pitot port is specifically designed to be insensitive to AoA while the AoA is sensitive.

 

[Dynon101]

THANKS for the reply!!!

I gotta admit someone at Dynon was really thinking when they came up with this system. Please thank that person(s) for me!!!

I look forward to using my Dynon...some day...to dream!!!

 

[rogersmart]

I went through the AOA calibration yesterday performing 3 stalls 1 with flaps and 2 clean. Each time I pressed the Stall button after the recovery, then hit save once after I completed the stalls.
My understanding is that whatever my greatest angle of attach achieved is the value saved, not a combination of the 3 stalls.
Is this correct?
If so then what is the affect of being clean or with flaps? Is the saved value accurate for one configuration but not the other?
Since the Skyview knows my flap position, is flap position a component in the warning?

 

[Raymo]

 
Since the Skyview knows my flap position, is flap position a component in the warning?

Flap position is not part of the algorithm today. It is in the newer AFS system (Dynon owned). I helped a friend with his AFS install and he would have preferred the SV solution versus having to install a flap position sensor, which he opted not to do at this time due to the amount of work it would take in his F-1 Rocket.

 

[swatson999]

 
Since the Skyview knows my flap position, is flap position a component in the warning?

Flap position is not part of the algorithm today. It is in the newer AFS system (Dynon owned).

And I've never liked this aspect of it. AOA is a function of flap position (more flaps means decreased critical angle). Even if Dynon just added a simple two-position calibration (no flaps and full flaps) I think it would be better than nothing at all.

Not taking flaps into account means you either calibrate for cruise, which means its inaccurate during landing and take-off, or you calibrate for full flaps, which invalidates it for all other flight regimes.

It seems such a simple thing to do, I don't understand why after all these years Dynon hasn't added it.

 

[rogersmart]

So if the AOA does not use flap input, then I need to recalibrate using on a clean stall. If I go through the calibration procedure again, will this remove the previous calibration and save the greatest angle of attack from the current flight only?

 

[Raymo]

So if the AOA does not use flap input, then I need to recalibrate using on a clean stall.  If I go through the calibration procedure again, will this remove the previous calibration and save the greatest angle of attack from the current flight only?

I believe it will remove the previous calibration. You normally do more than one stall (or close to it) and tell the SV system to record that value.

I did one stall dirty and one clean during calibration. When on final, in the flare, I hear the tone come in as a near touchdown. I try to hold it off as long as possible from there. It has been very windy here in the southeast US lately so my landings have been a little faster to accommodate the gusts so I haven't heard the AOA as often.

After calibration, do some slow flight and stall practice to see if you need to re-tweak the settings. Good practice!