AoA Calibration

[jnmeade]

According to Dynon's earlier post, "the calibration is ultimately railed by the deepest stall as the AOA probe sees it" - "the red zone will be tied to that deeper stall". So there are variations on stall warnings that depend on how and where you calibrate. As stated, I'm wanting to know how to calibrate to warn me of an impending stall while maneuvering when I least expect it.

Also - what's with the "oscillate the aircraft + or - 5 degrees 4 times" procedure before performing stall? What does that do

Your statement is internally inconsistent. Dynon say says you can calibrate the AOA based on various stalls, then goes on to say the AOA is based on the deepest stall (greatest AOA). Dynon does not give any way for you to have more than one AOA baseline nor is there any way for the airplane to know that you are landing or in turbulence. It only senses AOA..
You can't have your cake and eat it too.
You can adjust the audio output, but again, only one setting. There does not appear to be a way for Dynon to give an audio warning when you are approaching a stall and not give a warning at the same AOA when in the flare.

 

[airguy]

According to Dynon's earlier post, "the calibration is ultimately railed by the deepest stall as the AOA probe sees it" - "the red zone will be tied to that deeper stall". So there are variations on stall warnings that depend on how and where you calibrate. As stated, I'm wanting to know how to calibrate to warn me of an impending stall while maneuvering when I least expect it.

Also - what's with the "oscillate the aircraft + or - 5 degrees 4 times" procedure before performing stall? What does that do?

My comment about the airplane not knowing the difference was meant to be taken as "not knowing the difference between a flare over the runway, and intentional maneuvers at altitude" - both look exactly the same to the wing and AOA system. There is no way to automatically selectively mute the AOA when 10' over the ground, versus 5000' in the air.

 

[cbretana]

It's not that it remembers just "one stall", but that the calibration is ultimately railed by the deepest stall as the AOA probe sees it. IE, if you do a stall clean and a stall completely dirty, they might look relatively the same at the AOA probe, but more likely the one where you have your flaps out will allow a higher AOA before it goes critical. So the red zone will be tied to that deeper stall. That make sense?

Don, Doesn't that depend on the flaps? I mean, putting trailing edge flaps out changes (increases) the angle of incidence of the wing. Yes the "new" wing will be able to go to a higher AOA than the clean wing, but that's measuring AOA relative to the Chord line of the wing, which has been tilted up relative to the Fuselage Reference Line (FRL). The AOA system doesn't know where the FRL is, it only knows the pressure difference between the two holes on the probe, which is isn't moving. So won't the AOA system see that as a lower AOA?

 

[Rhino]

According to Dynon's earlier post, "the calibration is ultimately railed by the deepest stall as the AOA probe sees it" - "the red zone will be tied to that deeper stall". So there are variations on stall warnings that depend on how and where you calibrate. As stated, I'm wanting to know how to calibrate to warn me of an impending stall while maneuvering when I least expect it.

Also - what's with the "oscillate the aircraft + or - 5 degrees 4 times" procedure before performing stall? What does that do?

Probably just establishing a baseline for how the airflow changes with attitude changes.

 

[swatson999]

Don, Doesn't that depend on the flaps? I mean, putting trailing edge flaps out changes (increases) the angle of incidence of the wing. Yes the "new" wing will be able to go to a higher AOA than the clean wing, but that's measuring AOA relative to the Chord line of the wing, which has been tilted up relative to the Fuselage Reference Line (FRL). The AOA system doesn't know where the FRL is, it only knows the pressure difference between the two holes on the probe, which is isn't moving. So won't the AOA system see that as a lower AOA?

What does angle of incidence or fuselage reference line have to do with anything? AOA measures angle of *attack*, the angle between the wing and the relative wind. And, at least for the Dynon system, it doesn't know anything about flaps...be they Fowler flaps, slotted flaps, plain flaps, split flaps, etc. It doesn't know about leading edge slats or any other high-lift device. It doesn't know the attitude of your aircraft, be it landing or upside down. It only knows the pressure differential and the calibrated values at which that differential starts indicating loss of lift up to the full stall warning sound.

 

[andresmith76]

And, at least for the Dynon system, it doesn't know anything about flaps...be they Fowler flaps, slotted flaps, plain flaps, split flaps, etc. It doesn't know about leading edge slats or any other high-lift device. It doesn't know the attitude of your aircraft, be it landing or upside down. It only knows the pressure differential and the calibrated values at which that differential starts indicating loss of lift up to the full stall warning sound.

The Dynon system may not know anything about flap position, but lowering flaps does change the wings camber and stall characteristics. Flaps add more lift, requiring a lower angle of attack to maintain level flight. Adding lift also allows you to increase the angle of attack before a stall occurs. This is emphasized in Dynon's previous post "more likely the one (stall) where you have your flaps out will allow a higher AOA before it goes critical. So the red zone will be tied to that deeper stall." Dynon's AoA system is tied to where you calibrate the stall - with or without flaps - and that calibration point can be altered by changing flap position.

How Do Flaps Work?

What actually happens when you lower your flaps? You increase lift and induced drag for any given angle-of-attack. Here's why it happens.

www.boldmethod.com

 

[swatson999]

The Dynon system may not know anything about flap position, but lowering flaps does change the wings camber and stall characteristics. Flaps add more lift, requiring a lower angle of attack to maintain level flight. Adding lift also allows you to increase the angle of attack before a stall occurs. This is emphasized in Dynon's previous post "more likely the one (stall) where you have your flaps out will allow a higher AOA before it goes critical. So the red zone will be tied to that deeper stall." Dynon's AoA system is tied to where you calibrate the stall - with or without flaps - and that calibration point can be altered by changing flap position.

How Do Flaps Work?

What actually happens when you lower your flaps? You increase lift and induced drag for any given angle-of-attack. Here's why it happens.

www.boldmethod.com

Right. That's basic aerodynamics, taught in ground school and as part of your PP license. Nobody is arguing what the flaps do. The discussion was what the *AOA* system knows/does, and it doesn't know a thing about the wing camber, with or without flaps. It knows the pressure differential at which it was calibrated. If you did it without flaps, then that's one value. If you did it with half-flaps, that would be another. If you calibrated with full flaps, then that's where it'll warn of a stall. But, and here was the point, the AOA system knows nothing about your flap position or aircraft attitude or airspeed or anything else. ALL it knows is angle of attack based on pressure differential, so asking it to warn you that you're about to stall when you're maneuvering, but not when you're landing, makes no sense. The plane doesn't "know" you're about to land, or that you're flying around doing Cuban 8s, or anything else.

 

[Dynon]

Oscillating establishes what "very low' angle of attack looks like. Some people prefer to use a fast cruise speed and skip the oscillations so that a fast cruise fills out the green. If you do a particularly deep pushover you can end up with a fast cruise that is missing a bit of the green. On the low AOA side, this is a matter of personal preference.

For those of you that understand the underlying aero principles. Yes, flaps can alter actual AOA profile of the wing, and the perceived AOA as seen by the probe. Our calibration procedure does not have a way to distinguish between the two. In practice, in most aircraft it does not make much of a difference, our customers have found.

BTW, Don, our support manager, isn't on the general "Dynon" account. When he responds, he does so with his own account.

 

[cbretana]

Bottom line here is this (the point I was trying to elucidate). Since the AOA system does not know about the position of the flaps (or any other aerodynamic device that changes the aircraft configuration), once you have performed the AOA calibration procedure, (no matter how you do it or what configuration you do it in), the system system will react the exact same way to the same pressure differential in the pitot/AOA probe, no matter what aircraft configuration you might be in.

That means that if the configuration process has settled on a pressure differential value to represent "stall" that occurred when you performed the AOA configuration step with the flaps down, then the stall warning system will not give you accurate warning as to when the stall will occur with the Flaps up, and vice versa.

Interestingly, if the aircraft only has trailing edge Fowler Flaps, deploying them increases the camber of the wing considerably, and raises the Stall AOA (As measured by angle between relative wind and new wing chord line), but as this chord line is angled considerably higher than the chord line for the clean wing) the actual stall occurs at lower angle of incidence (that's why the nose drops when you put down the flaps), and the AOA configuration process could, (probably will), register the stall as occurring at a smaller pressure differential between the two hole son the AOA probe.

As mentioned, this is probably not a critical distinction, but it should be understood.