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AOA Systems - Must They Account for Flap Position?

samgaddis

Member
In my 14A, I have the G3X with an AOA indicator powered by the Garmin GAP-26. I was considering adding the dash-mounted GI-260 indicator for better AOA awareness, but an instructor told me that pitot-based AOA system (not a vane) is less than ideal because it doesn't have a flap sensor and therefore doesn't take flap position into account.

This means that your AOA is either accurate clean or accurate with flaps full, but never with both.

AOA is important to me, so I am considering an upgrade to the RiteAngle system that does account for flap position. I am curious your general thoughts on the following:

  • Is my understanding of the difference between these two AOA systems correct?
  • If I got the RiteAngle system, would it replace the AOA indicator in my G3X?
  • With the RA, could I still use the GI-260 dash-mounted indicator for better AOA visibility?
  • What am I missing?
 
Those are two different systems, and very likely won't show the same AOA, especially if one is flap corrected and the other is not.

On airplanes with a plane piano hinge flap like the RV4-6-7-8-9 models the flap doesn't introduce a huge angle variation, just calibrate with flaps down and you can live with it, just know that it isn't accurate with flaps up.

The -10 and -14 are using a slotted flap, which does affect how the probe reads and you need to flap correct the AOA if you want it to be accurate. We (Onspeed team) learned that from looking at lots of data from a -4 and -10 using multiple different probe types.

Vane sensors are also affected by flap position. The issue is not the sensor itself. The flow around the wing changes when you deploy the flaps, more so with slotted flaps because the airfoil change is more significant.
 
That all makes sense and I’d love to buy an onspeed device - however, when I looked at these a month or two ago it seemed that it was a fairly DIY solution. If I recall, there was quite a bit of assembly and calibration that looked pretty daunting. And I’m fairly technically-minded.

What’s more, from the “hardware and software” page, it seems Onspeed only works with the Dynon EFIS, and not my G3X?

I’ve watched Onspeed videos and the solution seems great once it’s installed, but I worry about the getting it installed part. Are there plans to sell a more plug and play version of this?
 
I had to calibrate the AOA display on my AFS 5400 EFIS/Dynon heated pitot with flaps in both positions. The AOA display on the EFIS includes flap position. Works well flaps up or down, including the option of warning beeps that get faster as stall speed approaches. (Advanced Flight is part of Dynon, and my EFIS uses a standard DynonSV-network).

I'm surprised that the G3X won't accommodate AOA sensing relative to both flaps up and flaps down when using a pitot. I'd check with Garmin if you haven't already.
 
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Many years ago I had the RiteAngle AoA on my 9A and was never really happy with it as it was very sensitive to turbulence at low approach speed. Maybe it has been upgraded over the years and now performs better. Replaced it with the Dynon AoA which works well flaps up or down.

Fin 9A.
 
I had to calibrate the AOA display on my AFS 5400 EFIS/Dynon heated pitot with flaps in both positions. The AOA display on the EFIS includes flap position.
Same here, attached a Ray Allen POS-12 to my flaps and calibrated my AFS 5600"s as per the manual. AOA Donut turns green when flaps extended.
 
That all makes sense and I’d love to buy an onspeed device - however, when I looked at these a month or two ago it seemed that it was a fairly DIY solution. If I recall, there was quite a bit of assembly and calibration that looked pretty daunting. And I’m fairly technically-minded.

What’s more, from the “hardware and software” page, it seems Onspeed only works with the Dynon EFIS, and not my G3X?

I’ve watched Onspeed videos and the solution seems great once it’s installed, but I worry about the getting it installed part. Are there plans to sell a more plug and play version of this?

It's not buyable yet. We're working towards a production version. Calibration has gotten much easier now too.
It works with Dynon (D10/D100/Skyview), AFS (5000 series), Garmin (G3x and G5). The goal is to not be needing an Efis at all. Right now it's needed only for calibration.

Anyway, didn't mean to highjack your thread, just wanted to pass down some lessons we learned about flap effects while developing our own system.

Whatever you end up with, any AOA system is better than none just know how it works and what are its limitations.

Lenny
 
G3X AOA

This discussion is very timely. A few weeks back i calibrated my RV-14A G3X AOA. I did it per instructions which did not specify clean or dirty. I originally did it clean and the AOA indicator didn’t really work as I expected. Today I recalibrated using full flaps for all requested data points. I’m still not convinced it is functioning as I expect but it did provide indications at the various speeds I set. On my -9A I had an AFS AOA which worked great. It required the system be calibrated dirty and clean. Any comments from G3xpert or Lenny? Thanks
 
This would be one of my very few gripes about the Dynon system...it doesn't use flap position as an input to the AOA indication. I've requested it for years, but it never seems to make it onto the list of improvements.

I calibrated mine with full flaps, which is when I think it's most important (landing), and it seems "close" when doing no-flap stalls for practice, but I'm not really paying attention to that as much as the aircraft and stall buffet. Oh, well...it does work *great* on landing (always full flaps), so I can't complain too much.
 
Advanced Flight is part of Dynon, so I'm kind of surprised that the Dynon AOA doesn't have both flaps and no-flaps calibration points. My AFS system uses virtually all Dynon components (pitot, ADAHRS, and Skyview network). Only the EFIS isn't labeled Dynon.
 
I would think 1/2 flaps would give a margin of safety.

I think the Onspeed folks showed that the RVs have almost the same calibration curves for clean and dirty. That's reassuring in Dynon's case
 
AOA

Stall speed will be higher clean than dirty, but that is a function of the extra lift the flaps provide. However the angle between wing chord line and relative wind at stall is constant and is irrelevant of speed or flap position. The GAP 26 works on the differential pressures forward of thickest part of the wing (required to be within 25% of the total chord line, measured from the Wing Leading Edge (WLE), but could actually be forward of the WLE by 2"); these differential pressures are completely a function of wing chord line and relative wind. Note that there are four different available calibration points available for the G3X AOA.

Go have some fun with the AOA and you'll find that once calibrated, the alert setpoints will occur at the same stall angle for each of the flap positions, but simply at different speeds.

AOA is a wonderful tool for picking target approach speed, which will change at different aircraft loads and ambient conditions, or making a short approach or up hill landings to places like Johnson Creek (which is simply beautiful, btw). The speeds will change, but the stall angles won't.
 
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This would be one of my very few gripes about the Dynon system...it doesn't use flap position as an input to the AOA indication. I've requested it for years, but it never seems to make it onto the list of improvements.

Pretty sure you can calibrate the AOA for any flap position. Right out the Dynon installation guide:

“Refer to the list below of the stalls recommended to complete the user calibration. The general idea is to record stall data to find the lowest angle of attack stall. The list we have suggested is a starting point and produces a good calibration under most circumstances. However, if there are flight regimes not listed that you suspect would create stalls at lower angle of attack it would be good to record those as well.
 With full power, no flaps
 With no power, no flaps
 With full power, full flaps
 With no power, full flaps”
 
Stall will always, always, occur at the same angle of attack for a given wing.

Never said it wouldn’t!!
The given wing changes with the flaps deployed but on our airplanes the change is minimal. Start adding leading edge devices and slotted flaps, now you’re really changing that wing shape and thus the lower stall speed for that different shape. The only thing you’re doing when calibrating the system is setting the AOA window for your airframe.

Totally agree with Lenny that the stall speed on an -8 is virtually identical flaps up or down. Maybe 1 knot difference if I looked at it really close :D
 
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