GRT Derived AOA

[Dugaru]

The weather was perfect yesterday, so in a fit of EFIS nerd-dom, I decided to calibrate the "derived" AOA function of my GRT Horizon EX. For some reason I had never got around to that after installing it. The on-screen instructions were a bit cryptic but I think I figured it out. A few questions for anyone using this system:

1. Do you find that it's useful? How's the accuracy?

2. Did you configure it with flaps up or flaps down? There seems to be conflicting guidance in the GRT materials I've read. But I'm thinking since it's designed to be used during the landing phase, flaps down is the way to go.

Also, I was reminded that the -9 has an extremely benign stall.

 

[Tankerpilot75]

The weather was perfect yesterday, so in a fit of EFIS nerd-dom, I decided to calibrate the "derived" AOA function of my GRT Horizon EX. For some reason I had never got around to that after installing it. The on-screen instructions were a bit cryptic but I think I figured it out. A few questions for anyone using this system:

1. Do you find that it's useful? How's the accuracy?

2. Did you configure it with flaps up or flaps down? There seems to be conflicting guidance in the GRT materials I've read. But I'm thinking since it's designed to be used during the landing phase, flaps down is the way to go.

Also, I was reminded that the -9 has an extremely benign stall.

It’s been quite a while since I calibrated mine but I think I did it with flaps down. I often fly out to an air work area near the airport and fly stalls (both power on and power off) slow flight and steep turns. Since my GRT EX provides aural beeping sounds as I approach stall I use the stall warning sound more than the AOA indicator however I have noticed the AOA and approaching stall aural indications align very close with airspeed and aircraft feel. Definitely steep turns and aircraft loading do impact when the stall indications (visual and sound) start to appear so I guess i’s working as it should.

 

[clam]

From the HXr user manual... flaps extended per step 3.

Perform AOA calibration as follows:
1. Press NEXT > Set Menu > Primary Flight Display. Scroll to Angle of Attack (AOA) and select ON.
2. Scroll to AOA Pitch Offset and press the knob. The EFIS will return to the PFD screen and will show the pitch limit indicator and AOA indexer on the screen, with the right knob showing “ADJUST” and a value above it.
3. Start with the airplane at 5000’ above terrain. Slow the airplane and extend the flaps. Reduce power to idle and establish gliding flight.
4. Gradually slow the airplane and note the speed at which stall occurs.
5. Resume gliding flight and slow the airplane to within 1 or 2 mph of stall. Use the right knob to adjust the pitch limit indicator until it is on nose or bar pitch indicator. The screenshot above shows the pitch limit indicator nearing the attitude bars.
6. Press the “EXIT” softkey to end calibration.

I find the GRT AoA to be useful and accurate on landing, but a nuisance during aerobatics. The GRT (and Dynon) units in my airplane definitely do not replicate AoA vane accuracy, but they do provide a reliable stall warning. Very useful, for example, in the slow, wrapped up, DISTRACTED turn to a spot final at OSH.

 

[Dugaru]

From the HXr user manual... flaps extended per step 3.

Perform AOA calibration as follows:
1. Press NEXT > Set Menu > Primary Flight Display. Scroll to Angle of Attack (AOA) and select ON.
2. Scroll to AOA Pitch Offset and press the knob. The EFIS will return to the PFD screen and will show the pitch limit indicator and AOA indexer on the screen, with the right knob showing “ADJUST” and a value above it.
3. Start with the airplane at 5000’ above terrain. Slow the airplane and extend the flaps. Reduce power to idle and establish gliding flight.
4. Gradually slow the airplane and note the speed at which stall occurs.
5. Resume gliding flight and slow the airplane to within 1 or 2 mph of stall. Use the right knob to adjust the pitch limit indicator until it is on nose or bar pitch indicator. The screenshot above shows the pitch limit indicator nearing the attitude bars.
6. Press the “EXIT” softkey to end calibration.

I find the GRT AoA to be useful and accurate on landing, but a nuisance during aerobatics. The GRT (and Dynon) units in my airplane definitely do not replicate AoA vane accuracy, but they do provide a reliable stall warning. Very useful, for example, in the slow, wrapped up, DISTRACTED turn to a spot final at OSH.

Thanks! Good to know. I saw that in the manual and I think flaps down is the way to go. But the Sport EX installation manual (which apparently doubles as the EX installation manual…) says:

When in flight, in smooth air and at a sufficient altitude to safely stall the airplane, select the Set Menu, Primary Flight Display. Near the end of this menu, set “Angle of Attack (AOA)” to “ON”. New settings will appear below this setting when set to ON. We recommend setting “Pitch Limit Indicator” to “ON”, following this setting is “AOA Pitch Offset”. Change this setting to activate the calibration process, and follow the on-screen prompts. The prompts will include a step where you fly the airplane near stall speed. When performing this step, minimal power should be used, and the flaps should be in the retracted position.

 

[Ironflight]

Something to consider when doing your calibration - recent statistical analysis of stall/spin accidents by our EAA AoA Safety Subcomittee (Alan Wieder is the power behind the analysis….) shows that despite the fact that we (including myself!) have always talked about the base-to-final turn as the critical place to prevent stall/spin accidents, the truth is that the take-off and initial climb phase has twice as many accidents from stall/spin as the landing phase! A lot of this comes from attempted turn backs, but they also come from showy pull-ups after low passes, etc…. What this suggests is that if you want a WARNING of an impending high AoA to prevent a bad outcome, the “flaps up” case may be more appropriate - unless you routinely climb out with full flaps…..

Paul

 

[Dugaru]

the truth is that the take-off and initial climb phase has twice as many accidents from stall/spin as the landing phase!

That is extremely interesting!! I really would not have suspected that. Flaps up calibration it is!!

 

[BobTurner]

I’ve thought about this a lot. My theory is that it makes little difference. Most stalls begin just behind the region where the upper wing has maximum curvature - typically about 1/3 aft of the leading edge. This flow ‘knows’ very little about the flap position. Of course flaps increase lift so stall speed is lowered. But AOA? Where is Steve Smith? I need an expert opinion here!
I have both a measured AOA (Dynon D6 with two air pressure inputs) and a calculated AOA (GRT Hx EFIS). On landings they agree quite well. With aggressive maneuvering, the grt lags a bit - not surprising, since it probably needs a bit of time to calculate the flight vector from raw efis data.

 

[Vac]

As usual, Bob is correct. Apparently, he went to the same school as my wife ...It doesn't matter.

If a coefficient of pressure-derived AOA system is used to provide stall warning, the stall always occurs at peak coefficient of pressure. There are various means to determine coefficient of pressure, but the simplest is to divide one pressure by the other. In our system, we just divide the offset pressure by dynamic pressure. The "offset pressure" is the extra hole in the sensor, offset "x" degrees from the dynamic (pitot) pressure.

To prove Bob's point, let's look at a deceleration run from high speed cruise into a 1G, Flaps 0 stall:

Let's blow up the right side of the plot so the peak/stall is easier to see:

The two lines to focus on are the coefficient of pressure outputs of the airplane system. These are the grey line marked "cockpit angle of attack" (flaps up) and the yellow line marked "aoa computed using flaps 40 curve" (flaps down). It's the same coefficient of pressure inserted into to different calibration curves. Obviously, the correct curve (grey line) accurately measures AOA and the yellow line (wrong calibration curve) does not; but what's important is where peak coefficient of pressure occurs--both calibrations accurately capture stall. Obviously, the Flaps 40 calibration doesn't correctly compute angle of attack in degrees, but it doesn't matter in this case, the system still accurately measures when stall occurs and provides progressive warning.

If a coefficient of pressure system is used to measure actual body angle (difference between the relative wind and the longitudinal axis of the airplane in actual degrees), then an accurate calibration is required for each flap setting. We first noted this during ground effect testing. Operating in ground effect changes the critical angle of attack, but the effect is essentially nullified since peak coefficient of pressure still occurs at stall. Our system accurately measures AOA using coefficient of pressure, and I can observe the airplane flying out of ground effect on takeoff (momentary increase in AOA after rotation/lift off) and flying into ground effect on landing.

Here's a short video of AOA behavior during rotation and lift off. Since dynamic pressure is part of the coefficient of pressure we use to derive AOA, the AOA system provides adequate cues for takeoff and climb. In this case, I rotate/lift off in a "slightly fast" condition, but notice at the :08 second point in the video, there is a momentary "slightly slow" indication--this is the point the airplane is flying out of ground effect:

Paul is correct, most loss of control mishaps occur during takeoff/initial climb. They are also more "fatal" than mishaps that occur during approach and landing.

Fly safe,

Vac
FlyONSPEED.org

P.S. Bob, your observation of INS Derived AOA during maneuvering is spot on. It works well wing's level; but once you start to maneuver the matrix calculus starts to take up lots of "compute." If a simpler algorithm is used to determine flight path, that also degrades the solution during maneuvering.

 

[Scott Hersha]

I had the GRT Horizon 10.1 in my RV4, and used the derived AOA calculation. I found it to be unreliable and eventually turned it off. It worked as advertised sometimes, but I couldn’t count on it. On my new RV6 I also have the Horizon 10.1, and on this one I got the internal AOA option with the pressure port input. I calibrated it as suggested in the GRT install manual with a clean wing. In addition to that, I’ve installed an LRI (Lift Reserve Indicator) with the indicator mounted into my glareshield in my line of sight. This one was calibrated in the full flaps configuration. The indications on the EFIS are a PLI (pitch limit feather) starting to come in to view on the screen as I approach critical AOA, chevrons starting to come down in appropriate colors, and then a quickening aural pulse as I reach critical AOA (solid sound). The LRI is, as you can see below, an analog needle. Here’s how it works: in a normal VFR landing traffic pattern I may not see the feather at all until just before touch down, same with the aural warning. The chevrons will appear in the pattern if I’m below about 1.3-1.4 vs, depending on G loading. I’ll get the aural warning either just before, or in the flare if I’m on speed, which is about 50 - 53 KIAS unaccelerated usually. The LRI needle tells it like it is. On base or final at 65 KIAS, the needle is in the middle of the green band. Above about 70 KIAS the needle is pegged out on the right. This is normal coordinated flight, including turning flight. A perfect landing occurs when the needle enters the small white band as the aural warning starts. The LRI indications are very similar flaps up or down. On takeoff, I usually come off the ground tail low, clean wing, in ground affect (slightly), and the aural warning is beeping at me a little. It quickly stops beeping as I accelerate toward Vx. At that point the LRI needle is out of view on the right.
When I test the engine failure turn back option - at altitude - the LRI is my friend, and that’s where my focus is. As long as I keep the needle above the edge of the white band, even with a 45* bank angle, it won’t stall.
In the landing pattern at slow speeds, my focus is on the LRI, not the EFIS (IAS). LRI - “works fine, lasts a long time”.

 

[Dugaru]

Really grateful for all the observations and input.

 

[Canadian_JOY]

My experience is very much like Scott Hersha's... The GRT HX with "sensed" AOA plugged into the Adaptive AHARS works very reliably.

If you hear tone as you are maneuvering, you're closer to stall than you think - it's time to unload the wing. It's also very gratifying to use it as a "perfect landing" indicator. The first solid tone should mask the sound of the tires lightly chirping against the runway.

My AOA probe is simply a piece of 1/8" aluminum tubing which follows the aft end of the heated pitot tube downward from the wing, then makes a 30 degree bend forward and continues for another couple of inches at this "angled 60 degrees down from the pitot axis" condition. It cost almost nothing and works perfectly!

 

[clam]

…internal AOA option with the pressure port input. I calibrated it as suggested in the GRT install manual with a clean wing…

My HXr manual (legacy AHRS) for pressure port measured AoA says flaps extended calibration. The HXr manual hasn’t been updated maybe ever. Am I misreading or was there a change with the Adaptive AHRS?

In any case, I am in the Bob and Vac camp that regardless of flap position during calibration, the pressure differential AoA systems will still accurately present impending stall.

 

[Scott Hersha]

My HXr manual (legacy AHRS) for pressure port measured AoA says flaps extended calibration. The HXr manual hasn’t been updated maybe ever. Am I misreading or was there a change with the Adaptive AHRS?

In any case, I am in the Bob and Vac camp that regardless of flap position during calibration, the pressure differential AoA systems will still accurately present impending stall.

I don’t know if there was a change to the adaptive AHRS, but the latest Horizon install manual that I have recommends AOA calibration in the flaps up configuration - whether using calibrated or sensed AOA. I’m using sensed AOA on mine, tapping into the same 45* sense port from my LRI probe. The LRI needle stalls at the same point - right at the edge of the red arc, flaps up or down, and no matter the bank angle or G loading. The GRT EFIS indications are a little less reliable/helpful, and maybe that’s because of clean wing calibration, and where I’m noticing it is when landing. Maybe I’ll re-calibrate with flaps down to see if my results work better. Seems like I would get more conservative warnings. The pitch attitude, but not necessarily AOA, is lower at stall with flaps down, so on takeoff with a clean wing it seems like I would get critical AOA warnings earlier. Worth testing anyway to find out.

 

[Vac]

Pressure derived vs IMU (Inertial) derived Angle of Attack, points to ponder...

We spend a lot of time testing AOA . On our instrumented test airplanes, we have pressure-derived AOA, IMU-derived AOA and a separate air data boom that measures alpha (AOA), beta (yaw) angles, dynamic and static pressure. It's always interesting to look at the three systems relative to each other. My RV-4 has two separate pressure AOA systems, and two separate IMU-derived AOA systems in addition to the boom. What have we learned?

First, what's "IMU-derived" AOA? It's just the difference between where the airplane is going (flight path) and where it's pointing (pitch angle). Here's a Boeing diagram that shows how body angle is measured. Body angle is the difference between the relative wind and the longitudinal axis of the airplane. It effectively eliminates wing geometry effects (section blending, twist, etc.) and is all the "angle of attack" information the pilot needs to fly the wing.



Note the "(still air)" comment associated with flight path angle. Under stable conditions, unaccelerated level flight, pitch angle equals AOA. For years, this has been a standard flight test technique for calibrating and measuring AOA. Things become more challenging during maneuvering or unstable (turbulence/gust) conditions. The matrix calculus required to accurately derive all-attitude velocity (flight path) vector plus the filtering required to process a noisy MEMS gyro signal push the limits of a small processor. Not to mention the complexity of the AHRS coding required to get that right. In other words, under stable conditions (rolled out on final, trimmed, on glide path, on speed) the IMU-derived AOA is very accurate, but during maneuvering, say turning base to final, it may not be. Pressure-derived AOA doesn't suffer that limitation, it also is relatively easy to code and computes quickly.

Let's start with the base/final case. In this video, I'm just flying a normal (wide!) overhead to a touch and go. I'm not being particularly pure about maintaining an on speed condition, and fly most of the turn in what we call a "slightly fast" condition relative to on speed. On speed is used for approach/landing and equal to 1.73 L/Dmax AOA, which correlates with 1.25-1.35 Vs. It occurs with the ratio of actual coefficient of lift to maximum coefficient of lift is 60%, or what is sometimes referred to as a 60% lift condition. It's not a single AOA, but rather a small band (about +/- 1 degree) in a light plane that provides a cue equal to +/- 2 to 2 1/2 knots of desired AOA at 1 G. In other words a band about 5 knots wide. All AOA systems use a similar technique, but the range of body angle will change depending on the airplane. For example, an F-18 band angle band is narrower, but provides a similar IAS range. This is sufficient to catch a 3 wire aboard a pitching deck at night. It's also sufficient for driving auto-throttle input in the big jet I fly at work. More than sufficient for getting my RV-4 to the TDZ with just the right energy for landing.

Now let's look at how the three sources of AOA information compare in the video. Here's a plot that shows AOA and IAS:



Notice that the pressure Derived AOA and the AOA of the air data boom marry up nicely throughout, but the IMU algorithm is challenged even at the relatively shallow bank angles flown. The good news is that the phase aligns nicely and the IMU-derived AOA is "timely."

Now let's look at where folks get into trouble with the aerodynamic limit of the airplane. Most GA loss of control mishaps occur at "low" G, i.e., less than 2. Here's a video of a flaps 40 accelerated stall flown during a simulated base turn:

And here is a plot of the measured AOA during the maneuver:



In this case, the IMU-derived AOA detects stall, but notice the difference in relative accuracy compared to the other AOA solutions. We know this because the peaks are aligned.

Now let's look at a more extreme case, a skidding departure from controlled flight. This occurs if the pilot tries to "rudder" the airplane around the turn, but applies opposite aileron to keep the bank angle from getting too steep. A classic "uncoordinated" situation.

If we plot the data from this maneuver and once again compare our AOA solutions, we see some interesting behavior from the gyro-derived AOA:



In this case, I'm not sure what the IMU-derived AOA indication would provide, so that would be an interesting test point for some one with an IMU-derived AOA display/tone. We don't drive any of our displays with IMU-derived AOA, so have no way to assess performance real-time or using video de-brief. Our system uses the indigenous gyro for calibration only, and we derive AOA from the GNSS/INS reference system post-flight only. BTW, if you are an AHRS guru that can code matrix calculus and handle Kalman filtering, we'd love to buy you a beer and pick your brain .

Perhaps the GRT folks will chime in with some insights on their system.

Fly safe,

Vac

P.S., the ability to integrate pictures and video in the new forum software is awesome!