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Readability of AOA vs Airspeed

Ed_Wischmeyer

Well Known Member
So for all the discussion on AOA, here's what one vendor's system looks like in just slightly gusty conditions (9G14), shot in my RV-9A:

https://www.youtube.com/watch?v=mPGMKuT9NjI

I've checked the Posting Rules and didn't see anything against posting a link to my own non-commercial youtube video, so I think I'm safe. And I haven't got all the youtube video details straight yet, either, so this one is very definitely at the beginner level in terms of video production, channel management, etc.
 
The AoA is quite sensitive looking at this video.

It seems to me that if the airspeed was displaying a finer resolution similar to the AoA it would be jumping around as well.

Seeing that the airspeed indicator resolution is 0 to 200 Knots, the needle doesn't jump around very much. If the airspeed had a resolution of, say 60 to 90 knots, the airspeed needle would seem as skittish as the AoA.

I don't think it's a matter of reliability but of resolution.

Maybe someone can tell us how AoAs are calibrated and how their range is set.

PS: I like that you have the analog instruments displayed...very cool :D
 
The AOA is calibrated according to the book and works as expected. The AOA goes to steady beep at stall. And when you look at recorded data, it is similarly noisy.

From experience in other planes, my best guess is that the airspeed might jump +/- 3 knots if it were less damped. However, the point is readability -- and in this installation, the airspeed is much more readable. I have no idea what damping might be applied to the airspeed. And I think the issue is damping, not resolution.

Agreed that I like the audio and find the PFD display of little value. I never bought the glareshield indicator.
 
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The AOA is calibrated according to the book and works as expected.

From experience in other planes, my best guess is that the airspeed might jump +/- 3 knots if it were less damped. However, the point is readability -- and in this installation, the airspeed is much more readable. I have no idea what damping might be applied to the airspeed. And I think the issue is damping, not resolution.

Agreed that I like the audio and find the PFD display of little value. I never bought the glareshield indicator.

The airspeed in the video is jumping around quite a bit.

The only reason it's readable is the resolution is so low. If the scale of the airspeed was between 60 and 90 knots AND the airspeed readout was digital...I propose the airspeed would be as unreadable as the AoA.
 
Looking at the portion of the video after the descent from 1000' is commenced and before the flare for landing, pitch angle varies approximately 7 degrees while dealing with the gusty conditions. The AOA display appears to follow those changes in pitch angle. Vertical gusts will show up as AOA changes also. Airspeed will show little change if a pitch/AOA change is quickly reversed.

Cheers, David
RV-6A KBTF
 
Looking at the portion of the video after the descent from 1000' is commenced and before the flare for landing, pitch angle varies approximately 7 degrees while dealing with the gusty conditions. The AOA display appears to follow those changes in pitch angle.

What you didn't pick up on was flap extension at about 500 feet. That's when the AOA suddenly decreases at the same airspeeds because flaps give more lift at the same airspeed. Then again, I didn't point it out. :)

But regardless of the discussion, IAS is ever so much easier to read than AOA in this installation. Indubitably!
 
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AOA is pointless at cruise speed.
AOA is fine but AS and knowing your stall speed will keep you safe.
AOA is a nice to have. Some have mount flush in glare shield facing up at windscreen to reflect making a "HUD" without taking up Fwd viability
AOA you only care if it's yellow going on Red

To me AOA is for approach and short final. Can't imagine flying max performance short field min speed final/landing in extreme gusty conditions. AOA would be helpful, but if so sensitive it jumps around it would be useless. AS and extra margin above stall is the tried and true way. Even AS goes up and down in gusty conditions, and you add some extra approach speed up to a limit... half steady state wind and all the gust. However if that gusty go somewhere else.
 
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AOA and IAS damping

I just checked one of our Onspeed 50hz data logs where our sensor data is fused together with G3x Efis data. G3x airspeed lags about 2 seconds behind undamped dynamic pressure. I've also seen one Efis that lags 3.5 seconds. They have to do that to make it readable on the screen.

AOA isn't usually damped nearly as much as airspeed. As airspeed gets slower AOA becomes exponentially more sensitive. One of the things we're planning on doing with our Onspeed system is adaptive damping. AOA indicators are not wrong in turbulence, they are just unreadable. We're going to detect these random AOA excursions and tighten up the damping so that it provides a more flyable feedback.

Looking at a responsive and accurate in-flight AOA display ( ours ;) )it's also apparent that changing AOA doesn't immediately change airspeed, even if you take instrument lag out of the picture. You are a few seconds behind the airplane when flying indicated airspeed. Why would you ever want to use that as a primary reference?

We use a gaussian smoothing filter on our AOA signal, and it lags less than a 1/4 second at the moment. We want to know what the airplane is doing right now not 4 seconds ago. Yes, too fast in turbulence, but we're working on that. Thanks for the reminder! :D

Lenny
 
The only thing a wing cares about is AoA. You can go slower than stall speed and not stall and can be well above stall speed and stall. These stalls will be at the same AoA.

The readability issue in this setup compared to AS, is dampening and resolution. If the noise was filtered it would be better.
 
It seems like it might be beneficial to have both the raw and the dampened data. Have something that indicates the dampened number and have the raw going behind that. There is some utility to knowing how big and how often fluctuations are happening.
 
You are a few seconds behind the airplane when flying indicated airspeed. Why would you ever want to use that as a primary reference?

Pilots are trained to fly pitch and let the airspeed catch up. Chasing airspeed is never a good idea, so your rhetorical question is moot.

At my Oshkosh seminars, I showed how AOA and IAS both lag pitch about the same amount in gentle maneuvers. In a slow vertical S, AOA lagged a little bit more on the pull up and a little less on the recovery. Call it a wash.

But with a faster vertical S, AOA can lead pitch, so that really muddles the analysis. You can derive all this from the lift equation easily enough, showing that it is not system dependent.

Looking forward to seeing your results in some of the corner cases I sent to Vac. Those will be the real test.
 
Airspeed is pretty heavily damped

Hello Ed,
I watched the video several times. I have the same system (G3X) in my -14 and I don’t see the AOA oscillations shown with winds at 9G14. Honestly, that is only a 5kt gust. I am curious what your G variation was. What I noticed was a nearly constant 2.5 degree pitch oscillation despite fairly steady airspeed and steady VSI. The AOA tracked the pitch, that indicates that the Garmin system has damped the airspeed and VSI.

In the approach phase I don’t fly airspeed, I fly pitch backed up by AOA. Since the airspeed appears to be quite damped holding an airspeed would require the pitch and AOA to be out of sync as shown.

Respectfully,
Marvin
 
I watched the video several times. I have the same system (G3X) in my -14 and I don’t see the AOA oscillations shown with winds at 9G14.

I sent Marvin the raw data from the video flight so he can compare it with raw data from some of his flights. We will see what we will see!

And it's worth remembering that winds were **reported** at 9G14. There is a wind readout under the airspeed indicator, and it is set to display headwind and crosswind components. It seems to be pretty heavily damped as it doesn't get nervous about all the gusts.
 
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Looking forward to seeing your results in some of the corner cases I sent to Vac. Those will be the real test.

I am too. Vac's RV-4 is currently instrumented with a test boom, D10 Efis and our sensors (2 differential pressures, static pressure and a 9DOF IMU), all logged simultaneously and timestamped. So that should give us lots of good data to chew on.

Lenny
 
However, the point is readability -- and in this installation, the airspeed is much more readable.
...
Agreed that I like the audio and find the PFD display of little value. I never bought the glareshield indicator.

And there's the real problem. What makes the AOA more unreadable in your installation is the fact that it's buried in an EFIS display surrounded by a whole lot of stuff that isn't relevant when you need to be flying on AOA... Close to the ground, at low speed, getting into a tight strip, etc.

The glareshield indicator would isolate the AOA for you, which would make it a quicker reference. Still, keep in mind that any instrument on the panel will take time for your eyes to re-focus into the cockpit, and then back out again to the runway. That time, though short, may be just enough in a critical situation to be a problem. Hence the audio alert, which you can hear without taking your eyes off the runway, or HUD displays with infinity focus so you can see them and the runway at the same time. But such displays are expensive...
 
And there's the real problem. What makes the AOA more unreadable in your installation is the fact that it's buried in an EFIS display surrounded by a whole lot of stuff that isn't relevant when you need to be flying on AOA.

The glareshield indicator would isolate the AOA for you, which would make it a quicker reference. Still, keep in mind that any instrument on the panel will take time for your eyes to re-focus into the cockpit, and then back out again to the runway.

Uh, no.

The problem I was referring to was the low quality of the AOA data in turbulence. That problem will not go away by moving the display -- it will still be noisy data. Go watch the video again, please.

Also, a glareshield display is not a panacea. When turning base, chances are good that the pilot will be looking out the side of the plane at the runway, not looking forward, and the glareshield display will temporarily be out of the pilot's field of view.

And even on final, there's the phenomenon of cognitive capture that I described in a recent post. When pilots are stressed, even when looking through the expensive HUDs, pilots can tend to focus on the symbology or the outside view to the exclusion of the other when they need data from both. There are many well-known examples, including military target fixation. A glareshield display will be subject to this same phenomenon.

Your point about inside/outside focus transition time is, of course, well known. Usually that is discussed in the context of instrument flying, however.

Devices that work in benign conditions are always appealing, but it is rare that those devices are tested in the most demanding conditions. And if pilots learn to rely on those devices in benign conditions, and then find that they don't help in demanding conditions... or if pilots become dependent upon that device and it fails... or if it's not installed in the next plane...
 
Howdy Ed

I sent Marvin the raw data from the video flight so he can compare it with raw data from some of his flights. We will see what we will see!

And it's worth remembering that winds were **reported** at 9G14. There is a wind readout under the airspeed indicator, and it is set to display headwind and crosswind components. It seems to be pretty heavily damped as it doesn't get nervous about all the gusts.

Hello Ed,
I believe we have a miscommunication. I doubt I could find a comparable flight to compare data across our aircraft. I was simply interested in the G data from your video, the pitch oscillations seem more inline with higher wind gusts. From my perspective, your video shows the AOA matches the up and down oscillations of pitch attitude shown on the PFD. Stabilize the pitch and the AOA will subside and vice versa. The fact that the airspeed was stable does not mean the flight was stable if the airspeed data was damped within the EFIS system.

op1.jpg

The image above are three intentional stalls in the cruise configuration. The G3X AOA did a very consistent job of indicating the flight condition.

I surmise that you find airspeed more suitable for your flight parameters and that is your choice. I prefer stabilized pitch/AOA. We will simply need to agree to disagree hopefully without being disagreeable. This board is only of interest to me when it’s respectful.

I’m an AOA man through and through. My takeoffs and landings are shorter and more consistent using AOA. My aerobatics are more consistent because AOA is simply energy management without concern for weight, altitude, temperature, or G loading. AOA is AOA that cannot be said for airspeed thus we train normal stalls and accelerated stalls when referencing airspeed. What is your stall speed variation at 1G and 2G? I can pull 2Gs then stop the pull when my AOA indicates 1.3Vso I cross reference airspeed but I already know to unload the wings.

Now does that mean AOA is the bees knees of aviation? Of course not! It is a tool, nothing more nothing less. Surely you do not fly airspeed to the exclusion of all your other instruments to include outside references. Would you use airspeed independent of vertical velocity? Neither would I use AOA independent of other indications. There are flight regimes where I prefer airspeed and others where I prefer AOA, for instance I find no value in AOA during normal cruise flight; but for a loop I pull 3Gs then transition immediately to AOA, 1.3Vso is the same upright or inverted.

As for fixation on a gauge, I doubt AOA is the only available resource for fixation. Pilots are susceptible to fixation on many things thats why I strain to look at my wife’s eyes when she is talking. :)

At work I’m required to calculate a Vref for every landing. Oddly, that Vref seems to be at 1.3Vso on my AOA despite a fuel load weight variance of more than 40,000 lbs. that is a typical cross check I run for every flight. We do increase that speed for gusty days, but we are only allowed 1/2 the gust factor not to exceed 5kts. In other words very very very close to 1.3Vso.

Respectfully,
Marvin

P.S. - Vac is doing some very interesting work at www.flyonspeed.org. I will be adding his Gen3 to my aircraft at some point.
 
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Thanks, Marvin. I'm familiar with Vac's work and have even written a flight test plan for him. And I've also reviewed documents for him.

Since I don't have the ability to post graphics, I've sent you my plot of the data, and hopefully you can post that for me.

Thanks in advance!

Ed
 
I’m an AOA man through and through. My takeoffs and landings are shorter and more consistent using AOA. My aerobatics are more consistent because AOA is simply energy management without concern for weight, altitude, temperature, or G loading. AOA is AOA that cannot be said for airspeed thus we train normal stalls and accelerated stalls when referencing airspeed. What is your stall speed variation at 1G and 2G? I can pull 2Gs then stop the pull when my AOA indicates 1.3Vso I cross reference airspeed but I already know to unload the wings.

This and more. All the wing cares about is AoA. Best glide, max endurance, max range all flown at constant AoA. In light airplanes that weight doesn't vary much an airspeed is close enough, but it is all at the same AoA. Also, try flying through a temperature inversion at a constant airspeed, you'll be chasing airspeed. If you have a vane for AoA you can climb at a constant AoA through the inversion and maintain steady state. I grew up on AoA, now none of the airplanes I fly have an AoA gauge. The one I am building will have it.
 
The problem I was referring to was the low quality of the AOA data in turbulence. That problem will not go away by moving the display -- it will still be noisy data. Go watch the video again, please.
Ah, okay, you meant the noise in the instrument, not the location. Someone pointed out earlier that the reason you don't see it in the airspeed is that the range from min-max on the airspeed gauge is much greater than the range that the AOA operates over. The resolution is finer on the AOA, so you'll see more movement. I think it's partly that, but I think there's also some filtering going on digitally in your EFIS that smooths out the airspeed data. The filtering may be different, or not applied, on the AOA display.

Also, a glareshield display is not a panacea.
No, nor did I suggest it was. You won't be looking dead ahead a lot of the time when you want to reference it. But when you glance over at the panel, it's still easier to pick out a single AOA gauge on top of the glareshield, than it is to find the AOA gauge that's the same brightness as everything else on the display in your EFIS.

Devices that work in benign conditions are always appealing, but it is rare that those devices are tested in the most demanding conditions. And if pilots learn to rely on those devices in benign conditions, and then find that they don't help in demanding conditions... or if pilots become dependent upon that device and it fails... or if it's not installed in the next plane...
Well, we already have the issue of most planes these days not having the six-pack that most of us learned to fly with. Move back and forth regularly from a plane with one to a plane with an EFIS and you're probably asking for trouble in the long run... Your muscle memory won't take your eyes to the right place when it matters most.
 
So for all the discussion on AOA, here's what one vendor's system looks like in just slightly gusty conditions (9G14), shot in my RV-9A:

I find the AoA indicator on the G3X extremely usable. My setup is different with my PFD using ribbons and doesn't have traffic or EFIS information displayed. It's all what you're used to I suppose, but even set up like yours I wouldn't see an issue glancing down to check Airspeed and AoA.

I also fly in a lot of windy/gusty conditions, on the days I'm bouncing around I typically will see airspeed and AoA bounce around accordingly.
 
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