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-   -   AOA confession (https://vansairforce.net/community/showthread.php?t=178893)

McStevens 01-23-2020 09:39 AM

Never heard the phrase "unload for control" but I realize this is what I've been doing instinctively. Granted I've been using the stall horn as the indicator so I figure that's leaving a much greater margin than if I had an AOA, but is this not something everyone does?

Vac 01-23-2020 11:11 AM

More points to ponder
 
From the "it depends" department: "Unload" is a relative term, it just means less G than required for whatever it is you are trying to do. In most civilian flying, that means "something less than 1." For example, if I'm pulling a bit too hard in the final turn (riding the "slow" tone), easing the stick forward may induce a .9 G condition. In a more graphic example, if I'm in a 6G turn and want to grab some energy (go faster), 3G's is an "unload." A perfect unload may, in fact, be zero G since an airplane cannot stall at zero G. That recovery trick only works if the ground doesn't get in the way :)

Vanes vs Pressure. It depends on how well the computer algorithm works. A vane has the potential (pun intended) for better overall performance; but a good piece of software can match that performance for all intents and purposes. For example, at an L/Dmax condition (a critical performance parameter that a good AOA system should capture, our Gen 2 shows in absolute AOA of 7.54 degrees in the RV-4 with a standard deviation of .04 deg and standard error or .00 deg. At the same condition, the calibrated vane on the test boom shows a raw alpha of 4.41 deg, with a standard deviation of .03 deg and .00 error as well. The difference between angles is the delta between effective geometric AOA (what we learned in pilot training) and "absolute" AOA (what the wing sees in flight). We are seeing a worst-case error of about .2 degrees using pressure and .1 degrees on the calibrated boom; close enough for government work using a differential pressure source. We are testing with Dynon, Garmin, Alpha Systems, pop rivet, homemade and AFS style differential pressure sources and will publish results as we get 'em. Any of those configurations will work, but may require a different math technique to derive AOA.

Visual vs Audio. Ergonomics and "egonomics" at play. For eyes out, you can't beat an audio cue; but there are limiting considerations. For example, if task saturated, "tuning out" audio is a normal response. On the other hand, it's a pain in the butt to use a visual cue when you're looking over your shoulder. Perfect world, one of each can't hurt; so we are working with Vern to incorporate his nifty, inexpensive visual display. 2 "displays" (audio + visual) may also provide a degree of redundancy. Our system can display raw AOA in degrees (either absolute or geometric); but a graphic display makes more sense for the stick monkey. I'm a fan of the "standard" military configuration (doughnut + chevrons) because it's caveman simple. We added a simple floating bar with an intuitive L/Dmax cue; but totally get that some folks prefer chevrons so it'll have that capability as well. However, the audio capability will remain the heart of the system...you can bolt it into a round dial airplane. I don't have a visual display in the RV-4, but that's simply a personal preference.

Electronic vs air data AOA solution. You can derive AOA from an INS (accelerometers), i.e., you can remove the pounds of extra tubing in the airplane ;) We have an INS platform in Gen 2 for that purpose, but are using that primarily to automate the calibration process and, potentially, integrating that into the AOA algorithm for normal use. We are already integrating lateral G: the tone moves in the headset (sound field) with the ball providing a 3D audio yaw cue. This works with a stereo intercom or via bluetooth to a stereo headset. We learned in the military that directional audio cues work really well and it's nice in the base turn in the RV-4 to get a gentle nudge from the tone to adjust rudder pressure as well as stick pressure when I'm not looking inside. Our objective with Gen 2 is to demonstrate the utility of having intuitive AOA, sideslip and energy cues available to the pilot.

Stall warning. FAR23 stall warning requirements stipulate a minimum of 5 KTS of warning (natural or artificial cues). ANY calibrated AOA system will provide better progressive stall warning than a simple stall warning system. Actual "warning" with an AOA system is essentially pilot selectable. Since I fly a lot of aerobatics, I prefer an accurate slow tone with warning occurring about 2-3 KTS above actual stall so I can "ride" the aerodynamic limit if I want--purely a personal preference and since my airplane doesn't have to conform to FAR23, nor do I in this case. However, reaction to stall warning should always be the same, regardless of system installed or natural cues: unload for control.

Optimum turn performance. "Optimum" turn (best balance of turn rate, turn radius and energy) occurs ONSPEED. In real life, there are only two times when it makes sense to pull harder than ONSPEED. One occurs in combat; and since we don't face that unless the traffic pattern is really busy, the only other time I can think you want to approach the aerodynamic (stall limit) is in an emergency dive recovery close to the ground. In that case, it is appropriate to select maximum power, get the lift pointed 90 degrees to the horizon and pull right to the aerodynamic limit (stall warning) whilst hoping the ground doesn't get in the way. Since hope is never a good strategy, follow Ed's sage advice earlier in the thread to not get there in the first place by exercising good flight discipline. In general, don't ever pull harder than ONSPEED unless you're having some fun upside down.

I understand there's a lot here, so just refer back to the picture in my previous post--it's really that easy; don't pull harder than ONSPEED.

v/r,

Vac

gmcjetpilot 01-25-2020 12:55 PM

Quote:

Originally Posted by McStevens (Post 1402489)
Never heard the phrase "unload for control" but I realize this is what I've been doing instinctively. Granted I've been using the stall horn as the indicator so I figure that's leaving a much greater margin than if I had an AOA, but is this not something everyone does?

Well not all stall horns are equal, but stall horn means in general stall is about to occur. It may be conservative or not. Go up and practice some slow flight and nip at the stall horn. Then pull back and note when it breaks.

Every pilot at some point should have done slow flight or "min control" speed maneuvers and stalls and stall recoveries (including accelerated stalls) during training and check ride. GA, commercial and even military, have flown with out AOA.... Airspeed is our indication of stall or angle of attack (to critical) at 1G. Unloading is not a common term but it means 1G. Once you "pull G's" your stall goes up...

AOA calibrated and accurate is a great tool. If you can put it in your plane, it does add a bit of safety or situational awareness, but it is not required to fly safely.

Av8torTom 01-25-2020 04:56 PM

Degrees AOA
 
This is the AOA system that is now made by Advanced Flight Systems that displays degrees AOA. Again stall warning is just one SMALL reason to have AOA information, but without a degree readout all you have is a stall warner. AOA is about knowing precisely what the wing is doing in all aspects of flight.


BlndRvtr 01-25-2020 08:40 PM

Thanks Tom. I'll look into it.
Isn't Advanced now a part of Dynon?

George

Vac 01-26-2020 10:19 AM

Performance Cuing
 
Any AOA system that can be calibrated to provide performance cues (L/Dmax and ONSPEED) in addition to "slow" cueing is going to be more useful than simple, accurate progressive stall warning. The ability to ACCURATELY capture the aircraft curve (i.e., measure AOA) depends entirely on the sensor and processing that the computer does. AFS has multiple set points and the ability to capture two curves (flaps up/flaps down) I believe. For truly accurate AOA performance you need flaps up/flaps down curves, although, ultimately if the only use you have for AOA is approach and landing an accurate calibration in normal landing configuration is doable (with degraded performance at other flap settings). The type of flap fitted will really define the requirement for multiple curves. The plain flaps on the RV-3/4/6/7/8 have less effect than the split flaps on the -9/-10 and -14. Here's the difference between a two-point calibration and a multi-point calibration:



The shaded area is potential error. Vcc is Carson's speed. L/Dmax is best lift to drag ratio (bottom of the drag curve). Vprmin is min power required speed (ONSPEED, or bottom of the power required curve) and 1.1 Vs is where stall warning is set in this diagram. Note that each of these calibration points is a key performance parameter. This sketch is for a 23-series airfoil (RV-3/4/6/7/8).

If you have flight test data for your airplane and know what geometric or absolute alpha you are looking for, then a digital display can be helpful. If you don't have data; but have an accurate percent lift calculation (i.e., stall = 100%), then L/Dmax will occur at 50% lift, and ONSPEED will occur at 60% lift. Many systems have some sort of % lift calculation; but keep in mind that is calibration dependent and the 50/60% rules require that the system accurately measures 100% lift at stall. It also requires that the system accurately maps the aircraft curve which it may or may not do. Only way to determine is flight test.

The reason some AOA systems are simply good progressive stall warning is because even a basic, not-quite-right calibration will capture stall accurately although it may not express it properly in terms of % lift or actual AOA.

v/r,

Vac

Jeff Vaughan 01-26-2020 09:00 PM

Wow my head is spinning. Math and charts and graphs ugh. I guess many are interested in the details but after flying with the Onspeed Beta box for 18 months All I care about are the tones I hear ...Slow...Onspeed...or fast. This AOA is fantastic. I do not have ANY visual for the AOA other than airspeed so when I pull power fully off and put my flaps on I trim for the Onspeed tone. I do a quick check of the airspeed to make sure I am in the ball park and then fly the tone looking out the window until I grease it in. I love aural AOA.

bpattonsoa 01-27-2020 08:16 AM

I fly both my -10 and Rans with Dynon systems. The audio AOA is good but their on screen indicator is worthless. The wigit is so small that you can?t see it in a glance.

I have suggested the following firmware change several times, so here it is again.

When airspeed drops below a user selectable value, or when the aural warning is about to automatically come on, the AOA on screen expands to the size of the airspeed tape and is placed just to it?s right. It returns to normal size when airspeed drops below another user selectable speed, well below stall.

Or better yet, a remote indicator on the network for glare shield mounting as others have suggested.

vlittle 01-27-2020 09:35 AM

I've been pondering the flight safety issues for a long time, and have experimented with visual, audible and haptic indicators for AOA.

Under the assumption that I don't trust the automatic attitude recovery systems that are available (different debate), so how can we use technology to provide the pilot with the critical information, at the right time and elicit the correct response?

I think the OnSpeed technology comes the closest to fulfilling the requirement, primarily because it can be used in normal flight manoeuvres. This allows the pilot to develop good habits that will prevent upset situations, and should one occur, the instincts to recover.

Visual or Audio... how about both? Different people respond differently to audio or visual cues. For sportplanes, where we may be doing aerobatics or formation flying, or a full slip landing on a sandbar or short field, the visual indicators are less useful than the audio. However, many people disagree.

On my list of things to do is build a HUD laser-based AOA indicator. For straight-ahead landings, this may be useful. I have a long list of gizmos I want to build before I die, so give me some time for this. I will be collaborating where possible with the OnSpeed folks.

V

Av8torTom 01-27-2020 03:42 PM

Vansairforce classifieds
 
Quote:

Originally Posted by BlndRvtr (Post 1403109)
Thanks Tom. I'll look into it.
Isn't Advanced now a part of Dynon?

George

There?s one for sale now on this site


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