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Angle of attack indicator usefulness?

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  • Angle of attack indicator usefulness?

    I have not yet received my LSA quick build kit, but I'm trying to make a list of items for the panel. I have never used an AOA instrument, but they sound very useful for short field work. I searched the forum but could not find anything about them, and I wondered why. I would be very interested in the experiences of those who have used them in light aircraft. Are they extremely useful, or just nice to have? Thanks, Bob

  • #2
    Originally posted by bway View Post
    I have not yet received my LSA quick build kit, but I'm trying to make a list of items for the panel. I have never used an AOA instrument, but they sound very useful for short field work. I searched the forum but could not find anything about them, and I wondered why. I would be very interested in the experiences of those who have used them in light aircraft. Are they extremely useful, or just nice to have? Thanks, Bob
    This has been discussed many times, search using Google perhaps - enter this "search": "AoA site:http://bearhawkforums.com/forum"

    I can't say enough good things about them. They make flying much safer, close to edge of the stall. The cost and weight penalties are nothing compared to the benefits.

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    • #3
      I'll second Battson's comments. And many/most glass panels today include AOA indicators or have a low-cost add-on.
      One suggestion on electronics: wait until you are totally and completely ready to install them before buying. Electronics change too rapidly these days to buy and have sit on the shelf for a year or more. It seems like you get twice the functionality for half the price about every year or two, due to the fast development pace.

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      • #4
        What you both said is about what I thought, but never having used an AOA system I wanted confirmation. I once flew the time off on a Glastar for a friend. While it flew great, it could have really used an AOA system, I think, because the airplane had zero reserve for the landing flare at speeds way higher than stall. It could really build up a sink rate and, as a result, a lot of Glastars were damaged. However, a really good Glastar pilot could use power to land at a far lower speed than with a conventional landing. It would seem that an AOA system might be particularly useful for that airplane.

        As for not buying the electronics 'til late in the build, I learned that lesson the hard way on my first build, an RV7. That will not happen this time.

        Thanks much, Bob



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        • #5
          I've found the pitot-based Dynon AOA to be fairly useless for the 4-place, because it must be calibrated at a particular gross weight. Deviations from that weight make the alert go off early or late, and the 4-place can vary by around 900 pounds. This may be less of a problem with the LSA, since you'll be operating in a narrower weight window. The other reason that it's not useful is that Bob's planes just give so many other good sources of AOA feedback. I can't imagine being surprised by a stall in one of these planes, with or without an AOA gadgetry.

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          • #6
            Originally posted by jaredyates View Post
            I've found the pitot-based Dynon AOA to be fairly useless for the 4-place, because it must be calibrated at a particular gross weight. Deviations from that weight make the alert go off early or late, and the 4-place can vary by around 900 pounds. This may be less of a problem with the LSA, since you'll be operating in a narrower weight window. The other reason that it's not useful is that Bob's planes just give so many other good sources of AOA feedback. I can't imagine being surprised by a stall in one of these planes, with or without an AOA gadgetry.
            I don't think that's correct Jared,

            The wing will stall at the SAME angle regardless of weight. It always produces the maximum lift at the same angle of attack. Aerodynamically speaking, the wing has no idea how much weight it's lifting.

            Having landed many times at the stall, from dead empty to loaded at 2500lbs, mine has never missed the exact landing speed. The AoA stall warning varies by 9kts over the weight range - so it certainly works for me. It does work on any kind of aircraft, 4-place or LSA makes no difference at all. It's just a question of calibrating the correct stall angle, which is the same for any given wing. Of course changing the shape of the wing changes the stall AoA (flaps, deployable spoilers or slats, flaperons, etc)

            Maybe there is a leak in your AoA pitot line, which is causing your system to give bad results?
            Last edited by Battson; 08-09-2015, 04:25 PM.

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          • #7
            Was in necessary to calibrate it for different flap settings? As the flaps change the chord line they should be affecting the measured stall AOA also, methinks.

            Sent from my SCH-I535 using Tapatalk

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            • #8
              Originally posted by nichzimmerman View Post
              Was in necessary to calibrate it for different flap settings? As the flaps change the chord line they should be affecting the measured stall AOA also, methinks.

              Sent from my SCH-I535 using Tapatalk
              Yes different amounts of flap definitely makes a difference to the stall AoA, and you have the choice to calibrate any way you like.

              They suggest calibration with a range of different configurations, and the Dynon unit then takes the most conservative result of the 4 tests:
              1. no flaps, no power
              2. flaps, no power
              3. no flaps, power on
              4. flaps, power on

              Alternatively, if you are like me, you calibrate the computer to the same standard landing configuration (flaps, a little power) and it's very accurate in that regime. The prop wash changes the relative airflow over the inner part of the wing, so it matters.

              The critical AoA with flaps deployed is LESS than the stall AoA with flaps retracted, relative to the pitot tube. Relative to the effective chord, there is less change. But anyway, calibrating it with full flaps will make the warning more conservative in normal flight. So it's safer.
              Last edited by Battson; 08-09-2015, 08:02 PM.

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              • #9
                How does the calibration on the dynon work for best range AoA (i.e. best glide, L/D...)?

                Mark
                Scratch building Patrol #275

                Mark
                Scratch building Patrol #275
                Hood River, OR

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                • Battson
                  Battson commented
                  Editing a comment
                  I am not sure whether Dynon's software supports a best-range calculation? Perhaps Skyview does? The D100 series do not AFAIK.

              • #10
                The wing should stall at the same angle regardless of weight, but my hypothesis is that the Dynon does not measure angle consistently at different weights. Hearing that yours works better yields hope that maybe mine is just faulty. What is the tube's angle relative to the wing chord, and is it the L type or the boom type? As for flaps, I don't have an electric flap position sensor, so there is no way for the Dynon to know where the flaps are.

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                • Battson
                  Battson commented
                  Editing a comment
                  For me, the wing chord is close to parallel with the pitot boom.

                  I don't think the Dynon AoA software accounts for flap position, even with electric position indication installed? I understood that you just have to choose where to put the flaps when you calibrate it, or follow their advice in the manual (calibrate with multiple options).

              • #11
                since we're measureing alpha, weight is irrelevant. Differring flap settings do matter since you're modifying the airfoils effective camber. Most of us just calibrate the system with the flaps in the landing config. That seems a reasonable compromise. In the 8 I've rarely found the aoa usful, since I generally wheel land the airplane and we're not that close to a stall.
                David Edgemon RV-9A N42DE flying RV-8 N48DE flying Patrol #232 N553DE in progress ! Plans built.

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                • #12
                  Of course, if you pull hard enough on the stick you can stall at any airspeed Same also applies for very turbulent conditions in mountainous terrain, or windshear.

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                  • #13
                    Yup, having a flap switch would allow that to work more accurately, but it is still usable while manuevering. I find it interesting to watch during acro. One thing that is really needed with the dynon system is a remote indicator. Looking at the panel is less than optimal during landing. A bigger indicator up on top of the glare sheild would help quite a bit.
                    David Edgemon RV-9A N42DE flying RV-8 N48DE flying Patrol #232 N553DE in progress ! Plans built.

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                    • #14
                      Originally posted by dedgemon View Post
                      since we're measureing alpha, weight is irrelevant.
                      I'd agree that if we were measuring AOA, weight would be irrelevant. The Dynon system doesn't measure actual AOA, but rather approximates AOA based on pressure at a port, just like most airspeed indicators approximate airspeed by using differential pressure. When I calibrated at a heavy weight, and subsequently flew at a lighter weight, the alert came prematurely. There's certainly a possibility that there is just something wrong with my setup. I'm using the boom type of pitot, with the boom pointing about 5 degrees below the chord line.

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                      • #15
                        Hmmm, I'd go experiment with mine this weekend but the plane is down for a couple of weeks for a fuel servo issue. Might ask Dynon on their forum. They're generally very response. I guess I need to run through the math, but I dont see where weight of the aircraft would come into it. I'll try to remember to check it after I get the '8 up again.

                        This is a pretty good reference.

                        http://www.nar-associates.com/techni...ide_screen.pdf

                        There are some sensor linearity errors of course and that might be part of what your seeing.
                        Last edited by dedgemon; 08-11-2015, 05:12 PM.
                        David Edgemon RV-9A N42DE flying RV-8 N48DE flying Patrol #232 N553DE in progress ! Plans built.

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