Mechanical angle of attack indicator?

Discussion in 'Instruments / Avionics / Electrical System' started by cluttonfred, Apr 22, 2017.

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  1. Apr 24, 2017 #21

    cluttonfred

    cluttonfred

    cluttonfred

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    larr, a neat point on the port strut airspeed indicators, but I mentioned the Bacon Saver in my initial post and specified that I was looking for a cockpit indicator of some kind.
     
  2. Apr 24, 2017 #22

    Victor Bravo

    Victor Bravo

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    Funny part is that I wasn't kidding. If it would work on an old C-172 in the propwash, I'd have built one and put it on already. Works like a charm in gliders as many of us know.

    Matthew's original post mentioned a panel mounted instrument, so I am indeed guilty of thread hijacking on some level. To be 100% honest I have actually been working on the idea for something like what he mentioned, to put through FAA certification. I want to use a vane type sensor (smaller and less obtrusive than the Bacon Saver), and then send that signal to a panel mounted indicator. There are one or two other details I want to keep private, but that's irrelevant to this discussion.

    To address another comment in this thread, "who needs AoA?" is a matter of the type of flying you do. Millions of hours have been flown without any sort of AoA, millions more hours have been flown with a "stall warning horn" AoA device, and millions of hours have been flown (military and transport category) with a full AoA system.

    Apparently almost all of the people who have flown with them, in demanding flight environments (STOL bush planes, carrier ops, etc.) say that they are valuable. I have not flown any military or transport category airplanes (well, I did crash a full motion 727 simulator into the 405 freeway at LAX once), and I've never been a real bush pilot, so I cannot say from experience. But I am nonetheless convinced that AoA will allow a pilot to fly much more precisely when operating in a demanding environment, such as a <1000 foot landing strip with trees around it.

    Ramp Rumor has it that we have some very highly qualified people participating on this forum, ex-military folks and such... perhaps one of those pilots can comment from experience.
     
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  3. Apr 24, 2017 #23

    vtul

    vtul

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    Figured you weren't. Me neither!
     
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  4. Apr 25, 2017 #24

    lr27

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    Problem with all mechanical is friction. I know they have hydraulic bike brakes now, so maybe that kind of cable is the way to go to reduce friction?? Not exactly simple.. I think a mechanical system will require a bigger vane.
     
  5. Apr 25, 2017 #25

    larr

    larr

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    Well, let's take another stab at this - summarizing the previous posts.
    In order for an AoA vane to work it has to be in free stream air. On a single engine prop plane it has to be out of the prop wash and away from any air stream interference from the wing. This doesn't leave much to work with, and the reason I included the pics of the Bacon Saver mounting.
    The forces generated by a reasonably sized vane are going to to be very small and the distance to the cockpit is going to be quite large. The problem with a mechanical system is not just friction - the mass of the moving mechanical assembly will be considerable and well beyond the forces generated by the vane.
    On a multi-engine aircraft puting the vanes on the nose is completely adequate.
    eH5rR.jpg
    You could perhaps use a light pipe to conduct the image to the cockpit, but it's not going to be easy.

    The alternative is a LRI which provides roughly the same information in a different format:
    lri_probe.jpg
    No electrics and simple.
     
  6. Apr 27, 2017 #26

    lr27

    lr27

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    One solution to any oscillatory issues might be damping grease. Just make sure it doesn't get too stiff when cold or too thin when warm. Up to a certain point, the faster something moves, the more this kind of grease resists.
     
  7. Apr 27, 2017 #27

    Victor Bravo

    Victor Bravo

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    After all of this, and all the points that have been brought up, it seems clear that a purely mechanically operated AoA system with a panel mounted indicator is not really feasible in a "real world" application. The only way I can see to make it work without a lot of friction is a long shaft from the vane sensor to the instrument. This shaft would be hanging outside of the aircraft in all likelihood, like the pitch axis pushrod on a Pou du Ciel. So you could do it to make a point, but nobody may want to have a six foot long shaft extending from the cockpit.

    Pneumatically or electrically operated has too many advantages over mechanical linkages for this.
     
  8. Apr 27, 2017 #28

    Mad MAC

    Mad MAC

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    Couldn't one use a remarked ASI as a differential pressure gauge measuring across either a flat plate or upper / lower surfaces of the wing.
     
  9. Apr 28, 2017 #29

    Victor Bravo

    Victor Bravo

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    There was a trailing edge sensor invented for sailplanes, to measure the drag of the airfoil at different flap (camber) settings, so that the pilot could adjust the flap setting to the lowest drag position at any speed. The mechanism was very similar to what you describe, a series of pitot - static holes an inch or so above and below the trailing edge, with the lines plumbed to an ASI or manometer of some sort to display differential pressure.

    There is an STC AoA indicator that measures differential pressure between two flat faces of a large aluminum probe mounted underneath the wing. My friend has one on his 182. I think it's the "Lift Reserve Indicator"???

    Here's the LRI probe on the CH-701 "Green Lizard"

    4377814.jpg
     
  10. Apr 28, 2017 #30

    mcrae0104

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    Yes.
     
  11. May 1, 2017 #31

    lr27

    lr27

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    For airplanes not made of aluminum, something like Bluetooth might be handy in this application, assuming it's reliable enough. Maybe there's some small gadget that works off vibration to charge a tiny battery and power the sensor.
     

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