Flutter

Discussion in 'Hangar Flying' started by bmcj, Aug 9, 2019.

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  1. Aug 9, 2019 #1

    bmcj

    bmcj

    bmcj

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    i thought I would start this off with a wacky, yet serious idea...

    What is the lowest airspeed you have heard of that experienced control surface flutter? If you were trying to design a plane for the record of slowest flutter, what do you think that would be and would there be anything that could be learned from it?

    I know that this bottom edge is a little fuzzy (for instance, someone could rightfully say that a stick-free phugoid oscillation is a form of flutter), but I am looking for actual stick-fixed control surface flutter that doesn’t just mirror the flight path.

    OK, maybe “wacky” was to tame of a descriptor.
     
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  2. Aug 9, 2019 #2

    BoKu

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    What I've heard second- or third-hand is that this glider fluttered at just above aero tow speed. Given that the flaps and ailerons appear to be about 30% of the chord, I'm not surprised. There is just no way to mass balance surfaces that big.

    As to what can be learned: Make your control surfaces of reasonable size, make them torsionally stiff, and mass balance them if you want to go fast.

    Anyhow, the owner died before he could figure out any way of fixing it, and the glider and trailer were sold to someone who just wanted the trailer. The glider went to the dump, where an airstrip owner saw it and thought it'd be the perfect desert airstrip yard art.

    The story on RAS

    Location on Google Earth
     
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  3. Aug 9, 2019 #3

    SVSUSteve

    SVSUSteve

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    That's actually a reason why a primary concern with my design is flutter (part of the reason for the "spare" wings and tail thread). The flaps and ailerons are 30% chord. The flaps are less of a concern since they won't be used when the aircraft is going fast obviously but the ailerons, rudder, and elevator are all things I am trying to be extra cautious about.
     
  4. Aug 9, 2019 #4

    bmcj

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    Steve, even though the flaps will not be used at higher speeds, I think you should still have them vetted (flutter analysis) for the higher speed, at least in the stowed position. On the plus side, flaps tend to have a more rigid mounting than ailerons do because the flaps typically have to withstand higher air loads.
     
  5. Aug 9, 2019 #5

    SVSUSteve

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    I was planning on doing it but thanks for looking out for me. :)
     
  6. Aug 9, 2019 #6

    BJC

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    What was the opening in the nose for?


    BJC
     
  7. Aug 9, 2019 #7

    plncraze

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    Two stories: the prototype VP-1 had aileron flutter in its first flight but I think it was faster than BoKu's example. The second was the high altitude sailplanes where the mach number is more critical than speed IIRC.
     
  8. Aug 9, 2019 #8

    bmcj

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    I can’t speak for this one, but some gliders have their tow hookup in the nose, but it’s usually a smaller hole and the mechanism is normally visible. Perhaps it had one but was removed.
     
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  9. Aug 9, 2019 #9

    D Hillberg

    D Hillberg

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    95 mph a Lohle 5151 with loose control cables - when tight 135 mph
    [ yeah he sure did before the plane looked like a pigeon on both crashes. . . oh pooper! ]
     
  10. Aug 9, 2019 #10

    BoKu

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    Yeah, good question. Many gliders (mine included) have a hole at the very front of the nose for cockpit ventilation and also for the tow hitch. Conventional wisdom is that for a reasonably well-arranged hole of reasonable size, the drag penalty is very small and laminar flow is maintained regardless of the flow rate through the hole. This is why radial engine cowlings can be so efficient even when throttled at their exits.

    In the case at hand, the hole is way larger than typical. Maybe for Arizona Desert levels of ventilation air? I can't even imagine. Of course, that raises the question of how to exhaust all that air. You need somewhere between one and 1.5 times the exit area. And as we know now, it's important to keep the cockpit pressure slightly lower than ambient to prevent leaks across the canopy seal and other joints. So in order to use all the air that hole can deliver without affecting performance, you'd need an exit pathway with a hole the size of a dinner plate.

    --Bob K.
     
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  11. Aug 9, 2019 #11

    BBerson

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    That old glider in post 2 had so much thick bondo on the aft wings and controls. The wing center of mass was likely aft of the aerodynamic center (25%). Not good for flutter resistance on small chord.
     
  12. Aug 10, 2019 #12

    radfordc

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    My CGS Hawk had elevator flutter at 60 mph. It had a poorly designed elevator trim tab that created the condition.
     
  13. Aug 10, 2019 #13

    Pops

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    I had a low frequency aileron flutter descending from altitude and not paying attention and thinking of my true airspeed. Was over 200 mph true with a redline of 187 mph. Upon landing I found a aileron turnbuckle barrel has turned with the single wrap safety wire. I tighten and used the double wrap pattern. Now I always use the double wrap pattern.
     
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  14. Aug 11, 2019 #14

    pictsidhe

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    There could be an internal flow restriction. The big hole gives good towhook access, the restriction reduces the airflow to what is needed. It appears that the builder wasn't a complete idiot, so there is a likely a good reason for it. We just don't know what it is. Nose intakes can work very well with intake air at a much lower velocity than stream.
     

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