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Thread: Wake Turbulence, The Movie

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    Wake Turbulence, The Movie


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    skywagon8a's Avatar
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    Two good videos stewart. Notice that in virtually all cases the vortex rolled off the outboard ends of the flaps. One would think that perhaps something could be done to convert this to lift by trapping the outflow. Long ago I made a fence which was mounted to the outboard end of one flap on my 185. It extended 2" above and below the flap surface. I thought that this would provide a little extra lift on the flap. To test it I did some stalls at all flap settings expecting the plane to roll away from the flap at stall. It did not. The fence made absolutely no difference.
    N1PA

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    CamTom12's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Two good videos stewart. Notice that in virtually all cases the vortex rolled off the outboard ends of the flaps. One would think that perhaps something could be done to convert this to lift by trapping the outflow. Long ago I made a fence which was mounted to the outboard end of one flap on my 185. It extended 2" above and below the flap surface. I thought that this would provide a little extra lift on the flap. To test it I did some stalls at all flap settings expecting the plane to roll away from the flap at stall. It did not. The fence made absolutely no difference.
    That's actually just a secondary vortex. The main vortex is a byproduct of lift production. It rolls up from the full wingspan to approximately 2/3 of the span.


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    BC12D-4-85's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Two good videos stewart. Notice that in virtually all cases the vortex rolled off the outboard ends of the flaps. One would think that perhaps something could be done to convert this to lift by trapping the outflow. Long ago I made a fence which was mounted to the outboard end of one flap on my 185. It extended 2" above and below the flap surface. I thought that this would provide a little extra lift on the flap. To test it I did some stalls at all flap settings expecting the plane to roll away from the flap at stall. It did not. The fence made absolutely no difference.
    I did the same 'experiment' on a Citabria and then on a PA-18A. Clear Lexan (invisible?) 1.5" larger than the ends of the flaps all around and rivnuts for fastening or easy removal. In both cases the plane rolled away from the fence side when flaps were first deployed, indicating to me a modest increase in lift. I also extended the traditional flap seals outboard over the ailerons on the Cub (the Citabria came with factory aileron gap seals). Again a slight roll away from the side with them installed. I flew with both mods with no adverse effects for experimental purposes only.

    GAP

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    BC12D-4-85's Avatar
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    I might add that in very cold weather during temperature inversions at the surface and still air the descending vortices can be audible. After the large aircraft passes to land the vortices create a sound similar to a waterfall that lasts for several seconds (snap-crackle-pop). I assume it's an induced sound and reaction to still air and eventual contact with the airport surfaces.

    GAP

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    skywagon8a's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    I did the same 'experiment' on a Citabria and then on a PA-18A. Clear Lexan (invisible?) 1.5" larger than the ends of the flaps all around and rivnuts for fastening or easy removal. In both cases the plane rolled away from the fence side when flaps were first deployed, indicating to me a modest increase in lift.
    Interesting, were you on wheels or floats? I was on floats, so perhaps the floats masked the slight difference? I expected your result but was surprised when I could not find any.

    I'll tuft test my flap tips on the Cub to see what I can see.
    N1PA

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    BC12D-4-85's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Interesting, were you on wheels or floats? I was on floats, so perhaps the floats masked the slight difference? I expected your result but was surprised when I could not find any.

    I'll tuft test my flap tips on the Cub to see what I can see.
    Hopefully this reply isn't too far off the OP's topic...I look at it as harnessing the energy of the tip vortices FWIW. Modern commercial airliners often have tip devices so there must be some benefit worth the economic investment.

    My experiment was on wheels, skis, and floats for both aircraft as the tip fence or winglet was easily removable. It was most detectable at initial flap deflection up to 14-20* and somewhat thereafter (requiring differential aileron) below Vfe but less so approaching Vso...I assumed that was due to higher air velocity at the former and some boundary layer turbulence during the latter airspeeds, especially when full flaps were deployed (35-50*).

    I did tuft the tips and trailing edge of the flaps and noted a flow characteristic of some lateral blocking. I also plated the inner flap ends and 'thought' I noted an increase in roll over a single unit, but there was no real way besides differential aileron deflection to measure. No tail buffeting or unusual stick cycling was noted, nor was control at cruise effected.

    I discussed this experiment at the time with Gordon Mandell of the FAA engineering office in Anchorage. He concurred there may be some benefit to be gained and that any additional loading into the wing would probably be minimal...the effect of an effectively increased aspect ratio was also discussed but never analyzed or assumed. Note that he helped develop Crosswinds STOL kit I believe.

    In addition to the full aileron seals I also sealed the flap gaps on the Cub at the hinge points by attaching first a tape then metal covers over the tops. They were configured to allow rotation and bellcrank rod extension. The goal was to keep as much high pressure air under the flap as possible.

    GAP

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    skywagon8a's Avatar
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    Thanks GAP, your result is what I expected. I have no idea why I could find nothing on the 185. Perhaps the thinner airfoil caused a different airflow in this area????

    Gordon was a great help to me when I certified the brake boosters. I also worked with him installing flaps on a wood spar J-3. He certainly is an authority on these little things which we like to do to our planes.

    I'll explore this again on my Cub.
    N1PA

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