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Thread: tuft/stall testing (with video)

  1. #1

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    Default tuft/stall testing (with video)

    Had some fun with wool tufts on my Model 2 wing today. I've always been impressed with the stall characteristics of if and wanted to see just how the stall progressed out the wing.

    First thing everyone will notice is how the fabric, especially at the root seems loose and does a lot of oil canning. I had no idea this was happening until I saw this vid and quickly checked the fabric tension after this flight. It's nice and tight so I wonder if some of it is due to normal wing flexion and some is due to the violence of the slip stream where the prop wash interacts with the wing. Remember also that this plane was built in 1991 (by me) and at that time, Denney shipped the lightest Stits fabric available (1.7 oz) which I see now is reserved for ultralight use only. All Kitfox's since use the 'medium' weight 2.4 oz fabric. At least I think they do.

    Anyway, the 1.75" twist I built into the wing seems to be working and the stall is nice and linear as it spreads from root to tip.

    Next step. Vortex generators!

    Chris Carlisle
    Model 2, Rotax 582
    Sioux Falls, SD
    Last edited by carlisle; 10-11-2009 at 11:34 AM.

  2. #2
    Dave Holl's Avatar
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    Smile Re: tuft/stall testing (with video)

    Chris
    A very good video, interesting to see what happens on the top sursace of the wing! On another thread questions were asked weather rib stiching was necessary, well for me when you see the amount of movement of the covering in the prop slip stream there is no question must get my needles out!
    Dave
    Dave Holl
    Building Kitfox MK7
    Rotax 912ULS

  3. #3

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    Default Re: tuft/stall testing (with video)

    Dave...Agree that the 'flapping fabric' is a little disconcerting and knew this would raise the old discussion re rib stitching. If I had it to do over again, I'd probably do just that.
    However, in Denney's defense, I must say that the method prescribed in the construction man (gluing fabric to cap strips) appears to have held up flawlessly over the years and is very hardy. I was skeptical enough when I built the plane that I tested one rib to destruction by gluing on some fabric and trying to rip it off with my bear hands. It held fast and wouldn't come off without ripping the cap strip with it.
    Do the current Kitfox's being shipped recommend rib stitching?

    Chris

  4. #4
    Dave Holl's Avatar
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    Default Re: tuft/stall testing (with video)

    Chris
    Well glueing certainly has stood the test of time on your aircraft!
    The instructions with my MK 7 recommend rib stiching. I have a Jodel DR1050 aircraft which is low wing and I covered and rib stiched about 20 years ago (not polyfiber) and it is quite amazing how the fabric baloons between the ribs on the top surface of the wing, far more than on your video, but I do not see any of the rippling of the surface near the root, I think that is caused by the prop slipstream but I guess that it does not affect a thing with your aircraft and as you say may be because of the light weight fabric.
    Still a great video, how did you mount the camera?
    Dave
    Dave Holl
    Building Kitfox MK7
    Rotax 912ULS

  5. #5
    Administrator DesertFox4's Avatar
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    Default Re: tuft/stall testing (with video)

    Yes the current models should be rib stitched. We are using much more powerful engine/prop. combinations now than years and models past. Faster cruise speeds and the advent of the model 4 through model 7 wing demand it. You might get away with just gluing like before but the contact area of the cap strips may not be enough to retain the wing material/capstrip bond with the added lifting effects. Remember the current Kitfox models are carrying more weight than the early models which requires the added stresses of greater lift generation on every connection between fabric and aircraft structure.

    In reference to another thread that mentions over speeding an early model Kitfox that is surely glued only (VNE?), the pilot is possibly entering uncharted waters, or airspace as the case maybe, in regards to testing his fabric/capstrip contact integrity. Just as importantly and less discussed the integrity of the capstrip to rib connections strength. Are the early model capstrips both glued and stapled to the rib or only stapled? Not sure about that. Either failure point would ruin your day. Another reason to fly your aircraft within the tested and placarded limits.
    The spirit of the experimental 51% rule is educational in it's foundation.
    Get familiar with rib stitching and fly with confidence and have a skill few can discuss with aplomb.


    DesertFox4
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  6. #6

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    Default Re: tuft/stall testing (with video)

    Steve...I'm not surprised, given all your arguements, that rib stitching is now recommended. As I recall, the cap strips are in fact glued and stapled to the ribs but when I did my test back in '91 when I built it, as I said the cap strip came before the glue let go so your concern about underlying rib vs fabric strength is a valid one. I glued a sample of fabric to the rib, let it dry, and then put the rib on the floor with my foot through one of the lightening holes, bunched the fabric up in both hands and pulled real hard. The cap strip tended to tear in an oblique fashion but the strip-to-rib union held as did the fabric to wood bond.

    Vortex generators came in the mail today. I wonder if some of the flapping of the fabric seen in the video will be smoothed out with improved laminar flow from these. Can't wait to try them.

    The camera for the vid is a cheap web cam attached to a 5/8" diam hollow alluminum pole and shoved up through the Lexan skylight. I had to drill a hole to accomplish this but the original unformed Lexan is starting to look pretty ugly and I plan to replace it anyway with some smoked newer material this winter so a little hole wasn't really an issue.

    Chris

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    Default Re: tuft/stall testing (with video)

    http://www.youtube.com/watch?v=gor7LhsAILs


    OK, here's the new video with Stolspeed vortex generators added.

    Stall now happens around 30 mph instead of 35 and I can't get it to progress out to the tip of the wing, even trying to hold it (at) high pitch for a while with power on. The last stall does fall off to the right a little and this vid is of the left wing so I wonder if it didn't go ahead and go out to the tip on that side.

    Also note; the flexing/flapping/oil canning fabric is no more. I did take a heat gun to the entire upper surface after seeing the first vid but couldn't really tell that that made much difference, at least to the 'feel' of the fabric tension. I suppose the fabric might have loosened over the years a little. One has to wonder though if the vg's are at least partially responsible for smoothing out the boundary layer and therefore the fabric a little too.

    Guess I'll leave them on.

    Chris

  8. #8

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    Default Re: tuft/stall testing (with video)

    Mike...I'll try and do the G-loading thing. Weather here in South Dakota starting to turn flying unfriendly so opportunities are fewer. Now I see why so many people live in the desert southwest. A day when I'm not working, don't have extracurricular stuff, AND the weather is good is rarer and rarer.

    Also, the lines on the fabric are 'drawn' with a carpenter's chalk snap line. Very straight and easy to remove when I'm done.

    Chris

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