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About cr125r847

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  • Location Nowthen, MN

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cr125r847's Activity

  1. cr125r847 added a post in a topic Full Lotus Float Air Bladders   

    My searching found a bunch of manufacturers of the polyurethane film but no distributors. A local plastics film company tipped me off to a film distributor and I'm waiting to hear back to see if they will send me a sample and quote.
    Your "polyurethane air bladder" search brought me to the kite surfing crowd. Another hobby I know little about but apparently they use polyurethane air bladders on the kites so maybe there are some diy people that have leads to get the film.
    Anyway, thanks again for the ideas.
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  2. cr125r847 added a post in a topic Full Lotus Float Air Bladders   

    Thanks Chris. I have not heard of kayak float bags before. That is exactly the information I was looking for.
    Now I need to find out where I can buy the material without buying a 1000 yard roll.
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  3. cr125r847 added a topic in Avidfoxflyers General Hangar   

    Full Lotus Float Air Bladders
    I bought some older Full Lotus floats this weekend that have six of the rear air bladders needing replacement. Replacement bladders are available from Full Lotus for $124 each, but they seem like they would be easy to make for a lot cheaper. The bladders are just a single sheet 0.010" thick folded over and heat sealed on the three sides. There is also a circular reinforcement where the valve stem goes through the bag.
    Does anyone know what type of plastic these are made out of? I was originally thinking window vinyl, but they are stretchier and more rubbery feeling. My guess is they are either polyurethane or polyester TPU plastic film and I have found a few manufacturers of these types of plastic, but no distributors. It would be best if I could find an end user of the film where I could just buy a small quantity.
    It would be great if anyone has any info or leads. I'm planning on calling some of the companies that make the plastic to see where it can be purchased.

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  4. cr125r847 added a post in a topic Vortex Generators   

    You're right, it was a rude comment that should not have been posted. I shouldn't have lowered myself to name calling.
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  5. cr125r847 added a post in a topic Vortex Generators   

    Deleted my insensitive comment, but his impression remains. He had some really interesting testing but in my opinion could have presented it in a better way. There are some awesome videos on YouTube but maybe his are not for me. Thanks anyway for sharing!
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  6. cr125r847 added a post in a topic Vortex Generators   

    Maybe my comment was a bit harsh but I cringed when the grown man in the video said how scary it was to hold his plane in a stall. And then nothing happened. If he was scared the plane was prone to entering a spin, let's not be making YouTube videos and instead focus on flight testing the plane. I took his comment that he was scared to hold the plane in the stall. In that case, he'd best go talk to an instructor instead of the camera in the plane.
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  7. cr125r847 added a post in a topic Ribs and spar material   

    Pictures, or it didn't happen  I'm curious how you are building the ribs. I only have one wing to rebuild but if I were doing both I would have made composite ribs. 
    You must be just weighing the main part of the rib between the spars without cap strips or tails. I am just finishing gluing my ribs up and they weigh 9.3oz with without the flaperon tail and 10.3oz for the long ribs with the tail. The short ribs behind the gas tank are 3.4oz and 4.1oz. Total weight is 6.35lbs in each wing. I'm using the 5mm 10-ply plywood for the main rib and 3mm 5-ply for the cap strips. I'm thinking composite ribs would easily save 25% in weight and similar cost vs buying the plywood from Aircraft Spruce. 
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  8. cr125r847 added a post in a topic New Wing design.   

    I'm not an expert, but from what I understand, a sharp leading edge can get you into trouble with nasty stall characteristics and it's best not to mess with the leading edge unless you know what you're doing. Extending the trailing edge shouldn't make much of a difference. With that said, the KF4 uses the Ribblett GA30U-612 airfoil and the coordinates are posted on the internet. Why not plot the airfoil with a 2" longer chord and you will have a known airfoil and not be left wondering how your changes will affect the flight characteristics? I can plot it out in CAD if you need. Just get me the chord length.
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  9. cr125r847 added a post in a topic Getting started/setup for Welding?   

    I bought an AHP AlphaTig 200X last year and have had very good luck with it so far. I hate buying Chinese, but a comparable Miller or Lincoln would have cost 5x more and out of my price range.
    If you go the Tig route, I would recommend one that is capable of AC and DC. That way you can weld aluminum and any other weldable metal.
    Then take some time and watch Jody's videos on his weldingtipsandtricks YouTube channel. He does a great job explaining the settings and proper techniques.
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  10. cr125r847 added a post in a topic Avid STOL airfoil -some calculations   

    Agreed and I didn't intend to sway this into a "Cub Killer" conversation. I don't think a Kitfox will ever be able to compete with a Cub due to the narrow, high AR wing on the Kitfox. One of the compromises of having a foldable wing.
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  11. cr125r847 added a post in a topic Avid STOL airfoil -some calculations   

    Lift changes with the AoA at a constant airspeed, but the analysis I did was an AoA a sweep at fixed lift (750 lbs in this case). The variable is airspeed.
    What I'm after with the simulations is to compare the different airfoils and see what effect changing from the Avid undercambered airfoil will have while leaving the wing incidence as it is. There was talk a few years ago about how Kitfox increased the angle of incidence when they switched to the Ribblett 612. Would this need to be changed to make an Avid perform well with the 612 airfoil?
    It would also be fun to compare the Kitfox STi rib to see where it falls. Does anyone know what airfoil they are using?
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  12. cr125r847 added a post in a topic Carbon fiber rib tail reinforcement   

    I used a refrigerator compressor that i salvaged many years ago. They work great for this and are easy to find at a good price (free).
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  13. cr125r847 added a post in a topic Avid STOL airfoil -some calculations   

    Wow. I didn't know it was that much.
    I ran the analysis through an AOA sweep at a fixed lift. With the washout at 2 degrees, the velocity would be 97mph and at 4.5 degrees washout, the velocity would be 119mph! Also the LE of the wing tips do not start contributing to the lift until 7.5 degrees AOA. Interesting...
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  14. cr125r847 added a topic in Avidfoxflyers General Hangar   

    Carbon fiber rib tail reinforcement
    I thought I would share my method of reinforcing the rib tails on the wings I am rebuilding. I know other people have had good results with aluminum reinforcements, but that seemed like a lot of work and tough to make it look like it is not a band-aid afterthought fix.
    I ended up using 2" wide 12k unidirectional carbon fiber tape and epoxy. I originally was going to fold the 2" tape over the top and down the sides of the rib tail, but decided to cut the tape down the middle and laminate the 1" strips to each side. I vacuum bagged the rib tails using and cured the epoxy in a high tech cardboard box oven. Vacuum bagging and elevated temperature curing improves the strength and reduces extra epoxy, but really is not necessary for a part like this.
    In the end, I have around $40 in materials with a bunch left over for the other wing and other projects on the to-do list and around an hour of time. It is incredible how much stiffer the rib tails are with just one layer of carbon fiber. I suppose I could maybe break the rib tail over my knee, but I am not able to break the parts bending them with my hands.

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  15. cr125r847 added a post in a topic Avid STOL airfoil -some calculations   

    From what I have read, the USA35B is an old design and there are other more modern airfoils that could perform better on a Cub. I am in the process of modeling the Ribblett GA30U-612 that is used on the newer Kitfoxes in XFLR5 software. I have the wing modeled with an approximation of the standard Avid wingtips. The aerodynamics field is far from my specialty and am finding the learning curve to be steep. My plan is to model and compare the 30U-612, Avid undercambered, and speed ribs to compare the theoretical performance. Maybe I'll add the USA35B to the list as well.
    I still have a little ways to go before I get somewhat accurate data, but one thing I found interesting is how much the washout affects the lift toward the wingtips at low angles of attack. The first picture is at 5.5 degree AOA and the second at 1. I have the washout of the wing set at -2 degrees. I'll have to measure the actual washout on Avid wings, but I think I have it close.

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