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Thread: increasing gross

  1. #1

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    Default increasing gross

    anybody know what mods would be required to increase the gross on a model 111 from 1050 to 1200 utilizing the same wing?

  2. #2
    Super Moderator Av8r3400's Avatar
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    Default Re: increasing gross

    This is not really a practical conversion.

    The wing is different (spars and ribs) as is many of the tubes in the fuselage and lift struts in order to carry the weight. Why do you want to make this change?
    Av8r3400
    Kitfox Model IV
    The Mangy Fox
    912UL 105hp Zipper
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  3. #3
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    Default Re: increasing gross

    Kodi;
    There are lots of factors that need to be taken in consideration when increasing the gross weight of an aircraft. Here are just a few:
    1) Will the currently installed engine provide enough power for a decent climb with the higher gross weight on a hot day at your altitude?
    2) Will the landing gear handle the extra load at the higher gross weight?
    3) Will the change in G-loading on the wing reduce the designed in safety factor below acceptable minimums?
    Item 3 can be compensated by shorting the wings, which reduces the stress on the wings. For example, Denney stated that reducing the wingspan on the model-4-1200 by 3 feet would give it “aerobatic class” positive G loading (9 G ultimate) at a gross of 950 lbs, which works out to something like a new gross weight of 1400 lbs, assuming the original G loading safety factor is maintained (6 G ultimate at 1400 lbs). However, cutting down the wing will reduce the rate of climb and also increase the take off & landing speed, which requires further consideration of items 2 & 3 above.
    Having said all that, though the numbers work out on the calculator, cutting down the wing of a model 4 and going from a gross of 1200 lbs to 1400 lbs is a BIG change. However, cutting it down, upgrading to spring landing gear and going to a 1250 lbs gross (adding 50 lbs), that’s a much more reasonable change (4% increase in gross).
    Hope this helps
    Roger

  4. #4
    Senior Member cap01's Avatar
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    Default Re: increasing gross

    the model IV that i have is very early , 1993 . the first owner went to great lengths in the builders log to discribe how he had accomplished the mods to increase the gross weight from 1050 to 1200 lbs . the wing struts were replace with 1 in tubes . the lower wing strut carry thru tube has a solid rod installed inside it . the bungee gear was replaced with the grove gear . the web where the bungees attach is definately a weak point and common to have failures in that area , naturally the grove gear doesnt use that .
    i have never gotten the true story about these mods . weather denny offered them or the builders just came up with them on their own . the mcbeans wont approve of them .
    chuck
    kitfox IV 1050
    912ul warpdrive
    flying B , yelm, wa

  5. #5
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    Default Re: increasing gross

    Some of us have over grossed our planes on a regular basis (you know who you are), and though the insurance companies would disagree, if it’s a calm day and not too hot and the CG is ok, you will be fine. The Kitfox is not going to breakup in flight unless you over gross it AND do aerobatics. But, if you do have an accident and it has been over grossed, the insurance company might deny the claim. That’s one good reason for having a higher stated allowable gross weight on the paper work, even if you don’t intend to ever fly it at that gross weight on anything other than calm days. However, if you sell it, the buyer might not be so careful and then the higher stated gross weight might become a real liability issue.

    The solution? If you do have to sell it one day you could deregister the aircraft and sell it as parts, but you will take a major hit in the pocket book.



  6. #6
    Senior Member HighWing's Avatar
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    Default Re: increasing gross

    The mods Cap01 mentions are exactly what I have heard over the years - Lift struts and the rod through the carry through. The current owners haven't been too supportive of the idea, but I suspect the focus there has to do with they were not around when these mods were being done. Haven't heard mention of the landing gear issues until now though. In this case, however, I seem to inderstand that the upgrade to 1200 would include the existing Model III wing as part of the finished airplane. Every upgrade I have heard of had the Model IV wing. To me that would be the real question.
    Lowell
    Last edited by HighWing; 11-23-2011 at 01:42 PM.

  7. #7
    Super Moderator Av8r3400's Avatar
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    Default Re: increasing gross

    My question remains if the spars are the same from III to IV.
    Av8r3400
    Kitfox Model IV
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  8. #8
    Senior Member Geowitz's Avatar
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    Default Re: increasing gross

    Not sure if this helps or creates more questions, but my manual mentions newer longer spar inserts for all models. Here is the note...

    "Beginning in September 1997 Skystar began supplying a longer, stronger spar insert with all wing kits for the model 3, model 4-1050, model 4- 1200, XL, Speedster, Classic IV, and 1200lb gross weight vixens(pre series 5). Because of this change, the wing assembly portion of the manual which was supplied with your kit may not reflect the proper dimensions and hardware to use when installing the insert. Therefore, this supplement is to be used when installing these inserts.

    Distance from root end to spar insert (listed as part # 23900.000)

    Model 3 = 73 1/4
    Model 4-1050 = 73 1/4
    Model 4-1200 = 73 1/4
    Model 4 Speedster = 74 1/4
    XL = 73 1/4
    Vixen 1200 = 70 3/4"

    So...does this imply that at least after September 1997 the spar inserts are the same for all models? Were they the same before?

  9. #9
    Senior Member t j's Avatar
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    Default Re: increasing gross

    After 1997, Yes. The same before, no.
    Tom Jones
    Classic 4 builder

  10. #10
    desertfox1's Avatar
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    Default Re: increasing gross

    My early Vixen 1200 came with the light spar inserts and .058 spars.
    Shortly after that the spars were shipped as .065. I would assume
    model 3 and early 4's had the lighter spars, as well as the light inserts.
    Anyone know the spar wall thickness on the 1050 model 4's?

    Phil

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