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Thread: Getting 1200lbs out of a 1050

  1. #1

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    Default Getting 1200lbs out of a 1050

    When I bought my fox, I made a few, well, compromises with myself on my requirements for the "perfect plane" as I also needed it to fit my budget. One of the compromises was the gross weight. I bought a kitfox with a 1050 gross weight. I'm now finding out that I will need to carry 2 people's worth of weight (I'm going to assume an average of 200lbs each) and a decent load of fuel. My KF weighs a mere 590lbs with it's rotax 582. The 582 should be able to handle it, albeit slowly. I'd be overweight if I were using the 1050 gross weight so I need to figure out the easiest, cheapest, most efficient way to boost the gross weight of the kitfox. I saw some guy on barnstormers who "upgraded the spars" to increase the gross weight but I don't know if that can deliver. So, I guess my question is, can the kitfox 1050 handle 1200lbs on a regular basis, or should I upgrade? If so, what's the best route to take?

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    Senior Member Av8r_Sed's Avatar
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    Default Re: Getting 1200lbs out of a 1050

    I have not done this myself, so... beware free internet advice.

    I don't believe spar strength is an issue. I have heard others have increased the size of the wing strut carry through tubes, Added a strengthening reinforcement to the lift strut carry through and increased the diameter of the lift struts.
    -- Paul S
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    Default Re: Getting 1200lbs out of a 1050

    Quote Originally Posted by Av8r_Sed View Post
    I have not done this myself, so... beware free internet advice.

    I don't believe spar strength is an issue. I have heard others have increased the size of the wing strut carry through tubes, Added a strengthening reinforcement to the lift strut carry through and increased the diameter of the lift struts.
    This isn't exactly a response to you, but anyone who works for (or owns) kitfox:
    Are the struts they sell from the company all 1200 struts? I'm thinking that the carry through tubes themselves won't be susceptible to failing with 1200lbs gross weight but I don't trust the spars. My aircraft hasn't gone through phase 1 testing yet so I'll use it as a time to test to see how the struts work with the original tubes.

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    Senior Member Esser's Avatar
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    Default Re: Getting 1200lbs out of a 1050

    I know you aren't going to like hearing this but the best route to take is probably buying a Model IV 1200. There is a reason there is a premium price for those models.

    The spar carry through tubes do need to be stronger as do the struts and I'm pretty sure the thickness of the spar if it the same as the Model 5 1400#GW to the 1550. If it were just as easy as changing the struts everyone would be doing it.
    ------------------
    Josh Esser
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    Default Re: Getting 1200lbs out of a 1050

    I'm pretty sure the spars used .065" wall thickness aluminum tubes on all the early Kitfoxes. I believe the latest 1500 gross weight Kitfoxes use .065" wall as well. Different spar stiffeners though maybe. JImChuk

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    Senior Member PapuaPilot's Avatar
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    Default Re: Getting 1200lbs out of a 1050

    If you want to get the correct answer call the Kitfox factory. John or Debbie will let you know if it can be done, what needs to be done and the cost do do it.
    Last edited by PapuaPilot; 05-17-2017 at 09:18 PM.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
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    Default Re: Getting 1200lbs out of a 1050

    Quote Originally Posted by PapuaPilot View Post
    If you want to get the correct answer call the Kitfox factory. John or Debbie will let you know if it can be done, what needs to be done and the cost do do it.
    Thanks for the input, I'll give them a call.

  8. #8
    Senior Member av8rps's Avatar
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    Default Re: Getting 1200lbs out of a 1050

    The easiest way to provide safety margin for more capacity is to make a strap from some flat 4130 that will run between the wing strut attach points on the bottom of the fuselage. Dean Wilson (original designer of our planes) suggested doing that on early Avid Flyers when they knew they were going to be flown heavy (typically on floats). A friend of mine years ago did it to his plane and after I saw it installed I could see how effective it would be.

    When you think about excessive positive g-loads due to added weight on an airframe, as the wings get loaded and want to move upward, the wing strut attachment points on the fuselage is where most of that force is pulling. So if a failure were to happen there the strut would tear away from the fuselage, collapsing that wing (making for one very bad day ) but with that strap under the fuselage and attaching both struts together in effect, about the only thing that could happen then would be if the forces were so high that the strap would start to crush the bottom of the fuselage. But the struts would still be attached to the fuselage, so your wings wouldn't collapse. (You'd have one messed up Kitfox, but you'd have a much better probability of getting back on the ground than the alternative of trying to fly with just one wing ..)

    In my opinion the most probable failure areas in order would be the strut attachment area on the fuselage, then the rod ends where the struts attach to the wing, and finally the wing strut itself (and then only if rusted excessively at the bottom). The least likely failure area would be the wing spar. There have been a bunch of 1050 912 powered Model 4's (weighing typically 800 empty) flown on amphibs for years with no issues, and they didn't even have the safety strap. The 1st company demo Kitfox amphib was a 1050.

    The Model 4 is a pretty strong little airplane, so with that safety strap installed and some good common sense (like operating under max structural cruising speed in turbulence and doing all you can to keep operating weight's within a reasonable range), I personally wouldn't be afraid to operate a little heavy. But again, do what you can to minimize excessive stress to the airframe.

    With all that said, I just told you what the factory never will because they can't. We live in a litigous society so it would hardly be in their best interest to promote flying their airplanes over the designed gross weight. And some here may disagree or even flame me for my opinion. But I've been flying these airframes for over 3 decades and can tell you I'm ok operating them 10-15% over. But again, when I need to do so I do all I can to minimize airframe stress. I hope this helps.
    Last edited by av8rps; 05-18-2017 at 05:38 PM.

  9. #9
    Senior Member HighWing's Avatar
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    Default Re: Getting 1200lbs out of a 1050

    I agree wholeheartedly with Paul. Back in the day when the factory was still producing the IV, a good friend and mentor lived a stones throw from the factory. In a personal conversation there, what Paul mentioned is essentially what he was told. The difference was that the "suggested" method was to drill a hole on each side centered on the carry through tube and inserting a rod through both holes. As I recall he was told to weld the rod on both sides. I recall others using this method, but threading the ends and using a nut to secure the rod ends. Sadly, I don't recall the rod diameter, so can't pass that Info along, but someone might chime in on that. One way to approximate what would be needed would be to calculate the cross section of the 4130 in the tubes - 1050 and 1200 models and add that in the rod + a margin.
    Last edited by HighWing; 05-18-2017 at 08:54 PM.
    Lowell Fitt
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  10. #10

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    Default Re: Getting 1200lbs out of a 1050

    Thanks for all the support so far. I think I'm going to use the strap, and when enough information is available, I'll reinforce the carry through tubes. I think my struts themselves are fine as they're pretty much brand new. (Still 1050 struts though... ) and I'll give Kitfox a call when I've got the time to see what they have to say. If anyone has any more information, I'm very interested and would love to hear it.

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