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Thread: why the 850 gross weight for a model 1

  1. #1

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    Default why the 850 gross weight for a model 1

    So what is the reason the model 1 gross weight is set at 850 pounds and the model 2 950 pounds? almost identical in construction as far as tubing size and diameter. So why the difference? possible because power to weight ratio with the model 1 coming out when really the only choice in engine being the Rotax 532. I have ready all the published information about changes made from model to model like wing spar wall thickness and the addition of the inner spars, but am curious as to how Denny aircraft got to the 850 gross for the model 1 and the 950 gross to the model 2 and 1050 to the model 4. Anyone have any info

  2. #2
    Senior Member t j's Avatar
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    Default Re: why the 850 gross weight for a model 1

    I would say it is not a power limitation. A 532 is a lot of power for that size plane. My Classic 4 has a Maximum gross limitation of 1050 with a 503. That is a power limitation, not airframe.
    Last edited by t j; 10-03-2014 at 06:20 AM.
    Tom Jones
    Classic 4 builder

  3. #3
    Senior Member av8rps's Avatar
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    Default Re: why the 850 gross weight for a model 1

    Here's the best explanation I can come up with for the 850 lb gross of a Model 1 Kitfox;

    The model 1 Kitfox was essentially a copy of the A-model Avid Flyer. So if you take the time to learn Avids' history, it will explain why the early Kitfox has a lower gross weight than the later airplanes.

    So here goes...

    In 1982 the original Avid Flyer flew for the first time. It had an original gross weight of 764 lbs. And while that sounds like a really low gross weight, the Avid A-model prototype only weighed 364 lbs empty. So even with that low gross weight it still had a 400 lb useful load! (That's 110% of its empty weight, which is pretty uncommon in light aircraft even today).

    In early 1983 the Avid A-model was made available for purchase to the public. A short time later the Avid Flyer was upgraded from the 43 hp Cuyuna to the 64 hp 532 Rotax, which raised its empty weight to right around 400 lbs. So with some eventual airframe improvements, the Avid Flyers gross was increased to 850 lbs. (112% of EW)

    By 1984 Avid renamed their new 850 lb version officially the Avid Flyer model B.

    In 1984 the Kitfox model 1 was introduced for the first time to the public. The only obvious difference initially between the Kitfox and the Avid was the Kitfox had a different shaped vertical fin and rudder. So ironic or not, the Kitfox when introduced had the same 850 lb gross weight as the Avid Flyer.

    And of course, as the Avid kept increasing their gross weight as their new model empty weights kept increasing, so did Kitfox. Both companies competed heavily for the same market, so the improved versions just kept rolling out. And I guess the rest we will say is history...

    So to address the next question (that I perceived as); "Can I operate my model 1 Kitfox at the 950 or 1050 gross weight since the newer airframes don't seem that much different?".

    My simple answer - I wouldn't. Even though I've seen Avid A-models on floats operated regularly 200+ lbs over design gross, there is a big difference between a Kitfox model 1 and an Avid Flyer A or B model. So how is that?

    It's actually pretty simple as you can see the difference between the airplanes. Think about how the wings are held onto the fuselage of any high wing strut braced airplane. And then think about when the wings are being loaded heavily how much force is involved, trying to pull the struts out of the fuselage. So now that you are thinking about those forces, find any Avid and look at the attachment point where the lower wing strut attaches to the fuselage. There you will see a very stout welded up box type structure made of 4130 steel, where the wing strut bolt goes through and holds the strut to the fuselage.

    Now look at any early Kitfox. There you will see a flat plate welded to the fuselage tubes that has a hole drillled in it for the wing strut bolt. There's no question the flat plate is a lot easier and requires less labor to build than the 4130 box structure Avid used. And the flat plate is nowhere near as strong. Even to a non-engineer the difference should be obvious. (fwiw - Kitfox improved that area with the model IV and up).

    And even though Avid had the stronger wing strut attachment, for operations at high gross weights (eg; like amphib floats) they offered an additional 4130 rod / strap that ran accross the bottom of the fuselage essentially connecting the two wing struts together, offering an additional level of safety.

    If I owned a model 1-3 Kitfox, I would spend the time and money to have a part like that made for my plane. Even if I wasn't operating overgross, gusts alone can put a lot of stress on that little flat plate holding the wings on. If nothing else that additional reinforcing would give me at least a little extra peace of mind... But that's just me.

    And for the record, I'm not trying to berate early Kitfoxes here. I just really think early Kitfox owners need to understand the reasons they should keep their gross weights within the numbers originally specified. And while that is always a good idea for any airplane, I believe it is more important than ever for the early Kitfoxes.

    Fly safe...

  4. #4
    kitfox5v's Avatar
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    Default Re: why the 850 gross weight for a model 1

    You've got it going on Paul. I have 4s and a 5 but I always pondered those questions. Now we know the rest of the story.Thanks Eddie
    Flying a series 4 speedster
    Rotax 912uls whirlwind 75”

  5. #5
    Senior Member av8rps's Avatar
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    Default Re: why the 850 gross weight for a model 1

    Thanks Eddie.

    And since you own one of each, isn't it amazing the differences structurally between the 4 and the 5?

    I have a raw Super Sport kit in my garage, and a couple model 4's. And one day I actually took the time to do some comparisons of the structures, and WOW! The difference between the models is huge. The new generation Kitfox is a whole new Kitfox...

    Paul

    Quote Originally Posted by kitfox5v View Post
    You've got it going on Paul. I have 4s and a 5 but I always pondered those questions. Now we know the rest of the story.Thanks Eddie

  6. #6
    Administrator DesertFox4's Avatar
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    Default Re: why the 850 gross weight for a model 1

    Very informative Paul. Thanks for taking the time to write up such a comprehensive reply, as usual.


    DesertFox4
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    Senior Member av8rps's Avatar
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    Default Re: why the 850 gross weight for a model 1

    Thanks Steve. I try.

    And hey, I met your brother Tom today. He and his buddy Mark were LSA shopping in my neighborhood

  8. #8
    kitfox5v's Avatar
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    Default Re: why the 850 gross weight for a model 1

    Paul,
    You are so right. I've got to build faster. The older I get, the more room I need. Middle age is harder to keep the spare tire at bay. The 5 will give me the room and extra lift. It has the long wings and will put a 912uls on it. Maybe 912is if they ever get the bug out of it. Thanks again for you info and time.
    Eddie
    Flying a series 4 speedster
    Rotax 912uls whirlwind 75”

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    Administrator DesertFox4's Avatar
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    Default Re: why the 850 gross weight for a model 1

    Thanks Steve. I try.

    And hey, I met your brother Tom today. He and his buddy Mark were LSA shopping in my neighborhood
    Paul, I got the phone call last night from Tom saying he and Mark met you and that he was getting serious about one specific aircraft over near you.

    We also discussed a set of amphib floats that are for sale. I told Mark to buy them right away and bring them to Arizona. Anyways, thanks for looking out for them. They can get into trouble on road trips.


    DesertFox4
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  10. #10
    Senior Member cap01's Avatar
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    Default Re: why the 850 gross weight for a model 1

    thanks for the kitfox story . I'm not the original builder of my plane but i did complete putting the kit together and first flew her . it appears that she is a III/IV 1050 with mods to up the g/w to 1250 . the first owner went to great lengths in his builders log to describe the mods that he made to increase the g/w . the tube that goes between the lower strut attach fittings was opened up and a solid rod installed in the tube to connect the attach fittings . the lift strut diameter was also increased . it was evident that the first owner had intentions of installing floats . other than what info that i have about the mods in the builder log , there isn't much out there about the g/w increase or who came up with them .
    chuck
    kitfox IV 1050
    912ul warpdrive
    flying B , yelm, wa

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