increasing gross

[kodi]

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

[Av8r3400]

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?

[rogerh12]

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

[cap01]

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 .

[rogerh12]

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.

[HighWing]

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

[Av8r3400]

My question remains if the spars are the same from III to IV.

[av8rps]

The AVID STOL wing is essentially the same length as the Kitfox Speedster, so if you do clip the wings in your Kitfox the difference won't be as noticeable as one might think. Many people have found over the years that they like their Kitfox better with the short wing. The only time it won't work out all that well is if you decide to fly floats, or you have a Kitfox that is very heavy to begin with.

If I had a Model 3 and wanted to increase the ability to haul more I would just put on the larger 1200 wing struts, make a few gussets to reinforce the carry through spars in the cabin, and then build a 4130 flat "strap" that will go on the bottom of the fuselage between the wing strut attachments (providing significant additional strength to support the tension when the wing are loaded positively). And last, Id' put on one of the newer landing gears that are available. And if I did decide to cut down the wing, I'd make use of the old tips by making them be quick install wing extensions. That way I'd always have the long wing if I needed (or wanted) it.

Many stock Model 3's over the years have been flown on floats, and almost always they are flown over 1200 lbs. So while the factory can't tell you it is ok, those 3's on floats prove the Kitfox is stronger than one would think. And as far as I know, I've never heard of a failure of a Model 3 due to overloading. It might be worth your while to check NTSB data on that to be sure...

[Dave F]

The AVID STOL wing is essentially the same length as the Kitfox Speedster, so if you do clip the wings in your Kitfox the difference won't be as noticeable as one might think. Many people have found over the years that they like their Kitfox better with the short wing. The only time it won't work out all that well is if you decide to fly floats, or you have a Kitfox that is very heavy to begin with.

If I had a Model 3 and wanted to increase the ability to haul more I would just put on the larger 1200 wing struts, make a few gussets to reinforce the carry through spars in the cabin, and then build a 4130 flat "strap" that will go on the bottom of the fuselage between the wing strut attachments (providing significant additional strength to support the tension when the wing are loaded positively). And last, Id' put on one of the newer landing gears that are available. And if I did decide to cut down the wing, I'd make use of the old tips by making them be quick install wing extensions. That way I'd always have the long wing if I needed (or wanted) it.

Many stock Model 3's over the years have been flown on floats, and almost always they are flown over 1200 lbs. So while the factory can't tell you it is ok, those 3's on floats prove the Kitfox is stronger than one would think. And as far as I know, I've never heard of a failure of a Model 3 due to overloading. It might be worth your while to check NTSB data on that to be sure...

Good points Paul, and I wil add thast many 1050 Kitfox have been flown on float s over 1400 lbs.

So ask yourself-- would you want to start with a 600 lb plane and fly it at 1400 lbs

OR
a 1000 lb Kitfox and fly it at 1800 to 2300 lb gross?

Simple solutiohn build em light ands enjoy what they got to offer.

[Dave F]

does anyone have info on the model 1 that had a fuselage failure ..
was not too widely known.. very early tube between the gear was too light.