why the 850 gross weight for a model 1

[av8rps]

I once saw an Avid that was strengthened by welding that 4130 strap to the lower tube between the strut attachments, (for tension) putting another welded 4130 strap across both carry through tubes in the cabin (for compression), reinforcing the truss under the seat and forward of seat to the door frame (like you see on new kitfoxes), and beefing up the upper longeron tubes right behind the rear carrythrough spar (for compression). He also added gussets to the outer ends of the rear carrythrough spar. He said he had it computer analyzed by some company, and said the mods would safely increase the 1150 plane to 1400. But fwiw, he already had the big spars and struts, and the stronger outer wing strut brackets that mule williams sells. Cool thing is the Avid only gained 16 lbs.

But for me, before going through all that I would rather just operate the plane as it was designed (staying with design gross), or I would move up into a higher gross weight model and sell someone my old lightweight hotrod...

In spite of the weight carrying limitations, the early, super light Kitfox and Avids were a blast to fly! Everything is a tradeoff.

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 .

[SkySteve]

I once saw an Avid that was strengthened by welding that 4130 strap to the lower tube between the strut attachments

I got to thinking about this and since my model I has model III wings (installed by Dan Denny), I thought I would check to see if he had installed any cross tubing under the belly to add strength. None there, BUT, when I inspected it I realized the Grove main gear (for a model III), does, in effect, provide this mod by attaching both the front and rear attach points and the wing strut attach points to the main gear, across the belly of the plane to each other. See photos below (don't dog me too much about how dirty the belly of my plane is. It gets a lot of "dirty" on it where I land. Remember my plane is set up in the tricycle configuration so the main gear is in the aft position.

[dcsfoto]

one big thing is the tube that connects the wing spars "carry through spar"
has a thicker wall on the higher gross weight aircraft

[SkySteve]

one big thing is the tube that connects the wing spars "carry through spar"
has a thicker wall on the higher gross weight aircraft

Good point. I've discussed this with my A&P. I need to/want to replace my windscreen and in discussing this with my A&P, he has suggested that when we replace the lexan, we could, at the same time, weld an additional 4130 tubing to both the front and rear wing carry throughs, then cut out and replace the diagonal tubing at the top of the cabin, changing the design to that of the Model IV. His feeling is that would strengthen the cabin and wing root areas. Any thoughts?

[av8rps]

The spring gear does look to do what that 4130 strap did, but one would have to really look closely at how well the attaching bolts are installed to see if when the wings are subjected to high positive G loads, if the attaching bolts can handle the tension & shear loads, as I would think the gear attach is designed to handle compression loads. But I do agree that having a spring gear installed should certainly make the fuselage stronger in that area than would a stock bungee style gear. But it would really be interesting to see if a qualified aircraft engineer would agree with our theory.

I also agree with the comment about needing a much stronger carry through spar tube in the cabin so higher compression loads from the wings are handled as well as the tension loads are on the lower part of the fuselage.

I'm convinced you can modifiy older airplanes to higher gross weights, it just depends on how much time, effort, and expense you want to apply.

Unfortunately, when you stand back and look at an early model Kitfox and then you see a later model somewhere else, they don't look that different from eachother (especially a Model 1-4). Without having the different models side by side it is hard to tell how different they really are structurally. But I happen to own Avid SN1, another early Avid, a Kitfox 4-1050, a Kitfox 4-1200, a Kitfox Model 7 Super Sport, and a Highlander, so I've compared them to eachother many times. And I will tell you the differences are much bigger than most would think. It's very obvious even without having to measure things. That's the reason I said earlier that if I wanted more load carrying capacity, with the current availablity of Kitfoxes, I'd rather just upgrade vs having to tear my early airplane apart for all the mods it would take to make it as strong as the new models. But hey, if you have the skill, time, and money to do that, it certainly is do-able.

[SkySteve]

Av8rps,
All good points and I'm not really desirous of a weight increase. I've learned to pack well and live with what I have. And what I have is, well, what I have. My main point is that while I've done lots of mods to my plane and am very happy with them, as long as I have to tear something apart anyway, like replace the windscreen, I might as well make the plane stronger and hopefully better at the same time with just a little extra cost and time. I will say one thing though, when the time comes for me to sell ol' 85DD, someone is going to get one heck of a plane!

So, what the heck do you do with all those planes, anyway? Do you find them, fix them, sell them? Or are you a plane junkie and can't help getting just one more? No wrong answer here, there all good.

[avidflyer]

Steve, the first Kitfox project I got was a Kitfox 3 and it had a wing unfold while being trailered. Never did finish that plane, ended up robbing parts off it and eventually sold what was left. Included with that plane was a preassembled (welded) front and rear carry through plus the bracing between them. I believe it was bought from the factory. I think that might be less work if you were able to get that part from Kitfox. Cut your birdcage off, and weld the new one on and it would be good to go. Don't know if Kitfox sells this part now or not. Something to think about though. Jim Chuk

[SkySteve]

Thanks, Chuck. Good idea.

[av8rps]

Hi Steve,

I like your approach to upgrading your airplane, as well as your outlook about making the best with what you have.

Your Kitfox 85DD is a pretty historic airplane as I recall, as it is probably the oldest Kitfox in existence? Or is the original still around somewhere? Either way, I think it is really cool to still see it being actively flown.

What do I do with all those planes?? Hmmm.... not sure how to best answer that. I just always end up buying and never get around to selling is the best and most honest answer I can come up with. Plus, I do think I have a bit of airplane junkie in me. But for sake of conversation, lets just look at it this way; I have a lot of spare parts and its not too likely I will ever be airplane-less

Av8rps,
All good points and I'm not really desirous of a weight increase. I've learned to pack well and live with what I have. And what I have is, well, what I have. My main point is that while I've done lots of mods to my plane and am very happy with them, as long as I have to tear something apart anyway, like replace the windscreen, I might as well make the plane stronger and hopefully better at the same time with just a little extra cost and time. I will say one thing though, when the time comes for me to sell ol' 85DD, someone is going to get one heck of a plane!

So, what the heck do you do with all those planes, anyway? Do you find them, fix them, sell them? Or are you a plane junkie and can't help getting just one more? No wrong answer here, there all good.

[SkySteve]

Av8rps,
I don't know if 85DD is the oldest Kitfox around. I do know it was the first real factory demonstrator. It took first place in Oshkosh in 1985 and 1986, as the stickers are still on the side of the plane. Yes, it has a lot of good history. She flies great and gets flown at least every 10 days, usually more than that. Had her out in the dirt today and did 10 landings and I plan to fly tomorrow, then head to Scotsdale, AZ for a few days before attending the Copperstate Fly In!! You know what they say about perfect practice!

Good on you for never going plane-less!!