why the 850 gross weight for a model 1

[dalords]

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

[t j]

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.

[av8rps]

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...

[kitfox5v]

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;)

[av8rps]

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


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;)

[DesertFox4]

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

[av8rps]

Thanks Steve. I try.

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

[kitfox5v]

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:)

[DesertFox4]

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.;)

[cap01]

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 .

[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.

http://i237.photobucket.com/albums/ff12/KitfoxSteve/85DD/441BAED1-BEDA-4E32-96D5-FCF7A76856F2.jpg (http://s237.photobucket.com/user/KitfoxSteve/media/85DD/441BAED1-BEDA-4E32-96D5-FCF7A76856F2.jpg.html)

http://i237.photobucket.com/albums/ff12/KitfoxSteve/85DD/538ACED5-97CB-4ACD-90DA-141EA285817E.jpg (http://s237.photobucket.com/user/KitfoxSteve/media/85DD/538ACED5-97CB-4ACD-90DA-141EA285817E.jpg.html)

[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!!:D

[av8rps]

I admire you for keeping such a piece of kitfox history active, but it also sounds like its not that hard of a job for you to fly it regularly :D

I'd love to see a shot of those stickers on the cowl. I'm a nut about the history of the Kitfox and Avid, since I own the first Avid Flyer that Dan Denney and Dean Wilson used to get this whole thing started. I've been flying these kind of planes since 1986, and I just never seem to grow tired of them. As you already know, they are just so much fun.

Oh, and I love that pic of 85DD in the mountains with your tent pitched next to it. That one picture says it all about how capable, fun, and versatile these Kitfoxes are.


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!!:D