Getting 1200lbs out of a 1050

[pileofgeorge]

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?

[Av8r_Sed]

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.

[pileofgeorge]

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.

[Esser]

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.

[avidflyer]

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

[PapuaPilot]

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.

[pileofgeorge]

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.

[av8rps]

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 :eek:..)

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.

[HighWing]

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.

[pileofgeorge]

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.

[WWhunter]

Are you flying insured? Do you think your insurance will cover you if there is an incident and you are a little over weight? Granted, people do this all the time and get away with it but it is a question you have to answer yourself and be able to live with that answer. Yes, sounding like negative ninny, but as they say $hit happens!

I have been in this game long enough to realize it is generally much more cost (and time) effective to sell what you have if it doesn't fit your mission and buy something that does. Your wallet and conscientious will thank you if there is a problem. It seems that once one starts modifying things, it is a never ending slope. Modify one place and another area needs it also...on and on... JM2C.

[redbowen]

I agree with Hunter. Take the opportunity to upgrade to an airframe that is designed to meet your mission.

[pileofgeorge]

I agree with Hunter. Take the opportunity to upgrade to an airframe that is designed to meet your mission.

This airframe has everything I need with the exception of the gross weight. When I started poking around, I found that the original owner has already reinforced the carry through tubes so I'm going to go with the reinforcement of the spars. I'll be able to get it insured at 1200 (for a higher premium) when I'm done building. Thanks for the advice though. I'm just not ready to sell my current project and start the whole plane buying process all over again.

[cap01]

in the builders log of the kit i obtained partially completed the original owner mentions mods that he had made to increase the gross weight to 1250 . the struts have been replaced with a larger diameter rod . a solid rod was installed inside the cross fuselage tube between the lower strut mounts . i have talked to kitfox and they don't endorse the weight increase mods . the other weak area that does fail is the tubing in the area of where the bungees attach. i have the grove gear so that eliminates that weak area . i have seen reference to the mods being done for use with floats which also wouldn't use the bungee attach points

[jdmcbean]

For clarity sake:
The Model IV 1050 gross to 1200 gross modification mentioned and apparently in your logs was offered by Denny Aerocraft and was done for float operations ONLY.
Once floats were removed and returned the gear it did not apply and was once again a 1050. This was equivalent of adding the 10-15 percent gross increase when on operating on floats... very similar to what happens on many of the certified aircraft. This has been true to my personal knowledge since 1999. This does not apply to the Model 3.
There is no test data to substantiate this modification. We also have not been able to find any aircraft "DATA" to support the automatic increase of gross weight by 10-15 percent because it is put on floats.

The mod offered by Denny Aerocraft was to allow the 1050 gross to operate at 1200 when on floats ONLY.. Please note that it was 1200 not 1250.

All that being said.. it is an Experimental. Please be safe!



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[cap01]

thanks for the clarification, been trying for a long time to get the straight poop . the 1250 was mentioned in the builders log entered by a previous owner.

[WWhunter]

Thank you for posting that Mr. Mcbean!! I see too many people doing modification that they 'heard' were a fix for their problem. Granted, it may have worked for someone, until it doesn't. I see the 'it won't happen to me' attitude all the time.
What I can't understand are the guys trying to stretch the limit of something that is carrying their very life into the air. Sure, go ahead and take all the chances you want by yourself, but the first time you are carrying a passenger and something happens, you better be prepared for the ensuing lawsuits and condemnation that will come!
Just my opinion!!!

[snowpaw]

Does anyone have the instruction and pictures (both rod through carry through tube and strap between strut attach points) of the modification from 1050-1200 for float operation?
The reason for asking is that I intend to operate the plane on floats and would like to make it as rigid as possible, not to exceed the gross weight limit.