Kitfox 4 1250 lbs gross is crazy ?????

[rogerh12]

Howdy all;

I just got a Kitfox model 4 (1200 classic) in an estate sale, and was thinking about building it with a FAA gross weight of 1250 lbs. Is this nuts? I figured I could install the heavy duty spring landing gear, the larger elevator and the stronger rudder pedals and then call it good and operate it at 1250 lbs. gross in the Normal category. I say normal because 1250 lbs. X 3.8G x 1.5 safety factor would put a load of 7125 lbs. on the airframe, which current is listed with an ultimate of 6G at 1200 lbs., or 7200 lbs, so it would be safe (I think). Please feel free to voice any and all opinions on this.

Roger

[t j]

rogerh12 said:

I figured I could install the heavy duty spring landing gear, the larger elevator and the stronger rudder pedals and then call it good and operate it at 1250 lbs. gross in the Normal category.


Please feel free to voice any and all opinions on this.

Roger, your logic for increasing the maximum gross weight does not convince me to try it.

[HighWing]

A thought on landing gear. My opinion is to not count in heavier duty landing gear to do anything to increase max gross wt. In the day, I put Hammerhead gear on my Model IV. This was from a company, just like Avid and Kitfox that was once part of the Grove organization - partners, in fact. Hammerhead at the time was offering the gun drilled brake lines free while grove provided a groove in the trailing edge of the gear to glue in the brake line with a hundred bucks or so extra for the gun drilling. Being frugal (cheap).... The point, Hammerhead used 7075-T6 aluminum while Grove used a softer alloy - can't remember now which. Grove now uses 7075-T6 - and this is my opinion only - to eliminate the cost of the free re-bends and heat treating after hard landings they used to do - again my opinion only. The point again, 7075-T6 will not bend, yes it will flex, but in a hard landing if you exceed the flex, the fuselage will bend -as the suit guy says, "I guarantee it". I had an emergency landing three years ago in my Model IV. The landing gear survived unscathed except for scratches in the paint (and I suspect someone is now flying with it). The fuselage was destroyed (see picture). It was like breaking a stick over your Knee.

A week in the hospital, but we are now fine and I'm back to building another.

[jdmcbean]

Howdy all;

I just got a Kitfox model 4 (1200 classic) in an estate sale, and was thinking about building it with a FAA gross weight of 1250 lbs. Is this nuts? I figured I could install the heavy duty spring landing gear, the larger elevator and the stronger rudder pedals and then call it good and operate it at 1250 lbs. gross in the Normal category. I say normal because 1250 lbs. X 3.8G x 1.5 safety factor would put a load of 7125 lbs. on the airframe, which current is listed with an ultimate of 6G at 1200 lbs., or 7200 lbs, so it would be safe (I think). Please feel free to voice any and all opinions on this.

Roger

Not a safe plan.
Ultimate load is where structual damage occurred. So loading for +3.8 and -1.5 SUSTAINED requires essentially +6+and -3 Ultimate to allow for the FAR safety requirement. (not necessary in our application.. but a great guideline)

Also, keep in mind that the Model IV is the final evolution of the Model 1..

So the Gross weights have increased from:
Model 1 - 850
Model 2 - 950
Model 3 - 1050
Model IV - 1050
Model IV - 1200

And before anyone asks.. no... one cannot increase the gross weight of their 950 model 2 to 1050 or the 1050 model IV to 1200. There was never an "Approved" modification to allow the changes. Yes there was a written document by Denny Aerocraft that allowed a 1050 Model IV to 1200 when operating on floats only... certain changes needed to be made.. and when back on the gear it was a 1050 gross aircraft. To the best of our knowledge that modification was never structrually tested.

Before the Series 5 came into existance.. The Model IV was evaluated for another change not only in gross weight but also to allow for larger (heavier) power plants like the Continentals... It was determined that the IV had reached a evolutionary limit and would require to many changes to accomplish the desired changes and a clean sheet design was started for the Series 5.

There are other major factors in gross weight calculations.....

So while some have "gotten away" with flying over the recommended limits it is not a safe practice. and that holds true for Cessna's Luscombs, Pipers etc...

That being said.. The builder is the manufacture and most likely the test pilot.

[rogerh12]

John, thanks for the quick response and insight into the Kitfox family. I agree the models 5-7 do make a better platform for a heavier gross weight, but if you try to fit them into sport category 1320 gross, the price to get them light enough to fit goes way up, and the usable load goes way down with respect just modifying a model 4-1200 to handle a somewhat higher gross weight (or so my calculator seems to be telling me).

Of course as an experimental, the Kitfox 4-1200 does not need to comply with FAR Part 23. But just for an example, if the Kitfox was designed to comply with the FAA specification for NORMAL category, it would need to operate at 3.8g’s, and be built with a safety factor of 1.5 (as outlined in FAR 23.303), which corresponds to a ultimate positive G load of 5.7.
However, the Kitofox model 4-1200 has a specification for the ultimate load, specified as 6 g’s. This is what led me to believe that the Kitfox model 4-1200 could be built with a Gross weight of 1250 lbs, and still be operated safely and within FAR design loads for the normal category. Having said that, features of the airframe should be upgraded I think. Due to additional loads on the landing gear and tail, spring gear should be used and the larger elevator installed and upgraded horizontal struts installed. Of course, a gross weight increase of 50 lbs translates to only about a 4 % increase in total gross weight of the aircraft, however, any known weak areas of the design (gear and tail) could show accelerated degradation and I think the larger elevator is required to overcome the increased inertial load (pitch control). But why get the Kitfox certificated with a higher gross weight at all? Well for one, an accident in an over gross weight can void you insurance claim. That’s one good reason, two, no Kitfox has every broken up in flight, so is it really designed on such a fine 1200 lbs edge, and not one pound more?

Having said all that, am I still crazy to even contemplate this, or can reasonable modifications to the airframe create an airframe capable of handing the additional loads?

[enyaw]

Not a safe plan.



I'd note that these were the first words out of John's mouth (figuratively).

If 50 lbs makes the difference between go and no go, drill lightning holes whenever possible, make build choices based on wt. (engine, avionics, interior, paint scheme, etc). I'm sure there's 50 lbs to be had somewhere (cumulatively). If it were me I could save 15 or 20 lbs by dropping my excess wt.

[Av8r3400]

Roger-- It sounds like you've done a lot of thinking on this.

How did you come up with the 1250# requirement, anyway? That is so close to the "unmodified" 1200, maybe we can help you trim down to that number. :)

[rogerh12]

Steve;

Actually, that number is to offset the increase in empty weight caused by changing to spring gear, which I prefer, and also to change to a Tri-gear configuration, which adds weight via the nose gear install. Also, though I have the option of using the Rotax 912 I am also considering the Great Plains VW ( 2300 cc direct drive version) as they have a complete firewall forward offered, I have used their engines before and of course you can rebuild one for only about $450 in parts (vers Rotax $8000 I am told).

Roger