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

[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