KITFOX MODEL -4 AT 1250 Lbs Gross weight, Revised calculations !!!!!

[rogerh12]

Howdy all;

While at Oshkosh (which was great), I did some talking to other Kitfox guys and had a realization. A while back, I proposed building a Kitfox Model 4-1200 with a gross weight of 1250 lbs. I justified this because the current load specification of an ultimate wing loading of 7200 LbS (1200 lbs x 6 G’s) would become an ultimate wing loading of 5.76 G’s at a gross weight of 1250 Lbs, which still placed the kitfox within the “NORMAL “ category for wing loading as outlined in FAR Part 23. For example, if this was a certified plane and 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. As mentioned above, at a gross of 1250 Lbs, the model 4 would have an ultimate wing loading of 5.76 G’s. However, having said all that, a 1250 Lbs gross kitfox was kind of boarder line. But now I have a better idea:

I noticed that the model 4 speedster has a shorter wing, and that it is rated with an ultimate load limit of 9 G’s at an operating gross weight of 950 Lbs (for aerobatics). This corresponds to an ultimate wing loading of 8550 lbs. If I figure on using the shortened speed wing, the kitfox 4 at a 1250 lbs gross would now have a ultimate positive wing loading of 6.84 G’s, which places it well with within the NORMAL category with a safety factor of 1.8. In fact, with this level of safety factory, the kitfox 4 COULD be operated in the UTILITY category of 4.4 G’s, and still have a 150% safety factor.

Of course other changes to the airframe would be needed, most notably the landing gear.

What do you think, does this case hold water?

Roger

[DBVZ]

As the MFG of your kitfox, you can set the max gross. I saw an Avid Mk IV with the max gross at 1320 (with the heavy hauler wings). Many people increase the max gross on the W/B they submit, and have no trouble with the DAR in doing it. As for your calculations, I did not check that. But it sounds about right to me.

[jdmcbean]

Roger,
I am not sure where you are getting your load test data.. The Model IV-1200 has a 1200 lb gross weight limit as testing by the factory. They are rated at a +3.8 and -1.52 sustained loads at 1200. Being E-AB you are the manufacture of the aircraft and can establish the gross at 2000 lbs if you desire.. You may have to substantiate the test data to the inspector.
We are occasionally called by the FAA or NTSB asking what gross weight limits are on a given model. We have even been called by a DAR during an inspection.

The Kitfox was never "certified" or "tested" in the aerobatic category.

[rogerh12]

John;

Thanks for the input. A gross weight of 2000 lbs does sound good !!!

Ok, seriously, the load limits are outlined in a document I saw on a previous posting. It is from a Denney news letter outlining the evolution of the kitfox, starting with model 1 and ending with the model 4 speedster. This document outlines the speedster as bring “capable of positive G aerobatics, ultimate load limits of 9 G + and 4.5 G –“. This is what I am basing my calculations on. My bird has the grove gear, and model 7 noise wheel, so the gear should handle the extra gross weight increase.

The news letter is from Denny, so I guess it’s accurate, but the news letter is not dated and I only have a hard copy (anyone got a link to the downloadable version ????). A thicker windshield is the only other structural change noted, outside of the shorter wing.

Roger

[DBVZ]

If you have something that will scan and print-to-file as a PDF, you can do that and post the PDF. I use PrimoPDF for that (a free download), and it works well for scanned documents and forms that need to have a signature. Works for things I used to FAX, to people who do not have a FAX machine.