Load Factors for Kitfox 2

[mclayton]

I am trying to find out the load factors for the basic Kitfox 2: Ultimate Load, and Limit Load. I have read in several places that the Ultimate Load is 6 G. Assuming that means the load at which the structure will fail, I wonder what the limit load, the load that should not be exceeded without incipient structural failure is.

Does anyone have this information? Does anyone know where to find it?

[Dravenelle]

same for me
i see 6G ultimate only

David

[SkyPirate]

don't carve this in stone but I'd say 3G to be safe,..my old model 2 I did some mild aerobatics in it,..loops spins ,,wing overs ,,nothing major but I always kept it positive.

Chase

[mclayton]

Thanks for the replies. In general, I found after going back to some engineering texts, that Ultimate Load is the product of the Working Load and the Safety Factor. So if you pick a safety factor of 2, and an ultimate load of 6G, the working load is 3G. Generally, the structure will not fail for at least 3 seconds when the ultimate load is applied, and the idea is that the working load will not cause failure or permanent deformation of the structure. I would say that a safety factor of 2 is conservative.

[airlina]

Mike, I built a Series 5 and the load limits published for this model Kitfox are +3.8g and -1.52g with the ultimate load limits set 150 % above these at +5.7g and -2.28g. Can't tell you about the earlier models but on the Series 5, the factory back then said they wanted to produce an airplane that could be certified under part 23 standards which is where these load limits come from on normal category aircraft. Bruce N199CL

[mclayton]

Thanks for the info. I can't seem to find anything on the Model 2, other than opinions. An ultimate load of +6g would be similar to what you reported for the Model 5, and might represent a set of design requirements for all the Kitfoxes. I just don't know at this point.....

[SkyPirate]

The differences between the model 2 and the 5 as for construction of spars, the 2 has extruded i beam thru center of spar,all the way thru, the 5 has a builder inserted i beam at the strut mount , and i believe the diameters of the spars is different, the 2 is a little smaller? Not positive on that.
I do know as i said previously i have done mild aerobatics in my model 2, no passengers, the most "G" load positive i put on it was at the bottom of a loop, with out a G meter on board, keep in mind my speed was held below 75 mph , and i'd be willing to say i reached 2.+ G but not exceeding 3G, calm air always for aerobatic anything in the 2, as for thermals and G loads, the wings design reserves any rock hard thermal bumps in the sky, you get lift and allot of it to the point you will see flex in the wing outward of strut mount location, at worst 1" deflection , which isnt allot considering the distance from the wing tip back to the strut, and with proper washout the load is distributed evenly between fore and aft spars,

What's your plan that your concerned with the "G" loads?
If you would like to see an example of how much these type spars can flex under a load, there are 2 photo's in my profile photo album of a round spar load test, the spars in photo are smaller the kitfox spars

[mclayton]

I have several concerns...

First, I am rebuilding the wings, from scratch. This is because of the damage done to the aircraft in the accident by the previous owner. The leading edge spars were basically destroyed, and so I managed to get two new ones from John McBean. I also purchased the I-Beam inserts for these spars.

Having said this, the rear spars, which were OK, do not have the I-Beam insert, but rather a Z-shaped aluminum insert, which was riveted to the spar, as did the original leading edge spars. I understand that this was the technique used on the Model I, and in conversations with John, he indicated that his recollection was that the early Model II's may have been in a state of evolution from the Model I, and so could have used this type of construction. The data plate, by the way, indicates that the plane was a Model II. The aircraft was built by Richard Wagner, the founder of Wag-Aero.

I am not sure how to install the new inserts (where in the spar they need to go, and how they need to be fastened to the spar). Also, I have manufactured twenty new ribs to replace the old ones which were in very bad shape. I made some minor mods to these as well to increase the strength and to some extent the stiffness of the ribs, at a very small weight penalty.

Having said all of this, I was curious about the original design loads that were used, more to better understand the basic capabilities of the structure at the gross weight of 950 lbs. There just is not much info on the Model II. I also have the option of increasing the diameter of the lift struts as I go forward. I have already increased the diameter of the drag/anti drag braces from 3/8" to 5/8 inch, and am using the current fittings to fasten them to the spars, rather than squeezing the tube ends in a vise and bending them ( which seriously weakens the strength, and provides stress concentrations at the joint, which is bad). I can continue to make changes to the original design, but each change will increase the weight, even if by a small amount. Don't want to get carried away...

[SkyPirate]

Sounds like you have your plate full, from what your saying about your re build the specs on a model 2 wing , structurally, would not be an accurate example to go by, if your insert is an "I" beam versus the "Z" you have already strengthened the spar just because of the configuration of the insert, with a "Z" the weakest point is the radius or the back side of each angle of the bend, the "I" beam being equal in pressures implied at the top and bottom of the "I" less chance of distortion with exception to the "Web" , larger compression struts and diagonal braces would increase the wings longitudinal strength but will not increase the wings VNE, as long as your not modifying the airfoil your good with published VNE numbers , all said its still only as strong as it's weakest point, which in my opinion without the strips of 4130 inside of the rear spar pivot point , im not sure if my model 2 had them or not, it's been over a decade since i had that plane, if they are not standard on the model 2 , going by what you have or are doing now, i would install them as well, but keep in mind your putting 2 different metal materials aside each other and take steps to avoid electrolysis between the 2 dislike materials

[t j]

I am not sure how to install the new inserts (where in the spar they need to go, and how they need to be fastened to the spar). Also, I have manufactured twenty new ribs to replace the old ones which were in very bad shape. I made some minor mods to these as well to increase the strength and to some extent the stiffness of the ribs, at a very small weight penalty.

The proper dimensions and hardware for installation will depend on what you have.

For example, I rebuilt a model 4 wing a few years ago and at that time skystar only supplied one insert...a bigger and heavier one that in 1997 was supplied for the model 3, model 4-1050, model 4-1200, XL, Speedster, Classic IV, and pre series 5 Vixens.

If your strut to spar brackets are located in the same places on the spar as those of a model 3, I could send you the instructions that came with my insert kit. they have model 3 specific instructions. You probably should check with John Mcbean to be sure what you have first.

If it helps when you ask John, the instructions I have are part number 69256.000, Instructions; Spar Insert; M3-C4,
Spar Insert Kit part number 69255.000