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Fuselage landing loads (Kitfox IV) and cabane gear

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Posted

I have been working on my Kitfox IV project and am getting to the point where I will be making my cabane style gear. First of all, it will be about 2 inches taller and 84" wide to the outside of the wheels. I've been reading up on the reinforcement people have done to the fuselage. Most have added shear plates between the tubes. I don't really like that solution for several engineering reasons so I set about to do a bit of loads analysis. No numbers, but ....

In the first picture is the stock gear as mounted on the airplane. A hard landing (AKA dropping it in) will create primarily a vertical load if the brakes aren't applied. You can see that with the gear about 4 inches ahead of the front mount that the vertical load is resolved by 2 vertical loads and moments about the attach points. Since the forward mount point is in the middle of the door sill it puts a bending moment into the sill and if hard enough bends the door sill truss as we have all seen. The "fix most have used is to do the shear webs. they probably help a little, but shear webs only work in the tension direction so won't help with crushing  of the vertical tube above the front mount, and the original diagonal tubes are already oriented to take the load in tension. Bottom line is that the web will make the sill more resistant to bending, but the benefit in my opinion is minimal.

Now a tube on the other hand can take tension or compression, so adding opposite direction diagonals like the green lines in the second picture would do a few things. First, they would create an additional load path to the front and rear ends of the truss, getting the load directly into the meat of the fuselage structure. Second, the added diagonals will also make the entire sill area more resistant to bending. Third, instead of the one vertical tube at the forward mount taking all of the vertical load, there are two more load paths, alleviating some of the load on the vertical member. 

Going further, many of the aftermarket gear mount to the stock  locations, so any problems with the main gear are still there. I started looking at the mounts on the side of the fuselage and at least my airplane has the float mounts, including one forward of the stock mount at the next tube cluster forward. In the third picture I've sketched up a gear that mounts at the stock rear mount and that forward float mount. Using these 2 mounts puts the load directly into the fuselage at the forward and rear ends of the door opening, thereby taking all of the bending load out of the door sill. It also puts the landing load vector between the mounts instead of cantilevered forward of both mounts. This should help the front gear tube loads.

Finally, I took modified an orthographic projection of the airplane to show the stock gear and a cabane gear using the float mount. Personally I think the gear using the forward float mount looks better. so that is what I'm going to do. 

Stock gear.png

Stock gear mod.png

My Gear.png

3-view Model (1).png

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Posted

Actually, thinking about  it more, the moments with the stock gear will be resolved wit an upward vertical force at the forward mount and a downward force at the rear mount. In other words, the rear gear tube will be in tension and the forward gear tube will be in compression. This is even worse for bending in the  sill area.

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Posted

The other loads to consider is where the mass of the airplane is applied to these forces.  The engine is fairly far forward and could have a substantial lever arm with the stock gear.  Moving the attachment point forward will help with that.

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Posted

My thought is by moving the front mount forward you move to a position of less strength.  From my experience on my Avid there was a failure at the rear mount that caused/added to, the failure at the front mount.  My solution was to move the rear mount to the rear attach point of the tricycle gear at the wing strut attach point when I built my current gear.  Also installed the Roberts Rage Gear gear strut with the new gear and several other design changes.  This has proven itself by surviving an estimated 30 MPH forced ground loop to keep from going off a 15-20 embankment.  The plane did a 180 degree turn pivoting around the right gear when I stomped on the right brake. The only damage was to my nervous system, the plane was undamaged.

the pictures are of the gear after installation.  There was no engineering in the design just seat of the pants design from the school of hard knocks.

IMG_0878.JPG

IMG_0877.JPG

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Posted

My thought is by moving the front mount forward you move to a position of less strength. 

I don't get this statement. The front mount is at the door post and is far stronger than the 6" tall door sill. How do you figure that it is a "position of less strength"?

The loads are eventually reacted at the door posts regardless of where the actual mounts are. The difference is that the stock mount position requires the door sill truss to take the load in bending and transfer that bending load to the fore and aft ends of the door sill truss and into the door posts. Using the forward mount puts it into the forward door post much more directly.

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Posted

I should stay out of this, but here is something to consider.  With the cabane gear, when things get bent, it's often the seat truss is being pulled inward from downward pressure on the cabane, and the longerons are twisted.  The bottom section of tube of the seat truss closest to the longeron gets bent and is pushed up ward.  If you move the front connection forward to the front of the door, there is nothing to resist that inward pull and twist like the seat truss tries to do.   I still think if one wanted to make it stronger, he would tie the cabane to the seat truss with a verticle tube in the center of the cabane/seat truss.  And for that matter, infill the seat truss to the extent possible to resist that inward pull and twist.  If you can find them on here, there are a number of pictures of bent up fuselages and I believe they will show what I'm saying as far as what gets bent.   JImChuk

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Posted

I should stay out of this, but here is something to consider.  With the cabane gear, when things get bent, it's often the seat truss is being pulled inward from downward pressure on the cabane, and the longerons are twisted.

Thanks Jim, but I disagree. My cabane will have a tube under the fuselage between the mounts on either side of the fuselage. That will form a triangle and the cross tube will be in pure compression. this will take most of the inward load you are describing and the fuselage won't really see it.

The bottom section of tube of the seat truss closest to the longeron gets bent and is pushed up ward.

Of course. It has to if the door sill is bent. 

If you can find them on here, there are a number of pictures of bent up fuselages and I believe they will show what I'm saying as far as what gets bent. 

I have looked at many here and on other websites. The seat truss bending I have seen is either secondary to the door sill bending, or the result of hitting the limit cable on the gear.

As I see it, the stock design puts the door sill in bending, and due to the bungee arrangement also puts the seat truss in bending. Both the door sill and the seat truss are short in height and made from light tubing and it is not hard to see that a hard landing could easily overstress these lightweight short trusses. Using the forward mount takes all of the bending out of the door sill truss, and using a cabane takes all of the bending off of the seat truss. The lateral tube on the cabane will prevent any sort of twisting due to the mount pivot being hung below the longeron.

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Posted (edited)

So I went ahead and started on the new gear today. Got the legs on and they swing free. Still have to cut off the front tubes and put on the axle mounts. I will be using the 3/4" axles like the stock gear and the 8" Douglas wheels. Hopefully the Nanco N800 will become available again.

 

20201018_211446.jpg

20201018_193819.jpg

Edited by 109jb

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Posted

John,  You are pretty good with the drawings that you put up in your first post on this topic.  (I don't know how to do that)  Can you do one more drawing looking at the fuselage and landing gear straight from the front.  Please show with arrows like you did in the other drawings which direction the force will be applied  from the landing gear to the fuselage  when you land the plane.   Thanks,  JImChuk

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Posted (edited)

I think this is what you are asking for. 

Here is a drawing from the front. The red is the load at the wheels. The green is members in compression, Blue is members in tension. The purple is the resultant load on the part of the cabane where the shock struts mount (shock/springs/bungees not drawn).  This load is the one that would try to pull the points in the yellow circles together, but the added horizontal member will prevent the yellow circles from coming together.

The drawing were made in Autocad, and then the arrows added using just Microsoft Paint.

 

 

 

 

Front.png

Edited by 109jb

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Posted

On another note I haven't decided how I am going to go for the shock struts.  I'm considering using bungees instead of springs like Joey Myers on the facebook group does. The bars in his setup are both aligned  unlike the cub type gear that has them at 90 degrees and uses shock rings. For now I'll probably just use some conduit and make solid links so i can move it around.

121815715_10160276599054409_7633614022024644283_n.jpg

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Posted

Oh.  I forgot to mention that the other night I found a vendor selling gear doing just what I'm describing. It is the Shock Monster gear for the Kitfox IV. You can see a picture at this link https://store.tk1racing.com/ProductDetails.asp?ProductCode=80-2761  I can't afford $2200, so the couple hundred bucks spent on tubing will have to do.

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Posted

I think the bungees are a good idea in place of springs.  Easily adjustable by adding more wraps.  Lighter also.  I think wypaul's added triangulation in the cabane makes sense.  If the cabane is able to go be pulled down, it will buckle the cross tube at it's top which will allow the longerons to be twisted and pulled inward.  Just my take on it.  I'm no engineer, but can still form an opinion.  (right or wrong) I do know that Lowell's gear originally did not have the cross tube at the top of the cabane.  That was added to help beef it up some after some fuselages were bent.  He will still warn you not to abuse his gear, and that mainly it's meant to improve handling with the wider wheel spacing.  I have his gear on my Avid, and it's much better then the stock gear for handling.  JImChuk

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Posted

Yeah. I'm mainly looking for improved handling and to get everything gear related on the outside of the airplane because I plan to eventually get an amphibious set of floats. The stock bungees would be a pain to deal with for seasonal gear to amphib and back changes.  Also gets rid of the hole in the bottom of the fuselage.  If I can land in a bit more rugged terrain that's OK too but wasn't a priority. 

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Posted

So I got it welded up. Seems like it is going to work well. The height and width came out very close to plan. A lot was done in place on the fuselage so it is definitely a one-off.

20201023_195625.jpg

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Posted (edited)


looks nice but not the best placement for the lower bungee tube attachment. The auto cad drawing above shows a better way of doing it.

Edited by TJay

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Posted

Looks fine to me. The attachment in the cad drawing would produce a positive camber twisting force as weight is applied to the gear.

His bungee tube attachment is in line with the center of the axle and also gussets the front and rear gear leg tubes to the axle like other cabane gear I have seen for Kitfoxes and Avids.

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Posted

I decided on that attachment to keep the axle geometry stock so I can still use the wheel penetration skis I have. It was a hard decision and I am aware that the cub style bungee mount at the axle takes some of the bending moment out of the gear. I may see what I can do to accommodate both the cub style axle mount while keeping the ski mounting. The front leg on this gear is 1-1/4" x 0.058" wall so even with the current placement will take a very hard landing to bend.

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