3/4 .049 tube is what I used for the cabane truss. I didn’t “engineer” my landing gear but I did find a link on this site to an old article that gave some guidance on designing a cabane style gear, it explained how moving mounting points and angles varied the loads through the fuselage, I used what I learned in that article to design my gear. I haven’t seen any pictures of how the kitfox fuselage fails on a hard landing when equipped with a cabane gear but I suspect the lower longerons at the front mounting point will be pushed in towards each other.
One thing you could try is to put a couple of pucks in the lathe and machine a bit of an angle into the face of one end on a couple of the pucks say to produce an included angle of 150 deg. instead of perfectly flat.... for say 50% of the face of the puck. put them top and bottom of the puck stack and it might soften initial travel a bit.
I haven't drilled the pucks yet, this is my next job. The reasoning behind this is to give the material an additional path to displace to and maybe soften the rate. If you can add extra pucks to your build this step may be unnecessary,the current setup works fine but I feel just a little softer/more wheel travel may be an improvement. I was thinking 5 x 5mm holes worth an option to encrease up to 10 mm?
I am working on something similar. If you have any pictures of the truss, that would be great. Also, I am currently researching the geometry of the intersection points of the front landing gear tube and the strut tube (this intersection is an imaginary line drawn that intersects the 2 lines) to assure there are no buckling forces to the front tube under a hard landing. The geometry is important there. I will be happy to share my findings with you. If you have any drawings of your gear, I can check your gear for that as well.
Dusty, One more question for you. You said that you drilled the pucks like a revolver cylinder. Do you recall how many holes and what diameter you used? Thank you so much for your help and inspiration.
Thanks for your input Yamma-Fox. Just to be clear, when you said that when you spoke to "Tony at TMR he cited many examples of use in airboats used for wild rice harvesting where it held up great despite much more abusive use than aircraft." you are talking about the RK 400 clutch and not the TMR clutch, am I correct?
The rk400 clutch has held up extremely well in all the 120 - 140HP Yamaha applications for sure. Over a hundred of those being used over the past ten years and I haven't heard of anyone having problems, so that is a pretty good testimonial IMO. And in talking with Tony at TMR he cited many examples of use in airboats used for wild rice harvesting where it held up great despite much more abusive use than aircraft. I'd put it in a 670 without hesitation.
Yes I used 4 pucks,1 inch x 2inch 1.5 deep.if I were to start from scratch. I would use more pucks as you suggest.You will loose some material when parting off but you have plenty of spare. I am not sure on going to 2 inch deep pucks, it may work in your favour to effectively soften your rate which is possibly a good thing. The diameter to depth ratio I assume will change the characteristics of the setup. Regarding sag ,there is a small amount over the last 6 months which seems to have stabilised 10mm?(similar to the die Spring) If you need to preload you can make a simple Spring compressor. I can post a photo later today . I am interested to know how this works out for you cheers Dusty
Thanks for your reply Yamma-Fox. I had noticed that Tony's webpage is no longer up so I wondered if he was still in business. How many RK 400 clutches actually slipped from the Rotax 670 HP? I don't want to fix one issue, only to cause another problem with a slipping clutch, then I'm no further ahead and a with a lighter wallet. I asked Mohawk about the reliability of the RK clutch and his sprague clutch, and he failed to reply to my questions.
I took my inspiration from the stock design (Dean is a genius), the Lowell Fitt gear (a much higher compression/tension application then stock) another similar bush gear design (with a spring that has a short compression stroke), and a gentleman that was experimenting with the poly bushings (they never bottom out). I think the 2 key things to be careful of are the angle between the swing arm tubing and the strut tubing as well as where the intersection of the imaginary continuation of the 2 arms would occur (somewhere near the tire bottom). This can be seen by taking a pencil and continuing the lines of the 2 components until they cross) First, If the angle between the arms get too narrow, the tension on the strut and compression on the front swing arm tube become too high. Second, if the intersection of the imaginary continuation of the arms cross at too far an unideal point, you can buckle the compression arm (the front gear tube). Because this intersection moves, when the gear is cycling, care needs to be take as to where you design in this intersection. This would be controlled by the angle of the front swing arm tube, where it welds to the axle tube, and the angle of the strut tube, and of course, where it swivels on the axle tube.
Dusty: So I have a 36 inch long (914 mm) piece of the McMaster Carr polyurethane tube on its way. So I gather from your posts that the 4 pucks that are pictured are 50 mm OD (about 2 inches), 25 mm ID (about 1 inch), and 38 mm long (about 1.5 inches long. That would be 152 mm total length ~6 inches). My thought is to make struts that allow up to 200 mm total length, start with 4 50mm long pucks and see how it goes. This way, if too soft or too much travel I could insert a solid spacer to replace one of the pucks. What do you think of this plan, being the resident expert? Also, a question. Does your setup sag at all with the airplane empty.
Not sure if its a plus or minus on longer spring for stress on fuselage. Depends on how stiff the spring is I suppose. Longer spring would put more pressure on fuselage, but if it's not to stiff would bend more also, so which is worse??? It's still more leverage though, so on second thought, longer may be the worse option for stress. As far as the second bolt, I would put it as far back as you can just in front of where the two longerons meet. Maybe a plate tacked on top of the longerons for the top of the bolt to set on. On my Avid, I ground the head of the bolt, and it fits down in between the longerons. The bolt will tie the lower spring to the upper one which is held in place by the channel it sets in. JImChuk
The Mohawk sprag clutch failed in the testing phase, so that is out. As far as the TMR400 mine is working great but I am afraid that Tony at TMR may have passed away and that option also may no longer be available.
Getting the color on her. The gloss of the latex is definitely better than polytone. The white is really hard to tell but the blue has a little orange peel which was not unexpected. It is about what I was expecting and is in the perfectly acceptable realm for my tastes. If I ever decide to I could color sand and buff it but I doubt I ever will. Overall I would call it a success but in doing the fuselage I have learned a few things and will be modifying the process a little. The results that Malcolm at wienerdogaero.com gets can't be argued with, but at least for me a few changes to his process make it much easier for me. For my rudder and landing gear legs I made the following modifications which work very well for me. I am starting to thin the first coat of primer a bit more than he recommends in order to make it easier to get more complete wetting of the fabric. On the primer coats on the fuselage I used the wienerdoaero method and brushed each coat on sanding between each coat with 320 grit dry sandpaper. I still brush that first coat on, and on the landing gear I also brushed several coats, putting the primer on in one direction and tipping it at 90 degrees. Ultimately I went to spraying the primer on. The primer is still thinned and sprays nicely, leaving a slightly orange peeled surface. The orange peel is much easier to sand smooth than the brush marks from brushing/tipping. On my rudder I am spraying all but the first primer coat and it is going very well. You can get a smooth finish brushing and sanding, but it is a lot more work in my opinion. Spraying i was also able to reduce the sanding interval to 1/2 by applying 2 coats (one cross coat) at a time, waiting until the first coat tacks up and applying the crossing coat. I have also gone to wet-sanding with 400 grit which again is much easier for me. Anyway, here is crappy picture of the airplane after the first cross coat of blue was applied and had dried about an hour.
Just a thought. You might consider making the spring flat (tail as low as possible). Just high enough so the spring doesn't bump the rudder when it flexes hard. That will enable maximum angle of attack landing and taking off.
I am using a 582 so hopefully won't need to add weight to the tail. If I do, I would want it to be useful weight (springs, reinforcements, etc.). I am also thinking about upgrading the tail feathers to MkIV size with a trim tab so that would add some weight as well.