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Sockmonkey

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Might this be a viable arrangement?
eqRoiqM.png

You would get a more gentle spring.
 

rotax618

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Might this be a viable arrangement?
eqRoiqM.png

You would get a more gentle spring.
The springs are unnecessary, the tyres will be adequate ror suspension. Springs are heavy and concentrate loads. Best to use a light triangular pyramid shape tubular structure, will distribute the forces.
 

rtfm

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What about this? 25mm square tubing flush with the sides of the fuselage, obviating the need for cross-braces. The axle is an airfoil shaped 57mm thick plywood piece (57mm - 19mm x 3, and I have a lot of 19mm ply). The axle is both straight (easy to make) and an airfoil shape (low drag). Pretty rigid - but the tyres do all the heavy lifting. ???
1659774541720.png

1659774852406.png
 

addaon

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I’d definitely check side loads with that approach, seems like they’d be hard to react. And side loads in a cross-wind landing can be pretty extreme as you’re figuring out the limits of the aircraft, and your own limits.
 

addaon

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Not sure if this comment is helpful, but it might be, so here goes.

When I've been working through my design, I've found that it's super helpful to (a) figure out what others have done and (b) figure out how my requirements differ. Under the assumption that over the last century or so there have been some pretty bright people out there, if my requirements are the same, my solution should probably be pretty darn similar.

Now, I'm absolutely not familiar enough with ultralight landing gear arrangements to have opinions on what others have done -- the only ultralight I've flown more than a few hours uses my legs as the landing gear, and everything heavier I've flown has full suspension and, given my stick and rudder skills, could probably have used even more.

That said, here's what I see as your requirements:

Category one, loads: The AFB is intended to have ground operations typical for an aircraft in its weight class. This means similar vertical landing speeds, similar side loads in cross winds, similar braking loads (although those big tires will have impact there), etc. So standard approaches are the starting point here.

Category two, physical constraints: The big one here is the pilot's legs, which means that anything coming out of the side of the fuselage is at best inconvenient, and at worst unacceptable. This will disqualify some existing designs.

Category three, material constraints: I'm getting the sense that this is a pretty huge constraint, either due to availability or cost. This will mean that certain common approaches are off the table.

As a first step towards satisfying these, you seem to have settled on oversized tires and zero suspension travel. (I still question this decision, especially since you're also talking about streamlining your axle -- those tires will be more drag than anything else you can possibly do, as well as be heavier, and be underdamped in suspension. But for now, it's a decision, I don't need to understand it.)

So the remaining question is how you can react the necessary loads into the fuselage under the constraints given. If you don't need suspension travel, and your options are square steel tubing and 19 mm ply, it's pretty likely that a steel weldment is going to give you the maximum strength per weight for concentrated loads. The traditional triangulation here is the firat image in your drawing #718, but that puts the top tube right in the leg location.

Still, given that this is a standard approach to a nearly standard problem, my next step would be to make small perturbations and see if any work. Can you move that top tube so that, instead of being in the way, it acts as a foot rest? Or clear it just enough so that the front bottom brace can? Bending that top tube would limit its ability to take loads, but since the material is fixed size it's possible it's oversized already and you can bend it out of the way "for free" -- worth checking.

With more normal-sized tires, the standard approach would be something like Piper Cub landing gear, which basically inverts the truss geometry of #718 so that the vertical loads are reacted in tension under the fuselage instead of in compression into the top of it. It goes without saying that this is a super-well-understood, well-liked approach that's easy to make with tubing; but with your oversized tires it probably would be too high. If it were me (and I'm very clear that it's not!) I would definitely think about Piper-style gear and standard tires; with bungees on the Piper gear, I strongly suspect it would be lighter, lower drag, better ground behavior, and it leaves the sides of the fuselage completely free.

If neither of these check out, then I suspect you're looking at a truss geometry that I'm not familiar with (which probably is already flying; I just don't know my lightweight craft well). Everything I can find photos of is designed to give travel, so potentially not applicable. Not enough lightweights at airfields or museums near me to get inspiration that way, but that's often my next step. My hunch is that the best you'll do is to take the bottom fuselage struts from #718 and use them for the fore/aft and side loads like you currently do, but to beef them up enough to take vertical loads as well, and to tie the truss together with a single "axle" member. Basically the Piper Cub geometry, but taking a weight hit to make the truss shallow enough vertically to allow the big wheels, and hopefully getting some of it back by removing all the travel and using a single tube to lock it in place. [Quick edit: I suspect you might need another four small tubes here to brace the center of the axle, coming down from shared fuselage attach points. I'd definitely run the loads first without these members, but they'd be the first thing I'd add if the first check doesn't work.]

Beyond that... dunno.
 

Battler Britton

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And why not something in betwin? like the first Fleas, one, (or two) simple axle(s) , 4130 tubes, sandows, and big low pressure tyres, for prop clearence?

Maybe same as your last design, but with aluminium plates instead of your square tubes, and also, with 2 tubes, you could arange thoses tubes in X view from front, and gain some height
and002g.jpg
 
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OKDon

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Rubber shock strut on my Longster III. 1" OD lower tube, 7/8" upper tube. Makes for a real smooth arrival.
What rubber did you use for the Longster III gear? I 'm building very similar gear for a Woody's Pusher.

FYI - the A-frame gear are a fraction of the weight of either steel or aluminum spring gear.
 

rotax618

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What about this? 25mm square tubing flush with the sides of the fuselage, obviating the need for cross-braces. The axle is an airfoil shaped 57mm thick plywood piece (57mm - 19mm x 3, and I have a lot of 19mm ply). The axle is both straight (easy to make) and an airfoil shape (low drag). Pretty rigid - but the tyres do all the heavy lifting. ???
View attachment 128516

View attachment 128517
This arrangement has merit, lateral loads could be taken by crossed wires under the fuse. The axle should be a steel tube, could be reinforced by wood but ply would not make a good axle.
 

rtfm

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Hi guys,
I asked for advice and suggestions, because I value input from folks more knowledgeable than I. So thank you for your input.

Addaon - you've gone well above and beyond. I'll sit down this morning and work through your analysis more carefully. Thank you for taking the time to pen it. Quickly, though. could you direct me to some documentation on how to construct the Piper Cub geometry - it doesn't look that easy to build?
And you're correct about the airfoil-shaped plywood axel. No need for streamlining on a slow-flying Flea.

And Rotax618 - thanks for your opinion re: plywood not being suitable. Again, you're probably correct.

Battler Briton: Interesting. I might be able to adapt this idea - but my fuselage is already finished, so there is no way I can cut the required through hole for the axel. Besides, there is no way I'd have the necessary prop clearance. I'll play around with bonding on some stand-offs to the bottom of the fuse. Any idea what diameter/wall thickness that axel is? 50mm OD?
It just occurred to me that I might be able to combine my original idea of the side-mounted steel tubes with a HM14-style gear with bungees. Something like this? (suitably sized and reinforced, of course).
1659829110686.png

I still like the idea of the steel tubes bolted to the vertical sides of the fuselage. As for lateral loads on crappy landings, I'll see if I can check the physics of this, but my gut feel is that four steel tubes would be way more than required to react lateral forces.
 
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rtfm

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Question: What stops the HM14 axel from moving laterally? I can't see any retaining mechanism or bracket in the photo. I've gone in search of HM14 plans, but so far, haven't found any.
 
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addaon

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Some pictures in this thread should be helpful. Basically, the stub axle is held be three tubes. The upper two are hinged at the fuselage side, the lower one is hinged at the stub axle. The bottom tubes come together at another hinged joint under the fuselage, and use some mechanism (springs, bungees, whatever) to be adjustable length. All hinges can be made fixed if the bottom tubes are rigid and the tire does 100% of the suspension work. You'll note from e.g. post #20 in that thread that there's generally one more stay that triangulates the two upper tubes just below the hinges, to avoid that load going through the fuselage; depending on fuselage structure that may or may not be needed.
 

rtfm

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Thanks for the link, addaon. So, using rubber shock bushings, I can dispense with the bungees. Neat. Also, if I drilled a 5mm hole right through the axle retainer and the 50mm round steel tube, I could thread a 5mm bolt through the rubber bushings, and secure the axle from moving laterally, but not restrict its vertical movement. This is a terribly elegant and simple (not to mention cheap) solution. I'm really liking this concept.

In addition, I can machine a hardwood insert to slide into the end of the axle, which can accept the stub axle, and simply bolt the stub axle through the 50mm tube, using the wooden insert to account for the difference between the stub axle OD and the tube ID.

1659835326224.png
 
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rtfm

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Another quick question: would using a 50mm OD steel tube be stronger/weaker than a 50mm square tube of same wall thickness? A square tube would have a number of advantages: (1) it provides a flat surface for the rubber bushings to sit on, (2) easier to drill so that both sides have identical alignment (3) easier to fabricate the hardwood spacers for internal fitment.

Both are available locally of course. So just wondering if there was any advantage one way or another...
 
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