# Laminated wood landing gear

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#### davidb

##### Well-Known Member
I have been lurking in various threads to gather ideas for a laminated wood landing gear that would work on a VP2. I have done some construction on what I hope will be a successful design pending testing. I am soliciting critical comments--my pink body is more fragile than my ego.

I think the weak point of this gear will be the mounting of the KR axles to the wood legs. I have to figure out a bearing plate or sleeve design to spead the torsional load at that joint--the wood itself surely won't have the bearing strength. Ideas welcomed.

Some info: 1/8 inch (nominal) yellow birch laminated with West System epoxy, steam bent the wood to form and let dry before doing the glue-up, 7 inch radius bend, 2 inch thick, 6 inch wide tapered to 3 inch, 25 pounds (20 wood/5 epoxy).

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#### orion

##### R.I.P.
Nicely done!

The highest stress area of a bent type gear is actually at the bend where the loads are forced to turn the corner. Although I'd hate to cover that nice woodwork, a wrap of fiberglass, especially at the corner, would go along way to adding structural stability to the assembly.

#### davidb

##### Well-Known Member
Nicely done!

The highest stress area of a bent type gear is actually at the bend where the loads are forced to turn the corner. Although I'd hate to cover that nice woodwork, a wrap of fiberglass, especially at the corner, would go along way to adding structural stability to the assembly.
I had actually tried the fiberglass wrap on the first gear design I built--I wrapped 1 inch tape around the bend like wrapping bicycle handlebars because I thought it would help prevent any tendency for the layers to delaminate at the bend when flexing. Then I read that mixing materials did not have a cumulative effect on strength, i.e. one material would fail before the other so I didn't even concider fiberglass in this design.

As long as I wet out the glass properly with no trapped air, the wood still shows perfectly thru the transparent lay-up. If you think it would help I'll add glass.

This gear seems so rigid its hard to imagine I'll get the flexing that the "landing gear design" spreadsheet predicts for the values I input. Not sure if the ANC 18 data for 1 inch solid wood test pieces will correlate to a 2 inch laminate. I built the legs to a 47-48 degree angle figuring they would flex to the desired 45 degrees under the static weight of the aircraft. I suppose that will be the first verification of flexibility. I haven't yet put any stress on it because I wanted to be sure the epoxy is fully cured--I'll be back home in a few days and then I'll jump up and down on it.

#### davidb

##### Well-Known Member
Here's a few pics of the Ibis bearing plates--I'll fabricate something like that.

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#### davidb

##### Well-Known Member
I just ordered the axles--should be ready to start testing in about a month. This gear seems really rigid--I can't get it to flex at all when I jump on it with my 160 lb body. It will be interesting to see how it behaves under real forces.

In watching drop tests of similar designs made from aluminium, I notice the center section flexes downward conciderably. The way this gear is mounted, it too will be free to flex in the center section but that area is so "beefy" its hard to imagine much flexing in that area. I'm wondering if I should thin or narrow the center section.

I guess if its more rigid than expected, I can up the tire size for more shock absorbsion.

I've created a composite with un-known characteristics but I used care in the build-up so I think the material with behave homogeniously if that makes sense.

I will wrap the bend in fiberglass as Orion suggested, but if anyone thinks the shape could or should be optimized, I'd like to do that first. I'm not concerned with aerodynamics, just bending dynamics

#### wsimpso1

##### Super Moderator
Staff member
Log Member
That's a nice looking piece of furniture.

To make an efficient leaf spring, you have to design it to flex the same amount everywhere, which is to say, you have to design it to have nearly the same stress in all parts of the spring.

Bending moment of the gear is at maximum from one mount to the other mount, and decreases linearly as you go out to the tread contact. Shear is the same everywhere between the wheel and the mounts and zero between the mounts. Shear is generally a smaller actor than bending, so you should design to keep bending stress the same everywhere.

To get close to the same bending stress M/EI should be the same everywhere. Now bending moment is zero at the tread contact point and increases linearly to the mounts. You want EI to also increase linearly from the tread patch to the mount. If the leg is the same thickness everywhere, you just taper the width. If you tapered them in thickness, it gets a little more complicated. E is a constant here, and for a rectangle, I=(b*h^3)/12.

Now it gets tough to make the area around the axle attach fittings narrow enough, so the beam will generally be a little fat there. Look at any Cessna with flat spring gear for an example of what to do outboard.

Now if you know how to size the gear legs correctly between the mounts, you also know how to size them for the leg portions.

The other part of this is to make sure that your mounts do not restrict the beam from flexing.

I suspect that you will test it with the untapered legs. Go ahead and do it and get a reading on the spring rate, but if it looks to be too stiff, you can taper the legs the way I suggest and try it again. It should not carry any less load this way, but it should deflect more, which is what you may need.

I await your results. And remember, even the Volksplane is a lot heavier than you are, so how much it moves under your weight is not relevant.

Billski

#### davidb

##### Well-Known Member
Billski, big thanks for taking the time to reply and sharing your expertise. The gear is the same thickness throughout and the legs are already tapered in width. I plan to mount it against radiused blocks so the beam will be able to flex with less stress to the fuselage. I'm having trouble understanding what to do with the section between the mounting points. As you say, bending moment is at a maximum from one mount to the other mount. Right now I have that center section at a constant thickness and width. In general, should that section be tapered to be thinner or thicker in the middle?

#### gschuld

##### Well-Known Member
DavidB,

I am certainly not qualified to offer you the technical expertise that Billski or some others can regarding the fine points of landing gear design, but I do have a good bit of practical experience with laminated structures(I used to build wooden race boats, iceboats, etc.) Your lamination work appears to be first class, I congratulate you. Nice jig, etc. I might suggest that you contemplate a different shape for your gear legs. As others have mentioned, there will be a three distinctly different places where the gear will want to bend and each is very independent of the other.

-the area between the bends and axle
-the bends themselves
-the area between the bends

I agree that the bends will likely be the most highly stresses and the overall gear may be a challenge to more evenly distribute the load,stresspoints.

For a highly loaded wooden leaf spring type lamination, the most efficient and predictable one is one that has a gradual and tapered bending characteristics, such as a tapered composite fiberglass car leaf spring. If you were to shape the landing gear so it was flat for the width of the fuselage then bend in an even arch to the axle, you would have the start of this effect(bent Varieze gear below). For example, you could build one gear for testing. Perhaps a constant thickness for ease of construction if desired, and have the width constant (say 6") at the flat section then tapering down (say to 4") at the axles. Start with width dimensions that you are confident will be a bit oversized. You could test it, and remove material(width/taper) as necessary to achieve the desired flex and strength. Once you feel you are happy with it, go a little further and go ahead and prove that it is now too weak. Record your data, and glue up and shape the final one with the dimension that you are most happy with, assuming that you have decided that the chosen thickness and wood species used is acceptable. Another side bonus is that without the "hard point" of the fairly sharp bend of your current design, you may excuse yourself from needing to put any fiberglass on such nice woodwork, so you can varnish it and show off your handiwork Klaus Savier has run over 250mph with an 0-200 engine on this shape gear, so it should be go fast enough for a volksplane.

George

P.S. This would require 90 degree flange axles instead of the angled ones that you ordered though.

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#### davidb

##### Well-Known Member
Thank you George for a very thoughtful reply.

I should explain a few things. I've already put way too much time, effort, and money into this hunk of modern art. If it doesn't prove viable thru testing I'll give up the idea and buy the aluminum gear from Grove's. I have no desire to start the process of making another lamination. I chose the current shape to mimic the Grove gear. The fuselage mounting points are per the VP plans so if the wood gear doesn't work, I can just bolt on the aluminum gear without any re-work. I guess it boils down to me foregoing proper design (which I'm ill-equiped to do anyway) for ease of fabrication and mounting. Yes, I realise how stupid that is, but I had fun with the process.

So here's where I'm at now. The axles are mounted. The legs are tapered in width. The bends are not glassed but I'm willing to do that. The mounting area (just inboard of the bends) is sized to fit the mounting method and I don't want to mess with it. The center section is basically a 2 x 6 beam 30 inches long. I can taper or thin that section but I have no clue as to what general shape I'd be shooting for. The wheels (6 inch) and tires (17 inch o.d., 20 p.s.i.) will arrive this week.

At this point I'd be happy and surprized if it survived testing--optimizing doesn't seem like anything I could attain at this point.

I'll post more pics later. Any ideas or suggestions that might up the chances of this gear surviving stress testing that I can impliment from this point forward are greatly appreciated. Snide comments are welcome too.

#### davidb

##### Well-Known Member
Here's some pics.

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#### gschuld

##### Well-Known Member
As far as the chance for success for your laminated gear, it's got a shot! One thing that does jump out at me is the axle to gear leg joint. I believe that you will find the leverage put on those bolts less than 2" apart will tend to crush the thin backing plates that you have sandwiched on both sides of the laminate over time, loosening the joint. I would try to utilize to two bolt holes farthest away from the gear leg bends if possible. The ones right next to the axle itself. If you could tie them to the other side of the laminate you would be strengthening the joint significantly. If the wood was fit right up to the back of the axle itself that would help some as well.

As for you going to an aluminum Grove gear if this one does not work out, for $1,100 you could build and try a dozen wooden gear leg sets. I would never try to dissuade anyone from using a proven product on an airplane however. A laminated ash gear leg with bowed outer legs would only cost$100 tops for materials, leaving you \$1000 for ...other things!

I assume that you intend to stick with the 1040lb gross weight?

Assuming the lamination has a good west system bond, it may well be strong enough. A layer or two of carbon fiber or s-glass tape bonded to either side full length would also beef it up some if needed.

George

#### wsimpso1

##### Super Moderator
Staff member
Log Member
The section at the mount is the section you should also carry between the mounts. Simple as that. Shear load is gone between the mounts, but with a solid section, the metal will hardly feel the shear loads. It will sure feel the bending moments (which does not vary between the mounts), so no thinning between the mounts.

Using only the eyeball, (calculators and computers are much better tools to do this with) the axles and mounts look to be very flimsy. I would rather you not destroy the tet rig before finding out how strong the gear legs are... If you shoot me a PM with ALL of the dimensions of those blue parts including how far the tire center is laterally from something on the blue parts, I will run a quick strength calculation for you. If it proves wimpy, there are several simple ways to make a better one...

Billski

Billski

#### NorthwestJack

##### Well-Known Member
Ive been working on the same issue.
I designed a wooden landing gear out of laminated douglas fir strips. the max gross weight is 1200 lbs.
The landing gear is 1-1/4 thick , and it has an airfoil crossection.
leg to leg is about 6 ft. and max width is 18 inches tapering to 8.
the total weight is 28 lbs.
i did the initial testing by bending a 1/4 by 1 inch strip of fir to breaking and then extrapolated for the needed weight.
I used a formula for the max weight from a 1938 magazine (popular aviation) which gives

n= 3+ .133s ( where s is the wing loading) and n is the number to multiply the gross weight by to get the max design load to the gear.
One question that I have is : What should be the toe in (in degreees) for a landing gear to make it stable. I have not found any information in that regard.
Jack

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#### davidb

##### Well-Known Member
... One thing that does jump out at me is the axle to gear leg joint. I believe that you will find the leverage put on those bolts less than 2" apart will tend to crush the thin backing plates that you have sandwiched on both sides of the laminate over time, loosening the joint. I would try to utilize to two bolt holes farthest away from the gear leg bends if possible. The ones right next to the axle itself. If the wood was fit right up to the back of the axle itself that would help some as well.

I assume that you intend to stick with the 1040lb gross weight?

Assuming the lamination has a good west system bond, it may well be strong enough. A layer or two of carbon fiber or s-glass tape bonded to either side full length would also beef it up some if needed.

George
The four bolt holes around the axles are for mounting the brakes. That is also why they stand out 1/2 inch from the legs to allow room for the nuts. The wood is quite hard but thicker bearing plates would probably be better.

Yes, 1040 GW.

I'm still reading conflicting info on whether or not adding glass will have a cumulative effect of adding strength--just not sure it will help.

#### davidb

##### Well-Known Member
... so no thinning between the mounts.

Using only the eyeball, (calculators and computers are much better tools to do this with) the axles and mounts look to be very flimsy. I would rather you not destroy the tet rig before finding out how strong the gear legs are... If you shoot me a PM with ALL of the dimensions of those blue parts including how far the tire center is laterally from something on the blue parts, I will run a quick strength calculation for you. If it proves wimpy, there are several simple ways to make a better one...

Billski

Billski
No thinning it will be--thanks.

My eyeball sees very flimsy axles/mounting too. Will do on the PM--super thanks! I'll do pics with a ruler too.

#### davidb

##### Well-Known Member
Ive been working on the same issue.
I designed a wooden landing gear out of laminated douglas fir strips. the max gross weight is 1200 lbs.
The landing gear is 1-1/4 thick , and it has an airfoil crossection.
leg to leg is about 6 ft. and max width is 18 inches tapering to 8.
the total weight is 28 lbs.
i did the initial testing by bending a 1/4 by 1 inch strip of fir to breaking and then extrapolated for the needed weight.
I used a formula for the max weight from a 1938 magazine (popular aviation) which gives

n= 3+ .133s ( where s is the wing loading) and n is the number to multiply the gross weight by to get the max design load to the gear.
One question that I have is : What should be the toe in (in degreees) for a landing gear to make it stable. I have not found any information in that regard.
Jack
I was wondering about toe-in as well. I think Evans recommends zero degrees. Mine will have zero to maybe 1/2 degree toe-in.

1-1/4 inch seems a little thin for 1200lb GW but I've demonstrated that I don't know what I'm talking about. I think thickness is a bigger factor in bending strength than the width. I'd be interested to hear more about your gear like how you did the laminations and more pics.

#### davidb

##### Well-Known Member
Pics for Billski.

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#### NorthwestJack

##### Well-Known Member
+Question for Davidb:
Ive heard from some sources that toe in is required, but I dont know?
Also I believe my calculations are pretty close. Although you are right that thickness is much more important than width.
two reasons I did it wide are:
1. airfoil section has considerably less drag
2. the bolting of the gear to the fuselage is simpler and puts less strain on the wood if you were to hit a hole(ie. pushing back on the gear). I was curious on how your gear is prevented from going backwards if you hit something ?
Jack

#### davidb

##### Well-Known Member
... I was curious on how your gear is prevented from going backwards if you hit something ?
Jack
Good question. Even if it were per plans (aluminium) it's a long moment arm to magnify any aft forces on the mounting points. But if the gear shears off when I hit a pothole, as my son put it, "at least its more like motorcycle crash than an airplane crash."

#### gschuld

##### Well-Known Member
Well, Jack's gear design certainly will have much more resistance from deflecting aft under a strain(assuming the mounting bolts and fuselage mounting points are up to the task), but David's gear width at the fuse is a fairly common size for one piece aluminum gears sets. If the aluminum gear has proven to be successful in the past on the VP2, I would suspect that the same mounting system would work out OK. Depending on how it's mounted, crush plates might be a good idea for the mounting area so the wood does not crush under load.

George