Wooden spring/Wittman gear?

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Aviacs

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The possibility/puzzle of making a wooden gear for my Sonerai2 project is intriguing
(not a need - already have 2 aluminum sets, 5/8" as well as spare 1/2")

However, there seems to be a fair amount of speed in fairing the gear and brakelines.
Streamlined wooden gear legs could incorporate internal brake lines, just like the gun drilled aluminum versions, and maybe save a few lbs.?

I'm capable of doing the craft work, it's my business.
However, i have not seen how a set of spring gear is actually designed? Parameters? Cook book estimations of landing loads & how much extra margin to add?
Any insights or link to resources?

Thanks!
smt

PS: I did several attempts to search the forum re "wooden gear legs" and permutations with no hits.
Then i googled the world, and it sent me back here to this: Laminated wood landing gear which i am reading now.

Still interested in any further info and comments.
 
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don january

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My thoughts on this is how would you protect from moister ? if gear was covered with carbon fiber or fiberglass radius and legs would need to be very flexible and half the impact force would be slamming the glue joints. Don't get me wrong I would love to have wood gear like the one you gave the link to for my Taylor monoplane but to get to the strength the craft would need the weight would be very high. But in truth the stock gear for the T-mono are heavy little pigs. I've been kicking around the idea of motorcycle front forks cut down at the top for length with disk brakes. OH the joy of building something built 60 years ago. ;)
 

wsimpso1

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Pazmany's book on landing gear details the energy absorption and g loads in various directions that the leg has to stand.

Analysis of the gear leg for energy absorption and stresses is straightforward but kind of tedious. I might be able to help you run some numbers, but I suspect it will be very bulky compared to aluminum and steel legs by the time it will both stand the load and absorb the energy.

Billski
 

Aviacs

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A point i did not understand in reading your advice on the wooden gear above is this:

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.
Which metal is feeling the bending moments, and hardly feeling the shear loads? The fuselage?
If just the mounts, is that not trivial to accommodate? (not argument, curiosity/failure to comprehend)

My thoughts (apparently incorrect?) were that allowing the center section to flex "considerably" would reduce the instantaneous loads elsewhere & permit "longer travel" for the same gear dimensions.

IOW, my approach might be to design the mounts to roll & slide more but reduce center of gear section for weight and for flexibility. In practice, probably in thickness only so as not to reduce torsional resistance as much. skis were originally wrapped with fiberglass so as to improve torsional stiffness. perhaps that has merit for a gear, as well as for the cross-grain/cross glueline resistance to delamination that the OP discovered?

smt
 

wsimpso1

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A point i did not understand in reading your advice on the wooden gear above is this:



Which metal is feeling the bending moments, and hardly feeling the shear loads? The fuselage?
If just the mounts, is that not trivial to accommodate? (not argument, curiosity/failure to comprehend)

My thoughts (apparently incorrect?) were that allowing the center section to flex "considerably" would reduce the instantaneous loads elsewhere & permit "longer travel" for the same gear dimensions.

IOW, my approach might be to design the mounts to roll & slide more but reduce center of gear section for weight and for flexibility. In practice, probably in thickness only so as not to reduce torsional resistance as much. skis were originally wrapped with fiberglass so as to improve torsional stiffness. perhaps that has merit for a gear, as well as for the cross-grain/cross glueline resistance to delamination that the OP discovered?
Sorry, but the context was lost. Was that a posting of mine in davidb's thread on his wood experiments? Cite the post number within

I can make some assumptions that might be OK here. We were talking about the the gear legs here and in davidb's thread. So this was all about the legs and not design of mounts. And so are your posts, so let's guess it was there.

A full arch landing gear leg system goes from axle on one side to axle on the other, with two mounts. This is mechanically call four point bending. Two fixed inboard, two loads outboard, the shear curve drops to zero between the mounts. Moment starts small at the axle, increasing to the mount, then steady between mounts, then decreases to the other axle. The beam does see primary shear drop to zero between the mounts - this is vertical load carried across any cross section. Beam shear reaction is generally a small part of the what you need section for, and bending usually designs beams. Look at what is used to put up big buildings and bridges - wide flange beams with a relatively thin web holding them together.

Can you narrow the gear between the mounts? Maybe a little basing it upon the shear and bending moment diagrams, but you still have the interlaminar shear stresses in the beam to deal with. Here is what is happening while under pure bending. The top layer is compressed the most. The next layers down are compressed less and less until you get to the neutral axis, where there is zero length change. From the neutral axis to the bottom layer, the fibers get more and more tension and extension. The differences in length between each layer of wood fibers and lignen and epoxy are the intralaminar shear. Wood wants to shear parallel to the grain... which davidb saw when he started doing drop tests.

Additionally, in composite structures (wood is a natural composite material) this shear can develop edge splitting unless you do something to control it, like wrapping the whole thing is crossing plies or work with lower stresses...

This topic is big, and takes several one-hour lectures to cover, and then some guys just do not get it... To go through this in any detail, look in a Mechanics of Materials text book in the beam theory section and look up shear stresses within beams.

Billski
 

Aviacs

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Bill-
Your pre-amble was post #6
OP response & Q at #7
Your comment (my post above) included at post #12

I'm a semi-retired carpenter whose business was primarily millwork including a lot of curves and bent laminations. Bought WEST in C units only & used anywhere from about 2 to 10 gals year for 40 years. Periodically i drag out the books including those you mention for simple steel structure spans or cantilevers, but mostly cheated and used deflection tables, starting with the paper copies before 'net made it even easier. :) The most complex things i actually worked through designing were flat springs for 18th c antique furniture like the Roentgens built. Originals are amazing, forged all over, interesting reduced and tapered shapes sometimes in 3 dimensions, etc. (Like medieval steel crossbows from the century just before).

Think i can work through some of this if i decide to proceed.
My original Q was that i did not know the parameters and baseline requirements for landing gear.
Links in these 2 threads are a good start.

Reading these 2 also puts a perspective on the potential streamlining benefits if any - sections are going to be rather large.
Thanks!
smt
 

vhhjr

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Early in the development of a replica Facetmobile (FMX-4) a laminated wood main gear was considered. I designed and built a series of 1/2 size, properly scaled gear legs and we did drop tests on them. Several wood species were tested in two configurations. The bottom line was: Yes, you can make a wood landing gear, but it's not going to look like a metal one. We had very little success with gear legs that were straight with a sharp bend at the top. The stress concentration is just too much to keep the laminations together. Much better performance was found with curved legs. The gear legs were made from wood sheets from 1/16" to 1/8" that were water soaked and dried in a press with the desired shape. The lamination were the glued up in the press using Titebond III, T-88 and West epoxy. The press was wood blocks with the proper shape held together with large clamps and a section of fire hose as a pneumatic bladder. On one press we used a chain drive screw arrangement in place of wood working clamps. I have attached some info from the tests.
 

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TFF

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With the energy potential of a Cassutt, the gear will be massive if a margin of safety is kept. Not even running numbers, just visually wood is going to be big. An aluminum wing spar is about 1/8” thick give or take. A wood equal will be 3/4-1”. A wood longeron is probably 1- 1 1/2” square where. 4130 would be 5/8 .035. If I was to do wood it would not be landing gear.
 

vhhjr

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I failed to mention that we also included glass, Kevlar and carbon fiber in some of the test legs. That really complicated the analysis.

In the end they used tapered 4130 legs. Interestingly, the weight was about the same for the wood and the 4130 versions. The drag would have ben greater for the wood assuming the steel legs were streamlined.

Vince Homer
 

Aviacs

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VHHJR:

Thank you much for that post!
Lots to think about.

Noticing how wooden gear tends to fail in sheer makes me wonder if perforating the inner laminae on an (engineered? estimated? eyeballed?) pattern would add anything. letting the fiber filled epoxy act as cross-grain/shear blocking rivets internally. Move the failure mode of the laminae in shear closer to the failure mode of the remaining fibers in tension, with a distribution through the thickness to optimize enhancing where those forces are greatest.

Not sure i have the will or time left to explore that question.

smt

PS: for those that use firehose for pressure clamping - unless you have a surplus ("free") option, collapsible discharge hose is cheaper and comes in wider widths. Though if you go too wide, the pressure capacity is lower. IIRC working pressure for the 6" stuff here is 125PSI, dropping to 65PSI for the 12" dia size. Though higher ratings are possible depending on source.
 
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vhhjr

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Along a similar line. I read a procedure for making laminated archery bows that uses the fiberglass mesh normally used for drywall spackling as reinforcement in between wood layers. The writer claimed is was the best, and cheapest, material to use. I suspect it did exactly what Aviacs is talking about in providing a type of resin rivet between the wood layers. The resin has some elasticity, but in the normally thin layers can't give much without failing. The thicker resin "dots" would allow more slip between the wood layers. I will try it if I ever make more gear legs or tailwheel springs.

Vince Homer
 

vhhjr

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If you tak nice to your local fire dept. they almost always have some dead hoses. Ask for hose only with no fittings that way they don't have to be concerned about someone hooking up to a fire hydrant. I apply some GOOP to the inside of the hose end, fold over about an inch and using two strips of 1 x 1/8 aluminum rivet or bolt through to clamp the hose.

For the air inlet end I bandsaw a round plug from two glued up layers of 3/4" ply or from a 2 x 6. Get the smallest floor flange from your local hardware store, drill a hole thru the plug and attach the flange using screws or bolts and some sealant, such as GOOP. The hose should snugly fit over the plug. Smear the plug with sealant and slide it in the hose. I use two worm drive clamps and drill screw holes thru the clamp bands. I use a ball valve and an air chuck fitting so you can remove your air hose. Several sheet metal screws will hold it all together. The last thing you need is for the plug to rocket out at pressure.

Vince Homer
 

rv7charlie

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Sorry for the off-topic, but any updates on the FMX-4? I've wanted a 2 or 4 seat version ever since helping kill the grass around the original at OSH.
 

Jimboagogo

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Ibis was an all wood Canard homebuilt in Europe using wood for the gear as well. See the link above. You might find it helpful as it seemed to work just fine and was light weight to boot.
 

vhhjr

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Barnaby just moved to the Independence, OR airpark and intends to start building his latest design the 2-place FMX-7. He has a news letter about the aircraft. Check out his website for the address.

Vince Homer
 

Vigilant1

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Ibis was an all wood Canard homebuilt in Europe using wood for the gear as well. See the link above. You might find it helpful as it seemed to work just fine and was light weight to boot.
Good find.
I wonder how a laminated wood main gear bow would perform in absorbing energy in a high sink rate "event." The ductile deformation of a steel leg is hard to beat, but if a lot of energy is used to break apart the wood during the overload, it could perform very well.
 

vhhjr

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Have a look at the excel spreadsheet on my post of yesterday. We found even the failing gear legs absorbed a lot of energy.

Vince Homer
 

davidb

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Aviacs, my motivation for trying the laminated wood gear was to save money. I didn’t want to spend $1200 for proper Grove aluminum gear. I had time and enjoy wood working so I gave it a try. Your goal of saving weight and reducing drag might be achieved but probably not worth the effort since you already have aluminum gear.

I’m not familiar with the Sonerai but if the gear mounting is like the VP2, that will be the biggest issue. Take a look at the Ibis design previously mentioned. If you can modify the Sonerai to accept a shape and mounting method similar to the Ibis then it might be worth a try.

It’s been a while since I visited my old thread but if you have any questions on my fiasco I’ll give my thoughts.
 
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