Main gear attachment location theory

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geosnooker2000

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I'm looking at different main gear ideas for my design. I think I have settled on what I want to do, but would love it if you guys could critique this thought.
(4 place all aluminum, total gross of as far under 2200lbs as I can get)
I don't like the idea of spring gear because I DO like the idea of offering a retractable gear option.
So for simplicity's sake, I'm thinking of mounting a rotating tube attached along the longitudinal axis of the plane.
The front of the weldment would be bolted through the main spar. The rear of this weldment would be bolted through the rear spar (flap attachment zone)
The main gear strut would be welded to this. Retract version rotates down, fixed gear version is just fixed in the down position with fairings.
.... first question, is there an advantage torque-wise with the angle that the strut is welded to the rotating tube weldment?
.... I know Piper (PA-28) has had problems with this as far as a really hard landing busting up through the top skin of the wing, and I suspect that is also the source of their cracked main spar problem.
All of their landing force is on the main spar. The strut is literally bolted to the top and bottom flanges of the spar (back-side).
My proposal is to divide the load between the main spar and the rear spar. But I would be bolting through the webs, so they would need to be reinforced with doublers.
Anyway, I know that a moment will be caused by force x distance. wherever the wheel is relative to the main spar, contacting the ground at the particular range of landing angles will apply torque to the main spar, but it just seems logical to me that if the strut were raked forward, as opposed to straight, or even raked aft, it would be (somewhat) less of a twisting force, and more of a "pulling aft" force on the main spar. Or maybe it is the fact that if you rake forward, you must contact/connect to the horizontal piece closer to the rear spar to keep the tire in the correct position relative to CG and CL. Obviously that will put more load on the rear spar.
.... Second question, is it a bad idea to have an actual rear spar carry-through in order to carry some of this landing gear load through the cabin? There is plenty of room under the rear seats.
....Third question, it sure would be nice to get away with a 5" wide tire in stead of the traditional 6" due to the possibility of retracts. Are there weight restrictions/recommendations to aircraft tires? because I can't find them if there are.

Wing Landing Gear.jpg
 

Jay Kempf

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Pretty common setup. Your torque rod. But normally you want your gear to be attached to that torque rod at the main spar end with a triangulated drag brace going aft. With the current design, your landing loads are going to bend that torque rod. Best to put the landing loads into shear at the front attachment and have a drag brace to the aft attachment. You could make it trailing link with the pivot forward and the shock strut triangulated heading back up to the rear attachment as well. Nothing wrong with putting the landing loads into the root section of the wing box inboard. That is a strong part of the airplane. Just have to factor in extra braces or lams/layers for each load case.

As far as tires, I am sure there is a 5" tire that can handle your proposed MTOW. There are tons of versions of the same tire and many different profiles as well. More plies equals more load rating. The goodyear catalogue is a bit tough to read but it is all there. Aircraft Spruce has a bunch of specification information for tires but it is hard to search for you have to dig.
 

BoKu

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I'd suggest you go out and survey a bunch of tricycle retractables, find one similar in size and weight to your airplane, and then buy a set of those undercarriage parts from a salvage firm. Then adapt your airframe to the gear. You might also consider adapting the Falco F.8L gear to your design; it's for a lighter airplane but the plans are widely available.

As Jay points out above, loading that longitudinal tube in bending is an inefficient use of the material. You are much better off using a truss weldment that reacts the forces in tensile and compression components rather than pure bending.
 

geosnooker2000

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Thank you both. I Feel like the wheel placement puts it in an unusual position when compared with... say, a Piper, because my main spar is so far forward compared to the PA-28 wing. I like the idea of moving the attachment point closer to the MS, putting it in shear. But the wheel needs to be as far aft as I have shown it because of the (suspected) CG location. So is it going to cause a problem if it is raked backward to accomplish this?

Sorry Jay, I now see you were addressing this. Let me read it a couple more times and let it sink in... LOL
 

Jay Kempf

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Better but I would make the triangle even bigger so it isn't loading the center of the tube. When it is a triangle it doesn't need to have a vertical member up front. Just angle it right to the attachment. Put all the load on the spar attachments. That will be the lightest piece in the end. Might need an internal brace against buckling but that large triangle will be the strongest and lightest.

There is one other way to do this and that is to have the front leg hinge at the front attachment and have the shock attach to a lug near the rear attachment. Then the whole triangle is your trailing arm. As soon as you lift off it goes to max extension for retraction. Drag link attaches to the leg part with rod end connections. That would work great.

I stare at Bonanza gear a lot cause I happen to have one. Same sorta triangular setup even though those are retracted inward and also not trailing arm. Retracting inward allows more room for the larger tire.
 

WonderousMountain

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I would probably clear out the bay where it retracts.
Which honestly ought to work, but I'm nervous just
looking at it. Trailing linkage does seem to be fine.

~CK LuPii
 

Mad MAC

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How narrow is your track, I would be tempted to move the gear outward and make them swing inward (I guess you pick outward becuase of the local reinforcement of the walk way).

Mooney is reasonible a trailing link gear with rubber donuts which would be about the right size for this.
 

geosnooker2000

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Better but I would make A the triangle even bigger so it isn't loading the center of the tube. When it is a triangle it doesn't need to have a vertical member B up front. Just angle it right to the attachment. Put all the load on the spar attachments. That will be the lightest piece in the end. Might need an internal brace against buckling but that large triangle will be the strongest and lightest.
A) I'm assuming you are referring to the 30/60/90 triangle which makes the brace back to the horizontal torque tube, But if you are, I don't understand what (B) means.
"Up front" as I see it, is the vertical piece that comes down from the torque tube and turns 45 degrees and holds the wheel. If there is no "vertical member up front", what holds the wheel? I must not understand where you are talking about? Which means I don't know how to apply that to "Just angle *it* to *the attachment*. Which "It" and which "attachment"? Could you please sketch it?
There is one other way to do this and that is to have the front leg hinge at the front attachment and have the shock attach to a lug near the rear attachment. Then the whole triangle is your trailing arm. As soon as you lift off it goes to max extension for retraction. Drag link attaches to the leg part with rod end connections. That would work great
I think I get that... but I worry when you start talking about that kind of stuff, keep in mind, this has to fold up into the wing with a sheet metal door attached to the side of it. The wider you make the action, the bigger that door gets. Which is the reason I made the 30/60/90 triangle the size that I did. The deeper it attaches to the torque tube, the wider the door gets.
 

geosnooker2000

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I realize that if I mount the torque tube higher in the wing (which it needs to be), that a good portion of that angle brace will not need to be fared - the portion that remains up inside the wing. Unfortunately, I was using the extra room in the airfoil up front for lever arms to actuate the turning of the torque tube in the space above it. If I move it up as high as I can, it will about be in the middle of the main spar... no good for actuator arms...
 

geosnooker2000

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How narrow is your track, I would be tempted to move the gear outward and make them swing inward (I guess you pick outward becuase of the local reinforcement of the walk way).

Mooney is reasonible a trailing link gear with rubber donuts which would be about the right size for this.
I Will check Mooney out further. As far as the track goes, my track will be at least as wide, if not the widest I know of in the 4 place homebuilt market. The most popular ones, anyway. Figure that most of the other guys use weldments or spring gear that attach to the fuselage.
 

Jay Kempf

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A) I'm assuming you are referring to the 30/60/90 triangle which makes the brace back to the horizontal torque tube, But if you are, I don't understand what (B) means.
"Up front" as I see it, is the vertical piece that comes down from the torque tube and turns 45 degrees and holds the wheel. If there is no "vertical member up front", what holds the wheel? I must not understand where you are talking about? Which means I don't know how to apply that to "Just angle *it* to *the attachment*. Which "It" and which "attachment"? Could you please sketch it?


I think I get that... but I worry when you start talking about that kind of stuff, keep in mind, this has to fold up into the wing with a sheet metal door attached to the side of it. The wider you make the action, the bigger that door gets. Which is the reason I made the 30/60/90 triangle the size that I did. The deeper it attaches to the torque tube, the wider the door gets.
What you did is move the attachment of the leg and then put the triangle brace right back where you have it pushing on the middle of the tube in bending. You need to get that triangle to the rear attachment or move the rear attachment. Another spar or hardened rib attachment to shorten the horizontal tube.

Here's a pick of the extreme version. Trying to get you to stop thinking of the leg as vertical. Once you brace it doesn't have to be. There's a reason why most gear folds in but if you don't have room behind spars, seats, yadda... you have to improvise. That's the fun of design:
IMG_20210603_190741.jpg
 

rv7charlie

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For fixed gear version, the Bushby Mustang II looks kinda like that; might be worth a look (and also look at what they did a few decades ago to address some original weaknesses/cracks in the mount structure).

On swinging in vs out, the Globe Swift swings up/in, and it's just about the tamest taildragger you can find. (Please don't tell anyone & ruin my SuperAce cred).
The Swift is electro driven hydraulic for actuation (basically an inboard/outdrive tilt pump; but again, don't tell anyone...). Obviously moving the hinge point outboard increases stress on the center section spar a bit.

A friend of mine built a M-II many many years ago with trike retracts. The M-II uses a wing center section that's about 8' wide, with the original single seat M-I wings as the outer panels. I'm no structures engineer, but I suspect that running the center section out beyond the gear attach point should be quite a bit more efficient structurally, both in the landing load path through the spar and the outer wing panel attachment method (lower stress because the joint is farther outboard).
 

proppastie

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not to be real negative on homebuilt aircraft....I believe much of the performance increase with homebuilts is that they save weight on the gear.....as most G limits is airframe only "not the gear". The Chevys Fords, Cadillac's of the aircraft world have to be able to survive real abuse.
 

rv7charlie

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Unless the plane is going to cruise well north of 200 kts, it's unlikely there's much if any advantage to retracts vs proper fairings & aero. Compare Van's RV10, the Cirrus, & the Cessna certified version of the Lancair ES to the various certified retract a/c using similar engines. Weight gain from the retracts drives bigger wings, reduced useful load & more drag.

Does look cool, though.
 

proppastie

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Weight gain from the retracts drives bigger wings, reduced useful load & more drag.
as a former fixed gear owner and now Mooney owner....I really miss the simplicity of my Cherokee.....I do not miss the slow....but as regard work load and other factors I dislike the gear, cowl flap, EGT, gear speed limitation, red band in RPM, grass and rough field abilities, of the Mooney. At one time they were challenges to master now are somewhat overkill for the $100 lunch.
 
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