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Let's talk gear geometry

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Posted

This has been somewhat of a hot topic lately so i thought I would post some various gear designs to hopefully spark some discussion from someone much smarter than me on it. The discussion is on the strength of the cabane strutted spring gear.

 

Airdale gear

 

IMG_4329-1.jpg

 

Version two contains lighter springs a thicker wall tubing the addition of a cross brace between the front and rear leg where another set had failed.

 

photo8-2.jpg

 

New brace the first set didn't have

 

IMG_5560-1.jpg

 

HighWingLLC gear

 

gearleg5s.JPG

 

 

gearleg4.jpg

Super Cub Geometry on Pops Dory latest cub using springs instead of bungees

 

df9fb03867e81aafa96c6b69.jpg

 

 

Murphy Bush Gear

 

507af86c8e5bdc3584618c53.png

 

Rans Brand New "Monster" Gear

 

1.jpg

 

Roberts Bush Gear

 

IMG_2716.JPG

 

 

 

 

 

So it's obvious there are subtle differences between every set, but generally speaking they are all similar. So what makes one set work and another one not???

 

IMG_4708.jpg

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Posted

Great topic since I have been researching gear design for the last week.

John Roberts said the front gear leg takes the greatest load and is in compression so that is why it needs to be beefier and braced like your new version is to prevent it from collapsing.

Extending and widening the gear also requires bigger diameters and wall thickness to prevent tube failure on compression.

Roberts also said the gear and cabane attach points need to be designed so the forces are applied inline with the longerons instead of twisting the longerons.

That explains why most cub style cabanes use separate attach points from the gear leg.

I was surprized to see at the High Sierra fly in that Pops Dory was not using bungees on his new Cub.

As long as the springs have enough travel to prevent metal on metal for reasonable hard hits, they sure look like the way to go.

My off road car has 2 springs per shock.

When lighter spring bottoms out the stiffer second spring takes over for progressive action.

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Posted

How hard do you have to hit to bottom out the spring or the bungee gear? Taking the lead out of the tail and the 150lb engine off the front would be a good place to start. Like I said before, fly the airplane on to the runway! The hell with the full stall landings. Like Leni said once before, you are decending at around 1000ft per minute, If you don't judge the flare just right something is going to bend if you hit hard. I can't imagine the stress on the rest of the plane. My door latches easier on the ground than in the air. So the airframe must twist and turn while flying. It really must twist on a hard landing.

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Posted

I think the Pipers have survived the hard landings where the Avids/Foxes have gotten bent up because Piper longerons are a lot bigger and heavier than the Kitplanes - not engineer - just observing.

EDMO

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Posted

      Does any one Know what size tubing was used to build the Bush gear, I am not an engineer but I was an Industrial Technology Major in college and a lot of what I was taught was how to pick apart a design and modify it to work better, Tubing has different properties that can be taken advantage of depending on size and wall thickness, I have seen this gear and see one glaringly obvious but fairly easy to fix problem that will be the next point of failure.

      30 years ago I built a trailer that weighs less than 400 pounds and has hauled a 1 ton truck from LA to Des Moines with no problems by taking advantage of tubing that would do the job but not be any heavier than necessary, It is still going strong today, to bad I never get around to painting projects like that everyone wants to borrow my Backhoe trailer to haul their cars on and then complain about how heavy it was,

     In order to fix the problem I would need to Know the force that it came in at, a good guesstimate of landing speed and terrain would help and the size of the tubing, I have been working on the bush gear and can see the next point of failure that only the 1%ers like C5 are going to find, It is surprising that a 30 year old design for grass landing strips has been able to hold up to the forces imposed by the landings people are making now. Put the original gear on and if you can keep the prop out of the rocks I bet it does just as well. There is a guy on Barnstormers who has a Patent pending Air Shock design that I wish him luck with, Unless it is very heavy it won't work as well but I think the solution runs into the tubing and would like to know the thickness and the size, C5 gear failed where I originally thought they would, and If you just strengthen that you will have another failure in a similar place, By changing thee design slightly I think we could have the problem fixed but I would like to see more failures from you 1% ers so I can work on this problem.

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Posted

Joey

How much travel does the machined groove have on your spring strut before it goes metal to metal on the safety bolt?

From what I can see through the spring it only looks like 3 inches of strut travel.

Do you have any idea how many inches your tire will travel if you were to cycle your suspension without the spring in place?

I assume you are getting more travel at the wheel than the strut travels,  like a dirt bike has 12" of wheel travel and the shock only travels 2.5"

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Posted

Herman this is just a guess but I think 3" is about right. I will take pics when I get home and diassemble it for painting. These springs will hopefully allow for at least 2" of compression. Those springs on the first version would support a loaded 185. They had no travel. The weight of the airplane didn't even make them budge. You can see in the above picture of the unpainted gear there was only about 1/16" between each wrap of the spring. It was basically fixed.

 

Trackwelder I have a friend that is running the air rage shock system on his S-7. It is the Roberts Gear pictured above only it was retrofitted to use the airshocks instead of bungees. I can attest that he uses them to the max extent possible. Even his own home strip is very gnarly and ALL his landings are off airport on mountaintops in Idaho. He is running 31's too but he's had good luck with it so far. He started out with a Kitfox 1 a long long time ago and this is his 2nd S-7 and he said it's built so much better for that kind of operations.

 

Your right about the bungee gear. I think it'll take a pretty good hit before anything bends..the weak area is the seat truss. You are correct that if you beef one thing something else is going to fail IF you prang one in. The goal is find that balance where it'll take a pretty good licking and keep on ticking. The Airdale gear is built out of 1" tube with an .049 wall I believe. HighWing LLC used 1 1/4" for the front leg but 1" the rest of the way around. The Roberts gear uses 1 1/2" and is quite a bit beefier all the way around. It requires welding on the fuse to even install it. The Highlander Gear that Steve Henry had on Yehaw 1 and 2 uses 3 legs instead of two so the Cabane has it's own set of legs.

 

JustAircraft5.jpg

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Posted (edited)

     I don't think or at least didn't mean to say the airshock wouldn't work, but my math was in the ball park, I believe the next point of failure is the V at the top of the struts, if a person were to put a cross member through there it would triangulate it and take the seat truss out of the equation. Like I said the troubles seem to occur with the 1 % ers  and then only when they have reached beyond the limit, If you can build it I can break it, the only thing I had to really say about the airshock is that the patent won't hold up. I will never build a part to sell if someone else is making a living at it but someone will. I think the ultimate for now is the Highlander Superstohl with 18 inches of travel, other than the wings coming off I don't know how you could break it without trying. 

Edited by Trackwelder

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Posted

I took a close look at the black Highlander at Oshkosh this summer. It looked like to me the fuselage ahead of the verticle fin and the gear attach points had been repaired. The fabric had been patched and the black paint wasn't as glossy. I think those airplanes bend to.

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Posted (edited)

if a person could incorporate the shocks that are used in a soft tail Harley, they are designed for extraction loads not compression,..at the bottom of the stroke the oil works as a damper versus when you bottom out the spring it's solid,..the solid "Bang" of bottoming out is what instigates the deformation of the tubes, sending a ripple down the shock rod to the axle weldment at the wheel back up the forward gear leg,..take the "Shock or Bang" and add the fulcrum of the wheel center being close to 8 inches outward of the axle weldment junction,..which adds warpage to the forward gear leag like a bow,..bendind inward,..there is allot going on at once and the weakest link will deform and sometimes to the point of failure.

 

is anyone familiar with Challenger's fiber glass upgrade on the mains? if you could replace the front gear leg with a similar round fiberglass rod,..when the coil spring bottomed out,..this would transfer the "Shock" to the forward gear leg which would then dampen the forces that would be normally put on a tube,..yes it adds weight,..,..but now you have also added flex for when the spring gets compressed completely

Edited by SkyPirate

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Posted

Just thought I would put in my 2 bits. I built a Bush gear for my Mod 1 Kitfox. I used die springs in a housing similar to the bearhawk gear. Designed it to take 3 G's before spring would go solid. I did a very poor landing. The gear held up fine but the fuselage failed. I don't believe the springs went solid because the safety cable cable ties were unbroken. I have considered building new tension members with bungees. I believe an Aluminum Spring Gear or bungees are the more forgiving as they provide the most flex (not including the new Highlander gas strut gear). Here is a link to the drop test video I did on my gear; http://www.youtube.com/watch?v=fvQtKQ5gHVY

Dave  

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Posted

BTW This is what happened to the fuselage with the hard landing;

post-183-0-40373200-1383789205_thumb.jpg

post-183-0-42672300-1383789220_thumb.jpg

post-183-0-73540900-1383789237_thumb.jpg

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Posted

Dave, your gear, just like the Airdale and Highwing gear have the spring strut linkage making a strait line from the tire to the opposite side longeron.  This directs the force from the tire to the opposite side longeron.  

 

If you look at the Rans or Piper gear the link is not a strait line.  By breaking the plane of force it splits the force.  In this case to both longeron sides.

 

IMO this is the first design flaw.

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Posted

Av8r3400

That is not what caused the fuse failure. It was the compressive load of the gear leg not being directed to the center of the longeron due to the lug hole being directly below the longeron.

Dave

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Posted

Fox if you could have put a couple points about 3 inches long ,,1 at top of forward gear leg 90 degrees to leg and one at the bottom and then put a string between them so you could see it parallel with the forward gear leg from the front,..I wonder how much it would have slacked at impact

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Posted

SkyPirate,

Are you refering to the drop test?

Dave

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Posted

SkyPirate,

I believe it defintely would have gone slack at the outside. There is a bending moment on the axle which would transfer to the gear leg. This is a big contributor to bucking of the gear leg as shown in the picture posted by C5enginger. The best defence is to increase the diameter of the gear leg as I don't believe the tire could be any closer to the leg.

Dave

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Posted

DB thanks for sharing your information. This is the type of conversation I was hoping this thread would generate. I think the reoccuring theme here is don't make poor landings....I know mine would still be on there if I would have said naw....I better not land that spot that's at 6000ft on the side of mountain that's full of rocks and gopher holes, 20 miles from the nearest road, with all my camping gear on board, by myself......I made a laundry list of screw ups that day...I survived, damage was minimal, it makes a good story, and I learned alot so about all you can do is press on and try to not do it again..... :BC:  :lmao:  :huh:

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Posted

This discussion is driving me bonkers.

Why, it is making points for the Grove gear mainly due to it's simplicity in spite of adding approx. 12 pounds over original gear and who knows how abusive Grove gear is going to be to the longerons and bungee truss when the going gets really rough.

I really want Cub style gear (if it is really better than the alternatives for rock bashing) and I have spent weeks researching how to do it correctly on my Model 2 Kitfox.

I do not like the advice I have been given.

My Kitfox's longerons, seat-bungee truss and lower door frames need to be replaced with bigger tubing and more bracing and that is major surgery.

And why do I want to consider all this?

So I can Adventure fly to where airplanes were not intended to land.

What better thing to do with an airplane that I could almost drive there faster than fly.

My wish for Joey's thread is to come up with a gear design that requires minor surgery to the fuselage and will take much more bashing than stock gear.

FoxDB's post #14 regarding how the compressive loads are directed towards the longerons needs to be addressed.

Trackwelders post #8 regarding triangulating the Cabane V is right on IMO.

A simple tube across the top of the V would help both longerons to share the loads and minimize the longeron twisting I believe.

Notice Joey's 9th photo of the front view of a white Rans S-7 with Roberts Gear.

The gear and cabane get bolted to their own brackets so the compressive forces do not try to twist the longerons.

Both the Highwing and Joey's gear use the same bolt for the front gear leg and Cabane.

This makes the install very simple since new brackets do not need to be welded on the longerons.

This also becomes a compromise because I feel we really need bigger than 1 inch diameter front gear legs to prevent Joey's front leg collapse.

They say tubing diameter has more to do with compressive strength than tubing thickness.

My Kitfox brackets are 1-5/8" inside. It would be challenging to pinch down 1-1/4 tube and weld it to a slug and not exceed 1-5/8's width let alone room for the Cabane tabs to go inside the Brackets.

John Rodgers from Homedale has a unique style cabane gear he built for his Model 4 Kitfox.

His Cabane V if you can call it that, is welded to a reinforced seat truss and he uses 1-1/4 diameter front gear leg.

Anyone know how John Rodgers gear works.

Enough for now.

I am counting on you engineers.

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Posted

I have 2 of those Harley shocks sitting on the bench, they are heavy, and the wheel travel on a Harley is only 4 inches with the ratio included, I don't know how they would help, but I figured if we were going to talk about them, we should keep that in mind. A Softtail Harley weighs over 600 lbs depending on the model, and with passengers weighs up to 1200 lbs, and uses 2 shocks to support the load, they are designed for constant motion where we are looking for something that moves only occaissionally. Just some parameters to add to the pot,

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Posted

I'm a Harley rider and have thought of those shocks before too. They would need to be used with a bellcrank type do system to multiply their travel.

With a Grove retrofit, the bungee truss is no longer an issue for strength. As shown in Doug's "taxiing incident" the door trusses are where the failure will occure. That is what I did the modification I did to my mangy fox project. I doubt very much I will ever to any extreme stuff that you guys out west do, but this will add a ton of strength.

http://www.avidfoxflyers.com/index.php?/topic/709-classic-iv-restoration/?p=10266

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Posted

This discussion is driving me bonkers.

Why, it is making points for the Grove gear mainly due to it's simplicity in spite of adding approx. 12 pounds over original gear and who knows how abusive Grove gear is going to be to the longerons and bungee truss when the going gets really rough.

I really want Cub style gear (if it is really better than the alternatives for rock bashing) and I have spent weeks researching how to do it correctly on my Model 2 Kitfox.

I do not like the advice I have been given.

My Kitfox's longerons, seat-bungee truss and lower door frames need to be replaced with bigger tubing and more bracing and that is major surgery.

And why do I want to consider all this?

So I can Adventure fly to where airplanes were not intended to land.

What better thing to do with an airplane that I could almost drive there faster than fly.

My wish for Joey's thread is to come up with a gear design that requires minor surgery to the fuselage and will take much more bashing than stock gear.

FoxDB's post #14 regarding how the compressive loads are directed towards the longerons needs to be addressed.

Trackwelders post #8 regarding triangulating the Cabane V is right on IMO.

A simple tube across the top of the V would help both longerons to share the loads and minimize the longeron twisting I believe.

Notice Joey's 9th photo of the front view of a white Rans S-7 with Roberts Gear.

The gear and cabane get bolted to their own brackets so the compressive forces do not try to twist the longerons.

Both the Highwing and Joey's gear use the same bolt for the front gear leg and Cabane.

This makes the install very simple since new brackets do not need to be welded on the longerons.

This also becomes a compromise because I feel we really need bigger than 1 inch diameter front gear legs to prevent Joey's front leg collapse.

They say tubing diameter has more to do with compressive strength than tubing thickness.

My Kitfox brackets are 1-5/8" inside. It would be challenging to pinch down 1-1/4 tube and weld it to a slug and not exceed 1-5/8's width let alone room for the Cabane tabs to go inside the Brackets.

John Rodgers from Homedale has a unique style cabane gear he built for his Model 4 Kitfox.

His Cabane V if you can call it that, is welded to a reinforced seat truss and he uses 1-1/4 diameter front gear leg.

Anyone know how John Rodgers gear works.

Enough for now.

I am counting on you engineers.

The axial stress of pure tension or pure compression is strictly a function of the load divided by the cross sectional area of the steel material.  But a larger diameter tube with the same cross sectional area will be stiffer due to it's greater moment of inertia (fancy word for the steel area being further out from the nuetral axis of the tube).  In tension, the tube stays straight under load so it doesn't matter if the tube is large or small diameter as long as it has the same cross sectional area; but in compression, the tube wants to bend so the bigger diameter tube (greater moment of inertia) is stiffer and resists forming a "knee" bend.  The knee forms because of lateral instability under compression.   Each tube has a slenderness ratio that lets you determine the maximum laterally unsupported length the tube can have and still develope it's maximum allowable compressive stress.  You can put a lateral brace on the tube and cut the laterally unsuported length or you can select a larger diameter tube to keep it within the laterral length limit.  There are limits to keep in mind on large diameter thin walled tubing; it is possible to get the tubing diameter so large and the wall thickness too thin and you will develop crushing of the tubing wall like a pop can.

 

If you can keep the loads truely axial only, without any torsional loads or bending loads that night be caused by loads from something connected out of alignment or pushing sideways on the tube, the tube will be able to take the greatest load.  If it has to resist torsion or bending loads in addition to the axial load, the composite of the additional out of plane load will greatly reduce the allowable axial load component before it reaches it's stress limit.

 

Just my 2 cents for those who are working on new gear.

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Posted

I agree totally superavid,..if your going to go larger diameter tubes they should also be thicker,..if someone wanted to keep the same OD tubes but go thicker,..options cut out the tube and replace with thicker same OD tube or cut the tube at either furthest point  possible on one end or the other ,..but not so far that the inner tube can not be slid back past the cut joint and slide a tube inside then drill and  fill weld ,..then re weld  joint that was cut,..I'd stagger the drill holes so there is no linear stress on the outer tube

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Posted

Hi guy's it is very cool post......do you think it will be better to used a good grade of stell than 4130 ( i think 4130 it not very hard metal)

You can stay with same size but more strong,less weight..4130 is easy to bend..it is a great metal for aicraft frame ,but for landing gear???

Jean-francois...but call me Jf..lol

Building a kitfox 4 1200

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Posted

Oh...to...do you think it will be good to put a good rubber spacer between the spring and his stopper to absorb the hammer hit when the spring is fully load??? I think it's can really help .where i working ( undergroud) we have a very good red rubber ,they call that linateck..im not sure how is write..but it is really really strong...

Jf

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