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Magnums with Airdale Bush Gear

42 posts in this topic

Posted

Well, I took Randy's advice and stitch welded another tube under the seat truss tube. I used the same diameter tube but with a thicker wall. I attached the ends of the tube to my front gear mounts. It is strong as hell, so something else will bend before this does.

My question is...should I still reinforce the seat truss with glued in plywood? It would be easy enough to do at this point.

Thanks for the ideas!

Here are a couple of pics

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Posted (edited)

Why not reinforce sides with some sheet metal - better and easier than plywood?

EDMO

Edited by Ed In Missouri

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Posted

Here is what I did to reinforce a seat truss.  I had 3" wide .040" material to work with so that's what I used.  On a side note,  I still think your whole system would be stronger if you tied the seat truss to the center of the 'V'.  Jim Chuk

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Posted

Ed,

So you think the sides need it to? I thought they would need it, but most of the threads I have read just talk about the seat truss.

Jim,

Thanks for the pic, it is truly worth a thousand words.

So you are saying that you would add a vertical bar from the bottom of the seat truss to the deepest part of the "V", is that right?

Is that so the upward force would go into the middle of the seat truss?

I looked at your plane, but can't tell, are you using bungees? It looks like you have a taller gear too?

I appreciate the input.

Thanks,

Ron

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Posted

Ed,

I just found the thread and pics that Av8r3400 posted of his Kitfox, where he reinforced the sides with .032 sheet with 2" lightening holes. I like it a lot! Is this what you were talking about?

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Posted

Hi Ron,  you got the right idea on what I was trying to say about connecting the seat truss to the V.  I just think that by doing that, it strengthens the whole system.  Triangulation is what makes things strong.  Adding the verticle connector strengthens the V by adding the seat truss strength to it.  I think it would relieve some of the stress that is put on the fuselage where the V connects as well.  When the center of the  V is pulled DOWN in a hard landing, it acts to pull the sides of the fuselage togeather.  I've seen some pics of fuselages that were bent up from the bush gear, and you can see how the pressure acted upon them.  When the plane hits hard, the landing gear attachments are pushed up and in, and at the same time, the V is being pulled down which acts to pull the gear attachments togeather.  The pics I saw show how the bottom tube of the seat truss buckled inward next to the edge of the fuselage.   At any rate, that's how I see it, what does others think?    I had the regular Mk IV gear with bungees on my Avid when the seat truss bent.   (Someone before me had welded some tabs on the bottom of the seat truss, and caused a brittle spot on the bottom tube of the seat truss.  On a not real hard landing, it let go and the top tube of the truss bent.  I added the metal to the truss when I rebuilt it)  Hope I made sense with my explanation.  Jim Chuk

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Posted (edited)

Ron,  the photo you posted is exactly what I was talking about - some have used .040 or even .050 and the lightening holes - every thing helps - and it don't add that much weight, but a lot of strength.

I'm not an engineer, so I cant compare it to plywood, but I would use steel.

EDMO

Edited by Ed In Missouri

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Posted

Thanks guys, you gave me a lot to think about.

Jim, thanks for the explanation of the forces applied. I didn't think the V was getting a downward force, but now that you made me think about it, that makes sense.

Ed, I'm convinced and I ordered the steel today.

Thanks again,

Ron

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Posted

Ron,  when you think about the force on the V, think of this.  Take a long rope and tie it tight between two points,  Pull up on the center of the rope.  The straighter the rope is, the more pulling force you can put on the two ends of the rope.  It's basically a matter of leverage.  That would explain why some of these Vs are stronger than others, the straighter the V, the more pressure it can put on the fuselage.  My idea of a verticle connector would keep the V from flexing downward, it couldn't bend unless the seat truss bent with it.  Anyway, this is my opinion for what it's worth.  I remember this kind of coming up a while back and it just kind of died off.  Any other opinions?.......  Jim Chuk

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Posted

Ron, 

 

I don't think reinforcing the seat trus is necesary with the cabane type of gear.  It does not act to bend the seat truss like the bungee gear with the bungees wrapped around it in the center.  The cabane gear causes a compressive force on the seat truss rather than a bending force.  That is why the added tube at the bottom of the seat truss will help take the added compressive force.  Adding additional web reinforcement to the seat truss will just add additional weight.  What Jim did will be excellent if using bungee gear and what Av8r3400 did would be a perfect reinforcement if using grove spring gear.  If the angle of your cabane lines up pretty close with the angle of the tension leg of your gear (that the spring is connected to), the added compression tube you put in should do the trick as long as you don't have springs that bottom out.  If the springs stack solid, something is going to give no matter what.  That is the humble opinion of this structural engineer.

 

Randy

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Posted

Randy,

So, you feel that other than the extra bar that I welded in, I should leave the rest alone...seat truss and sides correct?

I am not an engineer so I appreciate the input. From other projects I have worked on it seemed like strengthening in one place often just transferred the force somewhere else, and taking the flex out through strengthening often led to ultimate structural failure. So I realize that there will be a lot of ideas that seem to contradict each other because it is a thin line we are walking, not to mention that this is completely custom and untested.

The suspension I'm using is basically rubber pucks that compress, I think somebody called it the Hydrosorb system? Anyway, it probably has a maximum compression, but it won't be abrupt like metal hitting metal when a spring bottoms out. Also, the cabane V and the suspension are lined up.

Thanks for the input everyone, this is very helpful!

Ron

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Posted

Ron,

 

If it were mine I would leave it as you have it. The best design is one with just enough strength and no more than needed.  Additional unnecessary strength is just additional weight.  Granted, this is just a qualitative and not a quantitative analysis, but I attached a simple sketch of the landing gear with the forces acting on the air frame shown by vertical and horizontal force components.  The tension forces act to pull on the connection joint and the compression forces act to push on the connection joint.  If you do a force equilibrium analysis on the joint you can break the forces into vertical and horizontal components.  If the joint is in equilibrium (not moving) the sum of the forces in the vertical direction have to equal 0 and the sum of the forces in the horizontal direction have to equal 0.  This shows the upward vertical forces acting on the joint from the compression leg are somewhat offset by the downward vertical forces acting on the joint from the cabane tension.  In the horizontal direction the horizontal forces from the compression leg and the horizontal forces from the cabane tension are acting in the same direction and add together.  This total of the horzontal forces has to be balanced by the total of the forces on the joint on the oposite side from the other gear leg.  You can see that these horizontal forces cause added compression in the cross member that you reinforced but do not add any bending to the seat truss and in fact the bending caused by the original bungees is not there anymore.

 

I hope that helps explain my reasoning on this but make the decision you are comfortable with.

 

Randy

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Posted

I still believe, just my opinion - nothing else, that:

 

The junction of the cabine (the center of the "X") should be either lower or higher making the "tension" element to be NOT in a straight line.

 

This is true of the Piper Cub gear and most other cabine style gear.  I believe that when this element is in a straight line all of the tension stress it exerts goes directly to the longeron mount on it's end.  This is where the twist damage is coming from in the Airdale design.  If the tension element is broken in the middle, this stress is then divided to both side mounts.  Negating the need or at least most of the need for additional cross bracing.

 

 

(Does that make any sense? Or am I just whistling Dixie?)

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Posted

Larry,

 

You're right that the depth of the cabane V changes the load on the airframe.  The deeper the cabane V, the less load it will impart to the airframe; the shallower the cabane V, the more load it will impart on the airframe.

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Posted

Randy and Larry,

You guys answered a question that I have always wondered...why is the cabane V always so low when you are trying to get higher clearance with a bush style gear? So, they are lowered to get rid of some of the direct force to the frame. Makes sense to me.

Randy,

Thanks for the drawing, it helps keep Mongo's brain from hurting :)

I am going to stick with what I've got, like you suggested. I can always beef it up later, or fix what I break. I don't think that will happen though. The guy that helped me design the gear geometry has been building cub gears for years and he really put a lot of thought into it as well, and his only additional input was that the cub had a larger diameter tube for a seat truss and that I should beef it up. Your stitch welded bar idea filled that bill!

Larry,

I've seen a picture of your vertical bar idea. I can't remember if it was on this site or somewhere else. I seem to remember that the person using it was very happy with the results! I'm going to track it down, and post it when I find it.

Thanks guys, I'm getting more out of this than I expected! I appreciate it!

Ron

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Posted

Well, it has been a long while, but I thought I would update this post. I decided to design my own gear but have somebody else build it because I wanted it welded right! I can weld, but I thought the landing gear was too critical for me to experiment with! That being said, I had to weld brackets on the front mounts to provide attach points for the cabane V. The gear measures 32" Tall and 73" from axle plate to axle plate. I stole rough numbers from Randy because I liked the AoA on his Avid.

Steve Furjersi at Experimental Aircraft Fabrication inc built the gear for me, and I bought his unique suspension system too. He is a great guy and he really helped engineer a cool gear for my Magnum.

I'm attaching pictures, so take a look, and throw spears if you have them, because I want all of your input!

 

Thanks,

Ron

 

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Where could a guy get this hydrazorb material?  Thanks,  JImChuk

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Posted

Well, it has been a long while, but I thought I would update this post. I decided to design my own gear but have somebody else build it because I wanted it welded right! I can weld, but I thought the landing gear was too critical for me to experiment with! That being said, I had to weld brackets on the front mounts to provide attach points for the cabane V. The gear measures 32" Tall and 73" from axle plate to axle plate. I stole rough numbers from Randy because I liked the AoA on his Avid.

Steve Furjersi at Experimental Aircraft Fabrication inc built the gear for me, and I bought his unique suspension system too. He is a great guy and he really helped engineer a cool gear for my Magnum.

I'm attaching pictures, so take a look, and throw spears if you have them, because I want all of your input!

 

Thanks,

Ron

 

post-545-0-70511300-1388265100_thumb.jpgpost-545-0-29202100-1388265166_thumb.jpgpost-545-0-10391700-1388265214_thumb.jpgpost-545-0-63353500-1388265255_thumb.jpgpost-545-0-71086900-1388265306_thumb.jpgpost-545-0-27314200-1388265352_thumb.jpgpost-545-0-78950500-1388265390_thumb.jpgpost-545-0-86525500-1388265431_thumb.jpgpost-545-0-97276700-1388265480_thumb.jpg

Where could a guy get this hydrazorb material?  Thanks,  JImChuk

Would also be nice to get some specs on the "rubbers" - I have the Polyurethane Products site where one guy got his "donuts" for his landing gear.  EDMO

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