Author Topic: Landing gear geometry  (Read 1189 times)

Offline Mark Kramer

Landing gear geometry
« on: September 09, 2019, 04:01:06 PM »
I am building a LEXL and am using the spring suspension. Drilled and mounted the vertical plates onto the fuselage as per plans where the gear legs will mount. The pivot points of the gear tops are horizontal, parallel to the lower longeron. Now, the v strut is tipped forward 12 degrees as per plans, meaning the pivot point at the top of the spring assembly is tipped 12 degrees downward to the rear. This difference in the hinge lines does not allow the gear leg to lift up. Is there something I’m not getting? Has anyone come across this situation during their build? I will try and post a pic. Also you will see I’ve been using wooden dowels to get the geometry correct, glad I did. Making me crazy

Offline scottiniowa

Re: Landing gear geometry
« Reply #1 on: September 09, 2019, 07:25:29 PM »
I am building a LEXL and am using the spring suspension. Drilled and mounted the vertical plates onto the fuselage as per plans where the gear legs will mount. The pivot points of the gear tops are horizontal, parallel to the lower longeron.
seems like your stating that correctly

 Now, the v strut is tipped forward 12 degrees as per plans, meaning the pivot point at the top of the spring assembly is tipped 12 degrees downward to the rear
Now you have lost me, perhaps I am not understanding your terms as per part of the landing gear.

. This difference in the hinge lines does not allow the gear leg to lift up.
If I read your first line correctly, you said your pivot points of the gear tops are horizontal and parallel, And I would call these points "your hinge line, or center of axis line"  So what are you calling the "difference" as you stated they were NOT different at the beginning.
  I will try and post a pic. Also you will see I’ve been using wooden dowels to get the geometry correct, glad I did. Making me crazy
I tried to see the full gear A frame down to the axle, could not determine this from your photo In fact could only see what hinge point... maybe I'm missing something?
best email address:  irondesignairparts@gmail.com

Offline scottiniowa

Re: Landing gear geometry
« Reply #2 on: September 09, 2019, 07:31:23 PM »
  I will try and post a pic. Also you will see I’ve been using wooden dowels to get the geometry correct, glad I did. Making me crazy
On second look at  your photo, it looks like your showing one pivot point down to the axle or one half of the A-frame, and then the tube down to axle as well that would have the shock spring, but showing without the spring. or SOLID, if I am seeing this correct, why/how  could it move? even with the full A frame, it would remain the same, no spring, no movement.
best email address:  irondesignairparts@gmail.com

Offline Mark Kramer

Re: Landing gear geometry
« Reply #3 on: September 10, 2019, 04:20:11 AM »
Sorry,guess my text and pic are a little confusing. I mocked up the front 3/4 leg of the a frame with a dowel. The other piece we’ll call the shock strut. I have not cut the shock strut to install the spring assembly yet as I wanted to see the geometry of everything first, so I ran it all the way into the v-strut with a bolt. Now if you grab the axle sleeve,
and lift up the front gear leg and axle sleeve, they do not travel upward together, not even close. If my front A- frame dowel was the real thing and welded to the axle sleeve I fear the axle would not lift. This I believe is because the gear leg pivot point and the shock strut pivot point (at the v-strut) are not parallel. The two pieces simply do not travel in the same arc.Sorry for my inability to easily convey the problem. I appreciate any help I can get.

Offline Mark Kramer

Re: Landing gear geometry
« Reply #4 on: September 10, 2019, 04:28:36 AM »
After looking at my last post I realized it still might not be clear. So let me add I realize without the spring assembly nothing can move. My mocked up gear leg is not attached to the axle sleeve, so if you grab both with two hands and lift, that is when you will see them separate and travel upward in two different arcs

Offline garygans

Re: Landing gear geometry
« Reply #5 on: September 10, 2019, 06:11:30 AM »
How did you align the 2 mounting fittings at station 2?  I ran a 1/4" rod through them back to the other 2 fittings at station 3, then welded them on. That set the arc for the main gear.  I did have a little binding from the different arcs as you described, I just reamed the holes out for clearance. It didn't take much since the gear doesn't move much. I don't know if there is a better way though.

Offline scottiniowa

Re: Landing gear geometry
« Reply #6 on: September 10, 2019, 07:40:30 AM »
After looking at my last post I realized it still might not be clear. So let me add I realize without the spring assembly nothing can move. My mocked up gear leg is not attached to the axle sleeve, so if you grab both with two hands and lift, that is when you will see them separate and travel upward in two different arcs
Mark,
Are you saying you are doing a mock up of your complete A frame down to the axle?  And that the two pivot points at the top of the A frame are IN LINE, and you still can't move this?  Never mind the shock strut for the moment  While I am not going to go into any details, there simply can't be two different arc's of travel if the center of axis on two pivot points are col-linear 

 The only way at this point you could create two different arcs is to have non=linear axis points i.e. two different plains or angles of axis

Perhaps a better way of saying this, is you would simply have all holes for the pivot of the A frame, line up perfectly, a correct sized straight rod will check this for you.  Go through the front holes and right on to the rear holes. Everything concentric.

Let us know. :-)
best email address:  irondesignairparts@gmail.com

Offline garygans

Re: Landing gear geometry
« Reply #7 on: September 10, 2019, 08:09:09 AM »
I think that he is talking about the strut connecting to the V-strut. That will have a slightly different arc than the main gear.

Offline Tom H

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Re: Landing gear geometry
« Reply #8 on: September 10, 2019, 08:21:28 AM »
Mark, I know exactly what you are talking about.  The prototype DE was designed with the gear legs straight down, right angle from the lower longeron.  However, a mod was introduced, which we called the DeLoach gear, which angled the gear forward from the longeron mount.  This introduced some weird angles and potential conflicts when the gear swung up/down.

I am not real familiar with the XL gear, but from your pictures it looks like your gear angles forward, too, like our DE.

The trick is to have all pivot pins parallel to each other, and parallel to the fuselage.  That includes at the top of the main gear leg where it pivots at the longeron, and at each end of the spring link tubing, both at the top at the V, and at the bottom attach point at the wheel axle.

You may have to fabricate your pivot bushings in the tubes at various angles.

It is hard to explain, but I hope you can get the idea.
Tom H
Stubby, a BDE
Treehugger, LEU

Offline Dan_

Re: Landing gear geometry
« Reply #9 on: September 10, 2019, 08:31:59 AM »
After looking at my last post I realized it still might not be clear. So let me add I realize without the spring assembly nothing can move. My mocked up gear leg is not attached to the axle sleeve, so if you grab both with two hands and lift, that is when you will see them separate and travel upward in two different arcs
How did you align the 2 mounting fittings at station 2?  I ran a 1/4" rod through them back to the other 2 fittings at station 3, then welded them on. That set the arc for the main gear.  I did have a little binding from the different arcs as you described, I just reamed the holes out for clearance. It didn't take much since the gear doesn't move much. I don't know if there is a better way though.
Some thoughts...

Unbolt the upper spring strut, bolt up the lower spring strut to the inboard axle mount.  Then hold the upper spring strut forward or aft to match the arcs as you move the axle up and down.  Then decide if you need to take a block of wood and a BFH to tweak that upper spring strut mount location.

I have marked with a red arrow what I am calling the upper spring strut mount...  See attachment.

The arc only has to match exactly at the mid point of the limited die spring travel. 


“A superior pilot uses his superior judgment to avoid situations which require the use of his superior skills.”

– Frank Borman, Apollo 8 Commander

Offline Mark Kramer

Re: Landing gear geometry
« Reply #10 on: September 10, 2019, 03:43:51 PM »
Hey guys, thank you all for the feedback. Scott,I have no problem with the hinge line at stations 2 and 3, gear leg pivots up and down fine. The problem is with the shock strut when spring suspension is to be used. The lower end needs to yaw fwd and aft when the wheel travels up and down. It can’t freely do this when the top is bolted between the two plates at the v-strut. I had a conversation with John B. today and he knew exactly what I was talking about. His suggestion is to install bearing rod ends, one on each side, at the top of the shock struts above the spring assembly’s. This will allow the lower ends to travel fore and aft as the wheel moves up and down. He feels as though most people don’t realize this problem even exists, and because of the limited travel of the gear up and down, bolt connections will wear in and things will be ok. I plan on using the bearing ends. I hope I have not created anymore confusion and thanks again for everyones help
Mark

Offline scottiniowa

Re: Landing gear geometry
« Reply #11 on: September 10, 2019, 05:03:58 PM »
Hey guys, thank you all for the feedback. Scott,I have no problem with the hinge line at stations 2 and 3, gear leg pivots up and down fine. The problem is with the shock strut when spring suspension is to be used. The lower end needs to yaw fwd and aft when the wheel travels up and down. It can’t freely do this when the top is bolted between the two plates at the v-strut. I had a conversation with John B. today and he knew exactly what I was talking about. His suggestion is to install bearing rod ends, one on each side, at the top of the shock struts above the spring assembly’s. This will allow the lower ends to travel fore and aft as the wheel moves up and down. He feels as though most people don’t realize this problem even exists, and because of the limited travel of the gear up and down, bolt connections will wear in and things will be ok. I plan on using the bearing ends. I hope I have not created anymore confusion and thanks again for everyones help
Mark
Excellent,  I will enclose a photo of how I do this rod end.  you can get the rod end types from any race supply house.  cost far less.
These work very well.  This load will be in   shear depending on the bounce...but a little goes a long ways.  I have done this for years.   Glad you got it figured out... Had I understood what you were meaning, I would have suggested this from the get go.
I do rod ends on both ends of the compression strut, but my set up is a little different.
best email address:  irondesignairparts@gmail.com

Offline Mark Kramer

Re: Landing gear geometry
« Reply #12 on: September 10, 2019, 06:37:26 PM »
Awesome! Is that a threaded insert in the tubing end? Did it also come from a race supply house? I have seen the ends, but not the threaded inserts
Thanks Scott
Mark

Offline scottiniowa

Re: Landing gear geometry
« Reply #13 on: September 10, 2019, 07:03:30 PM »
Awesome! Is that a threaded insert in the tubing end? Did it also come from a race supply house? I have seen the ends, but not the threaded inserts
Thanks Scott
Mark
Yes, both from race supply houses... I don't have the names in front of me, but when you think about it, there are 1000's of auto race cars compared to homebuilt aircraft. and the prices get lower because of it.  "competition"  

as you probably saw, we do both a plug weld and wrap around weld on this, but you can do,  so it is very smooth.  and when you think about it this "weld" is never being pulled upon.  The inserts are sized so that they fit most standard tubes.  depending on your needs of course,  if you don't find what you need at one place, search another or holler at me. And I can dig up a place or two that should have.

As I said before, depending on your situation, I do on both ends.  And that being said, you can often find or mill the end clevis for your axle verses welding tabs together.
Best of success.
Scott
best email address:  irondesignairparts@gmail.com

Offline scottiniowa

Re: Landing gear geometry
« Reply #14 on: September 10, 2019, 10:17:03 PM »
http://rodendsupply.com/parts/weld-in-threaded-bungs/

will get you in the right direction, or at least get you to what they are called.
best email address:  irondesignairparts@gmail.com

Offline scottiniowa

Re: Landing gear geometry
« Reply #15 on: September 11, 2019, 05:04:11 AM »
one more view of my lower end adaption of shock struts
best email address:  irondesignairparts@gmail.com

Offline Bob Wood

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Re: Landing gear geometry
« Reply #16 on: November 26, 2019, 10:12:12 AM »
I avoided all this extra work and weight by using 8 lbs of airpressure in the larger lawn tractor tires. I fly off of bumpy grass and it works great. I have no spring struts, the tires do it all. Took 5 lbs off the plane, even with heavier tires.

Offline scottiniowa

Re: Landing gear geometry
« Reply #17 on: November 26, 2019, 10:23:21 AM »
I avoided all this extra work and weight by using 8 lbs of airpressure in the larger lawn tractor tires. I fly off of bumpy grass and it works great. I have no spring struts, the tires do it all. Took 5 lbs off the plane, even with heavier tires.
A perfect, "simple is almost always better solution" 

The Super Breezy did exactly that... with tires alone. https://www.yakimaaerosport.com/aircraft/super-breezy/   Our options today for good, yet light tires,  as well as larger,  is far better than just 10 years ago.
best email address:  irondesignairparts@gmail.com

Offline jrbirdman47

Re: Landing gear geometry
« Reply #18 on: November 26, 2019, 11:10:13 AM »
Bob, are you using the Black Max wheels and brakes? I like the idea of less work/ weight.

Offline Bob Wood

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Re: Landing gear geometry
« Reply #19 on: November 27, 2019, 07:26:12 AM »
Yes, black max and the Turf saver ll tires. the 16''. We have a really rough paved runway which I only use to get out to the grass side runways. I wore out those lightweight stock small tires in one summer....The turfsaverll are great. Have enough beef to wear well. So now they are getting lighter with every flight!!

Keep it simple and light!! My 37 HP jumps off and climbs great. I made the weight limit with light paint and NICO cyl , and any other  thing I could SAFELY get rid of (struts for example)

Have fun
Bob