SeamusG

I'm looking to get your input on this frequently asked question.

What's your recommendation for mounting one piece aluminum or composite landing gear to the bottom of a fuse (tail dragger mostly)?
Things I'm looking for are:<ul>[*]bolt size and material (1/4-20 nylon, 8-32 metal, etc.),[*]nut type (hardwood block or blind nuts),[*]landing gear plate material (1/4"ply),[*]landing gear plate reinforcing material (tri-stock or other stuff)[*]number of holes and location (3 lateral holes, 4 - one at each corner, etc.)[*]size of aircraft (25, 40, 60, 90, etc.)[/list]Thanks for your input.
TTFN

Jetdesign

My Feedback: (8)

4mm hex bolts into blind nuts with loctite. For 1 piece gear, bolt the corners (need something fore and aft as gear will try to twist off of block). Was aluminum gear, now it's carbon fiber. Landing gear block is now 3/16 lite ply laminated with 1.5mm carbon fiber (from Hobbyking.com, cheap!). Dato grooves cut into fuse formers, CF slides into grooves. Carbon strips cut and glued to top surface along former. 1.10 size plane.


Something to consider: how easy is it to access landing gear block? Nylon bolts are used (well for weight I guess) but also so that the gear will detach in a hard landing. I find more damage caused to wings and such that it's just not worth it - if you can access the block fairly easily, just use metal bolts and repair the block if ever needed.

This is all my opinion based on my own experiences.

SeamusG

Thx Joe - hey, when did Cincy go metric? Just kiddin' ... Now I gotta go to my "conversion" program to figure out how a 4 mm translates to our (really screwed up) standard measurement system.

Em, nylon bolts might be used "IF the engine is mounted upright (no muffler hanging down) and IF the plane is a high wing trainer (wing damage minimal) THEN nylon bolts make sense because there isn't much that will get torn up on the bottom IF you land hard enough or run off the runway into junk that breaks the nylon bolts.

Also, landing gear that include wheel pants can cause quite a bit of damage to the bottom of the wing when (and if) the gear breaks free allowing the gear to rotate backward punching holes in the bottom of the wings. Especially true for low-wing, mid-wing maybe not so much, high-wing probably not an issue.

I've seen many "flattened" mufflers on side-mounted Saitos that landed "too" hard.

Your use of a ply / CF composite laminate as the landing gear plate is a first time for me. I've used your "dado" approach but have added some perimeter "help" using some tri-stock to keep the pieces from separating laterally.

Gray Beard

My Feedback: (-1)

I have used as small as 4-40 hex on 40% planes, one on each corner without problems so I gave up using the nylon 1/4-20. Today I use 4-40 on most all the planes I build.

Charlie P.

I have them with 4-40 metal on a .40 size (4 or six bolts), 8-32 Nylon on a .60 size, #6 metal on a 1.20 size.  It really doesn't matter so long as the attachment block is sturdy enough.  If your gear is secured too firmly it will tear out whatever portion of the fuselage is necessary when a hard strike occurs. 

SeamusG

Charlie - are you suggesting NOT to over-reinforce the attachment block (what Ithink Icall the landing gear plate)? Go ahead and leave it as the weakest link in a hard landing hit?

SeamusG

I found the attached "conversion"file that might help translating between metric and standard.

Hope this helps (people like ME) ...

So a 4 mm is effectively "the same as" bolt diameter of 8 as in 8-32.

Lnewqban

This attachment may help:

da Rock

At some point, the force of the hit is going to tear something off the airplane. If the hit is to the gear, then putting enough strength in the gear mounting area to withstand more than a hard landing makes sense. Add break away bolts and you've got two levels of response.

Balancing the two is the problem.

Using nylon bolts as step one can work without step two... or three. But if your ARF already looks weak, step one is solving that problem right off.

da Rock

If Charlie doesn't suggest that I will.

Do not over-reinforce the block the gear is mounted to.

If your ARF looks weak, then add some help. Spreading the support area around works far better than making the plate bulletproof. The plates are almost always big enough. How many times have you seen the gear tear the center out of a plate?

SeamusG

Do these suggestions go for kits as well?

Lnewqban

Any shock absorbing (at least nose to tail direction) gear is way superior to a rigid set up.

An elastic element (like rubber or a torsion bar) can absorb a good deal of the impact's energy, which will not reach the wood structure.

For a rigid set up, that energy will deform any plastic element (aluminum gear) and/or will break any fragil element (wood, glue, etc.).

Trike gear trainers survive a lot of landing abuse because they have that coil in the nose gear and torsion bars in the main grear.

Gray Beard

My Feedback: (-1)

When I was using nylon bolts my thinking was if I hit hard enough to break something I wanted it to be the bolts and not the wood. Over time I decided if I hit that hard I really messed up the landing so I went to the 4-40 metal screws and have been happy with them. I had more problems on even just bouncing in a plane and the nylon screws broke loose and caused more damage as the gear came off. I reinforce all my LG blocks with balsa tri stock and epoxy. I drill and tap the wood LG block and harden the threads with CA. If I have a real hard landing the threads in the block strip out and I can go to the next size bigger screw. Over the years i have gotten a lot better at landing too so it isn't an issue for me but I still reinforce the LG block and use 4-40s. Just in case, I have been know to have a brain fart or two even while doing something as easy as landing a plane. Sometimes I haven't a clue what I'm doing from moment to moment so better safe then sorry.

SeamusG

Hey GB - great idea about using the wood threads as a fail safe rather than using nylon bolts and blind nuts. For some reason I find hardwood blocks / plywood to be so much more satisfying to build with.

Yea, I'm starting to spend 70% of my first flight of the day shooting landings. That way the landing gear won't be the victim due to my poor piloting.

Gray Beard

My Feedback: (-1)

I have had the holes/threads strip out but I then have room to open up the hole and re thread one size bigger or I would use the brass inserts that epoxy in. It's been a long time sense I tore out anything on one of my planes but it has happened.
I built one of the CG Extras as a gift for a friend, I flew the maiden then handed over the controls to him. He didn't do any loops or rolls the first three flights with his new plane, all he did was touch and goes. His statement to me was, you got to know how to land um before you fly um. he had it down in no time at all. One of my old students does the same thing, if he bounces his landing at all then it's a touch and go until he gets a perfect landing. I land most my planes very well without bouncing but he can still land a lot better then I do. He just can't spot land so flying off a real field is tough on him. With 7 sq miles of flat you can land anyplace you like. I now paint a black line in front of us and he tries to set his wheels down on the line, a nice learning aid. There are a lot of ways to do the same thing but this is how I have ben doing it for years and it works for me. I fly with a lot of guys that think I'm nuts and they use the 1/4-20 nylon screws and are happy with the way they do it. If you don't like the small 4-40s then you can open it up and do it a different way. As long as you are having fun and your gear hangs on then you are doing OK.

SeamusG

The single most frequent repair for me is following the 1/4x20 nylon bolts breaking after a hard hit, an off-runway post landing roll (prairie dog holes) or me not catching a nylon bolt that has been compromised (cracked) leaving 1 or 2 holding the gear on.

da Rock

Try using nylon bolts and thread the wood to match. It's a match made in heaven.

There are a couple of places that will give without causing collateral damage. If the impact is a bit harder, the nylon can fail. If harder still, the damage was going to be significant anyway, who can predict what would or would not have been trashed.

An additional plus is that nylon and wood are good at resisting vibration, so you don't have to overtighten the bolts to keep them from walking out.

Noone thinks to mention that nylon will give a bit on impact. So some hits don't break anything. And the hits that do, wind up having a bit of the impact dissipate through the nylon's failure giving you less collateral. How much less? Jeez, who knows.

Your practicing landings is an excellent idea, btw. Excellent. Probably the best idea in this entire thread.

vicman

My Feedback: (10)

I ALWAYS use nylon screws into blind nuts. Size ranges from 10-24 to 1/4-20 depending on what I have in the shop that night.

SeamusG

Mumble, mumble, mumble - the upload gods are really ticked off with me for some reason ...

pic 1/2 - using wood blocks for the LT-40 rear wing mount and 1/4x20 nylon bolts
pic 3 - ditto for the Hog Bipe rear wing mount (not plywood as used on LT)
pic 4/5 - 1/4" real ply with 1/8" ply on the sides serving same role as tri-stock - just wanted to conserve on the space taken up so that the 4 blind nuts had a flat surface for mounting.

All Day Dan

My Feedback: (5)

I use 1/4 inch nylon bolts tapped into 1/4 inch plywood. The landing gear is 3/16 plate. The plane weighs 20 pounds. Experience has shown that the bolts will shear or pull out before the plywood plate starts to seperate from the fuselage. Dan.