Looking for info on landing gear options

[akflyer 1427]

to calculate the stress think of the plane at gross weight, you just hit a sinker on landing and smacked down at 500 FPM and hit a 6" rock

 

Or put 26 or 29" bushwheels on it to take the sink and the rock out of the equation since they absorb 90% of the impact not your seat truss or fuse.

 

[Luked 71]

That's kind of what I was thinking, but I just didn't know what kind of numbers are realistic. That's where feedback from you guys that have lots of hours in these planes is so helpful. You know what kind of a beating they can normally take.

 

If I use 500 FPM, I should be able to figure out the rest by calculating gear stroke, acceleration, and then force. From there the stress calculations are fairly simple. The 6" rock is a good thought too. I think I'll assume that if you're hitting those, you have some tires that can help out a little.

 

I have thought about beefing up the rear attach point though. Extending the gear leg length puts more force back there (especially when you hit big rocks). Moving the attach point back would help, but means adding more weight to create a new reinforced point on the fuselage. Adding more reinforcement to the existing point is probably more practical. Of course, I don't want to outsmart myself and make gear that's so overbuilt it just bends up the rest of the airplane

 

[akflyer 1427]

By adding the plates to the side of the fuse, and the gussets on the truss, I think you will have a winner.  I have smacked mine down pretty hard and hit some snow drifts during flat light, bent a few sets of axles, and have yet to bend the plane or the gear legs.  I think most stress is put o the plane when guys are first learning to fly them.  They are a VERY draggy plane so if you round out the flare at 10' and wait for it settle in like a cessna, your gonna end up landing it 8' off the ground.. the drop and plop method only works once or twice before its back to the shop for a fuse rebuild   I know a lot of guys got it beat into their heads by brand new instructors that you shouldnt have to touch the throttle after your turn to base if you fly the right approach.. well that may be in a 172, but it aint gonna work in an Avid or KF.  Too much drag and the slightest wind gust will put you short of the runway.  Elevator controls speed, power controls rate of decent.  99% of my landings are probly around 4200 RPM until the tires are inches off the runway.  I think I jockey the throttle more than I do the stick during landings adding bursts of power here and there to keep the site picture I want of the runway.  The only time I have not done this was when I had a total engine out, and when I had it overheat and pulled it back to idle at 3000' and dove for the runway.  That time I was on skis and had 4000'X 500' to get it down and stopped and I used a VERY aggressive slip to get it to come down even faster than normal (I had the VSI pegged at 55 MPH and the GPS was showing around 3200 FPM decent)

 

 

 

[Luked 71]

It sounds like the standard upgrades (steel gussets or plywood) have proved to be adequate. I wasn't happy with the way the gussets in my plane look (looks like someone did a rough job with a mig welder, and blew at least a couple of holes in the top tube), so I'm going to replace the whole truss. I don't know what is normal for the gussets, but mine has .040". They are pretty heavy. I modeled the stock truss on the computer, and then added the gussets. I don't remember the exact numbers, but they basically doubled the weight of the truss. I'm hoping to keep the weight closer the the stock weight.

 

As far as pilot skill goes, I'm sure I'll put the plane to the test. I just got back into flying recently after about 16 years away, and it wasn't too pretty at first. I'm a low time pilot to start with, and on top of that, I need to learn to fly a TD!

[dholly 718]

Luke-

 

IMHO side longeron reinforcement gussets are a must have, particularly if you ever plan to swap from bungee to spring gear (and I have the salvage to prove it). If you plan on using the wire limit safety cables on your bungee gear, there are recommended instructions to change either the upper or lower oem attachment point (can't remember which) that can be found floating around the Kitfox forums. This helps reduce bending the seat truss when the cables limit out and I recall thinking I needed to do it to my Mk-IV. Last, some of the Avid+ seat truss tubing was upgraded to 3/4" o.d., and a lot more bridge tubing added. Here's a pic that shows both front and back seat trusses on my A+ fairly well. Perhaps you can use as a pattern if you go the D.I.Y. route.

 

HTH.

Edited 1 May 2014 by dholly

[Luked 71]

Thanks Doug. That's really helpful information. I was considering going to 3/4" tubing on the top where it is in compression to increase resistance to buckling, but I haven't gotten down to the detailed calculations yet. Doing the calculations takes me a long time because I have to double check everything so many times to make sure I'm doing it right.

 

My current design looks a lot like your rear bungee truss (except flipped upside down). My idea is to put closer spaced diagonals with shorter lengths near the center where the bending moment is the highest. It turns out the Bearhawk uses a landing gear truss that is almost exactly this design. You can sort of see it in the picture below.

 

Thanks,

Luke D.

 

[IFMT 36]

I did the plywood glue in when I first put the wide gear on. I cant say that I pancaked it but the plywood will give way a little at a time. The seat truss mod is WAY stronger than the original. At the stage you're at with no fabric to cut off I wouldn't fool around. I think the whole thing cost me about $1200 without fabric. The truss is cheap. The tig welding up here where I'm at was $100 an hour. Make no mistake...it takes skill to put the new piece in right. The cost of peace of mind. Maybe you are skilled yourself or specialized welding is more affordable where you are at. I'll get some pics of that as well when I get to the plane. B

Edited 2 May 2014 by IFMT

[Luked 71]

IFMT, that would be great. I've been wanting to see what your upgraded truss looks like since you mentioned it. If you could measure some tube diameters it would really help too. Thanks a lot for your help.

 

Luke D.

[High Country 69]

Sorry to jump in so late I'm just finally getting caught up on some of the threads. to throw in an alternative option (which I chose to use on my project) the main gear legs off of a Ridge Runner http://realflying.com/ will fit perfectly on our airframes and they use the bungees like the cubs have under the plane rather than on the seat truss. all I had to do was fab up my own cabane and shorten the bungee struts 1/2" and it all bolted in real nicely just finished the mod this last weekend. This gear is just a tad longer than Brett's wide bush gear so it sets almost the same but replaces the springs with bungees. I'm planning to finish painting it this week and get it installed soon if you want pics just ask and I will shoot some for you.

 

-Robert-

[IFMT 36]

I took a quick look at the ridge runner specs. Very light empty and gross. I wonder if the gear would stand up to the heavier avid? I' pretty sure the airdale set truss is made of 5/8 tubing top, bottom and ladder supports. 

[Luked 71]

Hi Robert,

 

That looks like an interesting idea. Seems like there are tons of options with these planes. I'd like to stay with the Avid style bungee gear though. I guess I just like that look. I'm sure everyone would like to see some pictures though. I would definitely be interested to see what you did.

 

Luke D.

[akflyer 1427]

I too want pics.  the various parts laid out and showing how they go together would be nice!  I have thought about doing that versus using springs so you don't have that steel to steel action when you plop it in and bottom them out. 

 

[Luked 71]

Well, I'm still working out the details on my bungee truss and extended landing gear. To do it, I need some information about the gear geometry and movement. I made a simple diagram to figure this out. It looks like the prop could dig into the dirt by a little over an inch if the bungees are fully extended, and the tires deflect a fair amount (this isn't even a flat tire situation).

 

I may be using some bad numbers though. I can't find my construction manual (must have left it at the shop), and I can't download the one posted here because I get an error when I open it. So, I don't know the measurement for the safety cables. I tested some old aircraft quality bungee cord I have, and got about 57% stretch. That's what I'm using to determine full gear travel (I assume the safety should allow pretty close to full stretch). Anyone have the safety cable dimensions off the top of their head? I'd really appreciate it.

 

Thanks

Luke D.

 

Edit: I just realized my dumb mistake. I forgot to factor the amount the bungee is already stretched when installed. This will reduce the travel quite a bit. So, I guess a better question would be what is a normal or acceptable amount of bungee sag (the plane just sitting) at gross weight?

 

 

Other assumptions:

 

- 72" prop

- crank centered 7" below top motor mount bolts

- tires are my stock tires that came with my kit

Edited 29 Jun 2014 by Luked

[akflyer 1427]

If you get 2" of gear deflection your doing pretty good.  Those bungees are stretched out pretty good when installed.  I have hit plenty hard with my 72" prop and never even came close to a strike.  At gross I may have a max of .5" sag, but I like to keep the sag out of it as saggin gear looks about as dumb as saggy pants on half the kids these days

 

[Luked 71]

Good information. Thanks again. That makes sense now that I remembered to account for the bungee pre-load. I figured something wasn't right because I'm drawing it with the wider and taller gear, and I'm sure plenty of stock Avids have 72" props with no clearance issues.

[akflyer 1427]

I'm guess your an engineer?    you be way over thinking this one.  If your gear springs enough to get the prop, your going to have much larger issues to worry about because I would bet the rest of the fuse is going to be piled on top of you in a big smoking hole in the ground.  Short of the gear actually folding, chances are the only way your ever going to get a prop strike if by braking too hard and putting it on its nose... unless you do A LOT to upgrade your brakes, this WILL NEVER HAPPEN even if you try to push your feet through the firewall.

 

[Luked 71]

Ha, only a wannabe engineer. You're totally right that I am over thinking it. I actually like the process though. I also want to make the new structure as light as possible. I took two years of engineering, and then took some time off from school to  earn some more money so I wouldn't have to go into debt. That was almost 20 years ago. I've always regretted not finishing, and I've tried to continue learning.

 

The prop clearance isn't really what I was trying to figure out, I was just kind of brain dead there for a minute, and forgot to include the bungee preload. What I'm really trying to find out is the max load on the bungee truss during landing deceleration. Then I can use minimum sizes on the truss members to keep the weight down. That's the theory anyway. I'll run it past my brother when I get done. He's a real engineer. I know the weight savings probably isn't worth the time and effort. We'll see. 

[IFMT 36]

Here are some pics of the seat truss mod on my Mark IV.

[Luked 71]

Thanks a lot for the pictures! It's interesting seeing how it compares to the original. If you have a chance to measure the tube diameters, that would be great too.

 

Thanks again,

Luke D.

[High Country 69]

I too want pics.  the various parts laid out and showing how they go together would be nice!  I have thought about doing that versus using springs so you don't have that steel to steel action when you plop it in and bottom them out. 

 

Here are the pieces all laid out separately I forgot to take some after I got it all installed will try to get some in the next couple days

[akflyer 1427]

Thanks for the pictures!

 

I really think the cross bar on the top of the cabane vee is going to help.  Does your geometry keep a straight line between  the bottom of the gear leg, all the way through the cabane to the fuse attach, or is yout vee higher or lower at the center point?  Hope that makes sense

 

[High Country 69]

the "V" is higher. I used one of Bretts cabanes as a pattern just added the 1/2" cross tube. I'm just trying to get this thing flying at this point. but am planning to fab up a completely new gear and will make it a bit stronger, have it sweep forward and change some of the angles for better geometry. but for the time being I think this gear is going to work out well. I'm happy with it so far. If someone already has one of Bretts gears and wanted to convert it to the bungees it would be a piece of cake to fab up the bungee struts and just unbolt the original spring struts and replace them.

[ChrisBolkan 130]

That gear looks well triangulated and very strong, but how are you going to fit a kayak under the plane??? )