Sonex landing gear

[509sx]

One of the first things I did after purchasing my Sonex was screw up and perform a very bad landing. Although I have the TW endorsement, landing a Sonex took some getting used to. My bad landing bent both gear legs. Replacing them is a royal pain, as the motor mount must be used as a drill guide, which requires pulling the engine. I was tied out far from home and couldn't do this. Needless to say, getting the hole on center and at the proper angle without doing so is extremely difficult. Don't ask me how I know. I got the new legs on, but my field repair got the toe-in angle all wrong. Had I followed Sonex's advice on setting toe-in I would have been way ahead, but alas, I thought I had a better idea. For a year I taxied a little bit crabbed! This year's conditional inspection has me fixing that. I made the toe-in tool from 1" aluminum square stock.
From my bad landing I learned that shear tearout is the failure mechanism for the axles as they rotated about the gearleg in my bad landing. I would recommend two AN-3 or AN-4 bolts on the lower end axle attachment for this reason (but not a single AN-5!).
All of this led me to do a cursory stress analysis of the landing gear. What I learned is:
1. The AN-4 bolt is adequate for attaching the gearleg to the motor mount, however I believe that the bolt/nut can be torqued to the value appropriate for AN4 fasteners (50-70 in-lbs). The concern regarding combined shear/tension stress levels at failure does not apply; shear and tension can be safely considered separately. Once plastic deformation sets in, the tensile stress gets relieved, so shear is all that's left. Just don't over-torque the bolts.
2. I can't see around the results of my analysis saying the taildragger gearlegs are sadly underdesigned! We have a 10+ G wing coupled with a <1.4G landing gear. Land carefully and gingerly, and maybe they won't eventually yield and take a set. Arbitrarily upping the TOGW is (IMHO) dangerous in light of this. The gearlegs are splayed out ~40 degrees from vertical and swept ~30 degrees. The airplane sits on it's gear nose-up at about 8.5 degrees (no-load), and this helps by reducing the effective cantelever due to gearleg sweep. The gearleg's angle from vertical is almost 48 degrees with no load. That angle increases with aircraft all-up weight and landing G-level, increasing the bending stress at the gearleg root, where it emerges from the motormount. So what we have is a softening spring whose stress level goes up faster than in a linear fashion. I now always strive for a "tailhook" landing, where I try to just barely touch the tailwheel first. I never do wheel landings.
I'd love to hear from anybody else who has looked at landing gear strength in technical detail! Do carbon fiber rod gearlegs make sense?
Art Powell N509SX

[daleandee]

Hello Art,

Good to hear you are getting your aircraft sorted out and enjoying it. I do think it' would be helpful to others to point out a couple of things that may or may not be relevant to the information you have posted above.

Your airplane, like mine, has a Corvair conversion. Michael (the builder) and I had a lot of communication during the time he owned that plane. The Corvair conversion engine in your plane was built by Bill Clapp. I say that to make the point that I do not know who built the engine mount but it wasn't Sonex. I would think it came from Bill but that's just a guess.

I have a William Wynne Conversion on my airplane and the engine mount was built by Dan Weseman of SPA (Sport Performance Aviation). I used the stock Sonex titanium gear legs and Tracy O'Brien axles and brakes. My gross weight is set at 1250 lbs. My plane was flight tested to that and has flown at that weight a few times. In nearly 10 years and 300 hours I have not had a concern with the landing gear, the engine mount, or associated hardware. I fly mostly from pavement but have flown to grass fields numerous times over the years.

There is a service bulletin from Sonex to replace the bolts in the landing gear if they are still the stainless steel (I doubt this applies to you):

https://www.sonexaircraft.com/documents ... Change.pdf

You are not alone as there have been others that have had concerns with landing gear bolt breakage:

https://www.sonexbuilders.net/viewtopic ... 390#p17831

I have done wheel landings but I believe Sonex mostly recommends not to do them. Three pointers are easy enough but the tail wheel first does happen sometimes.

Thanks for the update.

[509sx]

Hi all,
As I stated in my post, I am not concerned about the AN-4 bolts attaching the landing gear to the engine mount; my analysis shows that they are sufficiently strong, even though they look smallish. My concern is with gearleg bending, and exceeding the 120ksi yield strength of the 6AL4V-O titanium gearlegs. The landing G-level required to do this is surprisingly low. Since the gearleg splay angle increases with G-load upon contact, and since computing the elastic deflection under load, and hence that splay, and the resulting effective cantelever is relatively complicated, I have used the zero-G gear deflection and cantelever value and looked at the gearleg bending moment and stress level, and was shocked at how high it appears to be relative to the titanium 6AL4V-O yield strength. It's less, but too close for my comfort. Unless I have missed something important, we don't have anything close to a 3G-capable landing gear! I'm sure lots of you guys are better pilots than I am, and grease her on more often than not. I'm getting better at this - slowly!
I also looked at getting the gearlegs heat treated. The process, called solution treating and aging, must be done in a vacuum in order to avoid the dreaded alpha-case, a brittle oxide layer that would form as the temperature goes above 1000F. This would have to be done after all holes were drilled, but is quite expensive. Having yield bumped up closer to ultimate might not be a good idea anyway; perhaps better they bend than snap! Is a 25% increase in yield worth it?
And yes, I have a Bill Clapp Azalea Corvair conversion engine, which I like since the starter and alternator are in the back, contributing less nose-down moment, and since I don't have to worry about a V-belt letting go. I too reckon the motor mount is also his. I carry 15 lbs of lead ballast in the tail, and have bent the stinger too! I have had numerous telephone conversations with Mike, the original builder.
I would love to hear from anybody else who has looked at the landing gear strength issue!
By-the-way, the shorter gearlegs of the tri-gear configuration in theory should be stronger.
Also B-T-W, I love the way the bird flies, and the security of that 10+ G wing!
Take care, all,
Art

[Kai]

I would like to comment as follows:

I am the originator of the Sonex gear leg bolt SB. We experienced four separate incidents of the original design AN4 gear leg bolts snapping, with just some 200 hrs on the standard gear planes. Built to original european ultralight regulations (FAI ULM), they all were fairly light- an empty weight of something around 270-275 kgs. And the legs occasionally bent after harder landings. The planes were operated both on grass and hard surface airstrips, which was all duly reported to Sonex: they quickly issued the SB. Great!

In one of the incidents the plane went up on its nose because the gear leg fell out; the others just resulted in the plane on its belly- minimal damage to the exterior. Not so funny were the splintered props and the need for crankshaft replacements (VWs and Jabs). Nobody wanted to fly a direct drive crank with a prop strike history.

Positively blocking the gear leg from falling out after a bolt snaps, is easily done by keeping the bolt in place by safety wire. The bolt will snap somewhere inside the leg drilling- making sure that the broken bolt bits stay were intended, effectively blocks the leg from either falling out or rotating 90 degrees.

This was all a while back. Since then we have not had one single incident of gear legs bending after what could be called normal three point or wheel landings. Not so when the pilot got things messed up: smack it on and you need to replace them- this has its merits. It’s better to replace a gear leg than to bend the airframe.

But our planes slowly increased their empty weight- the 4-bangers were replaced either by direct drive sixes, and more lately by Rotax 912 ULS or the designs from UL Power. And our safety wired AN4 bolts kept on snapping in there, often not discovered before the annual. Sooner or later we speculated someone would be up the well known creek without a paddle. Something had to be done- Sonex was not informed.

So Kerry will most likely strangle me (he is much younger, bigger and stronger than me) should we eventually again meet! What we did was to clear away all the mess around the engine truss gear leg bolt guides. Then the legs were fixed wery rigidly in place in their original positions- make absolutely sure they cannot budge in any direction. Then, armed with a LOT of patience, a fistfull of ø8mm (5/16”) rat tail files, and a strong quality drill, we set about increasing the hole diameter in leg and guide by running the drill reversed- the file going continuously in and out to get rid of the filings. Finally, after considerable work, our 1/4” holes had increased to 5/16”. True, it does not leave much material in the guide, but it is bolstered a little with a flat washer under the bolt head.

For bolts we use ø8mm 12.9 grade phosphated cap screws with unthreaded shanks. For nuts we use the longer type, which could mean that either a dent or a hole in the firewall is needed. Then the bolt is torqued to manufacturer’s specs.

We did the last one in 2013- after some 400 additional hours it’s still sitting there.

[Scott Todd]

Where does Sonex recommend to NOT do wheel landings? I make it a point to do one every other time. I also teach them for windy conditions. TW pilots should be proficient and comfortable with both types of landings.

[builderflyer]

Where does Sonex recommend to NOT do wheel landings? I make it a point to do one every other time. I also teach them for windy conditions. TW pilots should be proficient and comfortable with both types of landings.

Joe Norris (one of his many hats was to be the chief flight instructor for Sonex LLC) has recommended against wheel landings in a Sonex for many years. Personally, in the 800 hours I have in my Sonex, less than a half dozen landings have been wheel landings. But given the flat stance of the taildragger Sonex, most landings have an element of "wheel landing" in them. Given the tailwheel steering design of the Sonex, the plane is most controllable with the tailwheel firmly on the pavement. Making a true tail up wheel landing puts the plane in the most vulnerable position for a much longer time, that is with the tailwheel not in contact with the pavement. Also, I don't believe the Sonex flight training syllabus makes any mention of doing wheel landings in a Sonex.

But everyone has their own opinion, some stronger than others,

Art,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,Jabiru 3300 #261

[bvolcko38]

What would you think about a taper machined onto the gear legs? Thereby distributing the bend over the entire length of the gear, instead of concentrating it to a point at the motor mount.

[Kai]

No,

John M mentioned during my builders course in 2004(!), that one of the round stock gear leg material characteristics was its capacity to distribute load like a tapered rod. This takes a lot of costs out of the gear leg price.

At the critical point, you want the gear leg to bend- this is better than distorting the airframe or the engine truss.

Thx
Kai

[bvolcko38]

" round stock gear leg material characteristics was its capacity to distribute load like a tapered rod"
No offense, but this makes no sense.. I'm not an engineer. But how can a round rod have the same characteristics as a tapered rod?

[GordonTurner]

I’m with Bill on this one. I don’t see how this can be true.