ama62206

Hi, got the sbach at Joe nall 2013. Have a hacker 40 14s tallon90 and 3300 6s pack, all up weight of 5lb 2 oz. love the way it flies, but am not happy with the engineering relating to gear mounting. Second flight, landing on grass field, gear mount plate cracked loose. Eighth flight, again landing on grass, entire gear mount collapsed. These landings were not perfect, but also not overly rough, I hope OMP will work on redesigning this aspect of this plane. I am a long time AMA member, have actively flown for 30+ years, have several planes with 300-1500 flights on them, and I can say from personal experience that this design needs its gear mounting reengineered.[&o]

OMP

Why didn't you contact OMP about this issue? I've flown the prototype 60" Sbach for over a year on a grass field with no gear problems. The grass field that I fly on is a well maintained field and rather smooth. These planes are not designed to fly off of rough grass fields.

ama62206

The field I fly from is maintained by the park district, is mown weekly, was mown the day of the eighth flight, it is not a "rough grass field". This forum IS PRESENTED AS A CONTACT POINT ON THE OMP WEBSITE, I got here via a link from the OMP "contacts", in other words I was led to believe this is contacting OMP. The gear mount plate on the production model that I received is two layers of light ply, this is not the failure point, it is the skeletal structure of single layer light ply to which it is attached that is the failure point. These light ply members are about 8mm wide, they are not braced diagonally or reinforced. The extremely rigid carbon gear is very nice, but its rigidity causes all landing stresses to be transmitted instantaneously to the light ply framing. The carbon reinforcements that run lengthwise in the fuse consist of a pair of 4mm carbon tubes, and two smaller carbon rods that are imbedded in lightweight balsa longerons, there may also be others that are not in the vicinity of the gear mount. The carbon tubes were not attached to the gear plate even though they were touching it. The reinforcing rods in the longerons are attached only to the balsa longerons which are in turn attached to the light ply formers (skeletal). Perhaps the production unit is different than the prototype?. I am in the process of re configuring the gear mount to include some diagonal bracing, to include a brace to the wing tube and to include some shock absorbtion capability to reduce the amplitude of the shockwave delivered to the aircrafts framework. I believe these changes will add 1 to 1.5 oz to the flying weight, which is an acceptable weight penalty for the goal of less fragile landingear.

ama62206

Issue resolved, minor mods to fuse structure have proven to eliminate gear mount failures. Small carbon rods from firewall to gear mount location and 4mm carbon tubes from gear mount to wing tube. Carbon gear in silicone heater hose clamped in plywood channel created at gear mount location. Formers that gear mount is attached to doubled up with second layer of light ply. Gear clamps made of 3/32 aluminum about 1/2in wide screwed to ply channel. Approx weight gain 2.25 oz.

This could likely be accomplished with grommets at the original gear mounting points and the carbon rods and tubes, along with the former doubling on the two formers that the gear mount plate attaches to. Forgoing the clamp plates and silicone heater hose would minimize the weight gain required to reach the required strength.

Still flying at the same field, no more aggravation. If OMP chooses to incorporate this mod into further runs of this airframe they owe me an airframe for the engineering work. Video is available that documents the layout of the mods, email me if they are desired.

Buick McCane

I'm in the final stages of building my 60" Sbach and have discovered that the "bridge" between the elevators will twist. The elevator half that is connected to the servo is fine, however the other side will lose deflection. It seems that I have a few options:1) remove elevators and reinforce the "bridge" between the elevators and reinstall 2) install a second servo on the left side of the fuse. 3) Bend a piece of rod in a square shaped "C" and furrow out a channel on the underside of the elevators and use epoxy to secure it in place. The second choice would be easier but seems excessive for a 60" airframe. Has anyone else had this problem? I have sent an e-mail to OMP regarding this problem a few minutes ago but thought I would check in here for advice as well. Any suggestions would be appreciated. Thanks