LTS Gear Longeron Damage

[acrojohn]

The gear leg MUST be allowed to flex independently of the fuselage longerons, bolts, and bushings. Many amateur builders don't see the real issue and believe adding more metal for strength and heavier clamping is the solution. Tight clamping of the gear leg to fuselage is a guaranteed disaster.

You must additionally consider loads placed on the longerons from heavy braking that require vertical reinforcement at the bolt/bushing/cluster attach points to distribute these loads. As I wrote before, this was a very common problem on planes like the Pitts, Christen Eagle, and experimentals that used a Grove gear leg without proper reinforcement. Aviat Aircraft (Christen and Pitts) has done the analysis and engineering to generate a bullet proof fix. This is documented in a Service Bulletin at: https://aviataircraft.com/wp-content/uploads/2016/01/e364.pdf. The basic design of the Sonerai I is easy to adapt without doing everything that the service bulletin requires. Think of the required adaptation being functional sufficient, rather than identical to the SB drawing. The Sonerai II LTS requires a bit more work if you want to move the Bolt Bushings and vertical reinforcement forward and aft of the gear leg to sandwich the gear leg between the radius bars rather than bolting through the gear leg to the fuselage. Although I have witnessed successful results of the original through the gear bolt attachment. If the original design is used (bolts through the gear leg), I suggest adding the radius bars using ample countersinks in the gear leg to allow it to flex on the radius bars. Consider the required clamping action on the radius bars from tightening the bolt/nuts to only be tight enough to just hold them in place. Read the Service Bulletin.

John

 

[tls258]

I drew a another sketch of moving the gear attachment out to the outside of the longeron, which will eliminate pivot forces twisting the longeron. All solid landing gears like the Sonerai exert torque that flexes the gear down in the center. This is a torque that if not isolated from the longeron can cause failure as Wafer demonstrates. By moving the gear attachment outside of the box, all of the torque forces can move through the gear itself and flex it downward, which acts something like a Piper Cub bungee. While the torque forces are flexing the gear in the middle, most of the landing force can be directed into the fuselage where it can be absorbed without failing a critical part like the longeron. I've also attached a photo of the Cub system, which shows the pivots on the outside of the longerons and the energy absorbed by the bungees in the center of the frame. Any solid gear attachment system that creates an inside short arm lever by bolting the gear to an inside longeron boss will introduce torque to the longeron and potentially fail it because most of the landing force is going to be transferred to those bolt bosses. I think it is safer to eliminate applying torque to bosses on the inside of the pivot point (longeron). Even if the gear can flex, it will still be pulling down on those bosses and applying torque to whatever those bosses are welded, in this case, the longeron.

In this new sketch, the external bosses are similar to the Cub's, just heavy gussets with lateral gusseting. Gusset allows a heavy gear bolt to pass through to other gusset on the other side of the gear. Gear will be fit over the bolt. As a way to allow even more flexing and some security against gear slippage, and to attach the gear more or less directly to the bolt (and thus the frame) but allowing it to flex, a pivot pipe can be fit over the bolt. The pivot pipe would have an attachment bolt (or 2) welded to it and that would then be bolted through the gear securely using a loose fit hole. This allows the pivot pipe to rotate around the gear pivot bolt, and it eliminates nearly all the torque that wold otherwise be applied to the longeron. The amount of rotation is small, probably less than an 1/8 inch, but the torque is very high and must be isolated from the longeron. Most of the landing force will be absorbed through the additional gussets and the reinforced longeron that is wrapped and welded to the upright framing. This is not a torqued force, but is a direct compression energy when the gear is in contact with the frame and the force is coming up on the outside. All of the torque force created by the gear spreading on landing will be isolated within the gear and will cause it to flex downward under the center of the airframe, while the main compressive load is directed up and is absorbed in the airframe.

This type of design is fairly simple and would allow easy disassembly by loosening the pivot pipe nut(s) and then loosening and sliding out the pivot bolt. The pivot pipe would remain attached to the gear.

 

[JondorB1]

I've been following this thread with great interest since I'm nearing attaching the gear to my Sonerai 2LS. I appreciate the explanations and I now understand the physics of the problem. I also spoke to Robbie at Grove and asked if larger holes or deeper chamfers in the gear holes and inclusion of radius plates would help. He said no; it wouldn't solve the twist issue and would create another problem of weaker gear. He recommended moving the bushings outside the gear and using the clamp/radius plate setup. If you can't do that he said to ensure the gear is tight against the fuselage (bolts tightened to specified torque).

The question I have is: Is this a widespread problem? I ask all of you flying a Sonerai 2LS or LTS with bolt-thru-gear setup how many hours you have on your plane and if you've seen or experienced cracks in the longerons? We have Wafer's data (120 hours on the airframe, with a guess of ~200 landings). Thanks! Jon

 

[wbpace]

I was asked how I made my gear reinforcements, so here's a couple of pictures. The premise is this part must fit around a 3/4" round tube but have a square bottom, so it made sense that bending around a 3/4" square tube would be the ticket. I bought a short piece of Home Depot (or maybe it was Lowes) square tube. It had the desirable properties of being thick walled with relatively large radius edges. It happens that a 5/8" tube fits inside it nicely, so I used a bit of scrap thick wall round tubing and jammed it in to provide additional support and minimize distortion of the tube while hammering.

Then I clamped the two together in a vise, grabbed my heavy sledge hammer, and knocked over the innermost bend. Now there is springback, of course, so I took out the tube, reclamped it in the vise and a few "light" hits later had it folded over a true 90 degrees. For the outside edge, if I recall correctly I clamped the long side into the vise, put the tube between the top of the vise and the part to do the rest of the bend, then finished squaring it completely after removing the tube and a few more light taps.

When done, it fits snugly on the longeron.

Using a BIG hammer is important. My sledge is probably between 5 to 10 pounds.

Pictures of how it the square tube fits the part, the tube on top of the vise to do the second bend, and how I have one tube inside the other, are below. Maybe the internal tube was overkill, but I had the scrap, so why not?

By the way, I cut and ground the sheet to the final shape before bending, so I practiced bending on some small scrap pieces first to work out the order of operations. I didn't want to have to cut out that part twice.

Addendum: after posting the pictures, I see they are upside down and sideways. Sorry about that. I'll try to avoid that next time.