L-2 Shock Strut Question

[warbugdriver]

I was looking through the L-2 drawings assembled by Ron Greene and was curious if anyone knew the reason for the omission of the extra dog-ears on the upper strut tie between the L-2A/B and L-2M?

Drawing DCO-A530 is for the earlier L-2s while DCO-A569 is for the L-2M. The needed stretch to get the bungees on the early strut with the larger dog-ears seems it would be far more than needed for the -M part.

Dog-ears is hardly a correct technical term but I'm referring to what's shown as DCO-A528 on the DCO-A530 drawing.

[Ragwing nut]

Re: L-2 Shock Strut Question

The extra ears are sheetmetal screwed on, so they can be removed for bungee replacement.

[warbugdriver]

Re: L-2 Shock Strut Question

Mine are riveted and tack-welded on. In the photo, the left side is two screws and a rivet. On the right, fully welded.

Drawing DCO-A569 is the correct upper tie for the L-2M and the extra end-pieces aren't there. Sure would make life a heck of a lot easier to not have them.

[Forrest Barber]

Re: L-2 Shock Strut Question

Nice photo, the right side ( the photo) has been modified, My tool has no problem pulling over the plates, however they were made to rotate like the one on the left. There are several versions , i think I have them all at the airport will check for photos too.

[Ragwing nut]

Re: L-2 Shock Strut Question

Mine are riveted and tack-welded on. In the photo, the left side is two screws and a rivet. On the right, fully welded.

Drawing DCO-A569 is the correct upper tie for the L-2M and the extra end-pieces aren't there. Sure would make life a heck of a lot easier to not have them.

There should be sheetmetal screws where the rivets are to allow them to rotate.

Mike

[russtwa]

Re: L-2 Shock Strut Question

Is anyone sure of the size of the bungees. Many different sizes are listed in the different catalogs.

[warbugdriver]

Re: L-2 Shock Strut Question

Is anyone sure of the size of the bungees. Many different sizes are listed in the different catalogs.

Several sizes shown on various factory drawings. For example, drawings DCO-A571 and D-A52 show shock cords as item D-58 sized as 11/16 x 9-1/4" (750#).

Drawing DCO-A560 replaces shock cord part D-58 (11/16 x 9-1/4", 750#) with part D-513 (13/16 x 10, 1000#). Interesting that this drawing also references a proposed all-up gross weight of 1440 pounds.

Catalogs show 11104 (11/16 x 10-1/2) as correct. I have these on my L-2M and the suspension is nearly non-existent.

Some folks are using 11122 (11/16 x 12-1/4).

[Ron Greene]

Re: L-2 Shock Strut Question

I am reporting on the efforts of several L-2 owners in their quest for the correct bungee for the L-2 over several years. As Warbugdriver noted in his post, the drawings present some conflicting information and it isn't really clear what the factory intended to be used.

The 11104 rings will fit and are what have been traditionally shown in the catalogs. The problem with the 11104's is that they are stretched too tightly, giving the airplane little or no shock absorption except in very hard landings. Also, the 11104's are very hard to install because of the extreme stretch required, and their life is probably compromised due to the stretch. Instrumented flight tests have confirmed that the 11104 rings do not permit the landing gear to travel at all under normal or moderately hard landings. The only shock absorption that the airplane sees in normal operation is tire flexing.

We determined based on engineering calculations, ground tests, and flight tests that 11122 is probably a more optimum size. This would be a ring using the same 11/16" material, but with a 12.25" inside diameter. Unfortunately, this is not a standard size, so we were charged a $200 setup fee for the new size. After the setup fee, all subsequent rings are priced similar to standard rings.

After flying the 11122 bungee for 100+ hours the plane's owner reports that on deliberately hard touchdowns it still felt quite firm compared to a modern airplane, but it wasn't the hard jolt that the L-2 normally does. Also, there didn't seem to be any tendency to rebound. Deliberately soft touchdowns felt about the same as before, except that it was much easier to get a nice landing. Both wheel landings and three points were easier to do and look good. Tire pressure is much less critical with the 11122's because the bungees are now doing the work.

The other thing he noticed about the new bungee size is how much easier it is to install them. The amount of stretch required to get the 11122's on the struts is significantly reduced. This should contribute to longer installed life.

The address for the bungee's is:

SBC Industries, Inc.
2134 Porter Road
Scottsboro, Alabama 35769
(256) 259-6609

Hope this helps,

Ron Greene

[warbugdriver]

Re: L-2 Shock Strut Question

Considering the non-existent suspension qualities of the 11104's, I can't even imagine using 1192. Just getting them on would likely be a very risky procedure.

Not sure what the testing parameters were in the 1940s but the reference weights of 750 pounds are the same as today. Whether the rubber stretched easier to this weight before failure is open to speculation.

11122 sounds promising. I had meant to try them several months ago but continue to put it off. Might even be able to go larger still without issue? Something to approximate the suspension of a BC-12D would be nice.

I find I need to run very low tire pressures just to keep from knocking fillings loose on landing!

Do bungees loosen up over time? My 11104's probably have less than 10-15 landings on them and, to be honest, I really don't look forward to flying my plane because of the very harsh landings.

[Ron Greene]

Re: L-2 Shock Strut Question

Regarding bungees of the 1940's vs 2008, the rubber material could be
and probably was much different. We have no way of knowing how stiff
the bungee material of the 1940's was because any remaining material
from that time would have deteriorated long ago. But if the 1940's
material was far softer that may explain why today's bungees seem so
stiff.

Could you go larger than 11122? Calculations actually showed that
11124 (12.5" diameter) would have been slightly better, but we didn't
want to take such a big step all at once. If you install 11124's you
are probably OK but should realize that you are doing something new.

Roger on the tire pressure. If your bungees are too stiff then you
are relying on your tires for shock absorption almost 100%. 10 to 12
PSI seems necessary when you use the super-stiff bungees. Soft tires
still make for softer landings even with the softer bungees, but
pressures now can be raised to around 16-18 PSI without changing the
ride very much.

Your existing bungees will soften up very little in the first 90% of
their life. Just before the fail they will begin to feel nice and
soft.