THE "F" GEAR STRUT,PARTS 1 & 2
May/June, July/August 1993

Many comprehensive repairs and complete restorations as well, begin with a minor concern and balloon into a major effort as the problems unfold. So it was with a recent RNF annual. What first appeared to be a simple rebush exercise developed into a very interesting study, and eventually a complete rebuild--or more correctly, a remanufacture.

When the RNF appeared in 1930, its light, hydraulically damped gear was quite a modern innovation. It was only a few years earlier that bungee suspension was the standard, and even in 1930 many bungee-type gears were still in production. This continued throughout the thirties and forties, especially on the lighter types. So WACO was, in fact, quite innovative in providing a hydraulic gear on a light, inexpensive airplane. They even designed and manufactured the hydraulic strut themselves, and did not go to an outside vendor.

I offer this bit of history as a prelude because as we go forward, it might appear that I am a bit critical of the strut design. Although I point out the shortcomings, these are only lessons learned over time. The little strut actually works very well for all its simple design. Later hydraulic experience added to serviceability, leak integrity and corrosion prevention--and WACO learned these lessons as well. But in 1930 these things were yet to come. And in 1993, they are STILL yet to come in many Fs, as I was to find out.

The hydraulic strut, often called the oleo, connects the axle vee to the fuselage outrigger. It is positioned vertically, and connects to both the landing gear and the outrigger above with universal blocks at each end. These small blocks have two holes drilled at 90 degrees to each other, and provide both fore/aft and side to side motion for the strut, top and bottom. Inspection of these blocks, and the connecting bolts, is where the rebuild began.

The entire load on the wheels and axles is transferred to the fuselage through the little universal blocks, so it is important that they be inspected on an annual. They sustain lots of slamming up and down, so elongation of the holes is inevitable. And along with these loads and elongation can come cracking. I found two of the four on the airplane cracked, and all eight holes were elongated. The holes in the connecting lugs on the outrigger, gear vee and hydraulic strut(top and bottom)were also elongated--not surprising after 60 plus years of service!

If you consider this entire lash up, you can see that there are four bolts; two top and two bottom. It follows that there are a set of holes in the top lug on the outrigger, on the lug on the gear vee, and on the lugs on each end of the oleo; four sets of holes. Then each universal block has two holes, times two blocks equals four more holes. Where am I going with this you ask? I want to make the point that between the gear axle and the fuselage, the up and down loads are carried through connection that include these EIGHT holes(or sets of holes). Even .030 wear in each hole)set)could result in .240 total, or nearly a quarter inch slop, not to mention any additional bolt wear. The one I inspected was much worse.

Needless to say, we set about making a new set of universal blocks. This is a very simple exercise, really. If you have an RNF or INF, and have looked at these parts you know what I mean. Although it looks like you could cut a square steel block and drill the holes, it really should be a machine shop job. The holes should be accurately bored and should be reamed to .001 over bolt size for a perfect fit.

The holes in the connecting lugs on the fuselage outrigger and gear vee should be repaired or rebushed as well. This follows for the lugs on each end of the oleo. In our case we made new struts with new lugs, so this took care of the strut lugs. The outrigger and gear lugs were reamed and inserted with a bushing. This is one option, but very hard to do accurately on the airplane. If the airplane is in service, however, this may be the only option. If it is in full rebuild, perhaps welding the holes closed and reboring an accurate hole should be considered.

Once the entire gear/strut lash up is assembled with new bolts(a must)and properly fitted holes, the "slop"will be minimal and so will the wear. In this state, they should last for years-maybe even a lifetime, given the limited service we give them in modern antique aviating. But if they are worn, the wear becomes cumulative. More "slop"leads to more "slamming" as the gear goes up and down. And more "slamming"causes more "slop", and so on.

The lesson here is that if you can detect some "slop"or some elongation of the holes--FIX IT! The slamming can cause not only more wear, but cracking as well. And if one of the universal blocks lets go, your gear will either hang down in flight or will not support the airplane on the ground. In other words, the strut no longer connects the gear to the outrigger. A safety cable at the bottom you say? Good idea, but it won't help if the top universal lets go.

I don't wish to assume the role of the alarmist. I don't know of any failures of F gear due to universal block failure, but then I'm not privy to all the records either. At the very least, I think it's wise to inspect the blocks. My opinion is that they are not prone to failure, given their "beef", but it is a possibility nonetheless. In addition to the added safety of the properly fitted blocks, of course, is the satisfaction of a smoother quieter gear. I'd like to add that is this, as in any repair, be sure to rely on your IA for guidance.

We chose not to disturb the basic hydraulics of the strut, although I admit that I had my doubts as to its effectiveness. The direction of our work then, was to be (1)replacement of corroded parts,(2)improvement of seal characteristics, and (3)prevention of future corrosion.

The replacement of the corroded parts very shortly became a near complete remanufacture. The piston tube(Waco #8267)was badly corroded along its entire length. The first thought was to hard chrome and precision grind for a good seal surface, but the pitting was just too severe. Then an inspection of the bore of the cylinder tube(Waco #8264)revealed the same level of corrosion inside, especially at the bottom. Remanufacture to print was the only logical move, and we proceeded to make a new pair of piston tubes and a new pair of cylinders.

After completing the new tubes and cylinders we turned to the seal. First of all, there are a couple of glaring flaws in the design regarding seals, I must repeat here that when I criticize design, I refer to seal application NOT the hydraulics. We made the new piece exactly duplicates of the originals. The seals, however, are another issue. The first flaw is that there is no seal to prevent fluid transfer from the lower half(the operating portion)of the piston to the upper half. There is a "piston collar"welded to the piston tube just above the top metering holes, but no seal. We chose to put an "O"ring on this collar to reduce if not prevent passage of fluid to the upper part of the assembly. Actually, the groove for the "O"ring was turned into the collar before it was welded in position on the new piston tube.

The seal material of choice in 1930 was leather, and it was used both at the top of the strut and at the bottom of the piston as a "plunger"type seal. This bottom seal is of the type commonly used in hand-pump water pumps and is readily available. My seal supplier knew exactly what I was looking for and provided a replacement of modern material. The closest diameter we could find was 1.625"which is a bit small for the bore of the cylinder(1.652"). But it filled the bore and worked just fine. By the way, the depth is ½" and the hole size is 13/16".

The seal at the top was also leather and, because of the design, took quite a beating. It was held in place between two brass bushings, secured by the "piston retainer"at the top of the assembly. This is the cap, secured by two 1/4"bolts, that you can see at the top of the cylinder. The leather seal was "packed"by tightening the 1/4" bolts. These bolts and the cap also serve as the stop for the piston as it tops...that is, the weight of the airplane disappears on takeoff and the wheels drop to their extended position. So on each takeoff(or bounce)when the wheels slam down, the leather seal gets a good jolt and is "packed"some more. I thing eventually is "packed"to the point that it is the consistency of iron and probably seals accordingly.

Replacement of the top seal was an obvious requirement, but we needed to protect it from the jolt of full extension on takeoff. We cut a 3/8" ring out of tubing that would fit over just above the mounting lugs. A brass bushing was fabricated to fit the inside of the cap and act as the stop, similar to the original Waco #8255. It was made slightly thicker, so that six holes could be drilled and tapped for #10 screws. The ring brazed to the outside of the cap was drilled, and now the brass bushing/stop could be secured to the cap with six screws.

We selected a seal(1 3/8" x 1 3/4" x 3/16")that would fit snugly in the top of the retainer cap, pressed it into position, and secured the brass bushing/stop to the retainer cap with six stainless screws. This effectively provides an extension or topping stop so that when the strut extends to its maximum the seal is protected from the impact.

The original design has no provision for drainage at the bottom of the strut which accounts for the corrosion we found in the cylinder. The only way the strut could have been drained was to remove it from the airplane, disassemble it and turn it upside down to pour out the fluid. And this, of course, necessitates blocking up the airplane as the gear has no support with the struts removed. It's easy to understand that this operation was either skipped or overlooked at annual time.

Installation of a drain plug in the new struts was a must in my mind, and it really was quite simple. We tapped in 1/4" pipe plugs at the bottom similar to the filler plugs at the top. Waco must have seen the necessity for this as they put a similar drain plug in the F-2 and all subsequent models as well.

In order to simplify my description of the rebuild, I have omitted any reference to plating. The cylinder and its various parts were all cad plated, as were the universal blocks. The piston tube was hard chromed and the upper part was polished to provide a good working surface for the seal. All parts were baked after plating to prevent embrittlement.

We were fortunate to have springs that were not broken or damaged. Had the springs been broken, however, we would have obtained a new set from a spring manufacturer. We have done this in the past. The spring guys can measure the critical dimensions and identify the tension from a part of the spring.

Reassembly was routine, but not having any specs on fluid volume we guessed at the amount. Being concerned about moisture, we used a silicone fluid which holds the F strut moisture in suspension rather than allowing it to settle our at the bottom of the cylinder. We regularly use Type F transmission fluid which is a silicone.

The pleasant surprise came when the RNF was flown off a grass strip with a few bumps. All our concerns about the hydraulic characteristics of the struts were erased...the damping was just beautiful! A modern design just couldn't be any better.

In conclusion, I'd like to thank Joe Fichera for providing us with copies of some of the strut drawings.

And as always in this, as in any repair, be sure to rely on your IA for guidance.