I have used shock gears on all of my airplanes since 1971. This includes 3 NATs wins, 2 Walker Trophies and a second in the World Championships. These airplanes all had shock gears, in fact, some of the scores must be attributed to the extra advantage of shock gears on takeoff and landing. These takeoffs and landings have been appreciated by stunt flyers here and abroad and are generally considered to add to the quality of the takeoff and landing maneuvers.

Why use them?

I possibly lost the 1970 NATs because of a terrible landing on my best official flight. That Bearcat III bounced endlessly and there wasn't much I could do about it except praying for it to stop. When it finally did stop, I found myself in second place by only a couple of points. I decided then to bend every effort to see that didn't happen again. A simple wire and spring shock gear was devised for the Sea Fury. After a bad year in 1971, the Sea Fury went on to win in 1972 and 1973. I thought shock gears would become popular since demonstrating their advantage. Didn't happen.

How do they work?

The very first thing I learned when trying that first set of shock gears was not to use springs strong enough to support a large percentage of the airplane's weight. Strong springs made the airplane extremely "bouncy". Shock gears with strong springs were worse than gears with no shocks. It took awhile to come to some debatable conclusions about how the things worked. This is my interpretation of their function based upon my observations of thousands of landings with the gears.

When an airplane lands, at touchdown, two forces are exerted on the airplane. First there is rebound, or an upward force equal and opposite to the downward force exerted when the airplane touches the ground. Second, there is wheel drag while the wheels are spinning up to the airplane's ground speed. This is a rearward drag on the landing gear. The resultant of these two forces, upward from rebound and rearward from wheel spinup is exerted toward the general location of the Center of Gravity. If the landing resultant, at touchdown, is directed ahead of the CG, the airplane will rotate nose high, contributing to a bounce. If the landing resultant is directed behind the CG, at touchdown, the airplane will pitch nose down, perhaps breaking a propeller. The ideal location of the gear with respect to the CG is to have the CG somewhere between 15 and 17 degrees above and behind the landing gears. With a proper gear location, the airplane may land hard with little or no tendency to rotate in the bounce. It will skip flat, low, and settle easily back onto the ground.

OK, but where do shock gears enter into this? Actually, in my opinion, they don't. Their reaction to touch down would be exactly the same as any other stunt ship except that rebound is delayed until the shock gears "bottom out" as they hit their stops. This momentary separation of the rebound and wheel spin forces seems to allow them to act separately, slightly staggered in time, giving the effect of two small forces instead of one rather large resultant of a non-shock gear. Strong springs tend to close the momentary time interval making the gears behave more like non-sprung gears. In fact, "shock gears" work very well if we leave the springs out altogether. The only reason to include weak springs is to stop the gears from flopping around as the airplane maneuvers.

Why aren't they more popular?

The gears are simple devices made of wire, springs and brass bushings. They are a bit difficult to make. The "S" curve on the aligning guide wire is very critical. It must position the guide strut at exactly the correct distance from the landing gear load strut to pass without excessive drag through the brass bearings. It is a very fussy job to get that bend exactly correct and soldered to the load strut. Also it takes a jig and a fair amount of patience to attach the bearings to the movable strut with figure "8" wraps of light wire and solder. Actually, the job is a bit easier than it appears. Even poorly made gears will tend to "break in" and operate normally in service.

The very strong gears shown here are for a 70 oz, Saito 91 powered BBFB Bearcat. For airplanes weighing up to 60 ozs, including my Classic Bearcat III and Snaggletooth Mustang, I use 3/32" wire for both my load and guide struts to save a bit of weight. Shock struts add about .4 oz to each gear.

The point of all this?

I have redesigned the "Rabe shock gears" to simplify the assembly and alignment. Now, nearly anyone can accomplish this task easily. Basically, the figure "8" wire wraps were eliminated and precision machined steel fittings were substituted to locate the brass bearings. I also eliminated the critical "S" bends on the guide strut by using another steel fitting machined to exactly align the guide strut with the brass bushings. All parts are attached to a soldering jig of plywood and held on that jig by two simple twists of wire. These wires hold the wire struts and steel fittings in position for soldering. The fixed strut, movable strut, all bushings and guide strut fitting are all positioned on the jig and soldered in one continuous operation. Since all parts are perfectly aligned and assembled prior to any being soldered, the gear can be described as self aligning.

Here are the parts of a shock gear. Notice there are no more wire bends than any other wing mounted fixed gear. Anyway, we have the upper fixed wire strut, the lower movable strut, three precision fittings, two springs, four bushings, a soldering jig, and a plastic horn which is "cannibalized" to attach the gear door. Whoops, I didn't include a guide strut. Anyway, the guide struts on both "Standard" and "lite" gears are always a straight piece of 3/32" wire, about five inches long.

This shot shows the fixed gear with two of the steel fittings temporarily attached to the soldering jig. The two pieces of soft wire are twisted to hold the fixed strut and fittings in position. The steel fittings and the brass bushings are located by lines on the plywood soldering jig which are copied from gear plans.

This shot shows the gear with both the fixed and movable strut soldered. all parts were installed and aligned prior to any being soldered. The soldering was then done in one continuous process on both the fixed and movable struts.

After soldering, the jig wire twists are removed and the gear freed for installation onto the airplane.

With the shock strut soldered and ready for installation, it is time to attach the gear door. The door is cut from 1/16" plywood, drilled as shown on the plans and the drill holes countersunk. Parts of a nylon elevator horn are used to attach the door. The small plastic piece with two holes is used at the top of the door. The horn is cut as marked to free a piece of plastic and holes drilled 3/4" apart to retain the bottom of the gear door. 2-56 Machine screws can be driven through the 1/16" holes, but the screws are easier to install if a tap is run through first.

This is a shot of the gear installed on the wing missing its door, but otherwise finished. At this point the wheels may be adjusted for direction of track, if necessary, by heating just the bottom solder joint on the landing gear loaded strut. If done carefully, it doesn't affect the alignment of the rest of the solder joints. If, at this time, the gear is showing resistance to free movement, the likely cause is soldering paste residue in the bearings. This residue can be cleaned out with thinner to free the gear.

Now both shock gears are complete, aligned and installed. The shock gear installation is finished.

Now let's finish this BBFB Bearcat and see what its like to run that Saito .91. In addition to shooting these photos, a one hour video was made of the building of this shock gear. This video will be lengthened to include the assembly process for building Mustang style gears.

All of my gears are built with the load strut and guide strut on 7/16" centers with the fixed strut half way between. The strut travel is a function of the distance between the bearing sets. I use 3/16" for the wheel collar plus the 3/4" travel. so the distance between the bearings is 15/16".

I ran these gears for years, and thousands of flights, paying no attention at all to the gears, once installed, except to squirt a bit of fuel on them every now and again to clean and lubricate them. Since returning to stunt, and building the Classic Bearcat, on two occasions, I have lost gears off the airplane. I have no idea why it should happen now except that I am running different engine and propeller combinations which are probably operating in some harmonic of the gear causing more vibration than in the past. In any case, now, in addition to the wheel collar on the load strut, I also solder a safety collar of brass tubing on the guide strut in the same vertical location.

I haven't yet tried this new shock gear. It will be some time before the BBFB Bearcat is finished. Excepting the occasional loss of a complete strut, I have never had any mechanical failure of these gears. I'm not particularly concerned, but the weakest point of the gear is the load strut's ability to resist torsional loads twisting it at touchdown. I think the soldered steel fitting is more than strong enough to resist this torsional load. That particular solder joint is exactly the same as a flap or elevator horn joint, and probably stressed less. It should be plenty strong when made with soft solder as I doubt there is much of a twist at touchdown. Still, for my own large and heavy BBFB Bearcat, two washers were added to that solder joint. One was added above the fitting and the other one below the fitting to increase the thickness of the fitting and solder joint at that one point. It shows in the photos above. I sincerely doubt that it is necessary, but....

The gear assembly, with all my thoughts and concerns is covered in a video. I will have for these for sale as soon as I get more fittings and build a Mustang style gear for the video. The Mustang style gear is a functional opposite of the Bearcat style gear. It has the bearings on the movable strut instead of the fixed strut. Both work the same. Refer to the photo at the top of this thread. Interestingly enough, the Mustang gear has the wheel located directly below the load strut axis so there is little or no torsional twist at touchdown. On a Mustang gear, I would probably not consider adding to the load strut joint.

For what its worth, I only fly from hard surface now. In the past, I operated the Sea Fury's and Mustangs from grass when necessary to good effect. On those recent occasions, when a strut was lost, the airplane landed safely on the bottom of the gear door and only once tilted up enough to scratch the bottom of the cowl. Needless to say, on those two occasions, the extra drag of landing on the gear door placed the resultant behind the CG. I think the safety collar is more than enough to prevent any more loss of struts. Coincidently, the Mustang gear also has wheel collars, but that design also has a gear door retainer on the struts between the bearings which would prevent the loss of a strut if the wheel collar becomes loose.

For now, I add a loose safety collar to the guide strut when building the gear. The wheel collar is used to adjust the length of the gear. When the adjustments are complete, the safety collar is soldered. If the gear ever needs disassembly, it is easy enough to heat the solder of the safety collar to remove the strut. Like any airplane, there are possible failure modes everywhere. We simply use logic, craftsmanship and common sense to avoid those potential problem spots.

I have a final thought about wheel alignment. This applies, most particularly, to those that sit semi-scale tall on conventional two wheel main gears. The best wheel alignment for takeoffs and landings may not be what one would expect. All of my airplanes have the both wheels angled slightly toward the inside of the circle. These airplanes are launched with the nose slightly angled out to allow an initial track of the wheels tangent to the circle. Airplanes which sit noticeably nose high will pitch suddenly to a level flight attitude on release. This sudden nose down pitch will cause a left yaw toward the inside of the circle from gyroscopic precession of the prop. Without the "angled" wheels, you may need a quick step back to maintain line tension. With the "angled" wheels, and a bit of nose out release, the takeoff is completely normal.

Also, if you carefully watch airplanes rolling out on a hard surface, you may observe them trying to roll slightly out of the circle, restrained by a constant pull on the control lines. Look at the tires. Many wheels are observed noticeably "scuffing" sideways by the restraining pull on the control lines. "Angled" wheels seem to contribute to a longer rollout on landing. The longer roll may be attributed to the reduced drag of wheels not "scuffing". "Angled wheel" airplanes, on landing, may show some slight tendency to track into the circle, but this can avoided by walking to the left as the airplane rolls out, slightly pulling the inboard wing tip forward as the airplane slows. The effect is so slight I don't bother to mention it to people flying my airplanes.

My initial trim of the wheel alignment, when building, is to point the axle of the inboard gear toward the front edge of the tire on the outboard gear. I align the outboard axle to point toward the rear edge of the inboard tire.

When all is said and done, I would not build a competition stunt ship without the advantages offered by the use of "shock gears".

I expect these fittings will soon be readily available as a "cottage industry" product. You could certainly make your own, but for a reasonable cost, you can't beat fittings precision made by an excellent machinist. They appear to be a "nothing" item, but the machinist had to order a minimum quantity of steel stock, cut blanks, and build precision jigs sufficient to hold the steel blanks precisely while being cut and drilled. I expect fitting sets will be relatively cheap, but perhaps more than the appearance of the fittings would suggest. I don't know. That's up to Dirk, the machinist. I can say these are the easiest and quickest set of gears that I ever made. As long as parts are available, this is the way to build these gears.

Al

P.S. I (Al Rabe) just talked to Dirk Tollenaar, the machinist. Dirk says that if anyone wants to try these shock gears, he will sell six itsy-bitsy pieces (three to each gear) of precision machined steel taped to a piece of cardboard and mailed to your address for $13.95. If your gear is removable, why not give shock gears a try?

For airplanes weighing less than 60 ozs, I feel that the movable strut can be made from two pieces of 3/32" wire. For larger airplanes like my BBFB, I use a 1/8" load strut and a 3/32" guide strut. These parts are pictured above. This set will be the "Standard set". These pieces will work for both the Bearcat and Mustang style.

The "Mustang lite" and "Bearcat lite" sets are different. The (2)"Mustang lite" guide fittings have two holes of different size (one hole 1/8", and one hole 3/32"). The (2) "Bearcat lite" guide fittings have two holes of the same size (3/32"). The (4) bearing fittings in each set are also different. On a Mustang style gear the bearings fittings are soldered to the 3/32" movable wheel load strut. On the Bearcat style gear, the bearing fittings are soldered to the 1/8" fixed strut coming from the wing. The "Mustang lite" bearing fittings have two 1/8" holes, and one 3/32" hole. The "Bearcat lite" bearing fittings Have three 1/8" holes.