Herr Engineering J-3 Cub on DuBro Park Flyer Snow Skis
Modification Pieces for DuBro Snow Skis
Clamping everything in place while the epoxy dries assures alignment and flat ski bases.
Snow season has arrived here in Erie, Pennsylvania, already (13" on November 10th), and I didn't want to miss the chance to do some flying off of snow skis. Last winter I mounted a pair of DuBro snow skis to my Herr Engineering J-3 Cub and flew a couple times with them, but they were the standard model that are too big and heavy for this 1/2A-sized model.
DuBro's Park Flyer Snow Skis seemed like they might be a better choice for the J-3, so I ordered a pair. The size is just about right, but the vacuum-formed plastic was a bit too thin for me to confidently install them on the J-3. I decided that they would be perfectly useable with a little sturdying up.
As can be seen in the photos, there are two stiffening slots molded into the skis, so I epoxied a 3/32" x 1/4" spruce stick into each slot. Up inside the landing gear mount area is hollow, so I shaped a piece of hard balsa block to fit, and then drilled through-holes to accept 3/32" inside diameter (I.D.) brass tubing to fit the J-3's landing gear wire.
The plastic was roughened with sandpaper in all contact areas, and then cleaned with isopropyl alcohol. After a trial fitting, 30-minute epoxy was applied to all surfaces, wood was installed, and then the skis were aligned (a short length of 3/32" music wire was inserted between them) and clamped to a piece of plywood to be certain that nothing would move and that the skis would remain flat while the epoxy set.
I also modified the manner in which the skis are mounted to the landing gear with the supplied wheel collars and springs. The instructions provided by DuBro are in an illustration format that is not exactly unambiguous. I figured out that the vertical bump molded into the ski mount area is meant to contact the long collar bolt and limit the upward angle of the ski while in the air. The spring puts force on the back of the ski which forces the front tip up when no weight is on the airplane. If that bolt is not adjusted properly, the tip can raise very high and would produce a lot of drag, and maybe even make the model unstable in the air. The tricky part is that the bolt serves two purposes - the aforementioned in-flight angle setting and it is the mount for the spring.
My modification was to place the wheel collar with the long bolt on the inside of the wheel and use it to adjust the in-flight angle, and then I replaced the hex set screw in the outside collar with another bolt that was just long enough for the spring to hook onto. That gave me separately adjustable spring tension and ski angle adjustment. The additional weight of a short bolt is negligible, and makes the entire process much simpler.
The DuBro snow skis got their trial run today, and performed great! they might have worked out fine in unmodified format, but I'm confident that they will outlast the J-3. Only 0.2 ounces was added to the original weight of 1.0 ounces.
Posted December 9, 2018