New lessons, old lessons
By Gene Pape
Do you ever find that what you thought you knew how to do wasn’t really the right way to do things after all and spend years perfecting the new method only to then realize that the way you used to do it was the better way after all?
I designed a new Fast Combat model for the 1984 Reno Nationals. It was the first foam Combat model I’m aware of that used 1/8” x 3/8” spruce spars with the side cross section of the spars vertical and tapered from the root to the tip. The center was then reinforced with a bit of carbon fiber. This method was used with great success until I stopped flying for a number of years. When I started flying again, the design was dated in many ways with its wooden motor mounts and balsa fuselage. This was called the Underdog.
The 1984 Reno Nats Underdog prototype. Note the internal controls, Fox Combat Special and Top Flite wood prop. All photos by Gene Pape.
John Thompson (who used the design for several years while I was inactive) told me I should upgrade the Underdog, and I went to the Internet to learn how to do things. I took what I thought was the best of all the ideas and built a batch of 20 or so models that had fir spars cut to the vertical section of the wing and sanded to about 3/32” thick. I replaced the 1” balsa fuselage with a ½” poplar fuselage cut in the front for metal motor mounts. This served well, but I didn’t like the spar setup. Also, the trend was to use “arrow shafts” for tail booms.
This is part of the first batch of updated models. Note the external controls and Nelson .36 on a pickle fork mount. This model exactly as pictured won the Bladder Grabber in 2010.
My next generation of models used the same fuselage with the 1/8” x 3/8” spruce spars with the cross section horizontal to make spar notch cutting easier and perhaps add some crash resistance. This still worked fine, but I’m always looking for ways to make construction easier, improve crash resistance and improve performance. I then built a series of 4 four models, all slightly different including one with an oak engine mount and an “arrow shaft” tail boom. This was what I expected was going to be the perfect setup. It was the perfect disaster. The engine mount immediately shook loose from the spars. A lot of epoxy and a plywood firewall added in front of the engine mount cured that problem, but the plane flew awful due to flexing of the “arrow shaft,” which was the strongest one I could find locally. A minor crash a few flights later that would not have bothered my wood tail boom models broke the “arrow shaft.”
This is the next generation with flat spars.
One of the things that had been a nagging problem with all of the poplar fuselage models and the European models I’d had experience with had been engine mount crush. Talking with other modelers, I had led myself to believe that changing from poplar to basswood would cure that problem. I had also decided that cutting the fuselage blanks to the desired width for the engine mount would save me the step of cutting the motor mount portion of the fuselage and give me a flat spot to mount the shutoff. I had also done some experimenting with some laminating film for covering and decided to use it for covering instead of the proven FasCal. I built five models incorporating all of my great newfound ideas and hit rock bottom.
The covering using the adhesive built into it didn’t provide the needed rigidity to the inboard wing so any warp at all was immediately magnified by the strain from the leadouts. The models flew horrible. Without the added strength of the extra material behind the spars, the engine mount wasn’t rigid enough. This allowed the engine to vibrate which led to throwing prop blades and the mount to spar joint failing completely.
From the picture, you can't see much difference from the previous model, but there is a lot of clear tape and FasCal that was used to fix the problems.
With the lessons learned from this disaster, I built 10 new models using the methods I knew would work. These models had a ½” basswood fuselage with reinforcements behind the engine mounts and fiberglass tubes in the engine mount holes to prevent crushing. FasCal was used for covering. These models were great. Time to start experimenting again (I never will learn to leave well enough alone).
This plane fixed all the problems I had created for myself.
While my experiments with “arrow shaft” models had been unsatisfying, most everyone else swore by them so it was time to start fresh. After many emails and phone conversations with Lane Puckett, he sent me the plans for the Allenplane.
As I looked at these plans, I could see all of the things that were right about that model that I had been doing wrong, which were most of the things I had changed from my ’84 design. The motor block followed the wing profile behind the motor mount for about 2” and was ¾” thick wood in place of the ½” I was using. The spars were 3/16” x 3/8” with the side cross section of the spars vertical and tapered from the root to the tip. These heavy spars notched deeply into the ¾” thick motor block solved the engine mount coming loose problem. Lane also provided the information needed to get carbon fiber tubes that are strong enough to stand up to all the stress they will see. I built one of my models using these construction techniques and am finally caught up to technology that was available 15 years before I started this adventure 10 years ago. It is time to start catching up further.
This is the prototype for the next generation.
This is one of the Allenplanes I recently built. If I had been able to find plans for this model in 2008, it's likely I would never have bult any of the other Nelson-powered models. There is simply nothing wrong with this model
This page was upated Feb. 22, 2018