Building a Powered Bobcat

Here are some hints that may help anyone building a Bobcat glider, especially if you plan to power it.

The only modifications made to the airframe were to use a slightly wider firewall than specified. The top and bottom panels of the fuselage are not pre-cut, so it is no problem to widen it. This allowed room to mount the engine and make room for the fuel and pressure lines, and made the area inside the fuselage wide enough to accommodate a Du-Bro 2-oz tank.

My main concern was that since it was designed to have a carved balsa block up front instead of a 2-oz engine, that it would tend to be nose-heavy. I needn't have worried. I moved the battery as far forward as possible to just behind the fuel tank, and still had to add 1.5 oz weight to the front. So use a good thick heavy-grade ply for the firewall, with tri-stock reinforcements in the corners.

I used 1/8" wheel collars for spacers to move engine forward so as to to provide clearance between the muffler and firewall, and to help move the CG forward. I used washers to give it a little down and right thrust.

The throttle linkage has just a radical L-bend to connect it to the throttle. I couldn't get a Z bend inserted without removing the engine, and there isn't really enough room for a clevis. I bent the piano wire so it would spring in the proper direction, and it hasn't come out. The Norvel .061 engine has a metal throttle arm. I tried gluing a nylon servo arm onto the metal arm so as to avoid metal-to-metal contact, but was to weak. This configuration has caused no noticeable radio interference.

Here is the view under the hood. The foremost servo runs the throttle. The other two have cables going back to the rudder and elevator. I made a light-ply servo tray for the aileron and rudder servos instead of using the

The battery is to the left of the throttle servo. I had to carve out a little semicircle area in the servo tray and front throttle servo support to make room for the battery and padding.

I used FMA micro servos. They weigh about .45 oz as opposed to about 1.5 oz for a standard-sized servo. The plans say you can use standard-sized or mini servos, and since weight in the front isn't a problem, you could save a few bucks and use larger servos IF you don't plan to throttle it. But with the throttle servo there isn't a lot of room to spare even with the micro servos.

Micro servos are necessary for the ailerons, as there isn't enough room for anything larger, and weight is a consideration here.

Rather than run cables to the ailerons as per the plans, everyone recommended I just install two servos, one in each wing. This greatly simplifies building and aileron differential setup, and eliminates problems of trim change due to expansion of the cables and/or plane.

Glue thin plywood to the underside of the balsa, and attach the servo with servo tape. Two-foot aileron extensions are the perfect length. I used the holes I had originally cut for the cables, except for the two closest to the servo. These were cut so that the route to the center of the wing was a little shorter--otherwise the 2' extension would have been a little too short.

I removed the connectors from the extension and the servo so I could feed the wires thru the little holes. Be sure to put them back properly! Lock the connector together with a small wire-tie to keep them from accidentally disconnecting. Some people use shrink-wrap tubing instead.

As you can see, I covered over the servo. If you need to get to it later, just cut the covering away and apply a patch as I have done here.

I set the aileron differential up using my JR-642 programmable transmitter. I also set up spoilerons by setting up the transmitter for flaperons, and setting the travel in the opposite direction so the flaps extend up instead of down. I understand that they can save the day if you get caught in a strong thermal. I have used them already. In a hand-launch unpowered flight it was skimming over the ground and getting dangerously close to the lake. Deploying the spoilers dropped it to the ground pronto.

Here is how I hooked up the mechanical aileron linkage.

The specs call for a weight of 26 - 28 oz. Ours came in at 31.5. But, it was unflyable until I added 1.5 oz of lead in front of the fuel tank. The Norvel .061 engine has plenty of power on 5%-nitro fuel, but if doing it again I would consider a larger engine rather than use lead weights. However, the .061 weighs 2 oz with a muffler. But the Norvel .15 weighs about 6 oz, which would be too much. Maybe the .074.... I will also get a Du-Bro spinner for it, and could always add more spacers to move the engine forward.

The thing is incredibly loud with the provided muffler. A silicone exhaust deflector snout quiets it down considerably, and makes cleanup a breeze..

The only real criticism I have of this plane is that the 1/16" balsa trailing edge breaks easily when flexing the wing. Now just normal flying won't hurt it, but a hard wing-down landing will. This damage doesn't stop you from flying it, but it is easy to fix by trimming away the bottom covering and gluing things back together with CA.

Also, the fuselage in front of the vertical stabilizer is weak and subject to breakage. In a nose-in crash due to a loose elevator cable, the tail snapped off. I repaired it by epoxying about a 5" length of Dave Brown carbon fiber strap to either side of the fuselage, centered under the leading edge of the vertical stab. It has since had another nose-in mishap. The tail wiggled around a lot, but did not break off. I also beefed up the attachment of the horizontal stabilizer to the vertical by epoxying some Dave Brown carbon fiber "tape" to the joint. This "tape" is actually many individual strands of carbon fiber. It's messy, but cleans up by sanding after the expoxy dries.

Carbon Fiber is incredibly strong and lightweight. Had that held together, I think the wing would have survived unscathed had the trailing edge held. I repaired the wing, and glued a narrow strip of carbon fiber strapping to the trailing edges of the wing.

Thanks to our friend Jodi Lewis for her help in decorating this plane.