The Bobcat-75 Project By Don Giandomenico April 25, 2007
Owning a turbine powered jet model has been a dream of many RC pilots the world over. There is nothing quite like the sound and speed of a turbine jet streaking accost the sky. Unfortunately for many pilots owning a turbine jet will remain a dream as the expense of getting started in turbine powered aircraft can be extreme. Couple this problem with “nervous” stick fingers and you could have a short lived hobby with an exhausted budget to boot. I personally would like to own a turbine jet one day but I may have to “reduce” my fleet to help pay for the endeavor :0) Ever since the loss of my BVM ducted fan F-86 Sabre I have been gaining interest in getting another “jet” model. I had always admired my friend George Manning’s BVM Bobcat XL turbine plane and how well it handled accost a very wide speed envelope. The BVM Bobcats performance helped peak my interest in finding a similar sport plane. I eventually stumbled across a vender at the 2007 AMA show that carried some models produced by Nitro Models. One of the models I spotted was a scaled down version of the Bobcat that uses a 50 size glow fuel motor for power. I was intrigued by the Nitro Models Bobcat-50 so I picked one up for about $120 to see if there would be any fun to be had. My friend Andreas Blaser had also picked up an identical Bobcat model so we decided to build the ARFs with the same engines and parts so they could be flown in “formation”. We both were pretty excited about the project so we wasted no time and started ordering parts in preparation for the build. (More on that later....)
I was fairly impressed with the quality of the airframe upon initial inspection. The kit features a painted fiberglass fuselage and built-up wing panels, horizontal stabilizer and tail booms covered with an iron-on covering. The wing is supported by two spars one of which is an aluminum tube located close to the CG. There is also a “music wire” spar closer to the leading edge but I think it was intended to prevent wing torsion. The kit comes with a set of fixed gear wires and foam tires. A fuel tank and hardware pack was also included along with a pre-painted canopy. I honestly could not believe this was not a $200+ kit for what was included. Of course the model had not been built yet so I could not be sure what I was dealing with, I would have to wait and see.........
The kit was packaged very nicely with all the proper protection measures furnished with higher-end ARFs. I removed all of the plastic bags on the parts and inspected the kit for assembly.
Earlier I had ordered the engine and radio gear from Tower Hobbies. Andreas and I had decided to use a Super Tigre G-75 ringed two-stroke engine for power. I have had good success with the Super Tigre engines in the past and their modest price is a plus too.
For the servos we decided to use four Hitec HS-81MG’s (for the tail surfaces), two HS-425BB servos (for the throttle and nose wheel) and two HS-77BB low profile wing servos (for the ailerons). I opted to use a Futaba R148DP 8-channel 50 MHz PCM receiver along with a 1500 mAh battery for the radio gear in my plane. A Hobbico “Voltwatch II” was added to the system to monitor the battery voltage to prevent all of those power hungry servos from sneaking the power out of the battery. Having most of the parts together I started the assembly process and proceeded to research my first move. Looking through the assembly manual I noticed that there were a few things that were not explained very well. It seemed that the person in charge of producing the manual decided to leave out most of the text added more pictures to compensate :0? I pretty much figured on using the manual as a suggestion and go it alone. I decided to start out with the elevator and “split” it for redundant safety. I figured it would be a nice way to ensure that if one of the elevator servos goes bad on my Bobcat that the other will still be able to move part of the surface for auxiliary control. I cut the elevator in half with my razor saw to divide up the control surface in two equal parts.
I used some “dove gray” MonoKote to cover the exposed wood ends on the elevator which seemed to be a perfect match.
The next step was to hinge the elevator halves to the vertical stabilizer so I got out my Great Planes Slot Machine. I exclusively use Great Planes nylon pinned hinges in all of my aircraft and highly recommend them. On this build I used the GP medium pinned hinge (Cat # GPMQ3972) which is perfect for most models. I used the medium nylon hinge blades (cat # GPMR4016) in the Slot Machine to cut the proper width slot for these hinges.
I cut two hinge slots per elevator half and then used 7-12 minute epoxy to glue the hinges in the slots. (more on that later.......)
I moved on to the wing panels and prepared them for hinging. The Great Planes hinges are virtually bullet proof when installed correctly. I discovered them shortly after a few “CA hinge” failures that occurred on a couple of my models. I vowed to never use CA hinges on my larger models again :0/ To properly epoxy the hinges in place I start with one side first and glue all of the hinges in place allowing the epoxy to cure before moving on. I use a piece of “plumbers tape” (flat metal strapping) to apply epoxy to the inside of the hinge slot and then use a brush to lightly apply epoxy to the hinge tab (both sides on one end). I then insert the hinges into the surface and clean up the excess epoxy with 91% Isopropyl Alcohol and a paper towel. I use a drop or two of alcohol on the hinge to make sure the hinge moves freely before the epoxy can set up. The alcohol will dilute any epoxy in the hinge and prevent it from being glued in place. It is a good idea to “work” the hinge occasionally while curing to assure free movement later.
Once the epoxy has set 30 minutes or so I fit check it to the mating side of the surface. If it fits correctly I move to the next step, if not I adjust the slots to accept the cured hinges. I use the plumbers tape to apply epoxy to the slots and brush some on the hinges for assembly. The two are then aligned and joined making sure the hinges are seated as far as they can go. I usually leave about a 1/32” gap between the two surfaces for clearance, sometimes less depending on the geometry of the bevel end. I use the alcohol to clean up the excess epoxy and then place one or two drops in the hinge to reduce the chance of sticking. Once again I “work” the hinge a couple of times during curing to assure proper motion.
I proceeded to hinge the tail boom rudders in the same fashion. Notice the plumbers tape epoxy applicator. For the most part any flat strip will work but I like the plumbers tape because it has the holes in it to help apply the epoxy. I sometimes use 5 minute epoxy when there are three or less hinges to epoxy.
The Bobcat manual does very little to suggest a proper servo size/torque for the control surfaces so an educated guess was where I started. The rudders seemed to be manageable with around 30 oz-inces as well as the two elevator halves (30 oz each servo side). The ailerons are quite a bit larger though so I figured that a minimum of 60 oz-inches would be needed here to prevent the servo from stalling at high rates. To fill the need for the 30 oz servos I chose the Hitec HS-81 series servos. I used the metal gear ones which produce 36 oz-inches of torque and are very durable. I used two HS-81MGs for the elevator and two for the rudders. Each boom has two servos in it and each require a minimum of a 24” servo extension to get through the wing and into the fuse. I installed the rudder servos in the center of the servo pockets, only cutting away the covering to allow the width of the 81. (The servo pockets are a bit wider than a micro servo) Notice that I positioned the rudder servo horn towards the aft to shorten the linkage rod.
I used the stock control horns supplied with the kit which seem plenty strong. I trimmed the very end off of these to streamline the linkage. Notice the angle is parallel with the bottom of the boom.
I installed the control horn to meet up with the servo horn to have a parallel appearance. I used a Dubro 2-56 Kwik Link (Cat # 185+603) control rods in leiu of the stock hardware which I really did not care for.
The next step was to install the elevator servos. I installed these servos with the control horn forward so that the geometry of the linkage was a little more correct. I used Dubro 4-40 Kwik Links (Cat # 306+604) for these control rods so they would not flex under heavy load. Notice the control horn on the elevator is offset from the leading edge to compensate for the downward angle of the control rod.
For the aileron servos I used the Hitec HS-77BB low profile servo. It can produce 61 oz-inches at .18 seconds / 60 degrees travel (4.8V) which to me is impressive for a $26 servo. I installed the 77’s in each wing with the horn towards the trailing edge. I did have to widen the servo pocket a bit for these servos to fit. A 12” servo lead extension was used on these servos.
I used the Dubro 4-40 Kwik Links here as well to handle the torque. Notice the very cool keeper springs supplied with the Dubro 4-40 rods.
The next project was to install the landing gear wires in the wing panels. The wire slots on the wings needed to be cut open so I used a knife to make one cut down the middle of the slot. I looked for an obvious hole where the gear wire goes into and could not see anything. This was frustrating as I was not sure where the support block would be so I researched it by looking at photos and such to determine where the gear should be. I located the blocks and drilled my holes at 6“ from the trailing edge. Notice the wheel collar on the drill to keep from drilling through the wing.
I installed the gear wires in the holes and then screwed the support straps into place using the stock hardware.
After a quick strength test I noticed some serious flex in the left wing panel. It seems that the gear wire support block had not been properly glued to the wing rib. I had to pour epoxy down a hole made in the wing to secure the block to the top of the wing sheeting and to the adjacent ribs. Once cured the epoxy did the trick. It may have been a fluke or maybe a design flaw.
