Bobcat Project.

[John K (김종철)]

	Page 1

	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.

  It was time to mount the booms to the wing panels. The kit came supplied with some nylon bolts that were intended to hold the booms to the wing. I was not to keen on how weak they were or the fact they did not seem to fit the blind nuts very well so I used my own hardware. I found a few steel M4 x .75 metric bolts in my screw assortment that fit the blind nuts perfectly so I installed the booms with the steel bolts and some washers. I used a liberal amount of blue thread locking compound to keep them from backing out.

	I decided to thread the servo wires through the front hole in the wing root to be eventually run into the fuselage. I did have to trim a piece of balsa to fit them up front.

As far as I can tell there was very little thought in the design on how the wings were going to stay on the spar. I can only assume that the horizontal stabilizer itself would keep the two booms together and thus keep the wings from flying off :0P I wanted something a little more secure so I opted to install a blind nut on the front part of the wing root rib. This nut will be used with a cap screw inside the fuse to hold the wing to the fuse.

	Notice the 5/8” hole I made in the fuse for the servo wires to pass through.

	I used a “fender washer” under a M4 bolt to secure the wing to the fuse. I also used a liberal amount of blue Loctite on the threads to keep them tight.

I have heard of folks gluing the wing to the fuse using epoxy and even silicone which seems like a fair idea. Although I do not plan to break down the Bobcat for storage I still want to be able to replace a wing panel or be able to work on repairs in a modular form. This is why I like the blind nut setup :0)        At this point I installed the nose wheel bracket. I must say this was incredibly difficult task with the stock hardware! Nothing seemed to fit unfortunately although I did get it done :0/ I did notice that the stock gear wire was a bit short for the job....

	The Bobcat was really taking shape now and almost on it’s own feet. I used the stock hardware to bolt the horizontal stabilizer in place using the blue Loctite on the treads as well.

	The rolling airframe really looked nice and I was getting excited about wanting to fly the Bobcat!!!

The next step was to install the Super Tigre G-75 engine to the airframe. Because the engine will be mounted pointing backward I had to make a few adjustments. The muffler from the G-75 is a bit awkward and bulky as they are designed to be quiet. Unfortunately I could not stand using the muffler with the “square” end pointing into the wind as it just doesn’t look good. Besides it isn’t aerodynamic at all!!! Being the perfectionist I am I had to make a “new” muffler.

Now before I go any further I want to assure you that this muffler will work as-is with maybe adding an optional silicone exhaust tube to point the exhaust away from the fuse. There are also several after-market mufflers (like Mac’s Mufflers cat # 6900) that would work just as well. What I did to this muffler may be above and beyond what anyone would want to do, I understand :0)        I started out by buying some materials from McMaster-Carr supply. I picked up a 1-3/4” 6061 aluminum rod, 3/8 OD X .028 wall aluminum tubing and a piece of 3/8 ID X 1/8” wall silicone tubing (high temp). With these parts I planned to make two custom exhaust pipes for the Super Tigres. (One for myself and one for my friend Andreas)

	I cut the end off of the stock exhaust pipe so I could measure the ID of the pipe body and get started.

	I then cut a couple of blanks to be used as the muffler cap bodies off of the 1-3/4” aluminum bar. I also cut a couple of short pieces of aluminum tubing for the exhaust “stinger” tubes.

	I was now ready to shape the rod blanks into the end cap parts using my 7 X 10 metal lathe. I started out by machining the surface that will fit into the end of the muffler tube. I then bored a hole in the middle to allow me to shape the inside of the muffler cap.

	A 3/8” hole was then bored into the side of the muffler cap for the stinger tube to be installed later.

	The shaping process could now begin....

	After an hour or two in the garage I had finished the end caps and was ready to install them into the stock muffler bodies.

	I used some aluminum pop-rivets and JB Weld epoxy to secure the end caps into the muffler bodies. I also used some JB Weld epoxy to secure the stinger tube and exhaust manifold into the muffler. This makes the whole assembly very solid. After the modification I added about 2 grams of weight to the muffler. As a finishing touch I bead blasted the pipes for a finished look.

 The next project was to create the motor mount for the G-75. The Bobcat has sort of a universal mounting block system for the engine which was originally intended for a 50-60 sized engine. Unfortunately the G-75 has a bit deeper footprint over a 50 and needs a custom mount. Luckily my friend Andreas was kind enough to make two of these beautiful motor mounts (one for me) in appreciation for the custom muffler modification :0)
       Notice the stock engine blocks on the right, they are much shorter than the “Andreas” blocks. We had calculated about a 2-3 degree down-thrust needed for the plane to not to climb while accelerating. Andreas had built the angle into the blocks before hand....

	I used some 80 grit sandpaper to rough up the finished surface on the motor mount deck and epoxied the engine blocks to the deck. I then used some “dove gray” Top Flite LustreKote paint to match the fuse.

	I decided to use some blind nuts on the bottom side to hold the engine bolts. Longer M4 bolts were needed to replace the short stock mounting bolts supplied with the Bobcat.

	Notice the spinner nut and washer on the engine. I used a Dubro (cat # 731) 5/16”-24 spinner nut along with an OS .61-.90 Steel prop washer (Tower cat # LXCT76) to hold the prop securely. This will also aid in starting the G-75.

	Earlier I had purchased some silicone tubing for an exhaust deflector. My original intentions were to plumb the exhaust though a hole in the wing and point it towards the ground using this tubing. Later in the build my friend Andreas had come up with the brilliant idea of plumbing it through the center of the fuse so that is exactly what we did.

	With the exhaust tube in place I was able to install the throttle linkage using the stock push rod.

	The next step was to install the fuel tank. The stock tank (lower right) was a little too small for the thirsty G-75 so I decided to use the Hayes 16 oz Slimline fuel  tank (cat # 146) instead.

	The front “spar” wire unfortunately causes a bit of an obstruction for the fuel tank so a slight modification to the former must be made to fit this tank. Notice the top of the former hole has been raised a bit.

	Using some foam padding I made a fuel tank cradle that once in the fuse will hold the tank securely. Notice that the tank sits with the clunk towards the back. The fuel tubes are routed to the front of the tank.

	I mounted the muffler to the engine and installed the fuel lines as well. (Later on I would discover that the muffler gasket should be left out as it will most likely blowout anyway. I used a thin film of JB Weld on the flanges and then tightened the bolts quite a bit. Blue Loctite should also be used on the muffler bolts to prevent loosening.)

	I could now focus on the radio gear and install the throttle and nose wheel servos. Notice that the control rod for the nose wheel is at a diagonal. This is to allow maximum travel with proper geometry.

The stock nose wheel wire was a little short for this kit so I opted to buy a Dubro 5/32” nose gear wire (cat # 154) to get a little extra length so the nose does not sit too low. I also used a Great Planes Wire Axle (cat # GPMQ4281) so I could adjust the ride height of the nose wheel. The Bobcat has it’s main wheels quite a bit behind the CG so a short nose wheel gear will cause the jet to “jump” off of the runway on takeoff.        If the nose wheel is properly adjusted the jet will leave the ground in a predicable manner. Later on we would discover that 145 mm from the bottom of the wheel to the bottom of the fuse worked well as a starting point. (The mains were also 145 mm to the bottom of the wing.) Notice the gear wire is left long for adjustment. It can be cut once the proper ride height is set.

	You may have noticed that I decided to use different wheels than the ones supplied. I am not a big fan of huge foam wheels on jets so I toned it down a bit by installing a 2” wheel on the nose and two 2.5” wheels on the mains. (Dubro cat # 200T+250T)

	After doing some preliminary CG calculations I figured that the battery pack must be in the nose of the plane (probably to compensate for the large engine). I wrapped up the 1500 mAh pack and pushed it into the nose of the jet. Notice that I cut out the vertical brace of the nose wheel bulkhead. It really isn’t needed.

	I cut out two foam blocks to “wedge” the pack into the nose. It actually worked real well :0)

	My fellow modeler Andreas was kind enough to construct me a control panel to mount inside of the canopy. I used it to support my Hobbico VoltWatch II. I should be able to see the battery condition thorough the canopy.

	The receiver was next to install so I cut out another foam block to support the Futaba receiver which worked very well. Not only did the block support the RX it also hides the wires coming in from the wings.

I installed the radio switch on the left side of the aircraft along side a Ernst charging jack (Cat # 124) for ease of charging. (not shown)

	I installed the antenna wire on the leading edge of the right wing. I used some clear tape to hold it to the wing and it worked beautifully!
	I cut out the canopy using the molded cut lines and fitted it to the fuse with 8 small screws. It was a beautiful fit!!!
	With the jet complete I checked the CG and the nose needed about 62 grams of weight so I added some lead to the nose in front of the battery and brought the CG to about 255 MM behind the leading edge of the wing at the root (the manual suggests 260 mm). The total dry weight of my model came out to be about 8 pounds.
	I set up the control surfaces to around what the manual suggested but settled on dual rates. I used 15 mm up/15 mm down low rate and 20 mm up/20 mm down high rate for the elevator and 20 mm up/20 mm down low rate and 30 mm up/30 mm down high rate for the ailerons. I used a single rate for the rudders at about 28 mm (56 mm total). I used about -35% exponential on the elevator and -40% on the ailerons (each pilot may choose to use expo or not, I prefer a “soft” center on my controls)        I also added a mix to the radio allowing the rudders to spread out acting as a speed brake. I have the 8-channel program for the Futaba 14MZ so if anyone wants it you can contact me and I can e-mail it to you.
	To personalize the Bobcat I decided to add some decals to the jet. I found some Super Tiger decals online that looked great with this paint scheme so I cut out the tiger on the decals and installed them on the tail fins.
	After adding my favorite number on the nose and some creative decal use I dubbed the jet The Bobcat-75 and she was ready for flight!!!

I was very excited to test fly the Bobcat so I shot up to Rabbit Dry Lake one Sunday to see how she would do. I had put a couple of tanks of fuel through the Super Tigre at home a couple of days before so when I got to the desert the engine fired up with no problems. I adjusted the nose wheel servo so she taxied out straight and lined her up for a rollout. I applied full power and the Bobcat rolled about 120 feet before she rotated and climbed out.        I was immediately impressed with the similarity this jet had with it’s namesake BVM Bobcat XL. I banked left and the speed was picking up for a flyby. I was pleased to hear the G-75 pick up as it cleared it’s throat and started to sing. The bobcat was tracking at an average speed of 100 MPH by now. I was very happy with the roll rate and played with the rudders a bit to see if it would knife-edge. Although it would hold it a little this plane will probably never knife-loop :0) I did fly inverted and it was a little mushy but it flew OK.        The most impressive part of this plane is it’s speed and roll rate. It looks really cool on a hot pass when you pull up to a fast roll, looks very impressive. I tried some stall maneuvers and the Bobcat had no funny tip stalls or anything of the such. Although the nose would abruptly drop when I flew it too slow so keep this in mind when landing.

After some experimentation with a couple of different props I found that the APC 11x7 pusher prop seemed to push the hardest with the G-75 but needs to be turning at around 13,000 to get top speed. Andreas and I tried to cut down a Master Airscrew 14x7 3-blade to about a 13x7  but we could not achieve the same speed as the APC. The Zinger 12x6 wooden pusher also turned too slow.        One surprising problem I had not counted on was the nose wheel picks up rocks and throws them into the prop during takeoff. I have gone through several props and have put a few holes in the wing so far. The APC pusher prop seems to handle the occasional rock better than the others. My only suggestion would be to avoid loose dirt runways and FOD. I did add some packing tape to the trailing edge of the right wing to keep rocks from blasting through the wing.

A common problem that both Bobcats had was that the main gear wires would constantly bend backwards on slightly hard landings. This in turn made it harder for the jet to rotate on the next takeoff due to the mains being so far back from the GC. (BTW the mains are purposely set toward the prop to keep it from hitting the ground during rotation). I was able to fix the gear problem by adding some 5/32” support gussets to the stock gear wires. I affixed them to the stock gear using steel picture wire and silver solder.

	After the reinforcement the gear wires bent no more. The landings feel more solid as well as the tracking. The only improvement from here would be a retract conversion but I am not willing at this point to open that can of worms for a fun-fly prop jet. Although I have seen people converting these into turbine jets :0)

	See the Bobcat-75 in action: The Bobcat-75 Project Video

	From what I got out of the first few flights I can see that the Bobcat-50 is a great value for the money. Not only do you get a jet like platform with very cool looks you get the experience of flying a very unique pusher plane which can be quite different from flying a “tractor” plane. All-in-all I am very happy with the Bobcat and always look forward to buzzing the skies with it :0)

[GomCruise(타미한)]

이눔아 프로내요...

[Gary Ahn]

올가을 소풍떼비행은 밥켓인가......

[Philhkim]

이눔아 프로내요...

[이광영]

슬퍼다이겨 75 면 힘이 충분한가유~~~~~

[John K (김종철)]

조회수 죽입니다....ㅋㅋㅋ.

[니키 Skynv]

아 이런건 어디서 찾으셨어요.... 죽이네요...올가을 소풍떼비행은 밥켓인가......

[니키 Skynv]

갑자기 하나 사고 싶어 지네요....ㅋㅋㅋㅋㅋ