Electric Bobcat 50. The quest for a fast pusher jet. - RC Groups
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Dec 24, 2007, 01:58 PM

Electric Bobcat 50. The quest for a fast pusher jet.

So I've started building my Bobcat 50. The airframe was irresistable when they had it on special for $105 retract ready.

My goal for this plane is to make the fastest non turbine bobcat 50 around. Here are the parts going into it:

UH Premium Air Retracts
UH 100 amp HV controller
3x UH Zippy-R 4S (supposed to be korean cells) 4500 for a 12S 4500mah configuration
Hitec HS 225-MG in the wings
Hitec HS 125-MG modified into the Horizontal stab for elevator control.
Digi Polo 4 for Nose Wheel Steering
HS-55 or HS-81 for retract valve.
Rudders locked out at least for now (based on advice from other bobcat owners regarding usefullness at the expense of complexity, clutter, and drag as there are two of them)
Futaba FASST 2.4ghz

Motor wise I am going to be test flying the plane to using a UH HXT 63-64 aprox 290kv. This is a very cheap and VERY big brushless outrunner that I figured I could always use in a big Extra if I wanted. It is very heavy at a whooping 23 ounces however this weight should help meet CG. I will be using this motor to find out how capable the airframe is without spending much on the motor. If I'm satisifed with the results I will be moving to a Neu 1915/2Y 360kv.

These motors will be spinning a 12x12 prop under the advice of MarkF who also built a 12S pusher bobcat 50 and found the draggy airframe to need a lot of thrust to get some speed out of it.

On the HXT 63-64 with a 12x12 and 12S power the calcs are predicting 45 amps, 2kw, 132mph pitch speed. With the Neu 1915/2Y 73amps, 3kw (4 horsepower), 153mph pitch speed. Has anyone had luck putting 3kw into a 1915 Neu without cooking it?


Last edited by Z06kal; Mar 02, 2008 at 05:26 AM.
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Dec 24, 2007, 02:07 PM
I was pleasantly surprised by the kit. The airframe seems quite beefy for what it is. The fusalage feels very strong with a very thick glass/plywood former layup. The wings and tail sections seem strong for what they are, plywood and balsa with covering material. I plan to reinforce the horizontal when I cut open the covering for the HS-125MG mounted in the center.

FYI completely empty airframe is about 4.5 pounds. I estimate AUW to be around 10.5 pounds.

Here you can see the mounting blocks for the retracts. They look well glued and interlock into the wing ribs. I heard reports of some parts being poorly glued in this plane from the factory but I have not found that yet on mine.
Last edited by Z06kal; Dec 24, 2007 at 02:57 PM.
Dec 24, 2007, 02:23 PM
For the aerilons I cut my own horns out of G10 Fiberboard and slotted the aierons and epoxied them in filling the open slot space with epoxy. The wing is designed for standard size servos so I had to glue in a ply block to shorten the opening to allow the HS-225MG's to be screwed in. I did not use the rubber servo mounts as I wanted the servo as low in the wing as possible.
Dec 24, 2007, 02:29 PM
They have supplied balsa blocks for the retracts that you must glue over the plywood blocks that are interlocked into the wing. I had to notch mine in order to clear the nipple for the retract and give a place for the tubing to go. There are supplied grey plastic gear wells that will be going in and covering up all the visable internals.
Dec 24, 2007, 03:20 PM
Flying motor mount master
fly_boy99's Avatar
Not to detract from your endeavor here but don't you think that running a 290Kv outrunner goes against trying to be the fastest Non-gas Bobcat around?

I guess the other Bobcat owners are pretty slow.

12 Cell with 290Kv theoretically takes you to around 14k rpm. Which in the real world would work out to be around 10,500rpm.

With a 12x2 prop and 10500rpm you would be going 19mph which defeats the purpose.

Dec 24, 2007, 03:46 PM
Not sure where you get 12x2 from. The prop is a 12x12 square APC. At 10,500 that is 120mph pitch speed. I posted the calc predictions. Why an out runner? Because it is cheap and it has the KV I'm looking for. Why a 12" prop? Because you need the thrust to go that fast. Not too many inrunner other than Neu that have the power handling and KV. You can spin a smaller prop very fast but the plane won't go anywhere because the prop lacks the thrust to allow the airframe to attain the pitch speed.
Dec 25, 2007, 12:17 AM
Battery Puffer
Z06kal, your setup looks good. I wish mine would have had retracts. Make sure you post some video when its done.

Jan 09, 2008, 01:16 AM
Finished installing the retracts in the wing. If you notice I had to grind a hole at the bottom of the wheel wells in order to make room for the axle collar. I also had to cut some of the wing skin to make room for the spring on the strut. The wheel needs to sit as low as possible in order for the retracts to fully collapse and lock into place. I grinded the hub of the wheel a bit to make the axle and strut move a little closer to the wheel so the retract could fully close. A lot of trial and error. I glued the wheel wells into place however I used electrical tape to cover up and smooth out the seem between the wing skin and the wheel well. Not the prettiest but I'll redo it when I get around to it.
Jan 09, 2008, 01:21 AM
Flying motor mount master
fly_boy99's Avatar
lol I see you edited your original post.

Best of luck!!!

I need to find an F3D I can fly with a 1506 motor I have.

Originally Posted by Z06kal
Not sure where you get 12x2 from. The prop is a 12x12 square APC. At 10,500 that is 120mph pitch speed. I posted the calc predictions. Why an out runner? Because it is cheap and it has the KV I'm looking for. Why a 12" prop? Because you need the thrust to go that fast. Not too many inrunner other than Neu that have the power handling and KV. You can spin a smaller prop very fast but the plane won't go anywhere because the prop lacks the thrust to allow the airframe to attain the pitch speed.
Jan 09, 2008, 01:24 AM
Started on the nose retract. If you notice I also had to take a big slot out of the mounting block in order to clear room for the retract nipple and line. Because the big plastic nose wheel well is quite in the way of the pull pull installation I had to fab up a boom from some extra 1/8" ply scrap to put the servo in the exact position I wanted for the smoothest operation. If you notice the servo is tilted on its side and moved at an angle to maximize smoothness. From the bottom you can see where the wires for the pull pull exit and the final installation. Kinda finicky to get this right took a couple of hours to have it all working right.
Last edited by Z06kal; Jan 09, 2008 at 01:36 AM.
Jan 11, 2008, 12:51 AM
Moving along. These retracts sure do take long to set up! I must be into the retracts a good 10 hours. Unfortunately I had to cut a slot in the nose wheel well in order to extend the nose strut. I heard many reports that this airplane will not rotate correctly if the gear does not have the plane positioned with a nose up rake. Anyway rather than redo the main gear all over I took the easy way out and simply extended the nose gear. I have plenty of clearance now in the tail to swing any prop I want without worry.

Anyway. I quickly fabbed up a box using scrap ply to encapsulate the air tank. I then carefully measured and cut out my own retract/servo tray out of 1/4" ply to install my retract valve, servo, fill valve, and pressure gauge. I included pictures of the progress and finally the finished retract setup. All thats left is to hook the main gear up. I temp hooked up just the nose gear to pressure test and cycle the system. What a pleasure see it finally work after all those hours. Anyway here are the rest of the retract pics.

I quickly threw all the parts on my scale for a rough AUW estimate. The airplane is coming in lighter than I expected. AUW is looking like somewhere around 9-9.5 pounds. I'm hoping I won't have to add any lead to the tail to get correct CG. Gonna have to move the 12S lipos FAR back.
Jan 11, 2008, 02:24 AM
Registered User
epwierman's Avatar
Ahh,,,,,, a good swing swing.....

One of life's simple pleasures no doubt.

BTW.Impressive project.

Good Luck.
Jan 15, 2008, 02:04 AM
This build is going way over the ammount of time I was thinking it would. I hope it ends up being worth while...

Anyway I modified the horizontal to fit an HS-125MG for the elevator. I framed the servo into place using 1/4" x 1/4" hardwood. FYI it seems the CA accelerant/kicker reacted with the paint on the inside of the covering and made it start to color change. So make sure you don't get any kicker on the inside surface of the covering or else you will see it react and change color/break down.

Anyway HS-125MG dropped in perfect. I cut another access hole right at the edge in order to help pull the wire through exactly where I wanted. The wire needs to come out just before the end rib because the horizontal is supposed to sit flush against the inside of the vertical fin and if you run the wire out through there then the horizontal won't be able to slide all the way in flush with the vertical fin.

I had to drill a hole through the vertical fin in order to route the wires. You either need a very long drill bit or you need to make your own. I ended up taping my drill bit to the end of a carbon pushrod and used that to drill through to the slot for the horizontal. I then just fished the wire through and have it ready for soldering.

I taped back together the flaps in the covering used to access the horizontal internals.

I made my own control horn out of 1/8" G-10 fiberboard and slotted the elevator and epoxied it in place. I finished off the assembly using an extra servo cover I had from one of my old sailplanes.

All the covered surfaces were very badly wrinkled out the box so I went over everything with my covering iron and made it all ready for final assembly.

I began work on the motor mount. For a mount I am using a Dave Brown 90 size glow mount. Drilled out my prop and assembled the motor. Now I just need to line up everything and run the bolts through and mount them to the back of the fusalage.

Next up is to open up the space to fit the 3lbs of 12S lipos and design something to keep them secure. Despite the heavy outrunner hanging way in the back I'm expecting the plane to balance somewhat nose heavy even with the lipos pushed all the way as far back as they will go. We will see... I am really hoping I will be able to avoid putting any weight in the tail.
Last edited by Z06kal; Jan 15, 2008 at 02:25 AM.
Jan 15, 2008, 10:42 AM
Battery Puffer
Looking good. I hope your not going to run that epoxy stick. Use a 12 X 12 E prop. Also I needed a fair amount of down thrust and left thrust [ standing from behind the plane ]to get the plane to track strait. The plane needed a bit of up elevator as well so put some in before take off. These things will help with your maiden. The recomended throws for aileron were a bit much but elevator was OK. My C of G was toward the back of were they say. Good luck and I cant wait to see video.

Jan 15, 2008, 11:17 AM
Mark I could use some ideas for mounting the lipos and making them easily accessible. I'm thinking once I grind the bulkheads to slide the packs all the way back maybe just use a long bolt from one side of the fuse to the other inserted from outside the plane to hold the packs in place. If you have a cleaner idea I'm all ears.

By the way you notice a big difference between the glow props and the E props? I didn't realize that APC made a 12X12E guess I shoulda looked it up on the apc site.
Last edited by Z06kal; Jan 15, 2008 at 11:29 AM.

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