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The Electric Bobcat 50 - 1/15/2008 1:20 AM

z06kal

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I started a thread on another forum however it seems this airplane is much more popular here on RCU Extreme speed so here is my current project:

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
CC BEC for radio powered hooked up to one of the 4S packs.
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 for retract valve.
Rudders locked out at least for now (will have them wired up if I decide I really need 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 something else if I wanted. It is very heavy at a whooping 23 ounces however this weight should make it easier to meet CG with the big lipos going in. I will be using this motor to find out how capable the airframe is without spending too 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 it up well over 100.

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. I'm hoping for maybe 80-85% of the pitch speed in level flight.

You can see below starting the retracts in the wings. I had to slot the mounting blocks to clear the tubing for the down nipple. For the ailerons the HS-225MG's required me to put some 1/4" x1/4" blocks in the standard servo hole to fit this Mighty Mini servo. I did not use the rubber mounts but simply cut the brass sleeves and mounted the servo as low in the wing as possible. I cut my own control horns for the ailerons out of G10 fiberboard and slotted the control surface and dropped them in filling the hole with plenty of epoxy. I know not the cleanest job slotting the ailerons but they are not normally visible.

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RE: The Electric Bobcat 50 - 1/15/2008 2:07 AM

z06kal

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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 however you don't want to pop any high G turns with this airframe.

FYI completely empty airframe is about 4.5 pounds. I estimate AUW to be around 9.5 pounds.

Anyway moving on....

Finished the retracts in the wings. 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 and get some matching color material.

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RE: The Electric Bobcat 50 - 1/15/2008 2:16 AM

z06kal

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Here is progress 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 to get the smoothest bind free 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.

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RE: The Electric Bobcat 50 - 1/15/2008 2:21 AM

z06kal

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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 (all the way back as far as they will go in the fuse).

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RE: The Electric Bobcat 50 - 1/15/2008 5:07 AM

Stick40

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Looks nice but that big outrunner will have lots of tourqe but lack the high RPM that makes the high speed possible. A EDF might be the best route to take.

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RE: The Electric Bobcat 50 - 1/15/2008 8:30 AM

z06kal

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That big outrunner should be turning over 11,000rpm. 290kv * 42 volts = 12,000+ As long as the motor holds together and the controller can switch fast enough shouldnt' be a problem. Don't forget 12" of pitch at 11,500 is 130mph. Doubt it could go very fast with a DF even with a 120mm fan. The fan's simply lack the thrust that this draggy airframe needs.

< Message edited by z06kal -- 1/15/2008 9:36 AM >

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RE: The Electric Bobcat 50 - 1/15/2008 10:23 AM

z06kal

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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.

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RE: The Electric Bobcat 50 - 1/15/2008 10:24 AM

z06kal

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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.

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RE: The Electric Bobcat 50 - 1/16/2008 10:11 AM

z06kal

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Moving Along....

Did I mention this plane has been a lot of work?

Anyway I finally fitted the 3lbs of lipos and designed a mount. I grinded the first rib behind the canopy opening first thinking I would fit 3 lipos side by side however this was unncessary as I ended up doing 2 side by side and 1 flat across the top in order to get the packs as far back as possible. The fuse tapers towards the tail however is quite tall so this was the config that allowed me to slide them back as far possible.

Grinding the ribs was a big pain as it was tight in there. I wish I had a dremel extension! Anyway you can see the pics of my sloppy rib grinding work.

For securing the lipos I cut out some 1/4" plywood trays that I taped the packs to. I then drilled out a hole for a long threaded rod to pass through and hold the front of the packs in position. Because the back of the packs are pushed into the taper of the fusalge they are very secure and tight and unable to move. I had to make a ply shim for the top pack so that it wouldn't have any room to jiggle. The long threaded rod has a nut on each end one under the fuse and one on top to hold it in place. Makes the packs a breeze to slide in and out.

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RE: The Electric Bobcat 50 - 1/17/2008 10:08 AM

z06kal

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Well I mounted the motor today. Per MarkF's suggestion I gave it a couple of degrees of left thrust and down thrust. The hextronic motor was feely very stiff and bindy and I became concerned that maybe it didn't work. I quickly rigged up the ESC to test it and the motor out. I ran the motor off a single 4S pack. Not surprisingly it wasn't very exciting about 86 watts and 4000rpm. After a few run ups the motor was turning smooth. Go figure.

Anyway I glued the wings on and finished hooking up the retracts. Did a few cycle tests. Looks like with that big air tank I have prolly something like 20 up down cycles if you pump it up to 100psi.

Nice to see it finally looking like an airplane.

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RE: The Electric Bobcat 50 - 1/18/2008 8:50 PM

z06kal

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I have been dry fitting the tail feathers. Incidence seems pretty close. There is about .25 degree of postive incidence in the tail. It is quite possible that this incidence is designed into the airframe so I will not mess with it. Looks close enough to me despite how easy slight building intolerances will set it off. Looks like this airframe is probably laser cut and the accuracy is actually quite good. I got an AUW yesterday. 10.3-10.4 pounds about 165-167 ounces. This makes for about 35.8 oz/sqft loading. It is a pretty big plane tho. I'm sure landing it will be real interesting but I do have a 600ft paved runway to play with. I hope just the retracts hold up okay at this weight. I'm gonna have to really focus on landing her easy so as not to blow the retracts out.

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RE: The Electric Bobcat 50 - 1/26/2008 6:36 AM

z06kal

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Okay well here she is. All done. Just need to program the radio. One final weigh in and she came in at a whooping 11 pounds flat. Wingloading comes out to be 38oz/sqft. HEAVY. I'm guessing full time to build around 50 hours? Anyway it is a lot to say the least

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RE: The Electric Bobcat 50 - 1/26/2008 6:38 AM

z06kal

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more pics

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RE: The Electric Bobcat 50 - 1/28/2008 8:12 PM

bob27s

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The project looks great. Nice attention to some often-overlooked-details.

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RE: The Electric Bobcat 50 - 1/29/2008 2:37 AM

AGR413

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Make sure you set up spoilerons, I think your especially going to need them on this heavy bird. Nice build can't wait for the maiden report.

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RE: The Electric Bobcat 50 - 2/24/2008 7:07 AM

z06kal

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I finally maidened my Bobcat today! When I got to the field I did one last power system run up test with fully charged packs. 1800-1900 watts 46 amps and 11,200rpm on the tach spinning an APC-E 12x12.

I was very nervous about the 11lb AUW. However this plane was a ****cat to fly even at 11lb at least with a bit of headwind. I taxi'd out and there was a storm moving in so I had about a 12-15mph head/cross wind. Despite not having any rudders just a steerable nosewheel the plane tracked completely straight I held a bit of down elevator to make sure I had steering authority as the plane accelerated nice and straight. Once I went to full power the plane almost immediately left the deck completely smooth with a very short take off distance.

The Bobcat was very very stable and flew very smooth and straight. Plane sounded pretty good all prop and no motor noise at all. Gear cycled perfectly. The plane looked great doing low passes with the gear up. With the strong head wind the plane was doing 110mph into the wind and 118mph out of a bit of a dive. Level cruising speed at full throttle is somewhere around 120-124mph. The plane doesn't look fast at all however due to its rather large size. It just cruises around nice and stable and appears to float at 120mph. If you want this plane to look fast and impressive its going to need to go 160-170mph (if the airframe can even handle it). The plane just cruises so nice and stable it feels completely docile at 120mph. I hit a roll at full rates and the plane looked like a blender on high with a perfectly axial super impressive fast roll.

When it came to land I was a little nervous. I tried the spoilerons and it pitched up violently when I had the spoilerons at high rate I lowered it and found a setting that seemd good. However I really didn't need the spoilerons with the head wind and 600ft runway. I did my approach and the airplane slowed down very quickly and was just very easy to land not at all like my pylon racers at similar wing loading which are just a nightmare to land because they don't slow down at all and just zoom right by.

I am super satisifed with the airframe for the money. The plane flys great and looks fantastic in the air. I had person after person coming over and telling me how great the plane looked and how impressive it was in the air. People were like it looks so good with that electric motor!

Anyway on the 2nd flight I hadn't noticed that the nose strut had vibrated loose and as soon as I took off and hit the retracts the strut and wheel slipped off and was just blowing in the wind held by the pull pull wires. I put the gear down and did an emergency landing in the grass. I settled the plane down on the mains and then let the nose drop it dragged for a few feet and stopped. Plane was completely undammaged other than a little scrubbed paint from the bottom tip of the nose. Just need to redo the pull pull as it ripped off on the landing and locktight all the set screws for the retracts.

Video to come.

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RE: The Electric Bobcat 50 - 2/25/2008 4:05 AM

Razor-RCU

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Nice!!!

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RE: The Electric Bobcat 50 - 2/25/2008 6:02 AM

Kmot

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That electric motor on there, almost looks like a micro turbine.

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RE: The Electric Bobcat 50 - 2/25/2008 6:04 AM

jetdryvr

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Nice Build Z06,
CONGRATS on the Maiden!
Erik

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RE: The Electric Bobcat 50 - 2/25/2008 8:20 AM

Troy-RCU

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Cool deal. I like your motor mount... very creative! Maybe see this at the next Revver event, eh!?

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RE: The Electric Bobcat 50 - 2/26/2008 8:15 PM

z06kal

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Troy what is the Revver event? I'm always down to go

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RE: The Electric Bobcat 50 - 3/1/2008 8:24 PM

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Glad to see this worked out so well. I have the same plane and a set of Spring-Airs that I was planning to power with a Tower 75. I started thinking about an electric option awhile back. How did you do on the CG? Were you able to shove the packs back far enough to avoid using ballast? I see you put the air tank in the nose...good idea. What kind of duration were you able to get?

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RE: The Electric Bobcat 50 - 3/2/2008 9:04 AM

z06kal

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CG is on the border of being tail heavy but it works fine. I have the packs shoved all the way back. Run time is actually very good I run down the batteries about 60% with a 7 minute flight with lots of time on the throttle. The vertical is pretty much unlimited even at 11 pounds but the plane needs some more rpms to be impressive speed wise. It only does about 115-120 in level flight and that is just too slow for such a big airplane to look like it's moving well. I'm getting pretty comfortable with the plane so I will probably be stepping up to a hotter wind motor. Idealy I'd like to spin that 12x12 at 14,500 instead of the 11,200 that I get now.

< Message edited by z06kal -- 3/2/2008 9:06 AM >

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RE: The Electric Bobcat 50 - 3/2/2008 1:31 PM

z06kal

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VIDEO UP FINALLY!

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