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#1 gtfreeflyer Apr 11, 2007 11:00 AM

Hobby Lobby Cessna 337 Skymaster Build Log w/Modifications
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This blog is dedicated to my SkyMaster and the modifications I've made to it.

Before I started putting this plane together, I was following a great thread on this plane and collecting ideas: http://www.rcgroups.com/forums/showthread.php?t=519407

Here is my build log... enjoy.

#2 gtfreeflyer Apr 11, 2007 11:14 AM

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Here is how it comes out of the box.
Also pictured are the following items I purchased separately:
-Two Frio 10 brushless motors rated at 150 watts each, 1050 KV.
-Two Apollo 25A ESCs. ($54 combo for motor and ESC from Towerhobbies).
-5 Hobbico CS-12 sub-micro servos rated at 40 oz-in torque (plenty!)
-2 9x6 wood Zinger props (I used to fly nitro planes and like these props!)
-1 9x6 wood Zinger pusher prop.
-2 carbon fiber tubes.
-2 carbon fiber strips.

The heavy battery, slow charger, and crap spare props are the items in the bubble wrap. They came with the plane, and I will not be using them.

Not pictured:
Align 3 cell, 2100 mAh LiPo rated at 22C to power the motors.
6V NiMH receiver battery for the electronics.
(I disconnected the BECs on both ESCs since they can not work together)

#3 gtfreeflyer Apr 11, 2007 11:25 AM

First things first:

-Throw the instructions away, but read them first for a few good laughs!
-Pull the motors out, throw them away.
-Remove all linkages from servos to control surfaces.
-Remove the panels on the bottom of the wings to access the servos.
-Pull the servos out, throw them away.
-Tug (very lightly!) on ALL the control surfaces, and they come out!
-Remove all the hinges and replace them with CA hinges.
-Stick your hand inside the fuselage and push on the front window to pop it out (for easy accessibility during building)
-Pull the two tailbooms apart carefully and take the stabilizer out so you can epoxy it back in. I don't trust the glue holding it in.

Okay, now the plane is gutted out and ready for modifications...

#4 gtfreeflyer Apr 11, 2007 11:30 AM

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I started by mounting the motors onto a plywood firewall, and then used epoxy to fix the firewall into the fuselage. The firewall sits nicely onto the foam without modifications to the foam. The foam seems to have the proper thrust angle built into it. I did have to cut the opening a little wider for the motor though.

#5 gtfreeflyer Apr 11, 2007 11:32 AM

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Next, I tried to figure out where to put the steering servo. I found this spot the easiest to get to. I cut out some foam to fit the servo here.

#6 gtfreeflyer Apr 11, 2007 11:34 AM

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The horizontal stab is pretty flimsy. I cut a slit in the bottom of it and inserted epoxy and a carbon fiber strip. It does not flex any more!

#7 gtfreeflyer Apr 11, 2007 11:37 AM

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The vertical fins are also pretty flimsy. I put a carbon fiber strip on the leading edge of each fin. The rubber bands shown are holding it in position while the epoxy cures. The fins are now rock solid!

#8 gtfreeflyer Apr 11, 2007 11:40 AM

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The forward ESC was dropped down into the pocket just aft of the motor. The aft ESC was mounted on the rear window (great for cooling!).

#9 gtfreeflyer Apr 11, 2007 11:41 AM

I cut out the air scoop on top of the wing to allow cooling of the aft motor.

#10 gtfreeflyer Apr 11, 2007 11:53 AM

That about does it for the fuselage, except for cutting some cooling holes for the front motor, and figuring out where the LiPo goes. I am using the current battery hatch for my receiver battery since it fits perfectly in there. I was also able to put a switch right next to it.

My receiver setup:
Both ESCs go to the throttle channel via a Y-harness.
The steering servo goes to the rudder channel via a Y-harness.
The elevator and aileron channels have extension wires coming out of them.

Both ESCs are connected to a Y-harness so I can use one LiPo.
My LiPo can discharge at 42 amps; I expect to pull 15 amps max from each motor.

#11 gtfreeflyer Apr 11, 2007 12:04 PM

That concludes my progress until now. I will continue working on the plane tomorrow.

Things to do:
-Cut cooling holes for the front motor/ESC.
-Mount elevator and rudder servos/linkages.
-Maybe I'll replace the metal wing struts with the carbon fiber tubes since I have not used them yet...maybe.
-Finshing touches? (I'm thinking of using magnets to hold the front window in so I can easily access the interior, and maybe that's where I'll put the LiPo)
-Balance the model without the LiPo to figure out where to put the LiPo (I read that the CG is 2.5" back from the leading edge).
-Measure max amp draw.
-Write about the maiden in this blog.

#12 warhead_71 Apr 11, 2007 01:09 PM


Originally Posted by gtfreeflyer
(I disconnected the BECs on both ESCs since they can not work together)

Can you just disconnect one of the BECs and run all the servos off of one? The Apollo has dual BEC anyway... it should be plenty to run 5 servos (The Ultrafly P-51 Mustang runs 6 servos off the same Apollo 25A ESC).

#13 gtfreeflyer Apr 11, 2007 01:36 PM

Was not aware of the Mustang setup.

This is from the Apollo 25A spec:
"Handles 5-8 cells with five micro servos.
Handles up to 10 cells (3 LiPo) w/ three micro servos.
Handles up to 12 cells (6 LiPo) w/two micro servos (25V max input).
Supports up to 4 micro servos & Gyro on 3S LiPo."

And this is from an email I got back from tech support after explaining my 5 servo, 2 motor setup to them:
"We would not recommend the usage of the subject item in the manner stated.
You would have to disconnect the BEC on both ESCs and run a 4.8V battery.
The servos would overload the BEC."

With those two pieces of info, I'm not going to test the BEC unless my model won't lift off the ground with the extra battery.

I did save weight by exchanging the motors.

#14 Vantasstic Apr 11, 2007 08:10 PM

I've run other ESCs in parallel to share the BEC load...I've encountered no problems. FWIW, I saw in Model Aviation Mag that Castle Creation is releasing a 10A switching BEC for ~$25. I've used UBECs in some of my larger planes, but the CC one looks like a real winner. You can even program it for 4.8v to 6v output. COOL!!!!

You're mods look very familiar to what I've done with mine. My flies perfectly fine with my Esskay 400XT motors. I'm sure you'll have even better performance with your setup.

#15 gtfreeflyer Apr 11, 2007 11:23 PM

The Apollo ESCs will burn eachother up if used in parallel with both BECs active. I talked to tech support about this. That's just the way they are designed. I've heard that CC ESCs do not have this problem because of a diode? in the circuitry.

I taxied my plane around today for fun. The thing turns on a dime! I guess I have a lot of throw in my nose wheel. The motors are super quiet and make that awesome brushless "whoosh".

Vantasstic, I drive out to Vegas several times a year. We should get a formation flight photo one day.

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