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Old 06-12-2013, 12:23 AM   #1
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Default Twin boom pusher camera plane

Nothing to show yet, but I'm gearing up to do a new foamie . I'm after a mildly aerobatic (fun to fly), 40" - 44" span plane on which I can mount a forward looking #16 808 keychain camera, with no prop in the view. I want to be able to use my extensive collection of 3 cell 1000 mAh batteries as well.

To accomplish this I'm thinking something like a twin boom pusher arrangement, with tricycle landing gear. A pusher is going to tend towards being tail heavy, so I'm thinking of mounting the motor ahead of the CG and using an aluminum drive shaft back to the trailing edge of the wing, through a bearing, to the prop; a couple degrees of down thrust built in.

I'm thinking low wing, mainly to keep the length of the landing gear struts down to a minimum. The rear wheels will be at the tail booms, well behind the CG so we don't have problems with the tail hitting the ground. The rudder channel will control the front wheel - not sure if I'll do rudder(s) or not. (My piloting skills have not yet progressed to extensive rudder use ).

More as things progress!

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Old 06-12-2013, 12:49 AM   #2
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3 cell 10,000 mah? or is that a typo and you mean 1000?

3 cell 10,000 mah would be on the heavy side for a 40 inch span plane.
3 cell 1000 mah would just about require you to use a Y to parallel a pair of packs if you want passable duration. (works fine)

With brushless outrunners, there is no real need to use the long shaft to the prop. The motor will be light and you'll be able to get the CG adjusted with battery placement. Don't make it more complicated than you have to.

You do not want the main wheels way back. If they are too far behind CG you will have issues rotating (getting the nose wheel off the ground) for take-off.

I suggest looking at the overhead view of a Cessna 336/337. Delete the front prop and put your camera in the nose. Adjust the side view for your low wing if you really want low wing... but the high wing mounting makes keeping the tail from hitting the ground easier.

I'm assuming you want to design your own, based on your post. Its always a good idea to look at similar successful designs.
NASA took a 337, replaced the front engine with radar and other equipment and put a turboprop in back. It worked fine. Almost made to order for your purpose.
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Old 06-12-2013, 01:05 AM   #3
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Originally Posted by dgjessing View Post
Nothing to show yet, but I'm gearing up to do a new foamie . I'm after a mildly aerobatic (fun to fly), - span plane on which I can mount a forward looking #16 808 keychain camera, with no prop in the view. I want to be able to use my extensive collection of 3 cell 10000 mAh batteries as well.

To accomplish this I'm thinking something like a twin boom pusher arrangement, with tricycle landing gear. A pusher is going to tend towards being tail heavy, so I'm thinking of mounting the motor ahead of the CG and using an aluminum drive shaft back to the trailing edge of the wing, through a bearing, to the prop; a couple degrees of down thrust built in.

I'm thinking low wing, mainly to keep the length of the landing gear struts down to a minimum. The rear wheels will be at the tail booms, well behind the CG so we don't have problems with the tail hitting the ground. The rudder channel will control the front wheel - not sure if I'll do rudder(s) or not. (My piloting skills have not yet progressed to extensive rudder use ).

More as things progress!
How about something like a Cessna Skymaster except with just the pusher motor? You can keep the gear short and the tail out of the the dirt. No gear loads on the wing or booms so you can keep them light. I'd mount the rear motor farther forward and maybe lengthen the nose a bit if needed to achieve the right CG.

See quorneng's excellent Cessna O-2 build for inspiration: http://www.wattflyer.com/forums/showthread.php?t=70578


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Old 06-12-2013, 01:56 AM   #4
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Originally Posted by fhhuber View Post
3 cell 10,000 mah? or is that a typo and you mean 1000?
Typo - 1000 mAh!

More later...

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Old 06-12-2013, 07:36 PM   #5
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Originally Posted by pmullen503 View Post
How about something like a Cessna Skymaster except with just the pusher motor? You can keep the gear short and the tail out of the the dirt. No gear loads on the wing or booms so you can keep them light. I'd mount the rear motor farther forward and maybe lengthen the nose a bit if needed to achieve the right CG.

See quorneng's excellent Cessna O-2 build for inspiration: http://www.wattflyer.com/forums/showthread.php?t=70578
That makes more sense, doesn't it? High wing would be more stable too... OK, new specs - thanks

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Old 06-13-2013, 04:42 PM   #6
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OK, I've managed to confuse myself regarding the thrust angle... With a pusher configuration, with the propeller (the source of the thrust) being behind the CG, do I get down thrust by angling the motor up or down? If it blows up (same as a front engine plane), wouldn't that tend to make it rotate on the CG and push the nose up instead of down?

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Old 06-13-2013, 05:58 PM   #7
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On a pusher its up on the incidence of the motor ,you can see it on this canard pusher pdf. I made this one to a 38 inch wing span out of dollar tree foam and balsa and it flew great with the incidence they had for the nitro one. Some pushers need alot more like a bd-5 foam plan i have calling for 25 % up incidence so your going to have to play with it to get yours right. BTW i switched one wire so my motor ran backwards so i could run regular props instead of having to buy pusher props. joe


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Old 06-13-2013, 08:25 PM   #8
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You can keep up/down thrust straight very easily.

The way I think of it : If the motor is in front of the CG then it goes by the front of the prop. If the motor is behind the CG it goes by the back of the prop.
(accounts for odd construction such as SeaWind where the engine/motor is behind CG but pointed forward)

Cessna C337 is a good one to look at. CG is between the engines. Spinner on front of front prop. Spinner on back of back prop. If the spinner is pointed down its down thrust.
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Old 06-15-2013, 12:03 AM   #9
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Originally Posted by fhhuber View Post
Cessna C337 is a good one to look at. CG is between the engines. Spinner on front of front prop. Spinner on back of back prop. If the spinner is pointed down its down thrust.
Makes sense to me!

The whole question is less important now though, as I've chucked the drive shaft idea in favor of a conventional motor mount, so it will be easy to change the thrust angle now.

We're off! -



Not a very good picture, but I've got the basic design worked out and the wing halves cut out and glued together. I've gone with a 42" wingspan, 7.25" chord, symmetrical airfoil - same wing as another plane I've done which flies quite well. I'm also going to use the same size tail planes, located the same distance back, so the only "iffy" thing is going to be getting the nose length right to make it balance.

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Old 06-15-2013, 02:45 AM   #10
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Were you able to download the 337 plans i sent you ? joe
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Old 06-15-2013, 12:45 PM   #11
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Originally Posted by road king 97 View Post
Were you able to download the 337 plans i sent you ? joe
Yes - thanks! Mine is going to be ~kinda similar, but I'm not making any effort to look "scale" .

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Old 06-15-2013, 03:31 PM   #12
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Originally Posted by dgjessing View Post
Yes - thanks! Mine is going to be ~kinda similar, but I'm not making any effort to look "scale" .
Ha Ha i could care less about scale but you could look at the motor incidence of a plan of a plane that we know flys. joe
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Old 06-15-2013, 08:07 PM   #13
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Two 1/8" sq. spruce spars glued into the wing:



And the motor arrived - a Power Up 400 Sport from HeadsUp. I've got the same motor on that other plane with the same wing, and it moves it right along .

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Old 06-15-2013, 08:11 PM   #14
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Do your own core cutting? Looks like a professional cut.
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Old 06-15-2013, 10:49 PM   #15
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Originally Posted by dgjessing View Post
Two 1/8" sq. spruce spars glued into the wing:



And the motor arrived - a Power Up 400 Sport from HeadsUp. I've got the same motor on that other plane with the same wing, and it moves it right along .
I agree you cut some nice looking wings. joe
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Old 06-16-2013, 11:26 PM   #16
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Thanks! Yes, I just recently got into hot wire cutting .

Today I put a coat of primer on the wing, and soldered up the unusual bits of the wiring harness:



Note the two battery connections on the ESC and the aileron servos connected together into a single plug. Yes, I know you can buy pre-made bits to do all this stuff, but I've got a soldering iron and I know how to use it .

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Old 06-18-2013, 01:00 AM   #17
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Here's the wing with the ailerons in place, and recesses cut for the aileron servos and wires:



... and with the packing tape covering complete:



(That's the underside of the wing.) I'm thinking the next step will be the tail booms and horizontal stabilizer. Then the fuselage, and sticking the whole thing together .

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Old 06-21-2013, 11:46 PM   #18
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Tail booms glued up, ready to be finished and covered with tape (they are just posed with the wing):



They each have a single 3/16" x 1/8" spruce piece laminated into the dollar tree foam bits.

And then I milled an aluminum mount/pivot for the front wheel:



I know, you can buy those things too, but where's the fun in that? I'm going to make it so it's an easy task to replace the 3/32" wire strut should it get mangled. Oh alright, when it gets mangled .

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Old 06-21-2013, 11:59 PM   #19
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Iam getting ready to mount my own landing gear in a build but its where you realy cant at the rear mounting spot once its covered and the front needs to have a working piston gear for rough landings or both could tear the fuse apart. The plate you made is nice and iam watching the booms you are building. I want to build this one soon and i have already been told the booms will fall apart if built like the plans. Maybe some angle iron is in order ! lmao joe


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Old 06-22-2013, 12:01 AM   #20
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Looking good I am very interested in your shaft extension idea. I've considered doing something similar, but was always concerned about vibration issues, coupling strength, mounting points etc. Full size planes like the Airacobra and such seemed to have been ok with this system; I've not come across it on a model before though. If it could be made simple and robust enough, I would use it on my nacelle mounted pushers. Watching with interest
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Old 06-22-2013, 12:20 AM   #21
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You'll gain some stiffness for the booms from your tape covering,

You'll gain a VERY big stiffening effect if you put one strip of strapping tape on the side opposite from that inserted hardwood or another strip of hardwood (or balsa)

Don't want full elevator to snap the booms...
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Old 06-22-2013, 12:23 AM   #22
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I did a shaft extension on a model once. Used a boat .15 glow power drive shaft for a .09 power airplane. It can work. Its annoying to set up though.

Its also heavy. I like to avoid weight and with brushless outrunners its easy to hide the motor.
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Old 06-22-2013, 12:54 AM   #23
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When i get around to building it i will sandwitch some carbon fiber inbetween some very thin veneer then wrap it in balsa with control rods in there also. Iam a little crazy this way, i am building a totaly different plane now but thinking of builds i might do months or years from now and how to build them. I like watching most builds to see and retain stuff thats a better idea than mine or combine both . Yea iam nuts but at least i admit it.lmao joe
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Old 06-22-2013, 01:37 PM   #24
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Dave

What's the purpose of the extra wires on the esc?

The booms just fit over the wing, not into it?

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Old 06-22-2013, 02:40 PM   #25
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Humm - you guys have got me worried about the tail booms. Maybe I'll do them over as a 1/8" ply / foam sandwich.

Yes, they just fit around the finished wing surface - hot glue.

The two battery connections are so I can use two batteries in parallel & double the flight time. I also think I'll be needing the extra weight in the front.

I gave up on the drive shaft idea for this one, although I'd still like to try it sometime. Just sounds like a fun engineering exercise.

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