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Old Jan 08, 2012, 01:54 PM
Vertical approach specialist
potshot's Avatar
Northeastern Illinois
Joined Mar 2009
1,082 Posts
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Need tips for location of prop-in-slot

I started a thread with this topic in the flying wings forum but got zero replies. Maybe I stated it poorly or chose the wrong forum. The main question is whether there are any guidelines for where to position the motor and slot in a prop-in-slot design.

For example, right now I'm working on a 20" w.s. foam delta for indoor use, built of 3mm Depron with a KFM2 step and running a 10 or 15g motor powered by 350 mAh 2S. Right now the motor would be right on the calculated CG or a tad behind it. How would this work out? (The parallel lines on either side of the slot are fins.)

I've seen several different approaches to the motor/prop location in these forums and am wondering if there are any guidelines for it. If not, tips from builders' past experience would he very helpful.

Thanks!
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Old Jan 08, 2012, 05:27 PM
Build straight - Fly twisty
Whiskers's Avatar
Australia, QLD, Little Mountain
Joined Feb 2010
4,172 Posts
Well, there are props on the front and props on the back which work equally well, so I think we could deduce that you can place the slot anywhere you like.
Many planes similar have to yours have been built, so it should be fine.
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Old Jan 08, 2012, 07:46 PM
Vertical approach specialist
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Northeastern Illinois
Joined Mar 2009
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Whiskers, the reason I asked is that in my first attempt I had the motor and slot too far forward and I would have had to put the battery and all the electronics behind the slot to balance it out. Seemed rather silly. So I thought maybe there was some rule of thumb to follow. Never hurts to ask, given all the great knowledge around here, yours included. Trying to learn.
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Old Jan 08, 2012, 08:00 PM
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150 mile house B.C. Canada
Joined Jan 2010
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I think its all about where your components line up . You dont want to have to extend wires around the slot to attach and get proper balance . Just lay out the plane mount the components and see where you need to cut it out .
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Old Jan 08, 2012, 08:09 PM
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Malaysia, Selangor, Kajang
Joined Jun 2009
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I designed my first prop-in-slot plane with the motor exactly on the CG. My reasoning was that since it was the heaviest element having it on the CG would make it easier to balance. I was wrong. I had the same problem you're having: had to push lots of things back to get the CG right.

Ever since then I've always built the slot with the motor just slightly behind the CG. Usually the motor mount/firewall ends up on the CG. That makes it easier to adjust battery position without being forced to mount the battery on top of the receiver and ESC.
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Old Jan 08, 2012, 08:15 PM
Build straight - Fly twisty
Whiskers's Avatar
Australia, QLD, Little Mountain
Joined Feb 2010
4,172 Posts
Indeed there structural reasons to locate the slot in a particular location, but not so many aerodynamic reasons.
Do as Wilfor says. It's the very best method to get the CG where you want it with no dramas and added weight.
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Old Jan 08, 2012, 09:20 PM
Vertical approach specialist
potshot's Avatar
Northeastern Illinois
Joined Mar 2009
1,082 Posts
Quote:
Originally Posted by slebetman View Post
... I've always built the slot with the motor just slightly behind the CG. Usually the motor mount/firewall ends up on the CG.
Makes sense to me.

Quote:
Originally Posted by Whiskers View Post
... no dramas and added weight.
Goals for life, too.

Thanks, gents.
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Old Jan 09, 2012, 01:29 PM
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Dave Barrow's Avatar
Ky
Joined Feb 2003
1,484 Posts
I like to build with the battery ending up on the CG, that way you can easily change the CG by moving it. It also lets you use batteries of different size/weight easily. Lay everything out on top using tape to get it all situated and balanced correctly before cutting any holes.
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