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Old Dec 09, 2014, 06:34 AM
Registered User
Netherlands, GE, Nijmegen
Joined Sep 2014
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Motors pointed inwards on multi engine plane?

Hi,

I'm building a small ( 30") 2 engined model (Aztec two step, from Outerzone). The Original model has 2 .020 cox motors on it. I want to equip it with 2 turnigy park250 electric motors in a counterrotating setup. The plan says that the motors should be pointed inward. I have seen more Multi engined plans with the motors (thrust) pointed inward.
My questions are:
Why are the motors pointed inwards?
Do I need to point the motors inward with the counterrotating setup?
If yes, how many degrees?

Thanks in advance.
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Old Dec 09, 2014, 07:54 AM
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walter3rd's Avatar
United States, MD, Towson
Joined Aug 2012
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i have heard just the opposite. someone in the know will chime in.
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Old Dec 09, 2014, 08:52 AM
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Springfield, VA
Joined Feb 2000
1,210 Posts
I have never seen twins or multi-engine models with motors pointing inwards--it is always out. This is a holdover from fuel engines, because if one engine quits, the outthrust will help overcome the drag of the dead engine, and hopefully help with controllability.

An engine out is not a great concern with electrics, especially if you are using one battery for both motors. in the case of a counter-rotating setup, I put both motors with no side thrust--just pointing straight ahead. However, based on the model design, you still might need down thrust or up thrust...Pete M
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Old Dec 09, 2014, 09:40 PM
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United States, NV, Reno
Joined Mar 2000
7,615 Posts
Agree. Especially with counter rotating engines (you want the prop to descent toward the fuselage and rise toward the wing tip on a counter rotating setup). So no side thrust at all. Down thrust per original plans but you are also probably turning a larger scale size prop with electric too so ensure you have clearance at the stock wing mount locations so you choose the right kv motor and prop combo.
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Old Dec 15, 2014, 11:12 AM
Registered User
Lawrence, Kansas
Joined Dec 2001
610 Posts
Can't think of any good reason for motors pointing in. A couple "semiscale" DC-3 kits had motors noticeably angled out, which my buddy who flew DC-3s found quite amusing. Angling motors toward each other just wastes thrust, makes no sense to me. I suppose it might have something to do with both props turning the same direction, but as others have said, it would increase problems in an engine-out situation.

I believe that glow engines can't be reversed (can only turn in one direction), whereas it is easy to reverse electric motors. So, assuming you can get matching right- and left-hand props, motors should be straight, not angled, because each motor will cancel the torque and P-factor of the other, and there is no "critical engine" as there is when both props turn the same direction.
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Old Dec 15, 2014, 11:35 AM
Kimbers Keeper.
Joined Nov 2014
107 Posts
Not sure if the motors originally did, but the Cox .020 motors are capable of running either
direction. Reed intake valves don't know direction.

I have several electric twins and all of them have the motors angled out. They are all CR.
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