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Author Topic: Twin Engine Power  (Read 665 times)
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njones68
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« on: January 04, 2017, 10:14:20 PM »

Looking for advice on how to power my Dumas Dehavilland Rapide. Couple of pics attached of the "bones", this is a nice kit, a real tribute to Pat Trittle's design skills. The original was powered by geared GWS DC motors, I'd like to go with outrunners but am contemplating running them turning in opposite directions to eliminate any "P" effect. I know the chances of loosing an engine on an electric twin are remote but would appreciate any input. Also what's the best rotation direction, outward or both turning inward, or would this make any difference? Your thoughts?

Nigel Jones, Salmon Arm, BC
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Twin Engine Power
Twin Engine Power
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ironmike
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« Reply #1 on: January 04, 2017, 11:41:08 PM »

Is this FF or RC?
I would choose outbd over the top for rotation.
I like the gws stick motors they swing a larger prop
then the outrunners do.
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Copbait73
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« Reply #2 on: January 05, 2017, 09:41:42 AM »

Mike, I'm familiar with electrics as inrunner, outrunner, geared and "can motor".
What is a stick motor?
Thanks
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Konrad
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« Reply #3 on: January 05, 2017, 10:23:16 AM »

"P" factor (asymmetrical thrust from a rotating blade) is not an issue. If wanting counter rotating props to balance out torque effects you will want the top prop blade rotating towards the fuselage.

Stick motor usually is a reference to "stick mount style" like what is shown in your photos.

Please be aware that Pat's structure is not suitable for high power. The IPS motors called out are "180 class" motors. But as these are set up in gear boxes they will turn larger props than "normal" out runners. So I'd look for a 250 class low Kv outrunner. I'd also change the mount to a firewall style with a box to bring the prop Shaft into position.
« Last Edit: January 05, 2017, 11:26:46 AM by Konrad » Logged

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ironmike
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« Reply #4 on: January 05, 2017, 12:37:36 PM »

My Beaufighter WS 38" flew beautiful
using (2) EPU (GWS) geared motors set up to
contra rotate outbd over the top. The setup was free flight.
I used a variable run time, power output controller
using a li poly battery pac.
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Konrad
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« Reply #5 on: January 06, 2017, 10:04:10 PM »

Nice Beaufighter. Is she from your plans?

There are no critical issues with the way the props rotate. Most twins have the top blade spin inwards to have the engine torque aid in lifting the dead engine in an engine out condition. Those that run the top blade outwards are placing the higher thrust side of the prop disks closer to the aircraft center line. And then there is the common practice of spinning the engines in the same direction to cut down on parts numbers making maintenance easier.  

Now an unusual reason Kelly Johnson https://en.wikipedia.org/wiki/Kelly_Johnson_(engineer)  had for running the top blades outwards on the P-38 Lightning was that he wanted the upwards deflected air off the inboard prop blades to aid in stalling the root of the wing first. In effect dynamic washout to help keep the highly tapered wing tips from stalling before the root. But this was assuming the engine were turning the props. So not really part of the OP’s original concern.

The British where more concerned with logistical support issues and ordered their P-38’s with the props turning the same way (“right handed”). This and the lack of turbo chargers made their P-38 (model 322B) a bit of a dud, more so than what the early P-38's already were.
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dputt7
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« Reply #6 on: January 06, 2017, 10:39:23 PM »

Just a passing comment, I found that turning the props out at the top must deflect the prop wash up to the underside of the tailplane and inturn need less down thrust.

As regard to using geared motors to turn larger props, I would have though the scale size props the Gipsy 6's would have turned to be about spot on with a direct drive brushless.
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Konrad
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« Reply #7 on: January 06, 2017, 10:46:13 PM »

Just a passing comment, I found that turning the props out at the top must deflect the prop wash up to the underside of the tailplane and inturn need less down thrust.

As regard to using geared motors to turn larger props, I would have though the scale size props the Gipsy 6's would have turned to be about spot on with a direct drive brushless.
Yep, the Gipsy 6's did turn a rather small prop. I have almost always found that the larger prop disk area helps enormously with the flight performance of our models. This is why geared electric models often have so much better low power performance than a comparable powered but high rpm set up, be it glow or electric.
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ironmike
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« Reply #8 on: January 06, 2017, 11:45:41 PM »

Right on Davey
Even on my rubber twins I have noted the need for less
down thrust when props turned outbd over the top.
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flyerinokc
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« Reply #9 on: May 07, 2018, 11:05:53 AM »

Nigel, what power system did you end up using?  I recently purchased this same kit and purchased a set of CF2805 1700kv Outrunner Brushless Motors (They come with aluminum stick mounts in addition to a standard plate. I chose 12 Amp ESCs with Switchmode BEC combined with APC 7 X 5E props they pull less than 9 amps with a 2 cell 7.4 V LiPo. Did you make any changes to the construction? I was considering swapping the 4 main fuselage corner stringers to bass to make it harder to break when handled and covered with my fat fingers.
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