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how to convert motor to engine size? - 9/28/2012 8:51 PM   
flybyjohn


 

Posts: 404
Joined: 1/14/2004
Last Login: 5/17/2013
From: Helena, MT, USA
Status: offline 		Ok, here it goes. I was looking to build a plane from electric plans for glow power. I read some plans that called for a e480 outrunner engine that is a 1020kv. It said that it would run best with a 3 cell lithium battery and a 11x6 apc propeller. Now I kind of know how this works, 1020 kv means the rpm should be 1020 rpm per volt which means that with the 11.1v 3 cell bat. it should rev to 11,322 rpm. This is a 31 oz 39" wingspan plane and I would normally put a 10 to 15 glow on this size plane, but holy cow, my 46 thunder tiger can only swing a 11x6 at slightly over 12,000 rpm and it pulls around 5-6 lb airplanes. The watt rating on this motor is 250w so that equates to about 1/3 hp. So how does 1/3 hp turn an 11x6 at 11,000 rpm? Can anyone tell me what I am missing here?

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RE: how to convert motor to engine size? - 9/29/2012 1:06 AM   
ckreef


 

Posts: 224
Joined: 1/21/2012
Last Login: 5/17/2013
From: Danville, GA, USA
Status: offline 		I can't answer your question technically but here is what I got that works for me.

My MiniStik currently has a WS of  36" and an AUW of 37oz.(just put it on the scale) I fly it with a OS .25LA.  It's a fast plane for it's size and good performance.

I would think a OS .25AX would work fine delivering about 3/4 hp and keep your weight down to a reasonable amount.  Just remember after converting your AUW  will be a bit over what was spec'ed for electric. Use whatever prop is recommended for your motor. Also use metal gear servos as I have already had to replace my nylon gear servos.

I would have loved to put a .25AX on the MiniStik but I used what I had. A .25AX would have made it a rocket ship - the LA motors are just low end motors.


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RE: how to convert motor to engine size? - 10/8/2012 1:06 PM   
optech



Posts: 521
Joined: 1/26/2002
Last Login: 5/17/2013
From: San Diego, CA, USA
Status: offline 		Flybyjohn,
Your calculations are assuming a 100% efficient system. Doesn't work that way in reality. Most outrunners like the one you described are actually not that efficient. In general, about 75% efficieny is about right for outrunner type motors. Gear driven inrunners are better at about 87% efficiency. There are always exceptions to the rules of course but the above figures should get you in the ball park.

This means your 11,300 rpm figure at 75% would be around 8400 which seems about right for that outrunner, prop, and battery you describe.

Mike

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RE: how to convert motor to engine size? - 10/10/2012 10:03 PM   
opjose



Posts: 12247
Joined: 11/26/2005
Last Login: 5/17/2013
From: Poolesville, MD, USA
Status: offline 		I prefer to utilize constant wattage maximums for a motor, then equate that to horsepower.

Doing that you can go back and forth easily, but you do need adjust for inefficiencies and advertizing hype.

e.g. 1 HP = 745.7 watts.

A powerful .32 glow engine puts out 1 - 1.2 hp, while a weaker LA type engine may only put out about .7-.8 HP.

It is a loose equivalence but it works pretty well for me.





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