Electric Stunt > Gettin all AMP'ed up!
Selecting Motor Size
Dick Fowler:
What sort of guidelines are you electric gurus using to determine the motor size for a given model?
Watts/lb. seems to be the common method for RC... how about for CL? A step by step description would be helpful.
Ron King:
Dick,
Step by step:
1. Read and learn as much as possible.
2. Get the Garden State CB DVD from Rich Peabody.
3. Figure out what others are doing.
4. Steal their ideas and run with them. #^
5. Adjust as you gain experience.
We have determined that 150 watts per pound will give you a decent stunt run.
An electric motor can be difficult to classify. By adjusting the available voltage and pulling more (or less) amperage, I can make the same electric motor stronger or weaker. This gives me a little more margin for error.
Take care,
Ron
Dick Fowler:
Thanks Ron, I appreciate all of the information you are posting on electrics.
I'm curious... do you pay much attention to cooling the motors on enclosed designs? The motors hang out on profiles so I imagine it isn't much of an issue, but what about full fuselage designs?
Also wonder if the you see much from the effects of heating in the motor during normal operation causing any significant increase in current flow as the temp. rises or is the normal voltage drop seen in the discharge curves of the batteries offsetting the problem if it even is of any significance?
Ron King:
--- Quote from: Dick Fowler on July 30, 2006, 04:08:26 PM ---I'm curious... do you pay much attention to cooling the motors on enclosed designs? The motors hang out on profiles so I imagine it isn't much of an issue, but what about full fuselage designs?
Also wonder if the you see much from the effects of heating in the motor during normal operation causing any significant increase in current flow as the temp. rises or is the normal voltage drop seen in the discharge curves of the batteries offsetting the problem if it even is of any significance?
--- End quote ---
Dick,
You've asked several good questions and I don't have good answers for all of them.
First, we do pay a lot of attentiion to the cooling of enclosed motors. If the motor temp gets much above 150 - 160 dgerees (F), then the magnets will demagnetize and lose their strength. ~^
You are correct about profiles. I haven't had any problems there, but still take care with the design and installation. Most of my profiles now run prop nuts instead of spinners to keep the front vents open.
My latest plane has a fully enclosed motor AND a spinner. I use a drilled backplate on the spinner and open the prop slots quite a lot to allow some air to bleed through the front. In the vent opening under the prop, I have an air deflector installed that directs much of the cooling air to the rear end of the motor. That seems to be the place where my motor heats up the most. These deflectors are common in the RC Pattern ships and seem to be the accepted practice.
I use a laser temperature guage and will check the temps as soon as the model stops rolling. Last week, on a 94 degree day, my fully enclosed motor read 86 degrees in the front of the case, 137 at the rear, and my batteries were 117 degrees after a six minute stunt pattern. These numbers are well within spec and let me know that I have the cooling slots and vent design correct for this model.
Speaking of batteries, they like to be warm. Last winter, I had to heat them up on the dashboard of the truck before they would develop much power. Batteries are very happy at 115 - 120 degrees.
I have an Eagle Tree system, but haven't run it yet. It would probably help us answer your other questions better. My limited empirical data tells me that motors run better when they are cool and batteries need to be at least 80 - 90 degrees before they provide good performance.
Take care,
Ron
Dick Fowler:
Thanks again Ron. I'm just putting together my first effort at an electric powered stunter (a Cardinal) and as you suggested, I'm going to beg, borrow and steal as many ideas as I can! ;)
One of the thoughts in the back of my mind regarding the motor temps was that it seems to me that if I don't try to keep the motor as cool as possible and the temp. rises and resistance drops and current flow increases, then it would seem logical that the discharge rate of the battery pack would increase leading to a drop in voltage and power towards the end of the flight. If this be true... then keeping the motor as cool as possible should really help to maintain a more consistant speed through the entire pattern.
Do you use a heat sink on the motor? I was thinking of incorporating some sort of ducted fan cooling shroud system around the outrunner motor. Pull cool air over the motor even at rest.
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