rainedave

My Feedback: (1)

I'm in the process of educating myself on electric power systems. I have a general understanding of how you can guesstimate the performance of a model based on its weight and the watts of your power system, e.g., 3 watts per ounce, or 60-80 watts per pound for average sport flying.

My question is what do the wattage specs that motor manufacturers publish mean? I was looking at a 480 that Horizon sells. It's rated at 275 watts. Am I to understand that this little 480 would actually provide good performance for a 3 1/2lb sport model (275w/3.5 = 78.6w per pound)? Or, is the 275w some kind of optimistic, ideal condition figure? Are such numbers applicable in choosing a motor?

I'm just trying to learn how to make sense out of what the manufacturers say about their motors.

Thanks,
David

proptop

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Hey David...
Well...I have an AXi 2808/24 which is virtually identical to that E-Flite 480 and 275 watts is pushing it...and I'd say a practical maximum for longevity sake. Mine will see close to 275 W. and 23.5A. with a 10 X 5E APC.

If, for example you'd want to fly a 3 1/2 lb. plane, you'd want a motor that would provide about 100 watts per lb. @ max power, so that you could fly @ reduced throttle, say 75% and that'd give you your 75W/lb. figure and not push it too hard. Believe me, 75 W/lb. ain't that much, and if you want "sporty" performance, 100W. or more per lb is the way to go

Then you figure what battery would that require to give me a potential energy of 350 watts at about 75-80% of the batteries full power. You want to give yourself some "fudge factor" so you don't fry something...

The ESC is another...go with an ESC that has some "head room" or if your motor will pull 30A. get at least a 40A. ESC so you're never at 100% duty cycle. Heat will build up quickly when you push these things too hard...

rainedave

My Feedback: (1)

Thank you, proptop! That's excellent, useable info. It looks like the E-Flite Power 10 BL (375 max) is more what I'm after. BTW, it's a little 42" Bridi Kaos I scaled down and built from plans that weights 43oz with a glow .15. I figure it will weight closer to three pounds (48oz) with batteries.

So, can you - or, anyone - recommend a software app that would let me plug in the number of cells/mah capacity, prop and motor in order to get a good idea what to buy, before shelling out the money on a power system? I don't really care if the first setup I buy isn't absolutely perfect, but I do want to make sure that what I buy is at least useable.

Thanks again,
David

rainedave

My Feedback: (1)

One other question. I was reading about the Jeti 40 ESC (Hobby Lobby site) and the description says: "BEC can run 5 servos on 6 cell battery, 3 servos if 12 cells."

Why does the number of servos the BEC can handle go down when the cell count goes up? I thought most BECs kicked in around 5v regardless of cells which is basically like having a 4-cell, 4.8v rx pack, right?

feathermerchant

Have a look at rcgroups.com there is more electric activity there along with 'sticky' posts that have god explanations. In the power systems forum there is a sticky of actual power systems and the planes they are on.

Your plane should not weigh more electric (LiPo batteries) than glow. My experience.
You should be looknig for a 300W or so motor.
Is the glow motor mounted on rails? If so you might have to remove them to mount the electric.
I'm looking at th eAXI 2514/12 turning a 9X6 or 10X6
For a sped control the Jeti40 should work. I use Castle myself.

Reason BEC servo capacity is inverse to cell count is that it must drop voltage from battery to 5V for your radio gear. Higher voltage drop (more celle) means more heat.
I know a Castle 35 or 45Acontroller would run your 3 or 4 servos OK. I have several setups like this.

rainedave

My Feedback: (1)

Thanks, feathermerchant. You probably know there's an Al Lehmberg design from around 1940 or so called the Feather Merchant. A beautiful plane.

Your BEC explanation makes sense now. I'll take a look at rcgroups.

Thanks,
David