View Full Version : E-confused

02-12-2006, 05:43 PM
:confused: Help!!!

I am looking into e-flite and have kadet ep-42. I have had some dealers say only look at the watts of a motor and others say windings. How should I decide on which motor to buy and batteries and esc'?

Wow am I confused!

Where is a good easy explanation of how to examine electric flight?


Dr. Kadet

02-12-2006, 06:27 PM
Perhaps you can find a plane similar to yours in this list of conversions.

The simplistic conversion is:
50W/lb - trainers
75W/lb - fun fly
100W/lb - aerobatic


02-12-2006, 07:49 PM
the Kadet EP-42 is supposed to fly on a geard 400 setup so I would bet it would fly great on a 400/480 sized outrunner like the E-Flite 450, AXi 2808 etc since it only weighs about 26 ounces perhaps even an SK 400XT would fly her.

02-14-2006, 12:06 AM
:eek:Thanks for the replies! I am even more confused than ever. Went to my LHS and the guy said just throw this 25.00 GWS brushless on there and you will be fine. No mention wattage batteries etc...

Guess I need to do more research.


Don Sims
02-14-2006, 12:54 AM
Do the research, ask questions, there are some super threads as "Stickies" here in this forum. Folks here want you to be successful in the hobby. Remember, we've been there, done made those errors, and want to prevent others from doing so.

Matt Kirsch
02-14-2006, 03:03 PM
drkadet, sounds like the hobby shop dealers were just as confused as you are. With respect to how to choose a motor, none of them were even close to helping you "learn to fish," and they were throwing you red herrings. :)

The shortcut way to getting the plane in the air is to do a couple of quick searches on this site, RCU, and even RCGroups. Copy what someone else is using.

This is going to sound counter-intuitive at first, but the first thing you need to look at when powering an electric airplane is the battery. The battery is where the power comes from; it's the electric engine's "combustion chamber" as well as the fuel tank.

Volts and Amps from the battery multiply together to become Watts. Watts is a measure of power, and it takes power to make an airplane go. There's a rather lengthy discussion on Watts and rules of thumb in another thread, but in a nutshell, it takes so many Watts to make a plane of a certain weight fly in a certain way. Just like putting an engine with more horsepower in a car makes it accelerate more quickly and/or have a higher top speed, putting more Watts into an airplane makes it accelerate more quickly and/or have a higher top speed.

A 10,000ft view:
Start with Watts, based on the airplane's weight and desired flying qualities. Next break those Watts out into reasonable values for Volts and Amps, using the constraints of the battery technology you're going to use (e.g. 3.7 Volts per LiPoly cell means you can't get 400 Watts by running 10 Volts at 40 Amps because 10 does not divide by 3.7 evenly). With the Volts and Amps, you can now find suitable batteries, ESCs, and motor/prop combinations.

You'll see a lot of people simply saying, "Use X motor and Y prop with Z battery." This isn't some sort of magic; someone, somewhere went through the process I described above, or used trial-and-error.

02-15-2006, 01:27 AM

You'll see a lot of people simply saying, "Use X motor and Y prop with Z battery." This isn't some sort of magic; someone, somewhere went through the process I described above, or used trial-and-error.

What Matt just said is some of us have spent a lot of time and money to get the answers that we give.
The LHS sometimes doesn't always have the same interests as you.
And then some of the info you get here will not always apply to you, but can be a guide.
I read a lot and hang around here, nice bunch a guys in general.

By George:)

Jeremy Z
02-17-2006, 07:05 PM
The simplistic conversion is:
50W/lb - trainers
75W/lb - fun fly
100W/lb - aerobatic

To add to hoppy's table:

150+ W/ lb - 3D (lots of thrust:weight)

To answer your original question, I would say that you have to look at both watts & turns, and here's why:

While it is true what I said above, it depends on whether the power rating of the motor (number of watts) is used for high torque at low RPM or high RPM at low torque. Higher torque will be able to swing a larger diameter prop. This will deliver more thrust, but at a slower speed. Thrust is good for acceleration, carrying loads, and all sorts of 3D type stuff. But lots of thrust doesn't necessarily mean speed. It is one at the expense of the other.

As noted above, the same power (watts) in a motor that is wound with fewer turns will mean higher RPM. But without the torque, you will be using smaller diameter props. Usually with a faster pitch speed. This will mean your plane flies faster, but will not accelerate as quickly, climb as quickly, or carry as much weight. You've given all of that up for speed.

If that confuses you even more, here's a quick example:

I have a 16.0 oz. Formosa. (1 lb.)
Set up with a 150W outrunner motor. (Himax 2812-0850) Please note that the '0850' part is the kv, which is how many RPMs it will turn at per input volt. For example, with 10V input, it will turn 8500 RPM. (no load)

Himax also makes two other motors in the 2812 series: 2812-0650, and 2812-1080. They are all 150 W. But they will fly the plane very differently. The 0650 model is more suited to either 3D flight or sailplanes, which require lots of torque for bigger props and faster climbs. It will draw less input current and turn at a lower RPM, but it has more torque. But top speed will be lower as a penalty.

I chose the 0850 because it is a good compromise, and I will use this motor in several planes. (the Formosa is alread its second plane)

The 1080 turns very fast. If you're after speed, and you figure you need about 150W, this is probably the way to go. However, it will not climb as quickly, and it will draw more current, which means shorter flight times.

Do you know how props work? A 10x4.7 prop is a 10" diameter, with a 4.7" pitch. This means that with one turn of the prop, it will propel the plane 4.7". (assuming no losses) So if you chose an 8x8 prop instead, it would move the plane further for each turn of the prop, right? But since the diameter is smaller, it wouldn't have as much thrust. (measured in oz or g) Starting to come together?

Go to www.maxxprod.com (http://www.maxxprod.com/) and download the PDF datasheet on the 2812 motor series. Print up the page with the performance chart for each model. You will see the difference. They show different props at different voltages. Then, they show the current draw. Armed with the knowledge that a larger diameter = more thrust and a higher pitch # = more speed, you will see how things work.

One last example:

Assume you have a 1 lb. plane. If you choose the 650 kv motor, it will easily do 3D, as it will have the torque to swing huge props, thus yielding lots of thrust. But the top speed will be relatively low.

Put the 1080 kv motor on that same plane it will be a LOT faster (assuming it is propped properly ;) ) But it will probably not accelerate the plane vertically out of a hover. It will take longer to get to speed and draw more current. Such is the price of speed.

The 0850 motor should be a good mix. I get reasonable thrust with a big prop, and reasonable speed with a small prop.

I hope that helped.

Jeremy Z
02-17-2006, 07:07 PM
Oh, I forgot to equate turns into that.

Fewer turns = higher RPM. This also means higher current draw and lower run times.

Some makers of brushless motors put turns right in their model designation. (such as Hacker) Others, (such as Himax) know that is not really what we want to know, so they put the kv in instead.