RCU Forums - Speed rating. What does it mean?

Well, unfortunately, you're not the only one to have asked this very same question. It is confusing for many electric newcomers who want to figure out what type of motor to get and why there isn't a standardized system to denote their power output/size like combustion engines.

Here is the general agreed upon watt rating for your flying needs as rated for *brushed* motors I understand (meaning brushless motors can get away with lesser specs as they are more efficient at using the power)

50-75 watts/LB - for trainers and slow flyer. Commonly used motor is the Speed 280/300/370.
75-125 watts/LB - great for sport/aerobatic flying - Generally Speed 370/380/400
150+ watts per lb - Great for 3D and vertical flying. Generally left to brushless motors for weight issues.

The term Speed is really just a name used by Graupner to label their motors I believe, which seems to have stuck. All motor sizes also have different windings so there are variants within that too, i.e. lower/higher rpms etc.

Here are some common examples of the "Speed" designated "BRUSHED" motors and their use:

Speed 280 / Mabuchi 280 sized - same thing. A common small motor found on many 1/18 "mini" r/c trucks, as well as "toy" r/c cars. Most planes lighter than 1lb use these.

Speed 300 - aka Mabuchi 370 sized motor, found in many R/C things, like airplanes ranging in the 1 lb weight, regardless of wing span (though usually not more than ~45 inches or so). e.g. Slow flyers, park flyers.

380 sized are a bit longer/wider by a few mm, more power and such, usually for faster applications, similar weight aircraft/size as above, some small r/c trucks.

Speed 400, found usually in planes of similar size as above, but again, faster, and I've seen them in up to 20-24 oz category.

A 480 sized is simply a longer motor for increased power etc. The Parkzone P-51 Mustang RTF and FW-190 RTF use those, I believe those are in the ~25oz area as well.

540 sized motors, yet again larger in dimension, are commonly found in many 1/10th scale electric cars/trucks/buggies in the 3 lb range. Haven't seen them used in planes, at least not from factory kits, lower turn/winding versions are often used as upgrades in the 2-4 lb airplane range.

550 sized motors again, are longer versions, I've seen them used in the 3-5 lb range of airplanes possibly 45-60" , like the Great Planes Super Sportster EP, or I believe the Multiplex Magister (nice big high wing foam plane).

EDIT:Brushed motors like above are literally as common as water and cheap like $10 (basic silver can, bushings, sealed endbell) up to $35, $50 or $90 for high performance 540 sized (usually with replaceable brushes, armatures, ball bearing equipped, adjustable timing). The hotter they are, the sooner they die. ;)

EDIT:Brushless motors I've seen as cheap as the famous Tower Pro series aka BP-21 or BP-12 outrunners (commonly used in light slow flyers upto aerobatic planes in the 1.5lb range) starting at $14 (overall good but cheap motors), all the way up to $50 for equivelant performance and $300 for your big "gas" planes. Almost all have ball bearings, and require no maintainence other than lubing the bearings time to time (no commutator or brushes to wear out).

Prolongued extreme heat (~140+ deg. F ) will kill both types of motor though.

EDIT:Electronic Speed Controls (ESC) is what you need to control the motors with. They translate your throttle input into motor speed, among other functions (supplying power to receiver and servos). You need one that can handle the amp draw, and voltage to get safe functionality Brushed versions are generally cheaper per amp rating (again from $14 for a basic 15 amp version and up to $100 for programmable ones) compared to Brushless which are usually much higher prices (again I've got a basic Tower Pro 15a that is $25).

EDIT: Brushless motors come in 2 different varieties: "Inrunner" and "Outrunner."
An inrunner is your standard motor shape/design. Usually in the higher kv rating of 2000-10000kv used with gearboxes for increased torque output.
Example: [link=http://www.electrifly.com/motors/gpmg5000.html]Great Planes inrunners[/link]

Anything over ~5000kv is generally too fast / low torque for plane use and used mainly in rediculously fast 1/18th scale cars and trucks with gear reduction of course. (Like mine down below :D )

"Outrunners" are higher torque/low kv motors, anywhere from 200 (big BIG power) - 2500 kv (ducted fan/hiiiigh speed plane), and don't really use gearboxes (sometimes 2:1 reduction or similar for higher kv motors). Example: [link=http://www.electrifly.com/motors/gpmg4505.html]Great Planes Outrunners[/link]

To make it easier to figure out your needs, some BRUSHLESS motor companies use the motor "Speed size" designation in the labels to help pick out your motor, such as the E-Flite brushless motor series; Park 370 (in- and outrunner), Park 480 outrunner, though compared to their brushed motor counterparts these motors are well over powered (and goodly so). Then you have their larger motors named like so: Power 10, Power 25, which means they are about the same as a .10 sized, or .25 sized nitro/gas engine respectively.

To further complicate things, many companies name their brushless motors according to their physical size, as in rotor or can/bell diameter/length, kv rating, # of stator poles (don't ask I'm no expert either) in outrunner type motors.

But you are pretty smart it seems :D You already know the Kv rating under load means nothing (well it still is useful) and that watts is usually what you want, as I listed above. However, the lower Kv motors are good for larger props direct drive, and higher kv motors are good for smaller direct drive props, or use gearbox for larger ones. Also, voltage from the battery drops under load, too. Nimh/nicad batteries are worse at voltage drop under load than Lithium Polymers in that respect.

Also you want to keep amps down and volts up with your setup, in order to reduce heat and stress on the power system you use.

Work out your watts, then find a matching motor for your application, an ESC (electronic speed controller) that can handle the amp draw and voltage of your motor, battery that can deliver the calculated voltage/amp draw (very important), then figure out which prop works with your setup, i.e. if you have a sport plane, you want higher speeds and thus higher kv motors with smaller props, while for 3D or slow flyers, you want low kv with larger props for more thrust instead of speed. Though not always, as you'll see.

As you may know: Watts = Amps x Voltage.
E.g. you have a 1.5lb (24oz) ready to fly weight, sport plane. You want say 100w /lb for decent sport performance, probably a smaller higer pitch prop.

So 1.5lb x 100watts = 150 watts total power. Ok. Lets see. The E-Flite Park 450 brushless outrunner (890 kv, by the way) direct drive, is rated for about 150-175 max watts @ 14 amps constant current, using a 11.1v 3-cell lithium pack. Since it is brushless you can afford to fall short a few watts and not have any problems (better efficiency). 11.1v x 14 amps = 155.4 watts. Oh, and you'll want a prop that can put the right amount of load on the system and yet harness the power. I am guessing from my past experience, an APC 9" x 7.5 E prop will do just fine. (just in case, the 7.5" is the pitch of the prop). I recall good strong climbouts, nice top speed. ...And then the crash..... long story.

Now you'd want to make sure your lithium battery can handle the amps comfortably. So you look up, (off the top of my head) a Thunder Power 3-cell 11.1v 1320 mah 12C (someone correct me) battery . The 12C means it can provide a constant amp draw of 12 x it's capacity = 1.320 amps x 12 = ~15-16 amps.

BTW, I think it's a good idea for your own safety/accuracy that you use a power meter of some sort, like the Astro Watts Up meter, or E-flite's Power Meter so you can measure the amp draw/watts of your system so you don't over amp any part of your equipment.

Whatever I've missed someone else can assuredly step in and add! :D I was in your shoes earlier this year. Practice and experience have brought me so far, and LOTS of MONEY (which I don't have). And of course, this very helpful forum! Thanks Al Gore for making the internet! :D ;). Ok bad joke.:)

Hope you found my thoroughly boring lesson somewhat helpful. :D