going brushless

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Why Go Brushless?
While electric helis aren't anything new, electronics miniaturization certainly is. As such, what used to be bulky, power-sucking, Richie Rich expensive hulks are now mass produced, affordable, microtechnological wonders. The most rampant trend has been offloading the traditional brushed electric motors in favor of more efficient brushless variants. So, what is the real difference between having brushes and not? For starters, the most obvious is that brushless motors are free of carbon brushes that wear out, electronically arc, and leave deposits and buildup over time. Also, there's no more need for a commutator lathe or a run-in period for new motor brushes. Not only that, but brushless motors are more efficient in that they convert more input power into work and generate less heat. Brushless motors produce more power out of a smaller can that their brushed alternatives. For all intents and purposes, brushless motors have a virtually unlimited lifespan when they are properly maintained-and there is very little maintenance to be done. In other words, it sounds like brushed motors should be banned from Earth! There is one caveat, however: brushless motors are more expensive, and they require specialized speed controllers.

Both brushless and brushed motors can benefit from the weight reduction and increased flight time that lithium motors and lithium batteries can provide, however the combination of brushless motors and lithium batteries offers the ultimate in weight reduction, power and flight time. Nonetheless, although brushed motors still have their place, brushless is now the ultimate electric flying experience.

While equipment selection is mostly a matter of personal taste and budget, there are still factors to consider when selecting your heli’s power plant. The electric heli’s overall size is loosely tied to motor mount size, much like their nitro-powered counterparts which are designed around engine displacement and mount size. Brushless motors designed for a 400-size heli like the Align T-REX are sized to the mounting dimensions of a 28mm diameter can, also known as a Speed 400.

The names Speed 300, 400, 480, and so on are part of an old nomenclature that does not directly apply to new brushless motors, although the names directly apply to new brushless motors, although the names have stuck as a way to define the general size. Most 400-size brushless motors offer both the mounting dimension of the 400 and 480, and feature a robust 3.2mm output shaft. The T-REX is not designed to fly with a brushed motor.

The Shogun is designed to fly with a Speed 400 brushed motor, but uses a smaller brushless motor with 20mm diameter can. Now that you have the model you want and know what class of motor size you can fit in it, you have to move on to finding a motor that will provide the power and efficiency your style of flying requires.

On to the nitty gritty of what makes brushless motors spin. The most common specification you’ll find is Kv. This number represents how fast the motor will turn per volt applied. For instance, a 1,000 Kv motor that is fed 10 volts of direct current would produce 10,000 RPM is an unloaded condition. The formula for figuring this out is: Input Voltage x Kv = RPM

That's Heavy
As the motor is loaded down by the cumulative mass of the head assembly and blades, it will slow down and draw more current, thereby increasing torque demand. To compensate for the general lack of torque of the stratospherically high motor RPM output, all the model
helicopters make use of gear
reduction. It takes weight
and mass to make torque,
so gear reduction-otherwise
known as torque multiplication-compensates for applications
using a high RPM motor. This same principle is used in high-revving
import cars that use a numerically
high first gear to help produce torque to overcome the weight and mass of the vehicle to accelerate it from a stop. Model helicopters achieve this gear reduction through the use of a motor pinion and the main gear. While the majority of helicopters are limited to one size of main gear you can change the motor's pinion to any one of several different sizes. So, how do you know what size pinion is the right


	one to use? You need to know some other numbers first, and then you need a little bit of math, then some flight testing to back it up. First, you'll need to determine what your optimum head speed should be, given the kind of flying you do, the type of blades you have, and what your heli can mechanically handle.

Figuring Head Speed

As an example, we’ll use the stock Align T-Rex 450XL CDE mini electric helicopter. A recommended optimum head speed for sport flying with the stock wood blades is 2,400 RPM. The T-Rex main gear has 150 teeth; our motor, an Align 400S, has a 2,800 Kv rating and is 80% efficient at converting voltage to work. We’re going to power the system with an 11.1V (average; actual voltage will range during flight from 12.6V to under 10V) 3-cell LiPo battery pack. This translates into the following formula:

(Target Head Speed x Main Gear Teeth) / (Motor Kv x Voltage x Percent Motor Efficiency) = Pinion Gear Teeth

With our information plugged into the formula (and rounding down) we get:

(2400 x 150) / (2800 x 11.1 x 0.80) = 14-tooh pinion

If you power the motor with a higher voltage battery, this will lower the required pinion size and effectively spins the motor at a higher RPM. These high voltage systems offer more power while reducing the current draw.

You can also change this formula around to determine what specifications will give you the head speed you want to obtain. For example:

(Motor Kv x Voltage x Percent Motor Efficiency x Pinion Gear Teeth) / Main Gear Teeth = Target Head Speed

This formula is not a rule; it is just a means to get you in the ballpark. Another thing to keep in mind is that the Kv value alone does not give any indication of efficiency, current draw, or performance. Unfortunately, there is no standardized way in which electric motor manufacturers advertise their motors’ specs. There are some more in-depth calculators available online that can help you experiment with different variables such a PCalc and MotorCalc. As with all helis, flight testing and trimming will ultimately rule out whether one pinion gives you the kind of performance you want over another, but this formula will help get you close.

Quality Materials

Having motor specifications from the manufacturer will help determine how efficient it is, how well it is made and roughly what kind of output you can expect given your application. Higher quality motors will have a substantially lower No Load Current than lower grade motors. The No Load current is a measurement of the amount of current it takes to spin the motor up with a bare output shaft; with no load. A motor built with optimum materials will give a 1 amp no load while a typical lower grade motor will draw 4 amps. The optimum materials ultimately yield a more efficient power-to-work output with less heat generation. To make an adequate comparison based on the No Load Current specification, group motors together based on similar weight and Kv rating.

Conclusion

While the information covered in this article represents only a fraction of the total story of brushless motors, for our models it should provide you a better than average understanding. All theory and calculation crunching aside we will be testing and reporting real world results with our motor tests in a future issue. Until then, check your size, your math, and your gears, and choose wisely.






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