Electric Flight

by Don Menk  menksters@msn.com

MARCH 2007

Starting out with your first electric model can be very confusing. I remember my first RC plane.  .049, 15, 40, 60—Confusing it all was—still can be. My best advise is get some assistance.

There are many kits, ARF and RTF models designed around electric drive systems. Most have recommended components to use, motors, speed controllers and batteries. In the beginning I would consider the manufacturers suggestions.

To summarize the state of electrics today, we have the inexpensive drive systems and the more expensive drive systems.

The cost of an standard brushed electric motor for a small airplane (18-25 ounces) can be as little as $10. Today we have available brushless motors to power the same weight plane that cost $25-$50.

Speed controllers for the motors are designed for brushed or brushless motors.

Newer battery systems such as Li-poly are available. These batteries, generally, can pro duce the same power as conventional batteries with a third the weight. Standard batteries can still be used. Other than the kit and radio components, the batteries will be your major expense. There are new terms you will learn such as amps, watts, volts, kv.

I find electrics clean, quiet, and convenient. Today we can fly electric models from 20 grams to 20 pounds and above. We are very fortunate in this area to have access to one of the premier electric hobby shops in the nation, New Creations RC located in Willis, Texas. They have the best customer service I have found. Not only do they have a vast inventory, the owner, Kirk Massey, can assist in model selection and appropriate power systems. So give electrics a try.

 

APRIL 2007

This month let’s talk a little about motor selection, review a few terms and also go into batteries in depth.

Today, with motor technology available, any of our models can be electrified. Big ones, small ones, short ones, tall ones, fat ones and round ones. I have built and fly models ranging from less then 1 oz. to aircraft over 20lbs. The first decision we must make is the motor we will need. Available to us are two basic motor types known as in runners, and out runners. The basic difference is the in runners can turn smaller props at high rpm or larger props with the use of a gear box (ugh). The out runners can turn large props without the use of a gear box. I have models which can turn 22" props with authority.

So what motor shall we use for a specific model. First determine the approximate weight of your model. Now that you know this value, your power requirements can be calculated by a formula known as watts per lb. 

50-70 watts per lb. will = a minimum level of performance.

70-90 watts will yield trainer, slow flying scale like power. 

90-100 watts sport aerobatic,110-130 watts advanced aerobatic,

130-150watts light 3d and fan units, and above this will give you enough power to recover kens Pluto lander.

I have a couple of models that produce close to 300watts per pound. also have some that produce less then 30 watts per pound. so as you can see it centers around the type of aircraft you select, the speed and flying techniques you will use. Of course the airframe integrity rules must be applied just as they are with gas or glow models. Most of us have seen 40 glow size models with 90 size engines on them perform but certainly not within the aircrafts design parameters. 

So now we know how many watts we need but what the heck is a watt or not.

watts are simply a measure of power output. Approximately 750 watts = 1 hp. The formula for calculating watts is simply volts x amps. now we have two new terms, volts and amps.

amps.- amps are the amount of power that is delivered to the motor. Amps are also referred to as current. Electric motors all have the maximum current that they can handle before they burn up. Amps are also used in a simple math formula with voltage to calculate a power measurement known as watts. 

volts - volts are an electrical measurement that we can think of as rpm at the motor shaft. When you see a term known as “kv rating” this is the rpm at the shaft of the motor per 1 volt. So the higher the kv rating the higher the rpm is. This is also included with most motors we use. Batteries are posted with the voltage they supply at full charge, so multiply the voltage by the kv rating and you get the Shaft rpm.

watts – watts are the total amount of power developed by the motor with the prop in place. 746 watts = 1hp. with electrics we don't really care much about developed horsepower because we use the watts per pound rule. Now, with the voltage kept constant (same number of cells), by changing the prop size we can add or subtract a little power just as we do with glow engines. Watts are calculated by multiplying volts x  amps.

 volts equal the number of cells in a battery and amps can be thought of as the diameter of the fuel tubing.  

To generate watts [power] we need a fuel source [battery] fuel tubing and regulator [speed controller] which can be thought of as a throttle servo, and a motor to deliver this power to our prop.

 These days I am using ly/po batteries. Under load a 3 cell battery will deliver 10 volts. Lets say the motor I select can deliver 10 amps. Multiply volts x amps and in this case we get 100 watts. I can now fly a 1lb model with sport type performance. 2lbs requires 200 watts and so on. The available motors are rated at the number of amps, or watts they will deliver. So, to increase your power you can add more cells which will increase the amps providing more power. Of course, as in gas or glow, the prop will change the equation as well.

Battery - batteries are our fuel tanks (voltage supplies). A 5000 ma battery can also be called a 5 amp battery, they are the same thing. Ma are used because our batteries come in ratings like 370ma,1320,2100,4200, and so on.

In this battery discussion I will be talking about lithium batteries. Each lithium cell produces 3.7 volts. So with a 3 cell pack we get 11.1 volts.  Under load a 3 cell ly-po provides 10 volts. The ma rating can be thought of as the size of the gas tank. so 2000ma battery give you twice the flight time as a 1000ma battery. Now the other thing we must consider with the battery is how much current (amps) can the battery deliver. The batteries are posted with a maximum discharge rate, for example 10c. So a 2000ma battery can deliver 20 amps of power. If you need 40 amps of power you would need a 4000ma(4 amp) battery. Many ly-po's can now supply 20c, so before buying a battery check the discharge rate. Should you discharge them at a rate higher then rated capacity they will puff up like a roasted marshmallow. 

 So now that we have a understanding of sorts about batteries what this 3s2p stuff about? well in brief the 3s means you have three cells wired in series. Wiring cells in series increases the voltage of the pack using the number as the multiplier. In this case it would be 3 x 3.7 volts giving us a 11.1 voltage cell. The 2p means the cells are wired in parallel. When cells are wired in parallel this number multiplier is 2 times cell capacity. Gee so now we have a battery that is rated as 2000ma 3s2p; it means the pack contains 3 1000ma cells in series, 1 cell in parallel giving us a battery pack consisting of 4 cells total. This may seem a bit confusing but after a bit of using these modern battery packs the mystery will clear up a bit.

At first, it all seems complicated but after a few electrics it gets easier. We are very fortunate in this area to have access to one of the premier electric hobby shops in the nation. New Creations RC is located in Willis. They have the best customer service I have found. Not only do they carry a vast inventory but the owner, Kirt Massey, will assist in model selection and appropriate power supplies. Give electrics a try, they are clean, quiet, and convenient. 

If some interest is shown by a few people about these electrons in motion I will write a review about  building a .40 size electric model from one of the following airframes. a rascal 40,  sig 4 star 40,  horizon pa-18 , or a sig kadet. all these are great models to build and fly using electric motors.......