Power Considerations: This should be the first question you ask. The answer is simple---50-60 watts per pound will give you a decent rate of climb (we're not doing acrobatics here!). How do you get to know the watts of your system? Read on...
Brushless Motor: If you are really new to the electric motor field, you will be amazed at the progress made in the past 10-15 years. I prefer an "inrunner" which is conveniently mounted inside the fuselage and behind the firewall - think of the older brushed "can" motors, but these are way more powerful and do not require gearing down. The alternative, "outrunner" is usually mounted in front of the firewall (unless you are very adept at reversing the shaft within the motor - not easy!). For all 2-4 meter woodies, I suggest a motor with a 5mm shaft. That is strong enough to sustain a hard landing without bending. Most of the propeller attachments (see below) work well with 5mm diameter shafts.
Motor Speed and Current Draw: Nearly every brushless motor will have a KV value - which translates into 1000RPM per volt. Because a sailplane moves through air relatively slowly, we want the prop to turn relatively slowly - hence lower KVs - it is fairly easy to obtain motors with KV's around 800-1200 - best not higher!
Batteries: Lithium polymers are now the standard, with 3.7 volts per cell. Almost all motors in woodies will require 2-4 lipo cells. Motor battery capacity ranges are wide - so depending on prop size/pitch you can optimize the current draw and run times. The capacity in milliamp hours and the peak drain is given for all batteries. For example, you can buy a 3000 milliamp hour (MAH) battery that can for brief periods safely produce 20 times its capacity (20C) thus 60 amps. 30C batteries can safely produce higher current draws - at higher cost and higher size and weight. There is really no need for that much power with woodies.
Speed Controller: Brushless motors have 3 wires - the usual black and red for plus and minus voltage, but a third wire which serves to sense to rotation inside the motor. This amazing device measures 100 or more revolutions a second and sends electronic sequences to the motor which allow it to vary its speed from zero to highest. The ESC (electronic speed controller) must be sufficiently robust to handle usual operating currents - typically 20-60AMPS. The ESC plugs into the receiver throttle port (usually #3) and interfaces between battery and motor. The ESC should be programmed with "brake on" so the prop remains horizontal and retracted and won't create drag by its freewheeling when the motor is powered off.
Receiver battery elimination: With 7 plus volts available from the lipo motor battery, it is easy for manufacturers to build in circuitry to provide 6 volts for receiver, servos, and whatever else you might be using. The circuitry is called BEC for battery eliminator circuitry. In 20 years of flying I have never had need for a separate receiver battery--- but some who worry about failure of the ESC/BEC system add a NiMH receiver battery.
Propellor: First, use a folding system. The 2 prop blades are attached to a yoke that attaches to the shaft (see figure 3). A spinner can be added. The yoke width should be sufficient to barely clear the edges of the fuselage front (typically 42-50mm). Second, use a diameter and pitch appropriate to power needs. For example: your plane, fully loaded, weighs 6 pounds. That needs 300 watts for climb well (probably 250 would do fine). Using 3 cells (11 volts) and drawing 30 amps will give you what you need. The greater the propeller diameter and the greater the pitch, the more current draw. You don't want too much propeller pitch - remember the plane is moving through the air slowly and there's no sense in trying to chew up more air. Typical prop combinations for sailplanes are 9/3-4, 10/4-5, 11/5, 12/6.
Bench testing: You can spend anywhere from $20-60 for a meter which can display the current draw and watts which your fully functioning system draws. I have been using AstroFlite Whatt-Meter for 15 years but it costs $60 and there are similar devices at half that cost - just be sure that your meter can measure up to 80AMPS. Otherwise you can simply bench test the motor by running it for 20-30 seconds --- that will tell you whether you are drawing too much current - things get hot quickly!
Mounting and configuring the airplane: The entire power system is likely to weigh about 16-24 ounces (motor 7-10; battery 7-12; ESC 1-2, prop, yoke, spinner 2-3). You should not have to use lead to balance, but can simply shift the battery, as needed. I build in 2-3 degrees of down thrust and 2-3 degrees of right thrust. So when I cut off the existing balsa nose piece, I try to build these adjustments into the remaining structure. Otherwise, one can use washers between motor and firewall. Most motors we use have a 36mm (about 1 3/8 inches) diameter - so to allow mounting screws some space on the firewall and to allow clearance for the thrust angles, you need about 50mm (2 inches) internal fuselage width and height.
Sky Bench Aerotech
Ft. Wayne, Indiana 46804