Dedicated to the Promotion of Electric Propulsion in all types of Aeromodeling

The Prez’ Sez’ for March 2012

Please visit the Innov8tive Design website.


Electric Power System Presentation

Key Points and Rules of Thumb

1.    A power system consists of a Battery, Speed Controller, Motor and Propeller
2.    Each component should be matched together for a good power system.
3.    Li-Po Batteries have 3 ratings: Voltage (Cell count), Capacity and C-rating
4.    Li-Po Batteries are 4.2 volts per cell fully charged, 3.7 volts under load, and discharged at 3.0 volts per cell
5.    Li-Po batteries should be stored long-term at 3.7 to 3.8 volts per cell
6.    Battery capacity is measured in mah or milli-amp-hours.  1000mah = 1 AH
7.    Battery capacity is similar to fuels tank size in glow engines.
8.    Discharge rate = current ÷ battery capacity.  A 1C discharge will last for 1 Hour
9.    To calculate flight time use 60 minutes ÷ Discharge rate
10.    Speed Controllers convert DC energy into 3-phase AC to power brushless motors
11.    Speed Controllers have 2 ratings:  Voltage (Cell count) and Maximum Current
12.    You cannot harm a motor by using a larger speed controller
13.    Brushless Motors are 3-phase and bearings are the only wear item
14.    Brushless motors have 3 ratings:  Size, Power Rating (or Current Rating) and Kv
15.    Different manufacturers use different numbering schemes for their motors
16.    Electric Motors are basically the exact opposite of Glow Engines
17.    Electric Motors are constant RPM machines, Glow Engines are constant power machines
18.    Glow engines can be damaged by using too small a prop
19.    Electric motors can be damaged by using too large a prop
20.    Glow engines push power into the prop while the prop pulls power out of an electric motor
21.    Prop selection is critical for good performance in electric motors
22.    The power that an electric motor puts out is determined by the prop used
23.    When selecting batteries, try to keep the Current to voltage ratio between 2 and 5
24.    Always use the 80% rule when calculating power systems
25.    Get the proper tools for Electrics, it will save you time and money in the long run
26.    Learn how to solder!  It is the most important skill needed for electric power flying
27.    Never trust factory Pre-tinned leads, always re-tin them yourself prior to soldering
28.    Only use rosin core solder!  Never use acid core solder

1 cubic inch of 2-stroke Glow Engine is equal to 2000 watts of electrical power
Ex. A .60 Glow engine = 1200 watts and a .40 glow engine = 800 watts

1 cubic inch of 4-stroke Glow Engine is equal to 1500 watts of electrical power
Ex. A .40 glow 4-stroke = 600 watts and a .90 glow 4-stroke = 1350 watts

Power requirements for Electric:  Power Gliders need 50 watts per pound, trainers need 75 watts per pound, sport models need 100 watts per pound, pattern planes and warbirds need 150 watts per pound and 3D and competition Fun-Fly planes can use 200+ watts per pound

Selecting a Power System for a Model

A Simple 5-step Process

1.    Calculate the required Watts of power needed based on the weight and type of aircraft
2.    Select an appropriately sized battery to provide the desired flight time
3.    Choose the correct Motor based on the number of cells you are using and prop size
4.    Select a Speed Controller to match the Motor being used
5.    Select the right prop to get the desired performance of the model

Step by Step example from the presentation, a 6 pound, .45 size Ugly Stick power system

Determining Power Required:  A .45 glow engine = 900 watts ( 2000 x 0.45), alternately a 6-pound model with pattern performance needs 150 watts per pound x 6 pounds or 900 watts

Selecting the proper size battery:  From the chart below, 900 watts can be done with a 4 or 5-cell Li-Po battery.  Higher voltage equals higher efficiency so we will use a 5-cell pack (18.5 volts under load).  To get 900 watts from 18.5 volts requires 900 ÷ 18.5 or 48.6 amps.  Since 2/3 throttle is typically ½ max current, we will use 24 amps for calculating battery size. Battery capacity is determined by flight time at power load.  For 10 minutes we need a 6C discharge rate.  24 amps at 6C is a 4000mah battery pack.  For 80% rule we increase battery size to 5000mah.

2-cell Battery – 100 to 300 Watts            6-cell Battery – 1,000 to 2,500 Watts
3-cell Battery – 250 to 600 Watts            8-cell Battery – 1,800 to 4,000 Watts
4-cell Battery – 450 to 1,100 Watts        10-cell Battery – 2,800 to 6,500 Watts
5-cell Battery – 700 to 1,700 Watts        12-cell Battery – 4,000 to 10,000 Watts

Selecting the proper Motor:  Find a motor that will deliver 900 watts on a 5-cell pack at 48 amps.  Remember the 80% rule and get a motor rated for 1125 watts if you need 900 watts.  Do not forget to take prop size into consideration.  Prop charts make this easy if they are available.

Select a matching Speed Controller:  The speed controller should handle the full rated current of the motor, even if you are not pulling that much current.  When in doubt, always go up one size larger.

Selecting the correct prop:  In many cases several props will pull similar current, but give different results.  For example, a 10×10, 11×8 and 12×6 prop will all pull about the same current, but give dramatically different results.  The 10×10 prop will give the highest speed, but the least thrust and would be suitable for a pylon racer type plane.  The 12×6 prop with give a lot of thrust but greatly reduced top speed.  This prop would be good for a slow plane like a Piper Cub.  The 11×8 prop would provide a good compromise between speed and thrust, and would work best on a sport model such as an Ugly Stick.