Guests: Brad Hensley and…..Rufus Leaking. Ye olde editor—is this for real??? I need to meet this person.

PUT DAVID’S PICTURE HERE – PLEASE Ye olde editor: Lawrence sent me a very good picture of David all spiffied up in coat and tie. Unfortunately, his/ MSN Photo software does not seen to be compatible with my wonderful (if you believe that I have a bridge that is for sale) Windows Vista software. More whining will follow.

J From Lawrence Pendelton: Ye olde editor: Lawrence sent me a neat picture of a bunch of the gang at the flying field—another victim of our software incompatibility.

J From Jack Brown, a retired airline pilot and old friend: F 35 & F-22

By

Russ Petersen

February 23, 2009

This following material¹ is intended to be a non-technical introductory guide to understanding and selecting model airplane electric power systems. Some liberties have been taken with precise definitions in order to provide practical and workable terms useful for developing electric power systems for model airplanes.

How electric motors work – An electric motor consists of a fixed magnet, a magnet that is free to rotate around the fixed magnet and a timing device that can turn one of the magnets on and off. The rotating magnet may be energized with the application of electrical current. By taking advantage of the inherent opposite-pole features of magnets and by arranging the two magnets properly the electric magnet can be switched on an off to cause mechanical rotation to occur.

Scale flying power requirements – 70 Watts per pound of loaded airplane.

Sport flying power requirements – 100 Watts per pound of loaded airplane.

3D flying power requirements – 130 or more Watts per pound of loaded airplane.

An example:

Assume that you have a glow powered kit (ARF or otherwise) that is intended for a 40 sized 2-stroke motor and is expected to weigh 5 pounds when ready to fly. Assume that you intend to fly the plane with an electric power system as a "sport" model, so it must do standard aerobatics easily and let’s assume that we would like it to be capable of flying 10 minutes on a typical flight.

Assume that the weight of the plane with an electric system installed will be similar to the weight that would exist with glow power. This is usually close enough to work out the electric components that would be needed to fly such a plane. Our airplane manufacturer says the plane will weigh between 4.75 pounds and 5.25 pounds, so we will guess that 5 pounds is a good estimate of its weight with the electrical power system installed.

Wattage required (sport flying): 100 X 5 = 500 watts

Possible amperage and voltage (Lipo assumed) combinations to generate 500 watts at takeoff:

2 lipo cell pack: 2 X 3.7 = 7.4 Volts, 500/7.4 = 67 amps required

3 lipo cell pack: 3 X 3.7 = 11.1 Volts, 500/11.1 = 45 amps required

4 lipo cell pack: 4 X 3.7 = 14.8 Volts, 500/14.8 = 34 amps required.

5 lipo cell pack: 5 X 3.7 = 18.5 Volts, 500/18.5 = 27 amps required

And so forth.

Something should be said here about weight of the packs as they increase in size. The following information is based on Hyperion CX 18C/30C Lipoly cells (http://www.allerc.com/index.php?cPath=3_4_93):

Notice that the weight per cell is around 4 ounces in these packs. If we add a cell to a pack, we also add a nominal 3.7V to the pack, which if we assume a 30 amperage load produces 3.7 X 30 = 111 additional Watts. Under our sport assumption of 100 Watts/pound of power required, the 111 Watts would add a weight carrying capacity of 1.1 pounds. So, we gain about 3 to 4 times more power than we do weight for each additional cell added. So, unlike glow or gas power options, usually an additional cell in electric flying is more than covered by the additional power provided.

Assume that we want to use a 40 amp speed control which is rated to handle up to 4S voltage. For best efficiency, this would mean that we would need a 4 cell lipo power set-up in order to safely develop the 500 watts necessary to fly the plane as a sport flyer. Also, this 4S pack may provide too much voltage to use a BEC on the speed control so we may need a separate BEC or we may need a separate on-board battery to power the receiver and the servos. If we had chosen a 3S pack, we probably could have used the BEC in the speed control to deliver power to the servos and the receiver. The BEC in most speed controls currently available will only work in 3S or smaller voltage packs. A limited number (called "switching BECs") are becoming available that can handle more voltage and still provide proper voltage to a receiver.

We also need a battery with a discharge capacity (C rating) that will meet or exceed the expected 34 amps provided in our calculation and that will provide the needed 10 minutes of flight. Let’s assume that for an average flight we will require 50% of takeoff power or 250 watts of power (50% if full power). That means that we would be drawing about ½ of the 34 amp takeoff power or 17 amps on average during a flight. So, 10mins/60mins X 17 = 2.8 amps for a flight. This means a 2800 milliamp capacity battery. Remember that the battery discharge rating (C rating) is a function of the Mah capacity of the battery. (Required discharge capacity {takeoff amperage rate} = (C) (Mah of pack in amps). Or, to solve for the needed C, C = (takeoff amperage rate)/ (Mah of pack in amps) = 34/2.8 = 12.1. So our battery needs a minimum C rating of 12.1, needs to be a 4 cell lipo with a minimum of 2800 milliamp capacity. Probably a 15C 3000 4S1P pack would work nicely in this application. Alternatively a 15C 3000 4S2P pack consisting of 8 lipo cells each with a 3.7 volt and 1500 Mah capacity would provide the same thing. A table provided at the end of this paper provides this calculation in a simplified format.

Lastly, we need to select a motor that will be capable of delivering at least 500 watts of power. I would usually choose a 600 watt motor in a situation like this to provide some slack. One nice thing about electric motors is that the weight penalty paid for installing a slightly larger motor is usually quite small, and the slack provides some flexibility in final set up. I have a preference for out runner motors as well, since they run slowly enough to eliminate the need for a gearbox to obtain efficient propeller speed.

After the power system is installed, set-up will always require running the power system with a variety of propellers to obtain the takeoff amperage that was planned for the system. Propeller loading (and therefore amperage draw) can be changed by altering either the diameter or pitch (or both) of a propeller. Don’t skip this important step before you go flying. Establishing the proper amperage rate will protect your entire power system from damage and will give you the sport performance that you planned for at the beginning of this exercise. One should be sure that the propeller selected is stressed for the rpm intended. For example, APC notes in the safety section of their website (http://www.apcprop.com) that the maximum rpm for their thin electric props is equal to 190,000 divided by the prop diameter. This also includes folding propellers. The maximum rpm for a slow flier prop by APC is equal to 65,000 divided by the prop diameter. This means that a 10 inch diameter thin electric may be run to a maximum rpm of 19,000 while a slow flier prop of the same length could only run a maximum of 6,500 rpm.

The easiest way to take the amperage measure of a system is with a watt meter, several of which are available on the hobby market. I use a meter sold by Astro Flight, (http://www.astroflight.com/) but many other good meters are around. The meter is temporarily installed between the battery and the speed control and when the power system with the propeller installed is run a full throttle, the meter will provide the amperage information you require to make a final propeller selection.

After setting a proper amperage load to deliver planned watts, some flyers also prefer to use a tachometer to verify the actually rpm and thrust that the system is generating at the propeller. One should remember that such an rpm measure is static and that propellers unload in flight between 10% and 25% so the static rpm is only an approximation of the rpm that would be obtained in flight. One thrust calculator has been provided by the Licking County Radio Control Club at (http://www.lcrcc.net/thrust_calc.htm). Using the RPM obtained at full throttle in this calculator will provide an estimate of the amount of thrust that your power system will provide.

So, in summary our calculations have yielded the following choices: