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====================================== May 2002 =========================================

By Don Hart

I’ve spent some time over that last few months reading all the articles in the model magazines about electric powered flight that I can find. I even bought an "E-Gull" ARF model. While it appears that electric flight is both practical and popular, there’s still a lot I have to learn. My wife likes the idea of quiet powered flight (that makes the investment in new models and power systems easier to manage).

Electric powered flight is practical for everything from ultralight indoor fliers to giant scale planes. I’ve watched Bob Benjamin fly his 1/4 scale Taylorcraft in scale competition and was very impressed with the performance. While the sound is not exactly scale-like, the Taylorcraft is no underpowered floater. It can fly in windy conditions as well as any glow or gas powered model. It’s also aerobatic.

First you need to determine what size model you’d like to fly and what you expect for performance. A small slow flier can be lots of fun, can be flown in a small area (or even indoors), but cannot be flown in windy conditions. A glow powered model, such as the Sig "Kadet" or GB "Sky Tiger", can be converted to electric and fly as well or better than the glow powered original if equipped with a well matched power system.

One approach is to buy a complete ARF system with plane and electric power system included. Some ARFs, like the "Firebird" and "E-Gull" even include a 2 channel radio system, battery packs and charger for $90-$120.

Hobby Lobby sells many electric powered models covering the full range of electric powered flight. Hobby Lobby also sells complete "combo" packages that include all the required components for the model of your dreams. Tower Hobbies has recently begun including electric powered planes and power systems in their catalog. There are also many other sources available on the internet.

The power output of an electric motor is measured in watts. The actual output depends on the efficiency of the motor and the losses in the various electronic and mechanical components. This formula assumes a working voltage of 1 volt per cell in your battery pack and 70% efficiency from your motor.

Watts = number of cells * current.

Information on current draw of typical motors is available in the Hobby Lobby catalog or from Ezone.

To get the expected performance from different kinds of models use the following guidelines:

Glider = 30 watts per pound

Sport aerobatic = 50-60 watts per pound

Pylon racer = 80 watts per pound

For a battery pack, Ezone recommends "one cell for every 35 square inches of wing area for sport planes. One cell for every 50 square inches of wing area for trainers."

An electronic speed control (ESC) plugs directly into the throttle channel of your receiver. It controls the speed of the motor by varying the voltage to the motor. A simple on/off switch would work, but an ESC is much more practical.

Graupner, APC, GWS and others have props specifically tailored for electric powered flight. These include props for everything from direct drive high-speed racers to gear drive systems on low power slow fliers.

You hear the terms "indoor", "slow flier" and "park flier" a lot. These small e-powered planes fall into the following categories: indoor - under 8 oz, Park flier - 16-18 oz using 280 motors and up, Slow Flier - in between. Slow fliers can be flown in fairly small areas, but park fliers are faster and require large open areas.

"The first numbering scheme is very simply based on the physical size of the motors. The most basic sizes are 360/380 and 540/550. These motors are 36 or 38 or 54 or 55 millimeters long. The German company Graupner coined the Speed xxx naming convention and in the process rounded things up a bit. But basically if you measure the length of a standard ferrite electric motor in mm and add a 0 you have its approximate Speed xxx designation (in fact most Speed 400s are actually Mabuchi RS380s but that's near enough). These are now widely used as generic names so a Speed 400 motor tends to mean any of that size, not necessarily one from Graupner."

"The other naming convention used by Astro and others, 05/15/25 etc, was originally intended to refer to the equivalent glow motor size. [A] 25 is in theory equivalent to a glow .25. The trouble with this is that electric motors are much more versatile than glow motors, which makes it a very vague designation e.g. an Astro 25 will fly most planes designed for a standard .40 glow motor. These designations are not based on physical size (e.g. 25 and 40 are the same size) but on the amount of power the motor is designed for."

To prevent radio interference from the brushes, fit two or three capacitors to inexpensive ferrite magnet motors. Solder a ceramic capacitor with the appropriate voltage rating (most are rated for 100 volts) for your motor between each terminal and the motor can. A third capacitor can be soldered between the two terminals. Use capacitors in the .001uF to .1uF range. The value is not critical.

Motor break-in improves performance of ferrite motors by 10-30%. This process fits flat brushes to a round commutator and prevents arcing. Run your motor at 1/3 to 1/2 the rated voltage with no load. For the typical Speed 400, run two alkaline cells until they are dead – you’re done. Some fliers run these motors in water at 5 volts to speed the break-in. Stop when you see the water turn slightly grey from brush wear. You must carefully dry the motor and use light oil to lubricate the bearings after using the water bath method.

NiMH cells have a greater capacity for same size and weight, but cannot be charged or discharged quite as fast as NiCads. They have higher internal resistance, so they deliver less current. NiMH cells work best where duration is preferred over power.

Small planes are very sensitive to wing loading. Fortunately, there are very good micro R/C components available at reasonable prices. Hitec makes submicro servos that are truly tiny for about $20. Micro receivers are available for $20-$70. These small receivers have limited range (less than 1000 feet), but are more than adequate for small planes.

One way to increase the efficiency of electric power systems is to use a gear drive. This allows the use of much larger and more efficient props. A gear drive also greatly reduces the current draw, resulting in longer run-time for the same battery pack.

If you want to really increase power, you can elect to buy a brushless motor and speed control. You’d also better have a fairly high credit limit on your VISA card. A Speed 400 sized brushless motor and speed control can cost well over $200. Of course, you can buy high powered cobalt motors for less than $50. These use standard ESCs, so the cost is not astronomical.

A minimum e-powered system and accessories in addition to your standard R/C system:

Motor and ESC appropriate for the plane

Micro receiver (optional)

Micro servos (optional)

Battery packs appropriate for the plane

Peak detection charger

I’ve had the "E-Gull" out a couple of times now. The e-power system flies the little plane fairly well, but it’s faster than I expected. It comes with two 6v 600mah NiMH battery packs. Each of these packs is good for 10-15 minutes of flight, depending on the throttle setting. That’s plenty of flight time for a lunch hour flying session.

While the little plane flies well, I’m not impressed with rudder only flight. The conditions were a bit too breezy, but I really wanted to give it a try. Flying in these bumpy conditions I got the "E-Gull" into a steep dive a couple of times, but without elevator control wasn’t able to prevent a near-vertical impact. It’s a good thing that it’s a really tough little plane. I ran one battery pack down, but had to quit flying when I broke the tail while flying on the second pack.

I hope some of this was helpful. See you at the flying field (or the back yard!).