The Electro-Ringmaster

By Dr. Floyd "Dr. Spark" Carter

Well, electric C/L stunt seems to be the wave of the future for some flyers.. Advances in motor and battery technology are rapidly making electric C/L competitive with glo engines. Although electrics are still not a substitute for a piped PA 60, there are areas where electric propulsion can do very well in PA Stunt.

I have been following the developments, and I decided to sneak-up on the technology by building a small C/L stunter. The Sterling S-1 Ringmaster design was chosen for ease of construction and experimentation. The profile layout puts all the electric components out in the open, where adjustments and changes are easily made.

The Ringmaster. I reduced my Ringmaster plan to 75%, giving a span. of 31 1/2" and wing area of 1.434 sq.ft. Construction is unlike any Ringmaster you have seen. The fuselage consists of 1/4" square sticks covered on both sides with 1/32" balsa a 45 degree angles. The wing L.E. , instead of being a heavy log, is built of 3 pieces of sheet balsa. Rear spar is 1/16" X 1/2", and the T.E. is 2 pieces of 1/32" X 3/4". The tail group is made like a free flight model, with a 1/32" sheet slab (!) outlined in 2 pieces of 1/32 X 1/8" bent around and then covered in silkspan. I now think that a 3/32" solid tail group would be just as light. The completed structure, with Polyspan on the wing and silkspan on the fuselage and tail group weighs 5.2 oz. (including the L.G. wire).

The electric flight components (motor and battery) weigh a total of 4.2 oz, giving an all up flying weight of 9.4 oz. with a wing area of 1.434 sq.ft, the wing loading is just 6.55 oz/sq.ft. One could obviously do well with a higher wing loading, and so a larger model would be in order with this motor and battery combination.


Components were purchased from Chief Inc., Grants Pass, OR. Here's what I bought:.

AXI 2208/34 Outrunner motor $64.95
PHX CSE10 Phoenix Motor controller 49.95
AXI 22002 Radial motor mount 14.95
THP 900 3C Li-Po battery (900 mAh) 42.95

From Windy Urtnowski:

Z-TRON Controller $30.00

How It All Works

The AXI motor is a 3-phase "outrunner". This is nothing more than a 3-phase permanent magnet motor whose commutation is supplied externally, instead of with internal carbon brushes. The wound armature is fixed, while the magnets on the stator rotate around the armature, and timed by the controller, and the rotating magnets are connected to the propeller. In my engineering days, this was called an "inside-out" motor, but the principle is the same as any permanent magnet motor. The necessary controller converts the battery dc into a timed 3-phase current drive in a "delta" configuration (3 wires to the motor).

The motor controller was designed to be used in R/C, where the receiver output would drive a throttle servo. Since there are no servos here, the Z-TRON must emulate the output of a R/C channel, with a 1.0 msec pulse to indicate "fast throttle" and a 2.0 msec pulse to signal "low throttle". Since there is no R/C transmitter in all of this, the Z-TRON has a potentiometer to adjust motor speed, which once set, cannot be changed during flight. Otherwise, it simulates the output of an R/C receiver. The Z-TRON has additional features, via a set of switches, to program total flight time, and other features, such as delayed start, etc.

For our use, the Z-TRON instructions indicate the installation of a small switch such that turning the switch to ON, will set everything in motion for launch. Additionally, the Z-TRON is programmed to give some warning before it times out, and so you can get ready for motor cut-off following the pre-set flight time.


My electric setup with a Zinger 8-6 wood prop turned 7000 RPM on the ground. This seemed to be equivalent to an 0.09 glo motor, but perhaps a smaller prop would allow the motor to turn faster and develop more HP. These experiments await.

"Dr Spark" is also known as Floyd Carter of Eugene, Ore., shown above with his Electro-Ringmaster. Flying Lines photo.

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This page was upated June 6, 2006