As you can see from many of the Sport Aviator review articles about electric-powered model airplanes, propulsion systems using electrons for energy have finally become practical. But the selection of airframes designed for electric power still remains limited. However, there are hundreds of ARF and RTF aircraft designed for glow power that can be easily converted to electric.

This article covers the conversion of Polk’s Hobby’s popular Dorothy II to electric power. But the techniques used here apply to converting just about any airplane with a beam motor mount like the one in the Dorothy II. Other glow-powered ARF aircraft use a firewall engine mount system. Converting a beam mount to electric is more difficult. Converting a firewall mount system usually just requires a plywood spacer which, combined with the motor’s supplied mount, positions it further forward so the propeller clears the fuselage.

In this project, I used an aluminum motor mount made from 1.5 inch angle bar stock that is available in any hardware store. I bought mine from a local True Value Hardware store. Aluminum is almost as easy to cut as plywood but is stronger and the angle stock provided a ready-made 90 degree bend.

The OS Max 40 LA-powered review model of the Dorothy II attracted a lot of attention at my local club, The Pine Barons R/C Club based in southern New Jersey. The Sport Aviator review article of this aircraft, The Dorothy II Basic Trainer ARF, can be found in the “On The Flight Line” Section listed under Basic Trainers.

In the original review I had asked the question “Could the Dorothy II be an ARF that bridged the standards of the older beliefs and yet still successfully develop the new breed of ARF trained pilot? “

The answer was a resounding “yes!” and I had expected the airplane to become a trainer for a new or younger club member. While my prediction wasn’t wrong, it was a little short of being right. Soon after the test flying session, I was approached by an elder of the club who inquired as to the suitability of the aircraft for electric conversion.

One short conversation and a deal was struck. If he provided the electric parts, I would do the conversion. Bob Emme was the gentleman in question. Bob flew P-51 Mustangs in WW II and has had a life-long love affair with anything that flies. He had the most interesting distinction of being one of the pilots who ferried back captured German airplanes to a base in France. Sometimes he would be flying airplanes that were of a more experimental nature and which didn’t have any manuals. Even if they had instructions, they were not in English!

On a more emotional note we discovered that his wife and the airplane had the same name. So armed with Bob’s never-ending spirit of adventure, his electric motor, battery and speed controller, we headed for the workshop.

FIRST STEPS

The first thing that was done was to give the airplane a new designation. The letter “E” was added to the name to help in all of the conversations and the naming of the computer files and digital photographs. The name “Dorothy II-E” became the aircraft’s new identity.

The good news was that we already knew that the airplane flew well and also exactly where the CG was supposed to be. The sort-of bad news was that we did not know if the electric motor would be big enough, powerful enough or even heavy enough. It could, of course, be too heavy?

The first step was to remove the glow engine and clean up the airplane with some paper towels and my wife’s Windex. Electric planes do not get “oily” like glow-powered models so it had to be clean from the beginning!

Photo 1

Next, Bob and I started removing the parts not needed for the Dorothy II-E. The following items were removed (photo 1):

Ø     Receiver switch with charging lead

Ø     1100 mAh NiMH receiver battery

Ø     Throttle servo and pushrod

Ø     10 x 5 APC glow propeller

Ø     Spinner nut

Ø     OS Max 40 LA and muffler

Ø     Fuel tank and tubing/filter.

Once everything was removed, it was much easier to plan the next conversion steps.

EQUIPMENT NEEDED

To complete the Dorothy II-E, additional equipment needs to be gathered and purchased. The radio system did not have to be changed. Any four-channel radio will do the job for this airplane, glow or electric powered. For this review, the Futaba 9CAP was retained. Futaba’s 9CAP also allowed the airplane’s radio program to be copied onto several other club members’ radios for subsequent flight sharing.

 
Photo 2          Photo 3

This is what was needed and used to convert the Dorothy II to an electric Dorothy II-E  

Ø     A 3-cell, HI-PO Lithium–Polymer (Li-Poly) 11.1 Volt battery like the PQ-4400 mAh battery shown in photo 2. I strongly suggest a battery with a charging guard like the Poly-Quest PQ PCM one (HIPO PQ440 SP36).

Ø     Electric Motor, Himax Outrunner, Brushless HC 3528-1000

Ø     Propeller adapter and a 12 x 8 APC propeller

Ø     Speed controller, Castle Creations Phoenix 60 Amp 12-volt capacity

Ø     Two female Deans style connectors 

Ø     Motor mount, supplied with motor or self made

Ø     4” of 3” x 1/16” balsa sheeting

Ø     Thin CAA super-glue and kicker

PLANNING THE CONVERSION

To convert the airplane to electric power, there were several design areas that needed to be considered:

Motor Installation

Because the airplane came with hardwood beams the decision had to be made whether to remove the beams and use the supplied radial mount or take advantage of the beams existing position and thrust alignment. The motor already came with a radial mount. It was an outrunner type of electric motor that needed room for the casing to rotate both in front and behind the motor mount.

Cooling

The electric motor needs to be kept cool. So does the battery, and more importantly, the electronic speed controller. The speed controller also makes a brushless motor work as an electric motor and in our case supplied the electricity to power the radio receiver and the servos. This saved weight by eliminating the usual receiver battery. (Ed. Note: It is possible, and sometimes desirable, to keep the receiver battery. In this case, keep the radio switch in place and connect the receiver battery as usual. If something happens to the motor battery, you still have control to glide the aircraft back to the runway. The Dorothy II is very light and can easily handle the extra receiver battery’s weight.)

 
Photo 6      Photo 7

Photos 6 and 7 show some of the areas that must be removed to provide cooling. The new air passages in the firewall allow the motor’s cooling air to escape while providing cooling inlet air for the battery and speed controller.

 
Photo 8     Photo 9

Photos 8 and 9 picture the exhaust air vents that must be cut in the fuselage. The general rule for all cooling air flow is that there should be about twice the area for exhaust as there is for incoming cooling air.

Battery Mount

The battery is a heavy component and needs to be securely fastened in place. It must not be allowed to come loose during