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Diary of a Junior 60 electric conversion

By: Barry Slater (April 2006)

Oil to electrons

Following my normal routine of fueling up, priming, connecting the glow cable, and running up the starter motor, I carefully carried the buzzing Ripmax Gambler sports plane over to the flight line and set it down onto the grass runway. Hoping the little blue OS40LA motor would not stall, I prepared myself for takeoff.

Sitting just ahead was an interesting looking plane, about to open the throttle. From memory it was something like a Limbo Dancer, or a Panic. I cast an eye over the brightly coloured plane, and in a way that is difficult to describe, it looked purposeful. Whilst admiring this machine, it was then that I noticed that his motor had stopped. I held back, waiting for him to step out and collect it from the runway so I could proceed. But he did not. Instead the prop suddenly whisked itself up into a frenzy and firmly dragged the plane upward into the blue yonder.

I was impressed to say the least. Obviously electric - but what a performance. My first RC plane, about 9 years ago, was an electric glider. It clawed itís way up into the air, puffing away on a brushed 540 motor and 6 Nicad cells. Eventually, after many flights, only level flight was possible from a hand throw, and that kind of fun does not last very long. Iíve been flying glow motor planes ever since, but clearly, as this plane demonstrated, significant developments have been made in the electric flight arena whilst I wasnít looking.

I donít remember my own flights that day, but as I wiped away the oil and grime using my scented baby wipes (!), I pictured this guy just landing, slotting it into the car, and going home to the hanger; pristine and ready for the next time. Looking back, it was then that something sort of clicked.

It was clearly possible to fly sizeable airplanes on electric power. The attractions were - well - attractive. The motor would start first time every time, it would not stall in flight, the model would come down clean, it did not need all the glow motor paraphernalia to start, and it had the added benefit of being quiet. Being an Acoustics Engineer living in a noise sensitive World, this feature was particularly appealing. In fact because our flying site is about half a mile from a residential area, there are restrictions on flying times - unless you fly electric! The appeal was growing. I know that some people like all the glow related rituals - the smell, the smoke, the noise - but I had discovered that I wasnít really one of them.

Dawn of a new project

So, I set myself a brief, did my research, trawled the internet, set up big Excel spreadsheets to compare parameters, and started buying Quiet and Electric Flight magazine, a dedicated electric (and gliding) flying magazine. What I wanted was to build a plane from a kit (that narrows things down a bit!), a plane that was very slow and basic so that I could learn the characteristics of the brushless electric flight system (which seemed to be the way to go) without trying to master another plane as well, and finally something that had a bit of charm.

To cut a long story short, I narrowed it down to either the Junior 60, a vintage dihedral high wing plane with rudder and elevator, or the Piper Cub. The Junior 60 won out, and the build commenced. It took about 8 weeks to complete, and being tight I decided to try it out with parts I had around me - an old buggy motor of fairly hot wind (16 triple I think), hooked up to a 3:1 gearbox with power from 8 SubC Sanyo Nicads of 1600mAh capacity that I had bought many years ago in an attempt to boost the performance of the afore mentioned electric glider. I installed the electrics and flew the 60 that same day - in essence it was nearly a total disaster. I did get the plane home in one piece, but the flights (or controlled crashes) clearly needed working on. In short I needed more power and more rudder area.

To be fair to Ben Buckle kits, this plane was originally designed for free-flight, then later it was used with a small diesel engine to putter around the sky on calm evenings. I, on the other hand, was flying it in a stiff breeze, off fairly long grass, and with insufficient power, all of which was entirely my own fault. I also found out that there was an electric version available which would have made life easier than converting the glow kit!

Back to the drawing board

So, the plane was grounded, and much time passed with other jobs around the house being carried out instead. I read up some more on brushless technology, Lipos, and the like, and before long it was time to open the wallet and get the scalpel out! I had heard good things about John Emms at Puffin Models so gave him a call. He recommended a Mega 20/30/3E brushless motor, coupled with a Jeti 40 advanced brushless controller and a maximum of 12x6 prop. As long as my existing 8 cell SubC Nicad pack was in good order it would be sufficient.

Parts duly ordered, I then set about enlarging the rudder area. I did this by fixing a piece of card to the fin and drawing around it. I then removed the card and sketched a rudder with about twice the area (mainly stretching rearward, but also vertically to try to keep the thing looking proportioned). I was also keen to keep it lightweight, being at the tail end of the plane. The originally rudder was a solid sheet, so I cut most of it away, keeping a hinged vertical strip into which I tenoned in some horizontal rearward pointing strips in the same style as the fin, keeping the whole thing looking homogenous. I figured that replacing a solid small rudder with a built up larger rudder would negate most of the weight gain. This process turned out to be quite tricky as it all had to be grafted onto the rest of airplane, rather than making parts on a flat plan.

Back to the patch

With all shiny new parts duly installed, it was off to the patch again. This time, however, things were different. Oh, there was still a stiff breeze, which grabbed the right wing on take off and cartwheeled the little plane off to the left on the first attempt, but I was determined. I picked her up, went back to the start, and waited for a lull in the wind. I waited and waited and waited, then suddenly there was a drop. Right, here we go. It was obvious there was plenty of power this time round, and I had also noticed that a bit of right rudder was needed to keep her straight; a combination of me gunning the motor, more wash from the much larger prop, and double the rudder area for the wash to twist into.

Due to the strong headwind, and my full throttle antics, she was airborne in less than 5 meters. With no elevator input on my part, she was climbing strongly at about 30 degrees. I backed off the throttle - and the nose gently dropped. Catching this slightly on the elevator and adding some power back in again allowed me to steady the ship for the first turn. This was uneventful and felt just right. Doubling the rudder area had given me the perfect amount of authority, and with 50% expo dialed in I was turning comfortably this time round. On the subject of rudders, I recently saw another Junior 60 which the builder had moved the rudder post forward, thus turning more of the non-moving fin into rudder, but keeping the same overall silhouette. I would recommend this modification to other builders. Anyway, back to the flying. The next few circuits were a bit up and down to be honest, as applying even small amounts of power would make her pitch up noticeably, and backing off would drop the nose again. I made a mental note to add some downthrust, along some right thrust which was also required. I also thought about moving the C of G forward a bit because the glide was neutral / a little nose up-ish.

It was soon time for a landing, and she came in fairly well, but despite full elevator for the flair out, it basically didnít happen. I made another note to double the area of the elevator as well, to match the larger rudder. I could already sense the hostilities should any traditionalist one get a good look at the deviations I was planning. Oh well!

At this point I should say that it was not my intent to make this into a 3D aerobatic ship with barn door control surfaces, but I wanted a bit more authority.

Home again

Going through my mental notes, the motor was raised slightly and downthrust added, together with some right side thrust. I donít have an accurate figure but it canít have been more than a few degrees for each axis. I then set about enlarging the elevator. In the kit this is a solid piece of balsa, so I cut a lot of it away, leaving a long strip still glued to the mylar hinges. Into this strip I cut some mortice slots, and tenoned horizontal balsa strips rearward, finishing off with a trailing edge piece to complete the built up elevators. All of this build work was done on the plane, using a flat surface underneath to keep the structure flat. With these modified feathers I found that I had added hardly any weight to the tail end, which I was pleased about. She was ready for another outing, so it was off to the strip.

Third time lucky

This time I had a bit of an audience. It was a still warm evening, and I was not alone in thinking I could sneak in a week-night flying slot. Slightly nervous due to the bumpy flights Iíd had so far, I placed her on the strip and prepared myself for the flight. This time I new there was plenty of power. I also knew I had enough authority, and that the thrust lines were more favourable. So I opted for a more graceful takeoff, slowly sliding the throttle forward to about ĺ, and waiting for the speed to build. A few moments passed, with very little rudder correction required this time, until she nosed up into the air. Much more satisfactory!

After a few settling circuits, playing with the trims incessantly to find the optimum setting, I set up a low and slow pass down the strip, right past my eye line. There is something wonderful about that moment when a collection of wood and electronics, assembled together into a certain order, magically defies gravity and hangs in the air, just there. With the transparent film covering it was possible to see all the structure, the electronic bits, and through the generous side windows I could make out the pilot. Ok, so he was marginally the wrong scale, and probably wondering why he was in a vintage plane rather than a jet, but still, Iím sure he was just glad to be flying - as was I.

The enlarged rudder and elevator were giving me perfect control, and at about Ĺ to ĺ throttle she held a fairly constant height. This plane turns beautifully, and since everything happens quite slowly, it quickly settles into a relaxing and restful experience. I heard a few complimentary murmurs from those behind me in the pits, and as I found out later, almost everyone gathered there had a Junior 60 tucked away somewhere. Had I started a local renaissance?

Barely 5 minutes had passed and it was time to land. Not because of the battery, but because it was getting late. My intention had been to get some air under the wheels, and prove to myself that it was almost sorted.

Out again !

There never seemed to be a decent weather window, so, being the impatient type I eventually went out into another stiff breeze and slightly overcast sky. The upwind leg was slow but sure, followed by a whisked away downwind leg - but the plane really needed a calm warm evening to get it into a nice slow groove. Idly running this thought through my mind I turned downwind and opening the throttle. The response was somewhat muted - had the battery gone already? It had only been 3, 4 or come to think of it probably 10 minutes of battling the wind. Yes, I probably had run it just one lap too far, and was now tasked with bringing it down at the end of the strip, into the strong wind with a definite ĎIím landing now!í glide angle. With a slightly early flair, and a hop skip and jump, she was down.

I did later try a bigger 3700mAh battery, with standard C cells, but the extra weight seriously effected the flying. I would not recommend going this route.

So summing up then, my second foray (after the glider many years ago) into electric flight was a total success. I can now confirm - along it seems with many other converts and new starters - that electric flight is a much more realistic prospect.

By: Barry Slater

See Also: Electric Flight, Model Aircraft Gallery.