JET-SET GO!

A LOOK AT AN EASY AND REASONABLY PRICED ENTRY INTO LARGE ELECTRIC SCALE HELICOPTERS

Written by: Martin Briggs

Around the mid 1990’s, I built myself a 46-size Graupner JetRanger. Instead of powering it with the usual O.S 46. option, I fitted a Graupner-badged, Plettenberg electric motor, driven by 20 x 2000 NiCad cells via a Kontronik ESC. By contemporary standards performance was good, but even then I wouldn’t have described it as exhilarating, or in any way comparable to the i.c. version. Mild aerobatics were possible, but flight times of around nine minutes for scale manoeuvres soon tumbled to five or six for a more aggressive flight style.
In addition to these problems I felt then, as I do now, that it’s impossible to create the necessary scale illusion with anything less than a 60-size model. Smaller helicopters just don’t have enough presence, nor do they have the ability to handle the average flying conditions we must tolerate in the UK. Given all of these disappointments, I left my thoughts of flying silent scale for a while, concentrating instead on some of Graupner’s 60-size i.c. scale models (which are very quiet anyway).

Uni-Star mechanics converted for electric sit below the Graupner fuselage on Martin’s JetRanger.

BUT NOW…
As time marches on, so does progress. Although 3D holds little interest for me, I couldn’t help noticing recently that some pretty hot electric models can be seen at flying fields I’ve visited. This rapid progress in electric flight is down to a couple of factors: the development of lighter and more powerful brushless three-phase motors, and the introduction of lithium polymer (LiPo) batteries, which are both lighter and more powerful than the previously used NiCad and NiMh cells. Charge and discharge rates were a problem with early generation LiPo’s, but not so much now - it’s still ‘best practice’ to charge them at low rates (typically 1C), but the safe discharge rate on many cell packs is now quoted at 20C. Imagine that… who would want to flatten his battery in just three minutes? Not me, that’s for sure.
I’ve been quietly watching the electric revolution, and whilst I can’t claim to have been entirely keeping up with it, I’ve certainly been making progress. About two years ago, I built another electric model from the Graupner range, this one being the slightly larger (50-size) Starlet-E. The Starlet is a cut down version of the German company’s Uni-Star 60 Trainer model, the differences being a shorter boom, a smaller and lighter canopy and fewer bearings in the main rotor grips. Otherwise, it’s much the same animal.
I started off with Kontronik’s ‘Tango’ brushless motor and ‘Beat’ ESC, powered by 24 x 2,400 NiCad cells. Once again this was a heavy package, the Starlet being a larger model than the JetRanger; but, with a far more powerful motor on board, I was able to enjoy longer flight times. This model was interesting (like Steve Davis), but still not outstanding in any way (unlike Steve Davis) and didn’t really hold my attention for long. Nine minute flights, with much less power on tap than an O.S. 50 SX-H, didn’t raise the hairs anywhere. I did fly it occasionally, but for most of the time it just sat at the far end of the model shelf.
By nature, I’m a cautious type. Although many of the early scares people associated with LiPo batteries were down to a lack of common-sense use, these fears contributed to my unwillingness to adopt this technology when it appeared; however, it did soon become clear that LiPo’s were the only path to take. It occurred to me that, if 3D fliers could obtain good performance levels through using less power to achieve longer flight times, Li-Po’s might provide that final part of the puzzle I’d been looking for. Hence, I acquired an 8S 1P 3,700mAh pack, which fitted nicely between the Starlet’s mainframes – however, it had to be placed well forward to maintain the correct C of G. Unfortunately, I never did weigh the Starlet when it was equipped with NiCads, but it was obviously a lot heavier then. With the LiPo, I’d achieved a dramatic loss of weight and a large increase in capacity, i.e. from 2,400 to 3,700 which, according to my internal calculator, amounts to more than 50%. Less weight and more power… how could we fail? Well, of course (with careful use, I keep reminding myself) we couldn’t!

The pod-and-boom electric version.

GRITTING YOUR PINION TEETH
Early flights showed great promise I but sensed that the motor, originally designed to be run on 24 volts, was uncomfortable with the 28.8 volts I was then attempting to drive it with. A hunt through my parts bins came up with the perfect answer - a 19T pinion to replace the provided 22T kit part. This change transformed the Starlet into a very satisfying machine and in the many flights that followed, I found it to be very versatile. Flight times could be anything between 17 minutes of scale flying and 10 minutes of continuous aerobatics - no, not 3D but continuous loops, rolls, reversals and the odd bit of inverted that my tired old brain can still manage! All this was done without discharging the battery pack to an unsafe level, with around 10 - 20% of its capacity remaining at the end of all flights.
This performance revelation spurred me on to go the scale route once again. Checking my Graupner spec-sheets confirmed what I had already guessed; the Uni-Star JetRanger was listed as the lightest option. With only minor modifications, the mechanics were upgraded to Uni-Star specification, and a JetRanger fuselage kit assembled to house them. The all-up weight of this completed model came out at an amazing 4 kilos, or just over 9.5 lb. in old money - and that without any attempt to save weight on the fuselage structure! The flight performance and times have changed very little since, even with the addition of the JetRanger fuselage. Now, for the first time ever, I could enjoying flying a large electric scale helicopter.

A CHEAPER OPTION
At this point, I felt the urge to convert all my Graupner scale models to electric. The only problem with this plan was the relatively expensive overall cost, and so I set out to discover if I could find a more economic route. As I mentioned before, I’ve been watching others (mostly fixed-wing fliers) and observing the success they’ve had with converting their model planes from glow to electric power; much of this has been possible through using motors from the AXI range of brushless ‘out-runners’, hence I was drawn to their catalogue in a search for a suitable power unit for my own application.
This time, I wanted to go for a 10-cell 5,000 mAh pack that could drive 66cm blades (the average size of blade I fit to my scale 60’s), at somewhere between 1,250 and 1,600 rpm, to suit both normal (scale) and aerobatic flight modes. The motor I chose, the 4130 / 16 had a Kv rating of 385; although it was rated for only eight LiPo’s, I had been assured by my supplier that it could handle 10s in the application for which I was using it. His reckoning was that the average current draw over my anticipated flight times would result in a total wattage that was well within its acceptable limits.
So, I had an alternative motor for much less than £85.00; now all I needed was a speed controller. Once again I was drawn towards a name which had been giving consistent results among fellow club fliers - Castle Creations. A look through the spec. sheets showed that their HV85 model would be more than adequate. In reality, drawing down five amps over a period of ten minutes shows an average drain of only 30 amps, which I knew could be comfortably managed by their HV45 controller;
however, I believe in having a wide margin of safety and felt that the extra-spent funds on an HV85 would be worthwhile. At a price of £122.95, it’s excellent value anyway.
The remaining item I needed was the battery pack, and for no other reason than my previous experience with them, I called FlightPower and ordered a 10s pack. They don’t actually make a 10s at five amps, and so I was sent two of their 5s five-amp packs instead; these have subsequently been charged as two packs but then plugged together in series to give me the required 37 volts at five amps. This was by far the most expensive item in the package, but I firmly believe there is no such thing as a good, cheap LiPo battery.

THE FIRST FLYING OPTION
Before committing this untried combination to a full-blown fuselage application, I decided to install it in a Uni-Star Trainer, the mechanics of which form the basis for most of Graupner’s 60-size scale models. Actually, an ‘Elektro’ version of this model is available, and this would have been a little easier for me to adapt but since I had a normal (i.c.) version to hand, I elected to use it. The Elektro conversion kit from Graupner is relatively cheap at approx. £11.00 plus £25.00 for the motor pinion but unfortunately, there’s only a 22T available - this was totally unsuited to my set-up.
Checking through the catalogues I decided that Vario had the answer, because they showed one pinion at 26T and another at 27. I immediately ordered both items from Vario UK, only to be told that the 27T item was no longer available. Then, I decided that 26 wouldn’t be right after all and decided instead to obtain my pinion by modifying a 24T clutch drum, and through using the 22T aluminium hub.
So, 24T it would be, giving me an overall gear reduction, to the main rotor, of about 9 to 1. In theory, we now had a motor Kv rating of 385; multiplied by a voltage of 37 and then divided by the gear reduction of 9 to 1, this equates to a rotor speed of 1,580rpm. Subsequent measurement with a reliable optical tachometer shows 1,560 rpm, using a straight throttle curve, so I wasn’t far out on my calculation.
In order to fit the AXI motor, an area of the side frames adjacent to the i.c. motor mounting points needs to be filed or ground away; a Dremel or similar is a handy item to have at this point. This may seem like drastic action, but after removing the necessary amount, there’s still more than enough material left should one wish to re-install a noisy, smelly and oily form of motivation!
A small modification to the electric motor mount is also necessary. This component has been factory drilled for a motor with three mounting holes at 120°, whereas the AXI has four holes at 90°, although only two are used. Finally, I had to ream out the bore of the pinion assembly from five to six millimetres, in order to fit the AXI output shaft. As usual, I mounted the receiver and gyro as far away form the motor and ESC as possible, using the gyro mount provided in Graupner’s Uni-Star kit to accommodate both items plus the switch. Then, I placed the ESC on what should have been the receiver tray.
The 4.8v receiver NiCad pack was stowed away in a convenient location at the rear of the mechanics. I had envisaged problems with mounting the LiPo battery but as luck would have it, with both five-cell packs taped together, the overall width was a hairs breadth slimmer than the internal width of the side frames. A cross support was installed to limit the height of the pack so that it wouldn’t come into contact with lower parts of the transmission, and Velcro was used to stabilise and strap it in place. The recipe was completed with a pair of 66cm SAB (FAI) mainblades.
A check of the total weight revealed we had a model that’s 4.8 kilos or 10.5 lb. in old money.

Martin modified a 24-tooth clutch drum to obtain the necessary ratio for his chosen 1,560 rpm head speed.

FlightPower packs aren’t the cheapest, but you generally get what you pay for with LiPo’s.

AND THE RESULTS WERE…
Right from the first flight, I knew I had the right combination. Sure, I wasn’t getting the power I’d previously had with an O.S. 61SX-HGL on 15% nitro, but it was close enough for my needs. Luckily, I was using a well proven and sorted model, so no real trim changes were needed; the beat of those blades, running at a crisp 1,560 rpm without being drowned out by exhaust noise, was music to my ears.
Now that I’ve got (at the time of writing this article) about 25 flights under my belt, I now know how far I can safely go with the battery pack, whilst always leaving around 20% of unused capacity. This equates to ten minutes of constant aerobatics or seventeen minutes of ‘scale’ flight. I could ask for no more.
Having seen both the JetRanger and the Uni-Star, my friend and clubmate Steve Tew decided to go the JetRanger route with the AXI option too, but differed in that he elected to use an 8s pack (2 x 4S packs from his Century Swift). Using my crude calculations, we decided that even the 26T pinion on his 8s wouldn’t give the desired results, so he made a 28T pinion from a four-stroke clutch drum to give about 1,450 rpm - he hasn’t been disappointed.
For myself, the next part of the plot is to install these electrified Uni-Star mechanics in my Agusta 109 Power fuselage, a helicopter which I consider to be the ultimate dream in electric scale.
To indicate the effect these models can have on fellow modellers who’ve had no previous exposure to them, I’ll relate the following. I was invited by a club to see their new and improved facilities and, at the same time, demonstrate a turbine helicopter. Normally, at such an invitation, I would stack the car up with models but on this occasion, mainly due to the after effects of flu, I just took the turbine Cuatro and my electric JetRanger with the intention of having a relaxed afternoon. Yes, as expected they were suitably impressed with the Cuatro but, they said, it was the performance of the JetRanger that really surprised them and further convinced the members that electric power was undoubtedly the future.

Steve Tew’s JetRanger has a slightly lower headspeed – 1,450 rpm – and runs on 2 x 4S packs borrowed from a Century Swift.