Dancing with Amps
Electrification of the Balsacraft Limbo Dancer
(originally published in the December 1999 edition of Sloping Off, the newsletter of the Christchurch and District Model Flying Club)
I always seem to be out of step with the seasons. Over the Winter I built two new slopers, ready just in time for the Summer round of electric fly-ins and now, as Winter approaches, I've finally got around to this year's electric project.
The reason for this phase lag is clear - indecision. I started thinking about the next electric model more or less as soon as I had finished the last one (August '98). Flushed with the success of the Stearman, I set about planning the next scale masterpiece - and it is still at the planning stage. Then, at the beginning of the Summer, I read that Nexus and the BMFA had somehow decided that a BMFA 'B' certificate qualification would be necessary to fly at any of the Old Warden events. Leaving aside the question of whether this is a good idea, it did get me thinking that it was about time I got myself certified (should that be 'certificated'?). However, as I gazed at the models around the room, I realised that none of them was really suitable.
So, the scale masterpiece got shelved, and a new decision process started. Seeing Steve enjoying his Limbo Dancer (see 'Sloping Off' March 1999) made me think in terms of a fun fly model and, after due procrastination and a study of Steve's plan, I concluded I couldn't improve on his choice, and bought one.
The box is surprisingly small, 9" wide by a mere 2" high, and just long enough to take standard 36" lengths of balsa. Inside, the bulkiest items are the engine mount and fuel tank. These were tossed aside with true electric flier's disdain, then hastily retrieved and weighed. I may as well get some credit entries on the balance sheet right away, especially since I was planning to fly on 16 cells weighing in at a solid 2lb!
The rest of the box is fairly tightly packed with bundles of balsa plus the obligatory bag of bits which, somewhat unusually, includes wheels which, even more unusually, are nice and light. The Limbo Dancer is unashamedly functional in appearance so no cowl, no canopy and, as power models go, not a lot of plywood either. The undercarriage wires are pre-bent and could perhaps be a little bit lighter. But then again, it is going to take the full brunt of that 2lb battery pack returning to earth.
So what about that balsa? I had the chance to inspect the kit before buying it and was favourably impressed. All the wood is usable, although almost inevitably, some of the stripwood, when released from its bundle, bowed somewhat. The only bits I opted to change were the fuselage sides (which I needed to reshape at the front anyway) and the rear underside of the fus, where I dropped the thickness from 3/16" to 1/8".
Power and Weight
The whole concept of fun fly models is built around high power to weight ratios and low wing loadings - not exactly traditional strong points of electric flight. Equally, they spend most of their time at low throttle, so I needed a power plant which would be efficient over a wide power range if I was to get anything approaching fun fly performance with a reasonable flight time. This meant only one thing - go brushless! This decision leads equally inevitably to significant wallet damage. Fortunately, the sums indicated that the Aveox 1406/4Y, suitably geared should cope (just), so I took advantage of West London Models long standing offer and bought the motor, plus an M160 controller and Kruse 2.4:1 gearbox.
So much for power, but what about weight? The box gives an all up weight for the ic version of 56oz, including five standard servos. Replacing four of these with Protech micro servos saved me 104g to add to the 86g in the bank from dispensing with the tank and engine mount.
Unfortunately the speed controllers for brushless motors are still rather bulky so I gained nothing here, compared with a standard servo and throttle linkage. My motor and gearbox though proved to be 164g lighter than the recommended MDS .38, I reckoned that swapping the wood for the fus sides saved me 36g, and making up a miniature Rx pack saved a further 25g compared with a pack of pen cells. Adding that lot up puts me 415g in credit. Well that's 7 cells accounted for!
What else can be done? I contemplated cutting some lightening holes but they do take time, and are they worth it? There were two huge lightening holes pre-cut out of each of the seventeen 3/32" ribs. Total weight of 34 discs of balsa? A measly 5g! The aileron servo mounting plates are fairly beefy bits of birch ply and the servo cutouts are already made and too large for my micro servos. The same is true for the fuselage servo plate, although this is, in fact, liteply. Casting these aside saved an instant 20g. Choosing a covering with no adhesive (in this case Fibafilm) saves perhaps another 50g - and hang on a minute, that fuel tank was empty! Full, it would weigh another 112g. So, that's another 182g accounted for but, with a 16 cell pack weighing in at 914g, I'm still going to be around 317g, or about 11oz, overweight.
Although there is no doubt the Limbo Dancer could carry this weight, I felt that a 20% increase in wing loading could change the character of the model. Also, the wing section is very thick and dragging that wing through the air faster is going to consume a lot more power. So, to ease the problem, I decided to stretch the span by one rib bay on each panel. This gave me 12% more wing area, containing the increase in wing loading to less than 7%.
Electric and i.c. Limbo skeletons. Count the rib bays to spot the difference
The decision to stretch the wing was made all the more easy once I realised that all the spars, sheeting etc. have to be spliced anyway, and there is more than enough material to build the stretched version. I even managed to make the two extra wing ribs from the offcuts in the kit.
The wing is very conventional, built as one panel, flat on the board, with parallel chord. I did wonder about that 2lb of battery slung under my stretched wing with only 1/4"sq. balsa spars but, after struggling with the weight equation, I was not about to start beefing things up!
The only change I made was to fit an aerial tube running from one aileron servo position out to the opposite wing tip. Mounting the receiver in the wing in this way enabled me to use a full length internal aerial, without it going anywhere near the high power electrics. Equally importantly, it created a bit of much needed space in the fuselage, where a few more changes were needed...
The first thing I did when I saw Steve's plan for the Limbo Dancer, was to offer up an eight cell battery pack to the side view and plan view of the fuselage. It fitted perfectly, and the space is readily accessible by removing that 15" chord banded on wing. The only snag is that the space was filled with fuel tank, receiver battery, receiver and three full size servos in line astern.
Dispensing with the fuel tank, and mounting rudder and elevator servos side by side towards the rear of the wing seat, still left me with too many bits. The receiver was duly dispatched to the wing, as described earlier, and the front of the fus subtly reshaped to enable the speed controller to be accommodated above the motor. I now had over 11" into which to fit my receiver battery and flight pack, giving about 1.5" of leeway for final cg location.
Building the fus is very simple - the firewall is the only former in it! The sides are held apart amidships by the servo tray and joined together at the rear. Joins between top, sides and bottom are reinforced with 1/2" triangular stock, enabling the simple box to be well rounded off, although the Limbo Dancer has no pretensions to graceful curves - at least not whilst stationary.
A bit of thought and a lot of fiddling was needed to finalise motor/gearbox and speed controller mountings, and details of the battery fixings had to wait until final balancing.
Flat and Flappy Bits
Tail feathers and ailerons are all simple flat structures, built up from a combination of strip and sheet. The wood here was all firm and straight, but as the tail took shape and I felt the weight I began to get concerned over the cg location.
The fuselage was covered in gold Profilm, the flying surfaces in translucent blue Fibafilm. As well as being light, Fibafilm does help stiffen open structures. Like Litespan though, it does not have the high shrink properties of the regular film coverings and it is hard to get a stable taut finish. I am hoping though that it will resist sunlight better than certain other coverings.
The Limbo Dancer comes with large and bold artwork in the form of stickers which, unlike many, just have to be used. That done, it's fitting out time.
Balancing the Beast
My fears about the heavier than expected tail proved well founded. This, combined with the fact that much of my weight saving was at the front end, meant that the flight pack had to go tight up against the rear of the motor in order to bring the cg forward to the recommended spot.
All up weight came out at spot on 5lbs, only eight ounces heavier than on the box. I was well pleased with this, until I learned that the ic ones typically come out around 4lb. It seems this is one occasion when the figure on the box is pessimistic. Nonetheless, with a wing loading of only 13.5oz/sq ft, the electric version can hardly be called a heavyweight.
With everything on board, the control throws were set up, a prop fitted (11" x 7" to start with) and the motor and gearbox given a test run. The motor was only run for a few seconds, just to check that everything was smooth, but it was long enough for me to realise that, whatever my worries might be on the first flight, lack of power wouldn't be one of them!
A few days after that ground test, the thought occurred to me that, if I fitted two seven cell packs instead of two eights, I could get the cg a bit further forward. So I set off to our field, armed with one 14 cell pack for the maiden flight and one 16 cell pack for take two.
Battery on board and range checks done, I was just checking the control surface neutrals when I found that a large amount of up elevator had appeared from nowhere. Investigation showed, to my embarrassment, that the threaded rods in the end of my carbon fibre pushrod were on the move, so it meant a quick dash home for some running repairs. Hardly an auspicious start!
With everything back together, the Limbo Dancer was at last lined up for take off. A last minute pause to compose myself, then ease the throttle open and... 30° climb out on a little over half throttle! After a couple of trimming circuits I remembered to start the timer (brain definitely was not fully in gear that day!). Three minutes of easy aerobatic flying followed, the model feeling very comfortable to fly. Because of the slow flying speed, those huge control surfaces, even unrated, don't seem to make the model twitchy, but, bang the ailerons over and watch it roll!
After a short cooling off period, the 16 cell pack was fitted, and off we went again. I even felt confident enough to experiment with the flaps this time, but once again forgot to start the timer until well into the flight. I don't like to run new electric set ups for too long initially, so landed after an estimated four to four and a half minutes of slightly more vigorous aerobatics, well pleased with this first outing.
My pleasure turned to astonishment when I later found that both batteries had over 1000mah left in them, indicating that eight to ten minute flights could be on the cards.
Subsequent tests have shown that, on 16 cells the 11 x 7 prop is turning at around 9000rpm and drawing 22amps at full throttle. This gives a calculated thrust of nearly 70oz. Now, if I can get hold of a 13 x 6, I might be able to have a go at this prop hanging thing that Steve is so keen on...
As an aeroplane to take the B certificate test, the Limbo Dancer will clearly fit the bill. I haven't tried spinning it yet, but all the other manoeuvres were completed in its first outing without fuss.
On top of that, it promises to be a lot of fun! I am not suggesting that the electric Limbo Dancer would be a threat to the ic version in serious competition, but it is certainly a viable alternative route. This was my first ic kit conversion and I shall certainly consider doing it again.
So, do I really have to scratch build that next scale masterpiece...?