Stearman Motor Swap
by Trevor Hewson
After ten years loyal service equipped with Speed 700 'can' motors, I felt that my Stearman deserved a motor upgrade to bring it into the brushless age. Its power demands are relatively modest, but it does use a large wooden prop, and, since I wanted to do away with the gearbox too, finding a suitable motor did prove somewhat problematic. Eventually I ordered (direct from Hong Kong, via eBay!) a brushless outrunner motor rejoicing under the designation EMGC3720.
This motor weighs in at a mere 159g, compared with 460g for the geared unit it would be replacing. Whilst weight reduction in a model is usually good news, the Stearman was deliberately built light to cope with the weight of NiCd batteries of its day and has already lost over a pound (480g to be precise) as a result of the LiPo revolution, so it really doesn't need to be any lighter. In any case, with restricted battery access through the forward cockpit, re-locating the battery any further forward was always going to be very tricky.
In spite of some encouraging figures in the advert, once I tested the new motor, it became clear that, on a 4s LiPo pack, it was not going to be happy swinging 13 or 14inch wooden props and I had to concede that it really wasn't the right one for the job. After a few days pondering the situation, I realised that this motor would probably suit the Capiche 50, currently powered by the much more expensive (and much heavier) AXI 4120/14.
I had previously ruled out motors like the big AXI for the Stearman because they wouldn't fit inside the existing motor tube but, when I offered up the Stearman's dummy engine to the AXI in the Capiche (see picture), I realised that the length of the 4120/14 was perfect for firewall mounting, so I began to think again. A quick test showed that, whilst the 14 x 8 prop currently on the Stearman was a bit too much, the AXI would cope with a 13 x 6 wooden prop. Admittedly it would give a full throttle current draw of around 50amps but, since I had bought a Hobbywing 60amp unit to go with the EMGC 3720, using that with the AXI could provide a viable solution for the Stearman, leaving the little 'Hong Kong special' to go with the existing Jeti 45a ESC in the Capiche. When I found that the gold connectors used for both motor / ESC connections were compatible I took this as a good omen, and promptly set about the job of de-commissioning two perfectly serviceable models. . .
This motor swap was pretty straightforward. The new motor is shorter than the AXI so it was stood off on discs of 3mm ply. This had the added advantage of bulking up the thickness of the firewall to the point that I felt confident to secure the motor mount with four wood screws rather than mess about installing another set of captive T nuts. So straightforward was it, that I quite forgot to take any more pictures!
The effect on the cg was though substantial and so the battery stowage had to be re-worked so that the battery could slide through the firewall under the motor into the cowl area. Even so, a couple of ounces of lead were needed to bring the cg back into range. After a couple of test flights, this has been reduced to one ounce, giving an all up weight of 4lb 3oz - a weight reduction of 5oz.
As far as power is concerned, the little EMGC 3720 is turning an APCe 13 x4 prop at 9,700rpm for a current draw of 31amps compared with the AXI setup turning an APCe 13 x 6.5 prop at 8,200 for 39amps. Theoretical thrust calculations predict that the new setup should provide 6lb 3oz of static thrust. That is 8oz more than before, in spite of drawing less current. All in all, a very satisfactory result.
The first job was to extricate the existing geared Speed 700 powerplant which is mounted in a rolled ply tube, which in turn serves as a mount for the dummy engine. When I looked at this tube more closely it reminded me that the bulk of the weight of the motor is taken directly by the firewall and that front mounting an outrunner within the tube would not be practicable without a lot of re-inforcement. This at least made me feel a bit better about having to demolish it to make way for the AXI.
This second picture shows just how long the geared setup is compared with the AXI. Although, at 351g, the AXI is more than twice the weight of the little Hong Kong motor, it still is significantly lighter than the 460g of the geared 700. However, as the picture shows, the AXI is mounted a good bit further forwards, so there was every prospect of the cg impact being minimal.
Once the motor tube had been demolished, there was much offering up and head scratching to work out how best to accommodate the four arms of the motor mount, together with the cable exit, within the 9-fold symmetry of the dummy Lycoming. Finally, a mounting plate was fashioned from 1/16in ply with three blocks to take small screws to secure the dummy engine. The plate also serves as a reinforcement for the balsa face of the firewall and I took the precaution of fitting a plastic cap from a Zap bottle in the centre just to make sure that the protruding motor shaft couldn't make contact with any wiring.
The motor is secured using 3mm bolts through the mounting plate into captive T nuts (and yes, installing them was a pain!).
Once in place, it all looks rather neat. The inside of the crankcase of the dummy engine did need to be relieved a bit to clear the motor's rotating case, but then it went on easily enough.
With a Zinger 13 x 6-10 wooden prop fitted, the motor was run up and I was pleasantly surprised at how smoothly it all ran. I recorded 8000rpm for 51amps at full throttle. The old brushed setup drew 28amps turning a 14 x 7 airflow wooden prop at a leisurely 5,700rpm so, needless to say, power is no longer an issue!
I was also delighted to find that the cg was still spot on, and that the radio range performance was also better than before, even using the built-in uBEC of the Hobbywing ESC. At time of writing, the model has only had one test flight on its new motor but, so far at least, I am more than satisfied with the conversion.