We have been in the process of developing a series of different E-power packages running from 1/2A sport to 65 equivalent size. One of the latest to fly was the the latter - in Strega. This was intended to be a proof of concept for a BIG power package. To build this we started with the good info shown here in the Forum, particularly the List your Set-up thread, plus factored in what we have been learning along the way.
The basic fan laws show that power increases as the 5th power of diameter - so for big props you better be ready! For starters we were looking to keep heat down by keeping reasonable current draws. The simplistic thought is that Current = Heat. Thus the heart of the system is the 6S battery pack. As far as we know this is the first (only?) 6S system that has been flown in ECL, and is quite possibly the most power system out there. It ws originally flown with 6S-2800 pack (about 15.5 oz) but we have downsized the pack to 6Sx2650. A nice by-product is that battery recharge times are shorter with the lower mah usage.
The motor is an Arrowind 3526 (equivalent to AXI & Scorpion designations) with a 685 kv. I know, the kv sounds a little high, but WAIT, more about that later! A Phoenix 60 was available so it was used. However, in practice even the static current has not exceeded 46A so it is really overkill - a Phx 45 or the new ICE50 would do just fine.
Of couse, a Will Hubin timer completes the set-up.
The Strega was built as a take-apart (removable wing) and that mod costs about 4 oz. I wanted a top access hatch for the battery so the top of the fuselage back to the bulkhead of the wing was removed. The Strega motor crutch mounts directly to the balsa fuselage side, and the doublers go AROUND the outside of the mounts. So instead of popping the mounts off the doublers like I usually do, a Roto-Zip saw was used to hack out the motor mount crutch. What was left was sanded flush with the doublers. The cylinder air ducting was unusable and in the way, so it was removed, but the air exits retained. The sub-floor was retained at the bottom of the fuselage sides (bottom skin is below that). A firewall for the rear-mounted motor was fabricated and installed, it juts into and is reinforced by the top block. Two small access holes (looking like gun ports!) provide access to the top motor mounting bolts. OF COURSE I clipped the ends off the flaps, and changed the control ratios to my liking.
Final weight is 79 oz (ugh), and we are flying it on .018x66 ft lines. Yes I know, they should be .021's but we did not have any handly and I am NOT worried about it! We originally flew it with a 13x6.5, but have graduated to 13x4 (much better!) There are two ar inlets, the first is aimed and baffled into the front of the motor, the second is routed to the battery and ESC. Must be working pretty well, we have not excced 105 degrees on the motor nor 100 degrees on the pack.
I flew the test flight: 1:45 of shear terror as the bird was moving at 4.9 sec/lap and pulling my fat patootie all over the circle. Before I landed I loudly announced that I was NEVER going to fly it again! Young Nephew Archie who still posseses youthful exhuberance not yet tempered by old age and treachery, cheerfully volunteered to take over. He slowed the motor by some 500 RPM and moved the leadouts forward 1/2" (!!!). The original 13x6.5 prop provided GREAT power but did not do a very good job of preventing wind-up of the heavy bird. Thus Archie moved on to a 13x4, cranked the RPM up to suit and now has it working pretty darned good. At higher RPM the motor kv is much better suited too.
Archie (Arch#3 in family shorthand) claims that even an old man can handle it - and proved it by having his dad (Arch#2) fly it in Muncie during the team trials. MOBA (Much Older Brother Arch) proclaimed that it ain't half bad, certainly the POWER is NOT a problem. Now Arch-2 and Arch-3 are conspiring to make me fly it again. However, my arm is still sore...
I know of at least one more E-Strega project in the works, this one sounds like it will be quite abit lighter. Hopefully Bob can post his pending success story within the next month or so.
My paradigm against large airplanes remains in place, however should I ever choose to go there I KNOW how to power it. Even at 79oz the poor Strega didn't even put up a fight - remember that bird also has a 2.75" thick wing. If I was building a Strega-sized Mythbuster, I would tailor the design for e-power and also take about a pound out of it. The power system would then just loaf along - and could probably downsize the batteries again...
Topic: E-Strega Flies! (Read 2687 times)
