My second Sig LT-25 electric conversion is powered by a brushless outrunner motor. This one uses the controller's BEC and a miniature FMA Encore reciever. Other than that, it's nearly identical to the brushed version. Interestingly, both planes weigh in at exactly 5 pounds, ready to fly. Performance is nearly identical with one exception: Flight times have increased dramatically. With a 10 cell 3600 mah NiMh battery using throttle management and flying a combination of basic aerobatics, touch & goes along with some lazy half throttle flying, flight times can exceed 20 minutes. That's about twice the flying time of the brushed version with the same battery.
I chose NiMh batteries over Li-Po primarily for cost considerations. Two 3600 ma, 10 cell packs cost about $100, less than half the cost of Li-Po's of similar capacity. Going to Li-Po would save about a pound but the plane flies so well at 5 pounds I don't feel the need.
1.
Power consists of a ElectriFly 35-48-700KV outrunner brushless motor and a Castle Creations Thunderbird 55 ESC driving an APC 13x8 electric prop.2. The kit was assembled with only a few changes. The muffler cut-outs at the nose were filled in & new firewall slots cut in the fuselage doublers 1 3/32" foreward of the original location. Seven 1/2" holes were drilled in the firewall, one on each side of the motor, three below and one on the motor's center line for shaft clearance. The bottom was trimmed to match the fuselage. The cut-out at the bottom of F2 was widened to clear the new battery box. Three lightening holes were cut in each wing rib, two behind the main spar and one in front using a sharpened 5/8" brass tube and the wing was made bolt on. The F2 former is high enough to allow for dowel holes and includes a lite-ply doubler on top. I added a 1/32 ply doubler on the oposite side and triangular bracing. At the rear of the wing saddle, 1/4" ply pieces, cut 1/2" x 1 1/2" with triangular balsa bracing under were glued to the fuselage sides and the front of F3, then drilled & tapped for 10x24 nylon hold-down bolts. I replaced the wing's center reinforcing tape with 2" giving added support for the 1/4" wing dowels.
3. A battery box of 1/8" lite ply, sized to fit the 10 cell battery pack and drilled with lightening holes, was assembled (Done first it can be used to size the F2 cut-out for a snug fit). The rear of the battery box was kerfed to key onto the servo tray which I shortened by cutting off the throttle servo portion. The servo tray and battery box were not glued in place until the plane was complete & covered. With battery and pushrods in place but not connected to the servos, the servo tray & battery box can be slid fore & aft to set the CG, then glued in place. I installed a scrap balsa suport under the front of the battery box. Battery access is through the fuel tank hatch with room to spare.
4. The motor was attached using it's spider mount, the kit's 6/32 blind nuts and 6/32 nylon bolts replacing steel. Hopefully they will sheer in a crash, protecting the motor from damage.
5. The ESC was attached to the fuselage side with foam mounting tape. Motor wires pass through one of the holes previously drilled in the firewall.
6. Covering material was removed from the bottom fuselage cut-out directly behind F4 as an air exit.
7. All servos are standard size and pushrods were done per instructions. The tiller arm assembly had to be moved aft, behind the fuselage doubler. Scap balsa was glued to the fuselage sides on a line with the servo tray for a glueing surface.
8. The original hatch can no longer be screwed to the firewall, which was moved forward. Scrap balsa was used to fill the gap between the hatch and the relocated firewall. A small piece of 1/8" ply, glued to it's underside serves as the forward attach point. I also CA's a plastic cap (cut down CA accelerator cap) to the back of the firewall to protect wiring from the motor shaft.
9. This LT-25 came out at 5 lbs ready to fly, not bad considering the 25 oz battery, battery box with it's added reinforcements and the bolt-on wing. Other than lightening holes in the wing ribs there aren't many places to cut weight from the airframe without seriously weakening it. On the plus side, the plane flies beautifully at 5 pounds. Take-offs are quick, under 100 ft, loops from level flight are big and round and landings are trainer-like. The plane is capable of basic aerobatics or you can putt around the field slowly at half throttle. Full power batery drain measured around 23.5A according to FMA's mv per amp shunt with the APC 13x8. With an APC 12x8 current drain droped to slightly under 20A. The plane flies okay with a 12x8 but with less authority.