|21gm/dm 2 (7oz/sqft)
|260gm (9.3 oz)
|Speed 300 direct drive
|Graupner folder 120mm x 60mm (4.7" x 2.3")
|10A with brake (no BEC)
|4 x Sanyo KR600AE NiCad (4.8V)
|2 x Cirrus CS-20BB
|Rudder, Elevator, Motor
|Balsa, covered with Airspan (lightly doped)
This is my second scratch-built design. My goal was to produce a light-weight plane that could be powered with a small motor. Originally I intended to use a Mabuchi FK-050SH motor rescued from a broken CDROM drive, geared down to drive a high-pitch prop. However, earlier tests on my "Sandfly" showed that this motor would not deliver enough power. Therefore I decided to fit a Speed-260 motor with direct drive to a Graupner 4.7" x 2.3" folding prop.
The Speed-260 was barely powerful enough to keep the plane aloft. Unfortunately after only three flights the motor overheated and melted its plastic endbell. Driving the 4.7" x 2.3" prop on 4.8V caused current draw to approach 3 Amps, three times higher than the motor's point of best efficiency. I then realised that a better motor was required, so I installed a Speed-300 unit. This motor draws about 5 Amps static, but is efficient at such power levels. The extra power available permits a good climb rate, and it cruises at half-throttle. Hand launching is easy, with plenty of thrust for a steep climbout. Flight duration is from 8 to 14 minutes depending on flying style.
Combining a Speed-300 motor and 4 x 600AE cells has proven to be a good match for this model. 600AE cells have excellent energy to weight ratio, and are less expensive than most smaller cells. The only problem I had was finding a speed controller that would cut off at the required 4.0 volts. I tried a GWS #100A, but it was heavy and inefficient (frame-rate) and had an awkward arming button. Therefore once again I built my own ESC , this time without a BEC (the receiver and servos run direct from the 4.8V motor battery).
Replacing the servos and ESC with lighter equipment could trim 15-20gms off the total weight. However, I doubt such a small improvement would be very noticable. An inline gearbox should markedly improve performance, as the motor could then drive a larger prop which would be more efficient at low speeds. I may try this if I can get a suitable gearbox for a reasonable price.
|a nice snug fit!
1st April, 2001. Found out air can be stronger than balsa.