|October 25, 2009
This is my 6 channel 1/48 scale Mustang P-51, built for the KIEF 2009 indoor electric meeting, where it won the award for "Best Micro Aircraft". It has a 6 channel radio controlling throttle, rudder, elevator, aileron, flaps, retracts. It also has wing-tip lights. The flaps and retracts move slowly over a 2-3 second period, controlled by the transmitter. The retracts lock in both the raised and lowered positions. You can see the mechanism in one of the videos below. They are based on a design published on RC Groups by cmartin.
Apart from the retracts the most interesting thing to me about this plane is the receiver. I built it especially for this plane. It is a 6 channel adaptation of the 4 channel Rabbit receiver that I designed and that Nick Leichty now makes and sells (see Microflier RC). It uses a different microprocessor that has 50% more pins than the standard one, and I use them all. The receiver uses a very similar frequency hopping spread spectrum technique for frequency independence. It is a hybrid in that it generates actuator output on 4 channels, for aileron, rudder, elevator and flaps, and generates servo output on two channels, for external brushless motor controller and retracts servo (also from Nick Leichty). All 4 actuator outputs are amplified for use with lower resistance actuators. It simultaneously generates an amplified output for a brushed motor in case I want to switch to that, but I don't think that will be used. The receiver autonomously generates a short pulse once per second that is used to drive the wing-tip lights. The whole receiver has the same footprint as a standard Rabbit, 0.25" square, but is thicker at about 0.25". It weighs about 0.2 grams.
Robert Guillot had been nagging me to make a scale model with a full fuselage, so when I told him I was thinking about a plane with retracts he suggested doing a 1/48 scale Mustang. He even sent me the design and graphics for the 1/72 model he had made. Two months and a lot of hard work later, here it is.
I finished the plane on the Monday before KIEF, intending to test fly it, but it then rained for two days after having been dry all summer. So the maiden had to be at KIEF. Given the high wing loading we knew it had to fly fast, so I didn't want to risk having the landing gear down for the initial flights. Nick Leichty did a fantastic job launching the plane for all of its flights at Kief. I flew it a half a dozen times, and while I was getting better, every flight ended involuntarily (aka crash). At that point Matt Keennon offered to pilot it, and immediately it looked a lot better in the air, with Matt's first flight being twice the duration of my best, though still ending in a snap roll into the ground. Matt really is an amazing pilot.
I went around to as many of the assembled experts as I could find to get their opinions on how to improve the stability. Where else could you get such a fantastic resource to help trim out an airplane? Everyone was very helpful. The most common suggestion was to fly it at full throttle. I implemented some of the suggestions and things started getting better. Matt was able to fly as long as the battery would last, doing very smooth, controlled figure eights, on full throttle. Remember, this motor can take a plane weighing nearly twice as much vertical.
During the noontime show we lowered and raised the landing gear in flight, but there was nowhere to do a wheels down landing. Matt made full use of the 400ft by 200ft main flying area and even made it look a little tight. Wheels-down landings will be difficult given the need to fly at full throttle. It has done an ROG with full flaps, but as of this writing it is yet to land on its wheels.
Update December 18, 2009
I've upgraded the receiver to 8 channels using the new Hip-Hop-2 system. Channel 7 is used to switch on and off the flashing nav lights. I'm still trying to decide what to do with the 8th channel. Thayer Syme suggested dropping a long range fuel tank, as these planes used to do. We'll see.
||9.25" (23.5 cm) 1/48 Scale
||1.6" (4 cm)
||14.8 sq.in (96 sq.cm)
||0.50 oz (14 grams)
||4.9 oz/sq.ft (15 g/sq.dm)
||Hexatronix 2 gram brushless outrunner with Turnigy TGY DP 3A 1S speed controller
||GWS 2.5" x 1.0"
||Custom 6 Channel Rabbit Hip-Hop FHSS, 0.25" cube, 0.2g (1/100 oz)
||MicroAct actuators on elevator, rudder, aileron, flaps
Leichty servo on retracts
||Balsa wingframe, 0.025" Durabatics covering
||Rudder, Elevator, Ailerons, Throttle, Flaps, Retracts