Richard Crossley's LFW 'Cato Engined' Butterfly

Introduction
I had been looking for an electric model to make use of the smaller sized indoor halls that I sometimes fly in. My intention was to use the little Voodoo 10 direct drive motor (included with this power unit is a very nice CNC machined Delrin mount and mounting screws), so the model had to be quite compact. I found a plan for the Dave Diels Cato Butterfly, and decided this was perfect to convert - it had a short nose, which meant that all the gear could go right at the front of the model, also the wing area is huge for such a tiny (13" inch span) model. This would mean that the wing loading would be low.

The LFW 'Cato engined' Butterfly was a pretty American design, and flew just after the first world war. It was intended to be a lightplane for the masses, but was never put into production due to much cheaper 'war surplus' aircraft flooding the market.

Installation
I built the airframe as the plan suggested, but decided to have the electric gear (Zombie timer/Motor/Cell) removeable on a pull-out balsa tray. This gives easy access to the switch on the Zombie and charge socket. The tray is held to the model by some tiny powerful NdFeB 'rare earth' magnets. This system works really well, as the delay feature on the Zombie means that you can press the 'go' button, slide the tray back into the model and wait for the engine to start.

I used a 20mah Lipoly cell, and I am just amazed by how small and light this is, it weighs in at about 0.8g! The Voodoo 10 gives (not surprisingly) 10 grams of static thrust. this means that it should fly a model of up to 20g all up weight. I am very pleased with the final weight of this model at just 14g, meaning it does not require full power from the motor to fly.

Flying
First flights were attempted at the recent SAMS indoor meet at Earls Court in January 2007 (brilliant hall, lets hope they do one next year). I soon noticed that I would require some more downthrust and sidethrust. I ended up with a full 8 degrees of right and down thrust to cancel out the torque of the prop. This did the trick though, and the results were outstaning - I don't think I have ever seen an electric model fly at such a low speed. It was quite possible to trot alongside the model when airborne! On one occasion I had the model flying slowly for almost 2 minutes, it just about hit the 50ft roof. The little cell worked well, and I reckon I would get about 3 flights before it needed a charge up.

Conclusion
SIDETHRUST! One thing I have learned after building quite a few electric models, is to build-in a lot of right thrust. If too little sidethrust is used then the result is that the model will 'wind-in' to the left. If you use opposite aileron or rudder settings to counter this, the model will then spiral to the right as the power drops off, or in the glide. The only option is to angle the thrustline off to the right, and quite a lot too - I have found the magic figure is 8 degrees.




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