When Atomic Workshop gave me a ‘Zombie’ to trial, my plan was to convert one of the large fleet of rubber powered models that I had in the attic. I chose a little ‘Keil Kraft’ Luscombe Silvaire. This is quite a dainty model, and although it is 21″ span it looks much smaller due to the long glider-like wings. All-up weight with rubber power was quite light at just 22grams.
I decided to power the model with a geared ‘pager’ motor, and purchased a 4.5ohm motor, Didel 48tooth spur gear and pinion gear, 1mm shaft and bearings from Graham Stabler. It only took me about 2hours to make the complete geared motor, I just used a small balsa block to space the shaft away from the motor, allowing the gear to mesh with the pinion – but not too tightly! The black 85mm dia. propeller was borrowed from my Knight & Pridham KP00. Amazingly this little power-plant only weighed 2.9grams with prop!
I measured the static thrust and was pleased to see that the motor produced about 10grams of thrust on full power, using a 145Mah li-poly Cell. This motor is a lovely little unit and does not sound as if it is being overworked. The KPOO unit also comes with a lower pitch white prop., I tried this and found it gave about 8.7g of thrust at full chat.
The tiny Zombie comes with a built in switch, to save weight. This is a great idea, but does mean that you need direct access to turn it on. In view of this, I decided to mount all of the electronics on a balsa tray which slides straight into the nose of the model. I have full access to all of the components, and the 10 second delay on the Zombie means that I have ample time to turn it on and then slide the tray back into the model before the motor starts. The Zombie and the battery where fixed to the tray with double-sided tape, and the motor was glued directly in to the tray with cyano.
I balanced the model by positioning the battery to the rear of the tray, which placed it just about under the main spar position. After an initial trial in the garden on low power, it became obvious that I would need some right-thrust as the model was turning left very tightly. I cut the motor out and off-set it by about 2 degrees to the right, while I was at it I also added about the same amount of down thrust (I remembered it needed quite a bit when it was rubber powered) Interestingly, the all up weight with 145Mah cell, Zombie and motor was now 23.5 grams, only a little over the weight of the original rubber powered model.
Its first indoor outing at Lowestoft sports hall was a complete success, after a few gently descending flights at about 50% power to unsure it was circling within the hall I wound the first pot of the zombie around to about 85% power for around 30 seconds, and the second pot was set to about 60% power for 20 seconds. Wow, this gave a wonderfully realistic (if rather heart stopping!) long flight that skimmed the rafters. Not bad, but power was shutting off when the model was still about 5ft off the ground. A tweak to allow the second phase to run slightly longer had the model descending to a lovely landing. As if by magic it taxied back to my feet and turned itself off!
A few pointers:
As a rough guide, your chosen motor’s static thrust should be about 1/2 the All-Up-Weight of the complete model. (10g thrust will fly a 20g plane with a bit to spare)
Trim the model on the second phase, allowing for a slow circling descent, when you are satisfied, kick-in the first phase for the climb.
The above set up is good for lightweight models with an airframe weight of about 12-14grams the weight of the tray and power package will be about 8-10grams
Don’t forget the right thrust and down thrust, particularly on short nosed models.
