I have a sport model (RCM Terrier, balsa/plywood construction covered with Monokote or Econokote stick/shrink film) that I built in 1984, and it has somehow come through multiple moves apparently intact. I have a radio for it that's still FCC and AMA legal, as far as I can find out (though I'm certain the transmitter and receiver batteries will need replacing), but back in the day, power generally came via burning methanol, often with a small amount of nitromethane added, along with oil to lubricate the small two-stroke engines; electric models were heavy and performed poorly with their nickel-cadmium batteries and "can" motors.

In this case, my old engine is an O.S. Max .25 (appr. 4 cc), plain bearing non-Schneurle, good for around 14,000 rpm with a 9dx6p (inches) Master Airscrew reinforced nylon propeller. The model has a span of 60 inches (~193 cm) and weighs about four pounds (1.8 kg) with radio, engine, and a full fuel tank sufficient for 20-25 minutes of flight; when I flew it regularly it was mildly aerobatic, capable of loops (inside and outside), spins, snaps and aileron rolls, etc. -- but a little underpowered for the style of aerobatics that was common thirty-five years ago.

I'd love to restore this model to flying condition, but glow fuel is harder to come by than it was then, and even when current was recognized as messy (unburned oil would coat the model after a flight), noisy (even with a muffler on the engine), and I'd have to replace the tank and all the plumbing, as well as the engine, after this much idle time (hasn't been flown since about 1986).

I'm aware that most modern R/C is electric, using lithium batteries and brushless motors. What size/type motor, battery, and controller setup would I need to get similar performance from this model to what it had thirty-five years ago?


1 Answer 1


I would suggest two answers:

Answer 1: The pragmatic approach: A good rule of thumb for estimating power needs for RC applications is to use 100 watts per pound for sport flying, 150 for aggressive aerobatics, 200 watts per pound for 3D flying. In our case with a 1,8 kg weight, that sound like about 4 pounds, I would suggest for sport flying at least a 400 watts system (Output power). To be more precise a 0.24 glow is providing about the equivalent of a 650 Watts brushless system. I thrust Hacker brusless motor and I would suggest: Engine: Hacker A30 10XL, 650 watts, Propeller 14×7 Battery 3S 2500mA, ESC 80 A (this systew will drain 50 Amps at full power, 7200 RPM at 80% efficiency). Please note that to get about 400 Watts output power you need a 650 Watts system due to the losses of the various components (see the graph for details).

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If you want 'more power' , I would suggest Engine: Hacker A40 10S, 900 watts, Propeller 14×7 Battery 4S 3000mA, ESC 80 A (this systew will drain 50 Amps at full power).

Answer 2: the Aerospace engeneering approach: Analyse the aerodinamical configuration of your aircraft using XFLR5 (https://www.xflr5.tech/xflr5.htm), then make your math to calculate the required power for level fligh, climb and turn at desired G. Choose your Engine-propeller-ESC system using one of the simulators available online, I suggest this one: https://rcplanes.online/calc_motor.htm or this one: https://ecalc.ch/motorcalc.php?usahacker .

Please verify your system for SAFETY, appropriate configuration, Center of Gravity (it will change consitently so posiition the battery accordingly), electrical connections, battery charge, Max current for each throttle setting (measure it !) etc. Please test your modified configuration on the ground to get familiar with it. Take care of your LiPo batteries using proper SAFETY, storing, charging procedures and install an onboard battery monitor checker to monitor the health of the battery and land with a proper battery reservoir. More info here: https://hackermotorusa.com/resources/rc-brushless-motor-application-guide/. Enjoy and fly safe.

  • $\begingroup$ Thanks for your comment, You are welcome. please stick at least with the 650W solution, providing 400W 'real' out of the propeller. I wouldn't recommend anything less. If you really, really really want it try A30 10L Bat. 3S 2500mA Prop 11×5.5, but I think is not worth for saving 5-7Eur. More from the factory here : hackermotorusa.com/resources/… $\endgroup$ Nov 15, 2021 at 16:41
  • $\begingroup$ Yeah, I reread and realized you were saying 400W output for a 650W system, which is probably about right. Might need more pitch (and trade off diameter) to get similar aircraft speed, though. 7200 rpm at 7 pitch is a good bit slower than 14000 at 6. $\endgroup$
    – Zeiss Ikon
    Nov 15, 2021 at 16:43

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