Electric power for scale models

Clean, reliable and vibration-free

Modelers around the globe have proven that electric-powered RC scale airplanes can be made to fly as well as their engine-- powered counterparts. For all but the very largest, propeller-driven scale models, an electric motor can deliver the same power as a gas or glow engine. The problem is that the "power density" of electric systems is not as good as that of engines, i.e., electric systems weigh more and may offer shorter flight duration than engines of equivalent power. Although electric models cannot yet match fuel-powered models in every aspect of performance (battery packs are still heavier than fuel tanks), by choosing an appropriate subject and using the straightforward guidelines presented here, you can be assured of a scale model that will perform just as well as it would with an engine up front. These guidelines also apply to sport models, from trainers to sport aerobats.

ADVANTAGES OF ELECTRIC POWER

E-power has many advantages; most obvious is its quietness. There are, however, several other very good reasons to use electric power, all of which relate to improved appearance and better, more consistent flight performance.

* Electric power is clean. This eliminates all of the problems of fuelproofing and all concerns related to post-flight cleanup without destroying surface detail.

* Electric power is efficient. Motors, operating through gear or belt reduction drives, are quite efficient for turning propellers that are very close to scale diameter. This is an advantage we enjoy only with electric motors and points out a shortcoming of model engines: they work best at high rpm (no practical means has yet been found to mass-produce reduction drives that will allow engines to turn big, slow props). Motors can be run flat-out on direct drive to turn small props at high speeds, but experience has confirmed that for the majority of sport and scale models, it's more efficient to turn big props at relatively low speeds (3,500 to 7,500rpm). Using a belt or gear speed-reduction drive with a motor is simple because the power transmission is free of vibration. What might appear to be a second-rate fix is actually a very efficient use of power. Keep in mind that in full-scale turboprop engines, a gas turbine (jet) is geared down to turn a big propeller at a practical rpm. As with our model-size motor system, the turbine works best going fast, and the propeller works best turning much more slowly.

* Electric motors fit better in scale models. We can build exact scale fuselages and engine nacelles that are never marred by protruding cylinder heads or mufflers. o Electric power is practically vibration-- free; an obvious advantage for radio systems. Electric power in all of my airplanes has allowed me to fly for years without any radio problems.

* Electric power is convenient. Moreover, you no longer need access to refuel, touch the glow plug and adjust the needle valve. You can also say goodbye to the problems of poorly adjusted mixtures that put you at risk of engine failure at idle. In fact, after you've set up an electric installation on the bench, you never need to adjust it, and starting is always automatic and immediate. At the field, you just charge the motor battery, turn on the radio and power switches, throttle up and fly.

GOOD E-POWER AIRPLANE SUBJECTS

Why do so many of the really successful electric scale jobs seem to be Golden Age planes? We have to accept that electric power systems still weigh more than engines of equivalent power, and they require careful power management (throttle use) to get really good flight duration.

The graceful airplanes of the '20s and '30s, WW I airplanes, classic light planes and many early WW II military planes relied on a lot of wing area to get good performance with engines that weighed a lot for the power they provided. Our electric-powered scale models will usually have the best possible mix of climb, speed and duration if we build designs that have generous wing areas and moderate flight speed expectations, and choosing from the wide range of those Golden Age subjects just about guarantees good results.

You can build an all-out heavy-metal fighter, fully equipped with flaps, retracts and a scale, paint-based finish and fly it with today's electric power equipment, but the margin of performance necessary to make a practical competition model or sport-scale plane that's relaxing to fly is used up pulling that high-wing-loading airplane at the speeds we expect from fighters.

PREDICTING PERFORMANCE AND PLANNING FOR SUCCESS

Unfortunately, very few scale models intended for electric power are on the market in either plan or kit form. This means that unless you design your own, you will probably have to convert a glow model to electric power. The following procedures will eliminate the mystery and guide you through this process with a good chance of success. The same considerations apply if you develop your own plans for an electric-powered scale model.