Direct drive vs gear drive: which one is better?

More and more model get released with direct drive on the market, even the bigger ones now. First the SAB Goblin Raw 420 was released with a pancake motor in the large league, then the Goosky RS4 followed it, and now the hot topic is the brand new Omphobby M4. Is the direct drive that good?

As usually in this hobby, there is no simple answer, instead, there are pros and cons on both sides. The direct drive uses much fewer parts, it's simpler consequently and hypothetically cheaper in total to manufacture, than the conventional drive trains using gears and pinion or pulleys and belt. When the helicopter crashes, there are much fewer parts to break, so even the repairing cost is lower.

As the technology developed, the direct drive broke through and took over among the small performance helicopters: Align T15, Omphobby M1 and M2 and so on; and there's no doubt, the trend continues, especially because the small parts of these small helicopters can break so easy, it's better to have fewer of them.

However, there are few facts which have to be taken into consideration. As Keven Schauz from Helicoach.de phrased earlier:

"Secret" behind motors is quite simple: power ≈ speed × torque. Higher torque for more power costs weight, higher speed for more power does not. Therefore, in many applications, a gear drive can provide significantly more power with smaller weight. Therefore, the trend within the area is currently moving away from large direct drives towards belt drives with smaller motors.

We asked Keven about more details to make this aspect clear

As I see, many people think a direct drive must be more efficient, as it has fewer parts, and they are not aware of the drawbacks. For smaller helis direct drives are great, but there's a certain point, where it gets worse, and then the power to weight curves move away from each other rapidly. It´s due to the increase in motor volume which is necessary to increase torque (which is necessary to increase power with a fixed maximum rpm), but increasing volume means a weight increase of factor 2 every time when the torque gets doubled.

Consequently, it's limited how large an efficient direct drive helicopter can be. The real question is where this limit sits on the scale. Well, the 380-420 class direct drive helicopters are out on the market, and they indeed provide a very good performance. For how long this trend lasts, how dominant standard will be, nobody knows apparently. And yet it's also certain, the gear drive or its equivalents cannot be written off, because very popular models like Align T-Rex 450, 470, XLPower Protos and many more in the class, and perhaps all in bigger classes will use these in the future too.

UPDATE: this story to be continued....

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