Hey Mark,

Naw, I think most of the guys are using 3 - 8611's on that size surface these days, even with a reduction linkage. If you were flying pattern only, I'd have no problem recommending just a couple JR's - because at the reduced deflections of pattern, the air loads are also reduced. I'd recommend only 2 servos if you're using Hitec 5955's, even if you do 3D. For each direct drive mount you save about 2 ounces on average. With that weight savings (4 oz) you can install a third servo (2.2oz) for good measure, still come out lighter than 2 servos with linkages, and put the $30+ you save on the linkages toward the cost of that extra servo - that I assure you is just for your piece of mind anyway.

We have found that whatever number and type of servos you used with linkages, that's the most you need with direct drive. My guess is you need extra torque when using a reduction linkage to overcome issues not related to air loads - like engine resonance and flutter suppression.

I have placed the drive on 39% extras and 53% Ultimates with no issues and no "extra" servos. You either have enough servo torque or you don't. Servo gearing is a more efficient method of increasing mechanical advantage than a ball linkage. This is because gears always have an optimum interface with each other, and linkages are only optimum at their center point. I have one Ultimate with a single 1800in/oz rudder servo...you can find the right servo for your application. In the '70's we were stuck with 26in/oz servos that absolutely depended on reduction linkages. That is no longer the case. I agree with you completely when you say if you use a weak servo you need the reduction linkage. I just disagree that you need to use a weak servo so you need twice as many of them. In other words, if you want to move your 50 ton beam, use a 50 ton jack - instead of a weak one that has to be rigged. Or, from another view, with a traditional setup you are adding a linkage that weighs almost as much as another servo just to increase the torque!

Next time you idle your plane take note of the amount of deflection your ailerons make as they shake. It's difficult to make an arguement that only servo gear lash accounts for the amount of aileron deflection you get when the aileron resonates to engine vibration. The more the aileron deflects when shaking, the larger the forces on your servo gears - at an exponential rate. That vibration does not stop when you are airborne, or when the surface is centered. Those are normal loads the control is continuously subject to, and they do cause deflections in the linkage. Direct drive reduces the resonant deflections of the aileron, thereby reducing the shock loads to the servo gears. This fact alone allows you to use a servo with less torque than you would need with a 1:1 linkage.

I have used soft engine mounts for more than 25 years, even on my sport planes. They protect the whole airframe from engine vibration, not just the servos. That's how I get away without using the servo grommets and still get much better than average servo life. Full scale planes also only isolate the engine, since that is the source of the vibration. They do not use any compliant rubber in the controls, mainly because it invites flutter - which is usually fatal. I feel that sometimes in this hobby we choose to reinvent the wheel. The boys with the money have worked out a lot of our problems for us - we just need to follow their lead.

Still, I have friends who are using the drive with no soft mounts - to save weight - and have no problems with servo failure either. I would have guessed they would have problems, but they don't. I think that says a lot about the quality of the system.

Julius, have you got any pictures of the system? I'm always fascinated by what the racers do, as they seem to evolve their own way of building...

John