CG for Best Performance


I was just pondering what the preferred CG location is for helicopters.

On conventional-layout airplanes the farther aft the CG moves the horizontal stabilizer has to produce less lift and accompanying drag, allowing faster airspeeds.

How's that work on helicopters? Where do you want the CG, and is there ever an advantage to shifting it laterally?
For the most part, I really only care that I'm in the envelope in a helo. The CG location in a helo has much more to do with controllability and cyclic travel authority than attaining higher airspeeds.

That being said, if I want to go faster in something like a Robinson R-22, having a CG in the front of the envelope means the helicopter will already want to be a little nose down, so I have a little extra forward cyclic to get more forward thrust. However, at some point, I could run out of collective to maintain level flight and I'll just accelerate in a descent or (and this is pretty unlikely in an R-22) I'll get into retreating blade stall.

In a bigger helicopter, like an H-60, with a stabilator, my forward airspeed is likely to be limited by a combination of power available and retreating blade stall. The stabilator will program up to keep the nose from being buried at high airspeeds, which will change the drag characteristics, but I'll run out of power from total drag before the specific induced drag from the stabilator becomes an issue. The CG location will have a neglible impact.

On the vast majority of helicopters, lateral CG is hard to exceed because they are so narrow. But there are some advantages in some of the lighter helicopters in having the CG slightly offset to the side of tail rotor thrust (vector, not the direction of air movement). To overcome translating tendency (the tail rotor thrust tries to push you laterally), the pilot adds opposite lateral cyclic, resulting in one skid hanging lower than the other. If you put the heavier guy on the side of tail rotor thrust, it can level the fuselage a bit. Some helos have control rigging and/or slightly offset from center masts to help level the fuselage in a hover. This can become critically important when performing slope operations because you can hit the limit of your lateral cyclic authority earlier on one side than another when hovering with a slightly tilted fuselage.
On airplanes, an aft CG requires less trim tab deflection on the elevator resulting in reduced drag (less elevator down-force). In helicopters, the limit to forward airspeed is the speed of the advancing blade.
As long as the CG is within the envelope of the helicopter, life is good. Yes, lateral CG on helicopters is tough to exceed, unless you are hoisting a couple of fatties, or hauling a bunch of “projectiles” that were poorly loaded… Makes life very interesting as you run out of lateral cyclic at low/no airspeeds.