Years ago, when I first joined the police aviation unit, I was fortunate enough to be partnered with a former Marine Corps CH-46 driver. As the new guy, I was very happy when this more experienced aviator would say, “You have the controls,” and give me command of the aircraft. As I made every effort to fly as smoothly and precisely as possible, there was always some wisdom he would readily share, and he’d deliver it in the most professional manner of any Marine. One day I was flying us back to base and heard a “BANG BANG BANG” coming from his side of the cockpit.
“What the heck is that?” I said.
My partner replied (as he continued to pound on the door with his fist), “It’s the SLIP BALL trying to get back in the aircraft. NOW STEP ON IT!”
I sheepishly complied, and he laughed all the way home.
He was referring to our “out of trim” condition evidenced by the off-centered ball in the inclinometer portion of the turn and slip indicator. In basic flight training, we all learn about the slip ball, and that we should “step on the ball” by adding the associated pedal to center it up and remain in trimmed coordinated flight.
Well, it’s 15 years later, and at this point I am not afraid to say that not everything that my partner says is 100 percent right, 100 percent of the time. In fact, back then his experience in the CH-46 may have been a contributing factor when he mocked me for flying out of trim. The point I am alluding to involves the often-overlooked “inherent sideslip” that all helicopters with a tail rotor experience when flown in ball-centered, straight and level flight.
The slip ball is a simple instrument, but can be misleading. It measures nothing more than side force or lateral balance. It shows the resulting direction of the gravity and centrifugal force vectors combined, with respect to the vertical centerline of the aircraft. Centering the ball while flying straight and level in a symmetrical aircraft such as an airplane or tandem rotor helicopter coincides with zero sideslip, with all forces and moments in trim. This is what you want if your goal is to minimize drag and maximize performance. But in a single rotor helicopter with a tail rotor, there is always an asymmetrical force pushing to one side: the tail rotor thrust. And we must cancel this force to avoid any lateral acceleration.
We can produce this cancelling force in two ways. We can slip the helicopter to provide an aerodynamic sideforce or “sidewind” against the fuselage that opposes the tailrotor thrust. This is what occurs “inherently” when we fly with a centered ball on tailrotor-equipped helicopters. So we end up flying a bit sideways. How much sideways will depend on gross weight and CG but its usually about three or four degrees. If your helicopter has a yaw string, you will see this to be true. A centered slip ball will not produce a centered yaw string.
But wait, didn’t we say a centered ball is how we know we are in trim with zero sideslip? Only in aircraft with symmetry as mentioned above. It’s true that all forces/moments are equal and opposite, i.e. “trimmed” with ball centered. But we are not in the ideal aerodynamic condition. Then how do we fly as streamlined as possible maximizing performance in helicopters with tailrotors?
The other way to oppose the tailrotor thrust is to tilt the main rotor slightly in opposition, as we do when cancelling translating tendency in a hover. In fact, a good approximation of what will provide minimum drag in forward flight is to fly with the slip ball in the same off-center position that it rests in while in a hover. A centered yaw string is equally as handy, except it has its limitations.
The issue of whether you should fly centered ball or centered yaw string is really dependent on the situation. If you are IFR or flying at night, the ball is, of course, your go-to instrument. If you are day VFR, or trying to minimize drag, especially in a low speed, high power climb situation, use the off-center ball or centered string if you’ve got one.
I continue to have the utmost respect for my partner and friend, the Colonel. He has taught me countless invaluable lessons. In fact, I have used his exact “banging” technique on newer aviators to start a conversation about inherent sideslip. But in that one particular instance many years ago, although our recollections may differ, I am going to close out by humbly saying, “Sir, I meant to do that.”