From the Ray Prouty Archives: Sideslip and Bank

By Ray Prouty | January 26, 2018
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A single-rotor helicopter, unlike an airplane, is not symmetrical and therein lies a possible source of confusion for those who fly both.

When a helicopter with counterclockwise (American) main-rotor rotation is in steady flight, the tail rotor/vertical stabilizer combination must produce enough sideforce to the right to balance main-rotor torque. This sideforce, in turn, must be balanced by an equal sideforce to the left to keep the helicopter flying straight forward.

Figure 29-1 shows two possible sources of balancing sideforce:

  1. A sideward tilt of the main-rotor thrust vector is obtained by a combination of lateral flapping and banking the entire helicopter to the left.
  2. A left sideforce due to fuselage aerodynamics is obtained by side slipping to the right (available only in forward flight, of course).

Thus, a single-rotor helicopter in steady flight will trim with a left bank, a right sideslip or some combination of both.

Fixed-wing facts

The symmetrical airplane, on the other hand, needs neither bank nor sideslip to tri in straight flight. The only exception is a twin-engine airplane with a dead engine. The trim situation, in that case, is essentially the same as for the single-rotor helicopter. In normal circumstances the pilot can trim the airplane by holding the rudder pedals in their neutral position or even by taking his feet off them.

Deliberately holding a sideslip angle with some rudder displacement is possible and, if the airplane is equipped for flight-test work with a sideslip vane and indicator, it can be done very precisely.

In most airplanes, however, the pilot only has an indirect indication of sideslip through his bank angle. For this he has two cues:

  1. The seat of his pants tell him which was is straight down
  2. The turn-and-bank indicator, or “turn coordinator,” shown in Figure 29-2.

The lower portion of this instrument is an inclinometer consisting of a ball bearing in a curved glass tube. The upper portion is the silhouette of the rear of the airplane whose bank angle is actuated by the precession of a gyroscope, thus indicating the rate of turn. When a wing tip is lined up with one of the marks, the aircraft is in a “standard-rate turn” of 3 deg per second, requiring two minutes to make a complete circle of 360 deg.

In non-turning flight, the position of the ball will indicate the bank angle since it responds only to the force of gravity. In turning flight, however, the ball is affected by centrifugal force as well. In a perfectly coordinated turn, the effects of centrifugal force and gravity will be exactly equal and opposite and the ball will remain in the bottom of the tube. The pilot will also feel no tendency to slide to either side of his seat.

If the ball is not centered during the turn, it is showing that the turn is not coordinated. The ball will be displaced to the outside of the turn if the airplane is “skidding” with insufficient bank angle and to the inside if it is slipping into the turn because of too much bank angle.

In both cases, the airplane has sideslip and the displacement of the ball can, therefore, be thought of as indicating sideslip, even though it is actually only showing the error form the ideal bank angle.

Because of this, the instrument is sometimes known as a “turn-and-slip” indicator. Even in non-turning flight, an off-center ball position indicates a sideslip since the tilt of the wing lift must be balanced by a fuselage sideforce that can only be generated by side slipping.

(I will leave it up to you to figure out what this all means if we were talking about flying-wing airplanes.)

The relationship between bank angle and sideslip at one speed for an airplane in straight light is shown on the top portion of Figure 29-3. The slope of the line is a measure of its “sideforce characteristics.”

Compensating for asymmetry

A single-rotor helicopter is slightly different from an airplane because it is not symmetrical and in steady, straight flight, zero-bank angle and zero sideslip do not coincide. This is shown by the lower line in Figure 29-3.

The bias also affects the ball position during turns. In contract to an airplane, a centered ball in a helicopter is not a sign of a coordinated turn but of one with some degree of sideslip. For this reason, when installed in a helicopter, it is incorrect to refer to the turn-and-bank indicator as a turn-and-slip indicator.

Knowing all of this, would it be a good idea to install the inclinometer with an initial tilt to nullify the bias so that a centered ball would correspond to zero sideslip in straight flight and coordinated turns? This would make it easier to fly at zero sideslip where the fuselage drag is minimum. A slight complication, of course, is that the required bank angle varies in flight because it is proportional to the tail-rotor thrust needed to balance main-rotor torque — and that changes throughout the flight.

Hughes test pilots recently evaluated the inclinometer tilt and turned it down. They objected to feeling a bank angle in the seat of their pants when the ball was in the traditional no-bank position.

A more fundamental fix is to tilt the rotor mast to the left during preliminary design. This was done on the Sikorsky S-64 Sky Crane to insure nearly simultaneous landing gear liftoff and touchdown.

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