DIRECTIONAL CONTROL

In addition to directional stability, the air­plane must have adequate directional control to coordinate turns, balance power effects, create sideslip, balance unsymmetrical power, etc. The principal source of directional con­trol is the rudder and the rudder must be capable of producing sufficient yawing moment for the critical conditions of flight.

The effect of rudder deflection is to produce a yawing moment coefficient according to control deflection and produce equilibrium at some angle of sideslip. For small deflections of the rudder, there is no change in stability but a change in equilibrium. Figure 4.25 shows the effect of rudder deflection on yawing moment coefficient curves with the change in equilibrium sideslip angle.

If the airplane exhibits static directional stability with rudder fixed, each angle of side­slip requires a particular deflection of the rudder to achieve equilibrium. Rudder-free directional stability will exist when the float angle of the rudder is less than the rudder deflection required for equilibrium. However at high angles of sideslip, the floating tend­ency of the rudder increases. This is illus­trated by the second chart of figure 4.25 where the line of rudder float angle shows a sharp increase at large values of sideslip. If the floating angle of the rudder catches up with the required rudder angle, the rudder pedal force will decrease to zero and rudder lock will occur. Sideslip angles beyond this point pro­duce a floating angle greater than the required rudder deflection and the rudder tends to float to the limit of deflection.

Rudder lock is accompanied by a reversal of pedal force and rudder-free instability will exist. The dorsal fin is a useful addition in this case since it will improve the directional stability at high angles of sideslip. The re­sulting increase in stability requires larger deflections of the rudder to achieve equilibrium at high sideslip and the tendency for rudder lock is reduced.

Rudder-free directional stability is appre­ciated by the pilot as the rudder pedal force to maintain a given sideslip. If the rudder pedal force gradient is too low near zero sideslip, it will be difficult to maintain zero sideslip dur­ing various maneuvers. The airplane should have a stable rudder pedal feel through the available range of sideslip.

DIRECTIONAL CONTROL REQUIRE­MENTS. The control power of the rudder must be adequate to contend with the many unsymmetrical conditions of flight. Gener­ally, there are five conditions of flight which provide the most critical requirements of di­rectional control power. The type and mission of the airplane will decide which of these conditions is most important.

ADVERSE YAW. When an airplane is rolled into a turn yawing moments are pro­duced which require rudder deflection to main­tain zero sideslip, i. e., coordinate the turn. The usual source of adverse yawing moment is illustrated in figure 4.26. When the airplane shown is subject to a roll to the left, the down­going port wing will experience a new relative wind and an increase in angle of attack. The inclination of the lift vector produces a com­ponent force forward on the downgoing wing. The upgoing starboard wing has its lift in­clined with a component force aft. The re­sulting yawing moment due to rolling motion is in a direction opposite to the roll and is hence “adverse yaw." The yaw due to roll is primarily a function of the wing lift coefficient and is greatest at high C/,.

In addition to the yaw due to rolling motion there will be a yawing moment contribution due to control surface deflection. Conventional ailerons usually contribute an adverse yaw while spoilers may contribute a favorable or “proverse” yaw. The high wing airplane with a large vertical tail may encounter an influence from inboard ailerons. Such a con­figuration may induce flow directions at the vertical tail to cause proverse yaw.

Since adverse yaw will be greatest at high CL and full deflection of the ailerons, coordi­nating steep turns at low speed may produce a critical requirement for rudder control power.

SPIN RECOVERY. In the majority of air­planes, the rudder is the principal control for spin recovery. Powerful control of sideslip at

high angles of attack is required to effect re­covery during a spin. Since the effectiveness of the vertical tail is reduced at large angles of attack, the directional control power neces­sary for spin recovery may produce a critical requirement of rudder power.

SLIPSTREAM ROTATION. A critical di­rectional control requirement may exist when the propeller powered airplane is at high power and low airspeed. As shown in figure 4.26, the single rotation propeller induces a slipstream swirl which causes a change in flow direction at the vertical tail. The rudder must furnish sufficient control power to balance this condition and achieve zero sideslip.

CROSSWIND TAKEOFF AND LANDING. Since the airplane must make a true path down the runway, a crosswind during takeoff or landing will require that the airplane be con­trolled in a sideslip. The rudder must have sufficient control power to create the required sideslip for the expected crosswinds.

ASYMMETRICAL POWER. The design of a multiengine airplane must account for the possibility of an engine failure at low airspeed. The unbalance of thrust from a condition of unsymmetrical power produces a yawing mo­ment dependent upon the thrust unbalance and the lever arm of the force. The deflection of the rudder will create a side force on the tail and contribute a yawing moment to balance the yawing moment due to the unbalance of thrust. Since the yawing moment coefficient from the unbalance of thrust will be greatest at low speed, the critical requirement will be at a low speed with the one critical engine out and the remaining engines at maximum power. Figure 4.26 compares the yawing moment coefficient for maximum rudder deflec­tion with the yawing moment coefficient for the unbalance of thrust. The intersection of the two lines determines the minimum speed for directional control, i. e., the lowest speed at which the rudder control moment can equal the moment of unbalanced thrust. It is usually specified that the minimum directional control speed be no greater than 1.2 times the stall

speed of the airplane in the lightest practical takeoff configuration. This will provide ade­quate directional control for the remaining conditions of flight.

Once defined, the minimum directional con­trol speed is not a function of weight, altitude, etc., but is simply the equivalent airspeed (or dynamic pressure), to produce a required yaw­ing moment with the maximum rudder deflec­tion. If the airplane is operated in the critical unbalance of power below the minimum con­trol speed, the airplane will yaw uncontrolla­bly into the inoperative engine. In order to regain directional control below the minimum speed certain alternatives exist: reduce power on the operating engines or sacrifice altitude for airspeed. Neither alternative is satisfac­tory if the airplane is in a marginal condition of powered flight so due respect must be given to the minimum control speed.

Due to the side force on the vertical tail, a slight bank is necessary to prevent turning flight at zero sideslip. The inoperative engine will be raised and the inclined wing lift will provide a component of force to balance the side force on the tail.

In each of the critical conditions of required directional control, high directional stability is desirable as it will reduce the displacement of the aircraft from any disturbing influence. Of course, directional control must be sufficient to attain zero sideslip. The critical control requirement for the multiengine airplane is the condition of asymmetrical power since spinning is not common to this type of airplane. The single engine propeller airplane may have either the spin recovery or the slipstream rota­tion as a critical design condition. The single engine jet airplane may have a variety of critical items but the spin recovery require­ment usually predominates.