This alternate philosophy that trusts active control may be used by some companies for future advanced aircraft design work; it will probably be used in any HSCT design. Some basic control will still be available even without active control in that pitch trim and rudder will still be mechanically activated. In the future, vertical tails will not be sized for Dutch roll, so long as the control system has sufficient authority to stabilize the airplane.
There is a limit to the instability that can be tolerated; the control system cannot be saturated. For this purpose, the rudder should be designed to return aircraft from a 10¡ sideslip disturbance at any altitude. For reliability, rudders may be split into upper and lower halves, with independent signals and actuators plus redundant processors.
The critical control sizing constraint is often VMCG, minimum controlled ground speed. In this condition, flight is straight and unaccelerated laterally. Nose gear reaction is zero. Aerodynamic moments must balance engine thrust with one engine out and creating windmilling drag, and the other engine at max thrust plus a thrust bump for a "hot" engine. If the moment balance is done about the aircraft center of gravity, main gear reactions caused by rudder sideforce must be considered. If the main gear reactions were ignored, rudder force would be underestimated by 15% to 20%. Alternately, the moment balance can be done about the main gear center, which lies in line with the gear and halfway between them. Engine thrust imbalance should be controllable with full rudder deflection.
VMCG is relatively independent of flap setting or aircraft weight because it is primarily a matter of balancing engine thrust imbalance with the rudder. Flaps may affect rudder performance sometimes because of aerodynamic interaction. Aircraft weight does not enter the moment balance because, when moments are taken about the main gear, there are no ground moment reactions and there are no inertial forces because there is no lateral acceleration. The engine thrust imbalance is constant because full thrust is always used for takeoff, regardless of aircraft weight. To determine a required VMCG speed, one would examine an aircraft in its lightest commercial weight. This would be the weight with a minimum passenger load to break even on a particular range, say a 30% passenger load. At low takeoff weights, more flaps will be used as a result of optimizing flap deflection for best lift to drag in second segment climb. The light weight and large flap deflection should reduce speeds for second segment climb and rotation. In establishing the balanced field length for this condition, VMCG should be set at the speed where second segment climb or rotation becomes critical. For aircraft such as the DC9 or DC10 this speed is about 110 knots. For heavier aircraft, VMCG is higher, 120 knots.
VMCA, minimum control airspeed, is usually not critical because dynamic pressure is higher, making the rudder more effective, the thrust imbalance is smaller, because of thrust lapse, plus the airplane is allowed to sideslip to trim. The VMCG condition is at zero sideslip; rudders may be double hinged to enable large lift coefficients to be achieved on the fin at this condition.
While VMCG is critical for 2 engine airplanes, on 4 engine airplanes VMCL2 may be critical. In this landing condition, 2 engines are out on same side of the airplane while the other two are at max takeoff thrust. The rudder is more effective since this is done at approach speed, 1.3 Vstall.
One airborne condition that might size the rudder is a crosswind landing decrab. This condition is at 1.3 Vstall with a 35 knot crosswind. The rudder is used to control an aerodynamic sideslip of 13¡ to 15¡. Increasing the vertical tail area does not help here because it increases the resistance to sideslip. If this condition is critical the proportion of rudder to vertical tail area should be adjusted.