part 2: Dynamic Lateral Stability
(Continued...)
There are two types of possible lateral dynamic motion.The first is called the "Spiral Mode". If a spirally unstable airplane,through the action of a gust or othe disturbance,gets a small initial roll angle, a gentle side slip in the same direction of roll is produced if rolled tp the right...slideslip to the right hand side. If the dihedal stability is low,the directional stability keeps turning the airplane qhile the continuing bank angle maintains the side slip and yaw angle.As this process develops,the spiral gets tighter and steeper until finallt, if motion is unchecked, a steep, high-speed, spiral dive results! The motion develops so gradually,however, that it us usually corrected unconsciously by the handling pilot,who may not be aware that the spiral instability exists.A combination of high directional stability and low dihedral,or roll stability leads ti a spirally unstable aeroplane.
The second lateral motion is an oscillatory combined roll and yaw motion rederred to as the "Dutch Roll" ( coined to mimick drunken Dutch sailors landing on their next port of call afyer a long spell on rough seas I suspect). The Dutch Roll may be described as a yaw and roll to the right, followed by a recovery toward the equilibrium condition, then an overshootibg of this condition and a taw and roll to the left, thenafter back past the equilibrium attitude, and so on. The oscillatory period is usyally of the order of 3 to 5 seconds.Damping is increases by large directional stability and small dihedral and decreased by small dirwctional stability and large dihedral. Although usually stable in a normal airplane,the motion may be so slightly damped that the effect is very unpleasant and undesirable!
Swept wing aircraft have large digedral stability even when they have no dihedral angle.This is because, the effect if an angle of yaw is to increase the swewp back angle of onw wing panel and decrease it for the other side of the aeroplane. The change in sweep alters the effective dynamic pressure normal to the quarter-chord line of the wing panel, increasing the lift on one side of the wing,lowering it on the opposite side, and producing a restoring rolling moment. Nevertheless, many swept wing aeroplanes have a considerable dihedral angle, not for aerodynamic reasons bit to give adequate ground clearance for the wing tips and especially wing -mounted engine nacelles during take off and landing.These aircraft may then have too much dihedral effect for satisfactory Dutch Roll damping.The technical name for Dutch Roll is Oscilatory lateral stability. The problem is solved by installing a yaw damper YD.The Yaw Damper is in effect a special purpose autopilot that damps out any yawing oscillation by apying rudder corrections instantaneosly.Some swept wing aircraft demonstrate an unstable Dutch Roll mode.If the Dutch Roll is very lightly damped or unstable,the YD becomes a safety requirement rather than a pilot and passenger design convenience.Dual YDs are required and a failed YD is cause for limiting flight to lower altitude and lower Mach numbers.At high sltitude there is reduced aerodynamic damping due to high altitude low air density.At the lower altitudes Dutch Roll stability is improved.
Part 3 ...to be continued. "Turning performance".