Aerodynamics of Flight

Cruise power Idle power Full power Thrust CG Below center of gravity Thrust CG Through center of gravity Thrust CG Above center of gravity Figure 5-26. Thrust line affects longitudinal stability. 5-17 Lift Thrust CG Lift Thrust CG Lift As this climb continues, the airspeed again decreases, causing the downward force on the tail to decrease until the nose lowers once more. Because the aircraft is dynamically stable, the nose does not lower as far this time as it did before. The aircraft acquires enough speed in this more gradual dive to start it into another climb, but the climb is not as steep as the preceding one. After several of these diminishing oscillations, in which the nose alternately rises and lowers, the aircraft finally settles down to a speed at which the downward force on the tail exactly counteracts the tendency of the aircraft to dive. When this condition is attained, the aircraft is once again in balanced flight and continues in stabilized flight as long as this attitude and airspeed are not changed. A similar effect is noted upon closing the throttle. The downwash of the wings is reduced and the force at T in Thrust CG Figure 5-23 is not enough to hold the horizontal stabilizer down. It seems as if the force at T on the lever were allowing the force of gravity to pull the nose down. This is a desirable characteristic because the aircraft is inherently trying to regain airspeed and reestablish the proper balance. Figure 5-27. Power changes affect longitudinal stability. Power or thrust can also have a destabilizing effect in that an increase of power may tend to make the nose rise. The increased a moment is produced to counteract the down load aircraft designer can offset this by establishing a “high on the tail. On the other hand, a very “low thrust line” would thrust line” wherein the line of thrust passes above the CG. tend to add to the nose-up effect of the horizontal tail surface. [Figures 5-26 and 5-27] In this case, as power or thrust is Conclusion: with CG forward of the CL and with an aerodynamic tail-down force, the aircraft usually tries to return to a safe flying attitude. The following is a simple demonstration of longitudinal stability. Trim the aircraft for “hands off” control in level flight. Then, momentarily give the controls a slight push to nose the aircraft down. If, within a brief period, the nose rises towards the original position, the aircraft is statically stable. Ordinarily, the nose passes the original position (that of level flight) and a series of slow pitching oscillations follows. If the oscillations gradually cease, the aircraft has positive stability; if they continue unevenly, the aircraft has neutral stability; if they increase, the aircraft is unstable. Lateral Stability (Rolling) Stability about the aircraft’s longitudinal axis, which extends from the nose of the aircraft to its tail, is called lateral stability. Positive lateral stability helps to stabilize the lateral or “rolling effect” when one wing gets lower than the wing on the opposite side of the aircraft. There are four main design factors that make an aircraft laterally stable: dihedral, sweepback, keel effect, and weight distribution.