Aerodynamics of Flight

airspeed. In this flight maneuver, the aircraft is in a steep capable of a wide range of speeds and altitudes. It is important climbing turn and almost stalls to gain altitude while changing for the pilot to remember that the maximum “never-exceed” direction. A lazy eight derives its name from the manner in placard dive speeds are determined for smooth air only. High which the extended longitudinal axis of the aircraft is made speed dives or acrobatics involving speed above the known to trace a flight pattern in the form of a figure “8” lying on maneuvering speed should never be practiced in rough or its side. It would be difficult to make a definite statement turbulent air. concerning load factors in these maneuvers as both involve smooth, shallow dives and pull-ups. The load factors incurred Vg Diagram depend directly on the speed of the dives and the abruptness The flight operating strength of an aircraft is presented of the pull-ups during these maneuvers. on a graph whose vertical scale is based on load factor. [Figure 5-55] The diagram is called a Vg diagram—velocity Generally, the better the maneuver is performed, the less versus G loads or load factor. Each aircraft has its own Vg extreme the load factor induced. A chandelle or lazy eight diagram that is valid at a certain weight and altitude. in which the pull-up produces a load factor greater than 2 Gs will not result in as great a gain in altitude; in low-powered The lines of maximum lift capability (curved lines) are the aircraft, it may result in a net loss of altitude. first items of importance on the Vg diagram. The aircraft in Figure 5-53 is capable of developing no more than +1 G at The smoothest pull-up possible, with a moderate load factor, 64 mph, the wing level stall speed of the aircraft. Since the delivers the greatest gain in altitude in a chandelle and results maximum load factor varies with the square of the airspeed, in a better overall performance in both chandelles and lazy the maximum positive lift capability of this aircraft is 2 G at eights. The recommended entry speed for these maneuvers 92 mph, 3 G at 112 mph, 4.4 G at 137 mph, and so forth. Any is generally near the manufacturer’s design maneuvering load factor above this line is unavailable aerodynamically speed, which allows maximum development of load factors (i.e., the aircraft cannot fly above the line of maximum lift without exceeding the load limits. capability because it stalls). The same situation exists for negative lift flight with the exception that the speed necessary Rough Air to produce a given negative load factor is higher than that to All standard certificated aircraft are designed to withstand produce the same positive load factor. loads imposed by gusts of considerable intensity. Gust load factors increase with increasing airspeed, and the strength used If the aircraft is flown at a positive load factor greater than the for design purposes usually corresponds to the highest level positive limit load factor of 4.4, structural damage is possible. flight speed. In extremely rough air, as in thunderstorms or When the aircraft is operated in this region, objectionable frontal conditions, it is wise to reduce the speed to the design permanent deformation of the primary structure may take place maneuvering speed. Regardless of the speed held, there may and a high rate of fatigue damage is incurred. Operation above be gusts that can produce loads that exceed the load limits. the limit load factor must be avoided in normal operation. Each specific aircraft is designed with a specific G loading There are two other points of importance on the Vg diagram. that can be imposed on the aircraft without causing structural One point is the intersection of the positive limit load factor damage. There are two types of load factors factored into and the line of maximum positive lift capability. The airspeed aircraft design: limit load and ultimate load. The limit load at this point is the minimum airspeed at which the limit load is a force applied to an aircraft that causes a bending of the can be developed aerodynamically. Any airspeed greater than aircraft structure that does not return to the original shape. this provides a positive lift capability sufficient to damage The ultimate load is the load factor applied to the aircraft the aircraft. Conversely, any airspeed less than this does not beyond the limit load and at which point the aircraft material provide positive lift capability sufficient to cause damage experiences structural failure (breakage). Load factors lower from excessive flight loads. The usual term given to this speed than the limit load can be sustained without compromising is “maneuvering speed,” since consideration of subsonic the integrity of the aircraft structure. aerodynamics would predict minimum usable turn radius or maneuverability to occur at this condition. The maneuver Speeds up to, but not exceeding, the maneuvering speed speed is a valuable reference point, since an aircraft operating allow an aircraft to stall prior to experiencing an increase in below this point cannot produce a damaging positive flight load factor that would exceed the limit load of the aircraft. load. Any combination of maneuver and gust cannot create damage due to excess airload when the aircraft is below the Most AFM/POH now include turbulent air penetration maneuver speed. information, which help today’s pilots safely fly aircraft 5-37