Aerodynamics of Flight

Level flight Medium banked turn Steeply banked turn Horizontal component Centrifugal force 5-22 component Vertical Centrifugal force Horizontal R component e s u lta n t lo a d component Vertical R e su lta n t lo a d Lift T T o o ta l lift ta l lift Weight Weight Weight design is necessary to obtain the best results from these two In comparison, the rectangular wing has a tendency to stall conflicting conditions. first at the wing root and provides adequate stall warning, adequate aileron effectiveness, and is usually quite stable. The second means of changing the planform is by tapering It is, therefore, favored in the design of low cost, low speed (decreasing the length of chord from the root to the tip of the airplanes. wing). In general, tapering causes a decrease in drag (most effective at high speeds) and an increase in lift. There is also Aerodynamic Forces in Flight Maneuvers a structural benefit due to a saving in weight of the wing. Forces in Turns If an aircraft were viewed in straight-and-level flight from the Most training and general aviation type airplanes are operated front [Figure 5-34], and if the forces acting on the aircraft at high coefficients of lift, and therefore require comparatively could be seen, lift and weight would be apparent: two forces. high aspect ratios. Airplanes that are developed to operate at If the aircraft were in a bank it would be apparent that lift very high speeds demand greater aerodynamic cleanness and did not act directly opposite to the weight, rather it now acts greater strength, which require low aspect ratios. Very low in the direction of the bank. A basic truth about turns is that aspect ratios result in high wing loadings and high stall speeds. when the aircraft banks, lift acts inward toward the center of When sweepback is combined with low aspect ratio, it results the turn, perpendicular to the lateral axis as well as upward. in flying qualities very different from a more conventional high aspect ratio airplane configuration. Such airplanes Newton’s First Law of Motion, the Law of Inertia, states that require very precise and professional flying techniques, an object at rest or moving in a straight line remains at rest especially at slow speeds, while airplanes with a high aspect or continues to move in a straight line until acted on by some ratio are usually more forgiving of improper pilot techniques. other force. An aircraft, like any moving object, requires a sideward force to make it turn. In a normal turn, this force The elliptical wing is the ideal subsonic planform since it is supplied by banking the aircraft so that lift is exerted provides for a minimum of induced drag for a given aspect inward, as well as upward. The force of lift during a turn is ratio, though as we shall see, its stall characteristics in separated into two components at right angles to each other. some respects are inferior to the rectangular wing. It is also One component, which acts vertically and opposite to the comparatively difficult to construct. The tapered airfoil is weight (gravity), is called the “vertical component of lift.” desirable from the standpoint of weight and stiffness, but The other, which acts horizontally toward the center of the again is not as efficient aerodynamically as the elliptical turn, is called the “horizontal component of lift” or centripetal wing. In order to preserve the aerodynamic efficiency of the force. The horizontal component of lift is the force that elliptical wing, rectangular and tapered wings are sometimes pulls the aircraft from a straight flight path to make it turn. tailored through use of wing twist and variation in airfoil Centrifugal force is the “equal and opposite reaction” of the sections until they provide as nearly as possible the elliptical aircraft to the change in direction and acts equal and opposite wing’s lift distribution. While it is true that the elliptical to the horizontal component of lift. This explains why, in a wing provides the best coefficients of lift before reaching an correctly executed turn, the force that turns the aircraft is incipient stall, it gives little advance warning of a complete not supplied by the rudder. The rudder is used to correct any stall, and lateral control may be difficult because of poor deviation between the straight track of the nose and tail of the aileron effectiveness. aircraft into the relative wind. A good turn is one in which the Figure 5-34. Forces during normal, coordinated turn at constant altitude.