Aerodynamics of Flight

GC GC CL CL Cruise Speed CG Balanced tail load Low Speed CG Lesser downward tail load High Speed CG Greater downward tail load Figure 5-24. Effect of speed on downwash. Thrust Normal downwash Reduced downwash Figure 5-23. Longitudinal stability. Weight CG Lift Thrust Weight that when the AOA of an airfoil is increased, the CL, by moving forward, tends to lift the leading edge of the wing still more. This tendency gives the wing an inherent quality of instability. (NOTE: CL is also known as the center of pressure (CP).) Figure 5-23 shows an aircraft in straight-and-level flight. The line CG-CL-T represents the aircraft’s longitudinal axis from the CG to a point T on the horizontal stabilizer. Most aircraft are designed so that the wing’s CL is to the rear of the CG. This makes the aircraft “nose heavy” and requires that there be a slight downward force on the horizontal stabilizer in order to balance the aircraft and keep the nose from continually pitching downward. Compensation for this nose heaviness is provided by setting the horizontal stabilizer at a slight negative AOA. The downward force thus produced holds the tail down, counterbalancing the “heavy” nose. It is as if the line CG-CL-T were a lever with an upward force at CL and two downward forces balancing each other, one a strong force at the CG point and the other, a much lesser force, at point T (downward air pressure on the stabilizer). To better visualize this physics principle: If an iron bar were suspended at point CL, with a heavy weight hanging on it at flow of air over the wing, the downwash is reduced, causing the CG, it would take downward pressure at point T to keep a lesser downward force on the horizontal stabilizer. In turn, the “lever” in balance. the characteristic nose heaviness is accentuated, causing the aircraft’s nose to pitch down more. [Figure 5-25] This places Even though the horizontal stabilizer may be level when the the aircraft in a nose-low attitude, lessening the wing’s AOA aircraft is in level flight, there is a downwash of air from the and drag and allowing the airspeed to increase. As the aircraft wings. This downwash strikes the top of the stabilizer and continues in the nose-low attitude and its speed increases, produces a downward pressure, which at a certain speed is the downward force on the horizontal stabilizer is once again just enough to balance the “lever.” The faster the aircraft increased. Consequently, the tail is again pushed downward is flying, the greater this downwash and the greater the and the nose rises into a climbing attitude. downward force on the horizontal stabilizer (except T-tails). [Figure 5-24] In aircraft with fixed-position horizontal stabilizers, the aircraft manufacturer sets the stabilizer at an angle that provides the best stability (or balance) during flight at the design cruising speed and power setting. If the aircraft’s speed decreases, the speed of the airflow over the wing is decreased. As a result of this decreased L ift CG Figure 5-25. Reduced power allows pitch down.