Aerodynamics of Flight

M = 0.50 Maximum local velocity is less than sonic Critical mach number M = 0.72 Maximum local velocity equal to sonic S u p e f r lo s w onic Subson N ic ormal shock wave M = 0.77 Possible separation Figure 5-64. Wing airflow. 5-45 )tneiciffeoc garD( C D M is expressed in Mach number. The V limit is usually MO MO associated with operations at lower altitudes and deals with structural loads and flutter. The M limit is associated with MO operations at higher altitudes and is usually more concerned with compressibility effects and flutter. At lower altitudes, structural loads and flutter are of concern; at higher altitudes, compressibility effects and flutter are of concern. Adherence to these speeds prevents structural problems due to dynamic pressure or flutter, degradation in aircraft control response due to compressibility effects (e.g., Mach Tuck, aileron reversal, or buzz), and separated airflow due to shock waves resulting in loss of lift or vibration and buffet. Any of these phenomena could prevent the pilot from being able to adequately control the aircraft. speed flight and/or high-altitude flight, the measurement of For example, an early civilian jet aircraft had a V limit of speed is expressed in terms of a “Mach number”—the ratio MO 306 KCAS up to approximately FL 310 (on a standard day). of the true airspeed of the aircraft to the speed of sound in At this altitude (FL 310), an M of 0.82 was approximately the same atmospheric conditions. An aircraft traveling at MO equal to 306 KCAS. Above this altitude, an M of 0.82 the speed of sound is traveling at Mach 1.0. Aircraft speed MO always equaled a KCAS less than 306 KCAS and, thus, regimes are defined approximately as follows: became the operating limit as you could not reach the V MO Subsonic—Mach numbers below 0.75 limit without first reaching the M limit. For example, at MO Transonic—Mach numbers from 0.75 to 1.20 FL 380, an M of 0.82 is equal to 261 KCAS. MO Supersonic—Mach numbers from 1.20 to 5.00 Mach Number Versus Airspeed Hypersonic—Mach numbers above 5.00 It is important to understand how airspeed varies with Mach number. As an example, consider how the stall speed of a While flights in the transonic and supersonic ranges are jet transport aircraft varies with an increase in altitude. The common occurrences for military aircraft, civilian jet aircraft increase in altitude results in a corresponding drop in air normally operate in a cruise speed range of Mach 0.7 to density and outside temperature. Suppose this jet transport Mach 0.90. is in the clean configuration (gear and flaps up) and weighs 550,000 pounds. The aircraft might stall at approximately 152 The speed of an aircraft in which airflow over any part of KCAS at sea level. This is equal to (on a standard day) a true the aircraft or structure under consideration first reaches velocity of 152 KTAS and a Mach number of 0.23. At FL 380, (but does not exceed) Mach 1.0 is termed “critical Mach the aircraft will still stall at approximately 152 KCAS, but the number” or “Mach Crit.” Thus, critical Mach number is true velocity is about 287 KTAS with a Mach number of 0.50. the boundary between subsonic and transonic flight and is largely dependent on the wing and airfoil design. Critical Mach number is an important point in transonic flight. When shock waves form on the aircraft, airflow separation followed by buffet and aircraft control difficulties can occur. Shock waves, buffet, and airflow separation take place above critical Mach number. A jet aircraft typically is most efficient when Force divergence Mach number cruising at or near its critical Mach number. At speeds 5–10 percent above the critical Mach number, compressibility Critical Mach number effects begin. Drag begins to rise sharply. Associated with C = 0.3 L the “drag rise” are buffet, trim, and stability changes and a decrease in control surface effectiveness. This is the point of “drag divergence.” [Figure 5-65] 0.5 1.0 M (Mach number) V /M is defined as the maximum operating limit speed. MO MO Figure 5-65. Critical Mach. V is expressed in knots calibrated airspeed (KCAS), while MO