Aerodynamics of Flight

Although the stalling speed has remained the same for our earlier in the chapter, the layer of air over the wing’s surface purposes, both the Mach number and TAS have increased. that is slowed down or stopped by viscosity is the boundary With increasing altitude, the air density has decreased; this layer. There are two different types of boundary layer flow: requires a faster true airspeed in order to have the same laminar and turbulent. pressure sensed by the pitot tube for the same KCAS, or KIAS (for our purposes, KCAS and KIAS are relatively close to Laminar Boundary Layer Flow each other). The dynamic pressure the wing experiences at The laminar boundary layer is a very smooth flow, while FL 380 at 287 KTAS is the same as at sea level at 152 KTAS. the turbulent boundary layer contains swirls or eddies. However, it is flying at higher Mach number. The laminar flow creates less skin friction drag than the turbulent flow but is less stable. Boundary layer flow over a Another factor to consider is the speed of sound. A decrease wing surface begins as a smooth laminar flow. As the flow in temperature in a gas results in a decrease in the speed of continues back from the leading edge, the laminar boundary sound. Thus, as the aircraft climbs in altitude with outside layer increases in thickness. temperature dropping, the speed of sound is dropping. At sea level, the speed of sound is approximately 661 KCAS, Turbulent Boundary Layer Flow while at FL 380 it is 574 KCAS. Thus, for our jet transport At some distance back from the leading edge, the smooth aircraft, the stall speed (in KTAS) has gone from 152 at sea laminar flow breaks down and transitions to a turbulent flow. level to 287 at FL 380. Simultaneously, the speed of sound From a drag standpoint, it is advisable to have the transition (in KCAS) has decreased from 661 to 574 and the Mach from laminar to turbulent flow as far aft on the wing as number has increased from 0.23 (152 KTAS divided by 661 possible or have a large amount of the wing surface within KTAS) to 0.50 (287 KTAS divided by 574 KTAS). All the the laminar portion of the boundary layer. The low energy while, the KCAS for stall has remained constant at 152. This laminar flow, however, tends to break down more suddenly describes what happens when the aircraft is at a constant than the turbulent layer. KCAS with increasing altitude, but what happens when the pilot keeps Mach constant during the climb? In normal jet Boundary Layer Separation flight operations, the climb is at 250 KIAS (or higher (e.g. Another phenomenon associated with viscous flow is heavy)) to 10,000 feet and then at a specified en route climb separation. Separation occurs when the airflow breaks away airspeed (about 330 if a DC10) until reaching an altitude in from an airfoil. The natural progression is from laminar the “mid-twenties” where the pilot then climbs at a constant boundary layer to turbulent boundary layer and then to Mach number to cruise altitude. airflow separation. Airflow separation produces high drag and ultimately destroys lift. The boundary layer separation Assuming for illustration purposes that the pilot climbs at a point moves forward on the wing as the AOA is increased. M of 0.82 from sea level up to FL 380. KCAS goes from MO [Figure 5-66] 543 to 261. The KIAS at each altitude would follow the same behavior and just differ by a few knots. Recall from Vortex generators are used to delay or prevent shock wave the earlier discussion that the speed of sound is decreasing induced boundary layer separation encountered in transonic with the drop in temperature as the aircraft climbs. The Mach flight. They are small low aspect ratio airfoils placed at a 12° number is simply the ratio of the true airspeed to the speed to 15° AOA to the airstream. Usually spaced a few inches of sound at flight conditions. The significance of this is that apart along the wing ahead of the ailerons or other control at a constant Mach number climb, the KCAS (and KTAS or surfaces, vortex generators create a vortex that mixes the KIAS as well) is falling off. boundary airflow with the high energy airflow just above the surface. This produces higher surface velocities and increases If the aircraft climbed high enough at this constant M MO the energy of the boundary layer. Thus, a stronger shock wave with decreasing KIAS, KCAS, and KTAS, it would begin to is necessary to produce airflow separation. approach its stall speed. At some point, the stall speed of the aircraft in Mach number could equal the M of the aircraft, MO Shock Waves and the pilot could neither slow down (without stalling) nor When an airplane flies at subsonic speeds, the air ahead is speed up (without exceeding the max operating speed of the “warned” of the airplane’s coming by a pressure change aircraft). This has been dubbed the “coffin corner.” transmitted ahead of the airplane at the speed of sound. Because of this warning, the air begins to move aside before Boundary Layer the airplane arrives and is prepared to let it pass easily. When The viscous nature of airflow reduces the local velocities on the airplane’s speed reaches the speed of sound, the pressure a surface and is responsible for skin friction. As discussed 5-46