Aerodynamics of Flight

Weight Gravity is the pulling force that tends to draw all bodies to the center of the earth. The CG may be considered as a point at which all the weight of the aircraft is concentrated. If the aircraft were supported at its exact CG, it would balance in any attitude. It will be noted that CG is of major importance in an aircraft, for its position has a great bearing upon stability. The allowable location of the CG is determined by the general design of each particular aircraft. The designers determine how far the center of pressure (CP) will travel. It is important to understand that an aircraft’s weight is concentrated at the CG and the aerodynamic forces of lift occur at the CP. When the CG is forward of the CP, there is a natural tendency for the aircraft to want to pitch nose down. If the CP is forward of the CG, a nose up pitching moment is created. Therefore, designers fix the aft limit of the CG forward of the CP for the corresponding flight speed in order to retain flight equilibrium. Weight has a definite relationship to lift. This relationship is simple, but important in understanding the aerodynamics Figure 5-11. The difference in downwash at altitude versus near of flying. Lift is the upward force on the wing acting the ground. perpendicular to the relative wind and perpendicular to the aircraft’s lateral axis. Lift is required to counteract the The greater the size and strength of the vortices and aircraft’s weight. In stabilized level flight, when the lift force is consequent downwash component on the net airflow over equal to the weight force, the aircraft is in a state of equilibrium the airfoil, the greater the induced drag effect becomes. This and neither accelerates upward or downward. If lift becomes downwash over the top of the airfoil at the tip has the same less than weight, the vertical speed will decrease. When lift is effect as bending the lift vector rearward; therefore, the lift greater than weight, the vertical speed will increase. is slightly aft of perpendicular to the relative wind, creating a rearward lift component. This is induced drag. Wingtip Vortices Formation of Vortices In order to create a greater negative pressure on the top of an The action of the airfoil that gives an aircraft lift also causes airfoil, the airfoil can be inclined to a higher AOA. If the AOA induced drag. When an airfoil is flown at a positive AOA, of a symmetrical airfoil were zero, there would be no pressure a pressure differential exists between the upper and lower differential, and consequently, no downwash component and surfaces of the airfoil. The pressure above the wing is less no induced drag. In any case, as AOA increases, induced than atmospheric pressure and the pressure below the wing drag increases proportionally. To state this another way—the is equal to or greater than atmospheric pressure. Since air lower the airspeed, the greater the AOA required to produce always moves from high pressure toward low pressure, lift equal to the aircraft’s weight and, therefore, the greater and the path of least resistance is toward the airfoil’s tips, induced drag. The amount of induced drag varies inversely there is a spanwise movement of air from the bottom of the with the square of the airspeed. airfoil outward from the fuselage around the tips. This flow of air results in “spillage” over the tips, thereby setting up a Conversely, parasite drag increases as the square of the whirlpool of air called a vortex. [Figure 5-12] airspeed. Thus, in steady state, as airspeed decreases to near the stalling speed, the total drag becomes greater, due At the same time, the air on the upper surface has a tendency mainly to the sharp rise in induced drag. Similarly, as the to flow in toward the fuselage and off the trailing edge. This aircraft reaches its never-exceed speed (V ), the total drag NE air current forms a similar vortex at the inboard portion of the increases rapidly due to the sharp increase of parasite drag. trailing edge of the airfoil, but because the fuselage limits the As seen in Figure 5-6, at some given airspeed, total drag is inward flow, the vortex is insignificant. Consequently, the at its minimum amount. In figuring the maximum range of deviation in flow direction is greatest at the outer tips where aircraft, the thrust required to overcome drag is at a minimum the unrestricted lateral flow is the strongest. if drag is at a minimum. The minimum power and maximum endurance occur at a different point. 5-8