FORCES AFFECTING FLIGHT

upper surface of the airfoil has camber to it, the flow over its surface is disrupted. This disruption causes a wavelike effect to the airflow. The lower surface of the airfoil is relatively flat. The airflow across its surface isn't disrupted. Lift is accomplished by this difference in the airflow across the airfoil. The shaded area of figure 3-5 shows a l ow-pressure area on the airfoil's upper surface. This low-pressure area is caused by the air that is disrupted by the camber of the airfoil, and it is the key t o lift. There is less pressure on the top surface of the airfoil than there is on the lower surface. The air pressure pushes upward o the lower surface. This difference in pressure causes the airfoil to rise. Now, you know that lift is developed by the difference between the air pressure o n the upper and lower surfaces of the airfoil. As long as there is less pressure on the upper surface and more pressure on the lower surface of an airfoil, an aircraft has lift. Lift is one of the forces affecting flight. Q3-5. What happens when the relative wind strikes the leading edge o f a n airfoil? Q3-6. Describe how lift is developed. FORCES AFFECTING FLIGHT LEARNING OBJECTIVE: Recognize the four primary forces acting on an aircraft. An aircraft in flight is in the center of a continuous battle of forces. The conflict of these forces is the key t o all maneuvers performed in the air. There is nothing mysterious about these forces—they are definite and known. The direction in which each acts can be calculated. The aircraft is designed to take advantage of each force. These forces are lift, weight, thrust, and drag. LIFT Lift is the force that acts in an upward direction to support the aircraft in the air. I t counteracts the effects of weight. Lift must be greater than or equal to weight if flight is to be sustained. WEIGHT Weight is the force of gravity acting downward on the aircraft and everything in the aircraft, such as crew, fuel, and cargo. THRUST Thrust is the force developed by the aircraft's engine. It acts in the forward direction. Thrust must be greater than or equal to the effects of drag for flight to begin or to be sustained. 3-3 Figure 3-4.—Angle of attack. Figure 3-5.—Airflow across an airfoil.