374 FORMS OF ENERGY lift of the left wing. When the angle of attack is increased to increase the lift of a wing, it is called "wash in"; when decreased, it is called "wash out." When the left wing is "washed in" to increase the lift, offsetting engine torque, the projected area of that wing is also increased, causing To correct this, the vertical stabilizer is the plane to yaw to its left. ofTset slightly to the left. Modem Airplanes Are Quite Stable. The ability of an airplane to return to its original normal position without effort on the part of the pilot or with the controls released is Fig. 166. When the airplane is disturbed longitudinally, i.e., when the nose is pulled up, the airplane loses speed because the downward force on the horizontal tail surface decreases and the tail comes up, placing the airplane in a diving attitude. As the speed increases in the dive, the negative lift increases on the tail, forcing it downward and placing the aircraft again in level flight attitude. called positive stability. Many modern airplanes will practically fly themselves; they will with the controls released. Fig. 167. When the airplane is disturbed directionally it will assume the new course, but it will not continue to turn because the vertical airfoil behind the center of gravity is greater than that ahead of the center of gravity'. recover from a dive or climb, a bank or a turn The stability of an airplane is accomplished by proper design features. These are the features built into the aircraft which give it stability. When a wang is dropped, it automatically comes back up again because there is an additional lift on the low wdng. When the airplane is disturbed longitudinally, i.e., when the nose is pulled up, the airplane loses speed, and the tail comes up, thus placing the aircraft again in levelflight attitude. When the airplane is disturbed directionally, it will assume the new course but not continue to turn. This is due to the fact that there is a greater amount of fin area back of the center of gravity than in front of it.
AERODYNAMICS 375 An Airplane Practically Flies Itself. It is said that a well-trained pilot is four times as safe flying in an airplane as he would be in driving an automobile but that a poorly trained pilot is four times as safe in an automobile as he would be in an airplane. The student pilot has a tendency to use brute strength and violent manipulation of the controls when an airplane goes into an unexpected motion, and in his excitement he may do the wrong thing. A safe rule is to leave the controls alone and let the airplane return to its normal position by itself, provided that there is sufficient altitude. The majority of airplane accidents are caused by flying so close to the ground that there is not time for the airplane to return to a normal position if something unexpected happens. A poor automobile driver is constantly turning the steering wheel back and forth and manipulating the brakes; a poor pilot likewise trys to do too much of the work which the airplane is designed to do for him. An airplane wall not fall just because the engine fails. Gliders often remain in the air for many hours, and the heavier airplanes can glide for some time in search of a landing field, provided that the original altitude above the ground was high enough. There are no such things as air pockets, and up- and down-currents of air are not serious except when flying too close to the ground or when the air currents are very powerful, as in thunderstorms or in mountainous country. Bumpy air does no more harm to an airplane than a rough sea does to a ship. 1. What is an airplane? STUDY QUESTIONS 2. What causes the dynamic action of the air upon an airplane? 3. How is the effect of propeller torque counteracted in the rigging (adjustment) of an airplane? 4. How could left-wing heaviness be corrected? 5. Why is the vertical fin often offset slightly? 6. What are the probable causes of nose-heaviness, and how may nose-heaviness be corrected? 7. What is parasite resistance? 8. Of what use is an adjustable stabilizer? What substitutes for adjustable stabilizer do some airplanes have? 9. What is an airfoil? 10. List the airfoils of an airplane and mention the useful purpose of each. 11. What are the four forces which act on an airfoil? 12. What is the angle of attack? 13. In flying out of a small field, which would be preferred, a small pitch or a large pitch for the propeller?