Forces Forces Acting on the Airplane

 While the airplane is propelled through the air and sufficient lift is developed to sustain it in flight, there are certain other forces acting at the same time (Fig. 3-4). Every particle of matter, including an airplane, is attracted downward toward the center of the earth by gravitational force. The amount of this force on the airplane is measured in terms of weight. To keep the airplane flying, lift must overcome the weight or gravitational force. The development of lift and thrust was explained earlier. Another force that constantly acts on the airplane is called drag. It is the resistance created by the air particles striking and flowing around the airplane when it is moving through the air. Airplane designers are constantly trying to streamline wings, fuselages, and other parts to reduce the rearward force of drag as much as possible. The part of drag caused by form resistance and skin friction is termed parasite drag since it contributes nothing to the lift force.

A second part of the total drag force is caused by the wing's lift. As the wing deflects air downward to produce lift, the total lift force is not exactly vertical, but is tilted slightly rearward. This means that it causes some rearward drag force. This drag is called induced drag, and is the price paid to produce lift. The larger the angle of attack, the more the lift force on the wing tilts toward the rear and the larger the induced drag becomes. To give the airplane forward motion, the thrust must overcome drag.

In a steady flight condition (no change in speed or flightpath), the always present forces that oppose each other are also equal to each other. That is, lift equals weight, and thrust equals drag.
 Another force which frequently acts on the airplane is centrifugal force. However, this force occurs only when the airplane is turning or changing the direction (horizontally or vertically) of the flightpath. Newton's law of energy states that "a body at rest tends to remain at rest, and a body in motion tends to remain moving at the same speed and in the same direction." Thus, to make an airplane turn from straight flight, a sideward/inward force must act on it (Fig. 3-5). The tendency of the airplane to keep moving in a straight line and outward from a turn is the result of inertia and it results in centrifugal force. Therefore, some impeding force is needed to overcome this centrifugal force so the airplane can move in the desired direction. The lift of the wings provides this counteracting force when the airplane's wings are banked in the desired direction. This is further discussed in this chapter in the section on Turning Flight. Since the airplane is in a banked attitude during a properly executed turn, the pilot will feel the centrifugal force by increased seat pressure, rather than the feeling of being forced to the side as is experienced in a rapidly turning automobile. The amount of
force (G force) felt by seat pressure depends on the rate of turn. The pilot will, however, be forced to the side of the airplane (as in an automobile) if a turn is improperly made or the airplane is made to slip or skid.

One other force which will affect the airplane during certain conditions of flight, and which will be frequently referred to in the discussions on various flight maneuvers, is torque effect or left turning tendency. It is probably one of the least understood forces that affect an airplane.

 Torque effect is the force which causes the airplane to have a tendency to swerve (yaw) to the left, and is created by the engine and propeller. There are four factors which contribute to this yawing tendency; (1) torque reaction of the engine and propeller, (2) the propeller's gyroscopic effect, (3) the corkscrewing effect of the propeller slipstream, and (4) the asymmetrical loading of the propeller. It is important that pilots understand why these factors contribute to torque effect.    One of Newton's laws states, "for every action there is an equal and opposite reaction." Hence, the rotation of the propeller, with a clockwise movement (as viewed from the cockpit), tends to roll or bank the airplane in a counterclockwise (to the left) direction (Fig. 3-6). This can be understood by visualizing a rubber band powered model airplane. Wind the rubber band in a manner that it will unwind and rotate
the propeller in a clockwise direction. If the fuselage is released while the propeller is held the fuselage will rotate in a counterclockwise direction (looking from the rear). This effect of torque reaction is the same in a real propeller driven airplane except that instead of the propeller being held by hand, its rotation is resisted by air.

This counter rotational force causes the airplane to try to roll to the left. It will be noted in the case of a real airplane that the force is stronger when power is significantly advanced while the airplane is flying at very slow airspeed.

 The second factor that causes the tendency of an airplane to yaw to the left is the gyroscopic properties of the propeller. Here, we are concerned with gyroscopic precession which is the resultant action or deflection of a spinning object when a force is applied to the outer rim of its rotational mass. When a force is applied to the object's axis, it is the same as applying the force to the outer rim. If the axis of a spinning gyroscope (propeller in this case) is tilted, the resulting force will be exerted 90 degrees ahead in the direction of rotation and in the same direction as the applied force (Fig. 3-7). That force will be particularly noticeable during takeoff in a tailwheel type airplane if the tail is rapidly raised from a three point to a level flight attitude. The abrupt change of attitude tilts the horizontal axis of the propeller, and the resulting precession produces a forward force on the right side (90 degrees ahead in the direction of rotation), yawing the airplane's nose to the left. The amount of force created by this precession is directly related to the rate at which the propeller axis is tilted when the tail is raised. The third factor that causes the airplane's left yawing tendency is the corkscrewing of the propeller slipstream, acting against the side of the fuselage and tail surfaces (Fig. 3-6). The high speed rotation of an airplane propeller results in a corkscrewing rotation to the slipstream as it moves rearward. At high propeller speeds and low forward speed,
as in the initial part of a takeoff, the corkscrewing flow is compact and imposes considerable side forces on the airplane. As the airplane's forward speed increases, the corkscrew motion of the slipstream loosens or elongates, resulting in a straighter flow of air along the side of the fuselage toward the airplane's tail.

When this corkscrewing slipstream strikes the side of the fuselage and the vertical tail surface at airspeeds less than cruising, it produces a yawing movement which tends to revolve the airplane around its vertical axis. Since in most U.S. built airplanes propeller rotation is clockwise as viewed from the cockpit, the slipstream strikes the vertical tail surface on the left side, thus pushing the tail to the right and yawing the nose of the airplane to the left.

The fourth factor which causes the left yawing tendency is the asymmetrical loading of the propeller, frequently referred to as P-factor (Fig. 3-6). When an airplane is flying with a high angle of attack (with the propeller axis inclined), the bite of the downward moving propeller blade is greater than the bite of the upward moving blade. This is due to the downward moving blade meeting the oncoming relative wind at a greater angle of attack than the upward moving blade. Consequently, there is greater thrust on the downward moving blade on the right side and this force causes the airplane to yaw to the left.

At low speeds the yawing tendency caused by P-factor is greater because the airplane is at a high angle of attack. Conversely, as the speed of the airplane is increased and the airplane's angle of attack is reduced, the asymmetrical loading decreases and the turning tendency is decreased.