Turning Flight
Before a glider turns, it must first overcome inertia, or its tendency to continue in a straight line. A pilot creates the necessary turning force by using the ailerons to bank the glider so that the direction of total lift is inclined. This divides the force of lift into two components; one component acts vertically to oppose weight, while the other acts horizontally to oppose centrifugal force. The latter is the horizontal component of lift. [Figure 3-25]
To maintain attitude with the horizon during a turn, glider pilots need to increase back pressure on the control stick. The horizontal component of lift creates a force directed inward toward the center of rotation, which is known as centripetal force. [Figure 3-26] This center-seeking force causes the glider to turn. Since centripetal force works against the tendency of the aircraft to continue in a straight line, inertia tends to oppose centripetal force toward the outside of the turn. This opposing force is known as centrifugal force. In reality, centrifugal force is not a true aerodynamic force; it is an apparent force that results from the effect of inertia during the turn.
Load Factors
The preceding sections only briefly considered some of the practical points of the principles of turning flight. However, with the responsibilities of the pilot and the safety of passengers, the competent pilot must have a well-founded concept of the forces that act on the glider during turning flight and the advantageous use of these forces, as well as the operating limitations of the particular glider. Any force applied to a glider to deflect its flight from a straight line produces a stress on its structure; the amount of this force is called load factor.
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A load factor of three means that the total load on a glider’s structure is three times its gross weight. Gravity load factors are usually expressed in terms of “G”—that is, a load factor of three may be spoken of as three Gs, or a load factor of four as four Gs. A load factor of one, or 1 G, represents conditions in straight-and-level flight, in which the lift is equal to the weight. Therefore, two Gs would be two times the normal weight. Gliders may be designed to withstand stress of up to nine Gs.
It is interesting to note that in subjecting a glider to three Gs in a pullup from a dive, the pilot is pressed down into the seat with a force equal to three times the person’s weight. Thus, an idea of the magnitude of the load factor obtained in any maneuver can be determined by considering the degree to which the pilot is pressed down into the seat. Since the operating speed of modern gliders has increased significantly, this effect has become so pronounced that it is a primary consideration in the design of the structure for all gliders.
If attempting to improve turn performance by increasing angle of bank while maintaining airspeed, pay close attention to glider limitations due to the effects of increasing the load factor. Load factor is defined as the ratio of the load supported by the glider’s wings to the actual weight of the aircraft and its contents. A glider in stabilized, wings-level flight has a load factor of one. Load factor increases rapidly as the angle of bank increases due to increase wing loading. [Figure 3-27] With the structural design of gliders planned to withstand only a certain amount of overload, knowledge of load factors has become essential for all pilots. Load factors are important to the pilot for two distinct reasons:
- It is possible for a pilot to impose an obviously dangerous overload on the glider structures.
- Increased load factor increases the stalling speed, making stalls possible at seemingly safe flight speeds due to increased wing loading.
In a turn at constant speed, the AOA must be increased to furnish the extra lift necessary to overcome the centrifugal force and inertia opposing the turn. As the bank angle increases, AOA must also increase to provide the required lift. The result of increasing the AOA is a stall when the critical AOA is exceeded in a turn. [Figure 3-28]
Rate of Turn
Rate of turn refers to the amount of time it takes for a glider to turn a specified number of degrees. If flown at the same airspeed and angle of bank, every glider turns at the same rate. If airspeed increases and the angle of bank remains the same, the rate of turn decreases. Conversely, a constant airspeed coupled with an angle of bank increase results in a higher rate of turn.
Radius of Turn
The amount of horizontal distance an aircraft uses to complete a turn is referred to as the radius of turn. The radius of turn at any given bank angle varies directly with the square of the airspeed. Therefore, if the airspeed of the glider were doubled, the radius of the turn would be four times greater. Although the radius of turn is also dependent on a glider’s airspeed and angle of bank, the relationship is the opposite of rate of turn. As the glider’s airspeed is increased with the angle of bank held constant, the radius of turn increases. On the other hand, if the angle of bank increases and the airspeed remains the same, the radius of turn is decreased. [Figure 3-29] When flying in thermals, the radius of turn is an important factor as it helps to gain the maximum altitude. A smaller turn radius enables a glider to fly closer to the fastest rising core of the thermal and gain altitude more quickly.
Turn Coordination
It is important that rudder and aileron inputs are coordinated during a turn so maximum glider performance can be maintained. If too little rudder is applied, or if rudder is applied too late, the result is a slip. Too much rudder, or rudder applied before aileron, results in a skid. Both skids and slips swing the fuselage of the glider into the relative wind, creating additional parasite drag, which reduces lift and airspeed. Although this increased drag caused by a slip can be useful during approach to landing to steepen the approach path and counteract a crosswind, it decreases glider performance during other phases of flight.
When rolling into a turn, the aileron on the inside of the turn is raised and the aileron on the outside of the turn is lowered. The lowered aileron on the outside wing increases lift by increasing wing camber and produces more lift for that wing. Since induced drag is a byproduct of lift, the outside wing also produces more drag than the inside wing. This causes adverse yaw, a yawing tendency toward the outside of the turn. Coordinated use of rudder and aileron corrects for adverse yaw and aileron drag. Adverse yaw in gliders can be more pronounced due to the much longer wings as compared to an airplane of equal weight. The longer wings constitute longer lever arms for the adverse yaw forces to act on the glider. Therefore, more rudder movement is necessary to counteract the adverse yaw and have a coordinated turn.
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