Gold Seal Online Ground School

Transcription

1 Aerodynamics Lesson Review Axes, Motion, and Control Surfaces Gold Seal Online Ground School An airplane is controlled around three axes: the longitudinal axis, the lateral axis, and the vertical axis. These axes intersect at the center of gravity, frequently referred to as the CG. The motion about the longitudinal axis is called roll and is controlled by the ailerons. The motion about the lateral axis is called pitch and is controlled by the elevator. The motion about the vertical axis is called yaw and is controlled by the rudder. In most newer airplanes, other moveable control surfaces exist on the trailing edges of the wings. These are called flaps. The main purpose of flaps is to allow a steeper approach to a landing without increasing airspeed.

2 The Four Forces of Flight The four forces of flight are lift, weight, thrust and drag. These forces are in equilibrium in unaccelerated flight. That is, lift equals weight and thrust equals drag. The Production of Lift A wing is an airfoil. In cross section, we identify its chord and camber. The chord is an imaginary line from the leading edge to the trailing edge. The camber is the curved shape. The relative wind is the airflow that is experienced by the wing. It is exactly opposite of the wing s line of travel. The angle formed by the relative wind and the chord line is the angle of attack. Lift is the upward force created by a wing that allows an airplane to fly. Lift is the result of several processes. Newer explanations from NASA differ from the traditional view of lift. This disagreement centers primarily on the reason that airflow is accelerated over a wing s upper surface. Regardless of the reason why, airflow is accelerated above the wing as it slices through the air. Lift requires a smooth attached airflow over and under a wing. If the airflow becomes disrupted or turbulent, a loss of lift occurs. 2

3 Bernoulli s Principle states that as the speed of a fluid is increased, its internal pressure decreases. Thus, when air is accelerated above a wing, a relative low pressure area is created. This is one of the primary causes of lift. The Aerodynamics of Turns Although the rudder yaws the nose of the airplane left and right, yaw is not the primary mechanism for turning a flying airplane. In order to understand this fully, an understanding of the term vector is needed. Vectors are used to describe amounts of forces applied in specific directions. Drawn as arrows, the lengths represent the magnitude of the forces. The lift vector points perpendicularly upward from the wing. The magnitude of lift can be controlled by the pilot. Lift can be increased (up to a point) by increasing the angle of attack. It can also be increased by increasing the amount of airflow over the wing (i.e. by increasing the airspeed). The gravity (weight) vector, however, is a constant. It always points downward toward the center of the earth with a force that we call one G. One G is one gravity unit. 3

4 In these two pictures we see the relationships of the lift and gravity vectors in level flight as well as in banked flight. In level flight, the lift vector fully opposes the gravity vector. In a bank, some portion of the lift vector s force is diverted into the horizontal. This redirection of the lift vector into the horizontal is what causes an airplane to turn. Airplanes turn as a result of the horizontal component of lift. The sideways lift force is what pulls an airplane into a turn. The rudder is used to coordinate the turn so that the tail follows the nose. In the vertical, a loss of force is seen. The lift vector no longer fully opposes the gravity vector. Back pressure on the yoke or stick is required to increase the angle of attack and, thus, the lift vector. Without this additional back elevator pressure, the airplane will fly in a descending, spiraling turn. When the gravity vector exceeds the lift vector, a descent results. 4

5 Left Turning Tendencies Propeller driven airplanes have a built-in tendency to turn away from their line of flight. In most airplanes, the propeller rotates to the right (as seen from inside the cockpit). The turning tendency in these airplanes is to the left. Left turning tendency is most noticeable when the airplane is experiencing a high angle of attack, high power, and a low airspeed. Slow flight is an example of a flight condition that maximizes left turning tendency. Pilots user rudder and aileron to counter left turning tendency. There are four reasons for left turning tendency: 1 Torque: As the propeller rotates to the right, an equal and opposite reaction is imparted onto the airplane. This causes the airplane to roll to the left. 2 Spiraling Slipstream: The spinning propeller produces a spiraling flow of air around the fuselage. This spiraling airflow impacts the vertical stabilizer on the left side, pushing the tail to the right. As the tail goes right, the nose yaws to the left. 3 Asymmetric Propeller Loading (P-factor): In conditions of high angle of attack, the downward moving blade on the right grabs more air than the upward moving blade on the left. Thus, more thrust is generated by the right side of the propeller. This asymmetric thrust yaws the airplane to the left. 4 Gyroscopic Precession: This factor is primarily related to tailwheel airplanes in the takeoff phase. As the tail rises, a force 90 degrees to the angle of travel is imparted. This yaws the nose to the left. 5

6 Load Factor In banked turns, passengers frequently sense an increase in their body weight. This is due to inertia and opposing forces. The airplane s structure also experiences increased weight. As was noted during the discussion of turns, some of the vertical lift component is lost in a bank. Unless replaced, a spiral descent results. The vertical component of lift is increased by increasing total lift. Back pressure on the yoke or stick causes an increase in angle of attack. Increased angle of attack causes an increase in total lift. As total lift increases, the vertical component of lift increases. Therefore, to maintain altitude in a banked turn, we must increase load factor. In a 60 degree banked level turn, a load factor of 2 is required. 6

7 This chart is used to calculate load factor based on bank angle in level flight. The curve starts at 1 G and climbs to infinity. The example of a 60 degree bank is shown here. Straight up from the 60, the load factor curve is intercepted at the 2 G height. At 70 degrees of bank, the load factor is clearly seen to be approximately 3 Gs. At 90 degrees of bank, there is no vertical component of lift at all. Thus, an infinite amount of back pressure will not allow the airplane to maintain its altitude. Ground Effect When an airplane flies very close to the surface, it experiences a cushioning effect. This is due to interference (by the ground) of the normal airflow patterns about a wing. It is said that ground effect alters the spanwise lift distribution of the wing. Ground effect affects an airplane when it is within one wingspan of the surface and is most noticeable when the airplane is within a half-wingspan of the surface. An airplane in ground effect can remain in flight at a slower airspeed and at a lower angle of attack than can an airplane flying outside (above) ground effect. An airplane landing with excess airspeed frequently experiences floating during landing due to ground effect. 7