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Physical Laws Effecting Aerodynamics


 

LEARNING OBJECTIVE:
Identify the physical laws of aerodynamics to include Newton's laws of motion and the Bernoulli principle.
Aerodynamics is the study of the forces that let an aircraft fly. You should carefully study the principles covered here. Whether your job is to fly the aircraft and/or to maintain it, you should know why and how an aircraft flies. Knowing why and how lets you carry out your duties more effectively.

LAWS OF MOTION
Motion is the act or process of changing place or position. Simply put, motion is movement. An object may be in motion in relation to one object and motionless in relation to another. For example, a person sitting in an aircraft flying at 200 mph is at rest or motionless in relation to the aircraft. However, the person is in motion in relation to the air or the earth. Air has no force or power other than pressure when it's motionless. When air is moving, its force becomes apparent. A moving object in motionless air has a force exerted on it as a result of its own motion. It makes no difference in the effect whether an object is moving in relation to the air or the air is moving in relation to the object. The following information explains some basic laws of motion.

Newton's First Law of Motion
According to Newton's first law of motion (inertia), an object at rest will remain at rest, or an object in motion will continue in motion at the same speed and in the same direction, until an outside force acts on it. For an aircraft to taxi or fly, a force must be applied to it. It would remain at rest without an outside force. Once the aircraft is moving, another force must act on it to bring it to a stop. It would continue in motion without an outside force. This willingness of an object to remain at rest or to continue in motion is referred to as inertia.

Newton's Second Law of Motion
The second law of motion (force) states that if a object moving with uniform speed is acted upon by an external force, the change of motion (acceleration) will be directly proportional to the amount of force and inversely proportional to the mass of the object being moved. The motion will take place in the direction in which the force acts. Simply stated, this means that an object being pushed by 10 pounds of force will travel faster than it would if it were pushed by 5 pounds of force. A heavier object will accelerate more slowly than a lighter object when an equal force is applied.

Newton's Third Law of Motion
The third law of motion (action and reaction) states that for every action (force) there is an equal and opposite reaction (force). This law can be demonstrated with a balloon. If you inflate a balloon with air and release it without securing the neck, as the air is expelled the balloon moves in the opposite direction of the air rushing out of it. Figure 3-1 shows this law of motion.

BERNOULLI'S PRINCIPLE
Bernoulli's principle (fig. 3-2) states that when afluid flowing through a tube reaches a constriction ornarrowing of the tube, the speed of the fluid passingthrough the constriction is increased and its pressure isdecreased.

AIRFOIL TERMINOLOGY
The shape of an airfoil and its relationship to theairstream are important. The following are commonterms that you should understand before you learnabout airfoils. Leading edge The front edge or surface of theairfoil (fig. 3-3). Trailing edge The rear edge or surface of theairfoil (fig. 3-3). Chord line An imaginary straight line fromthe leading edge to the trailingedge of an airfoil (fig. 3-3). Camber The curve or departure from astraight line (chord line) from theleading to the trailing edge of theairfoil (fig. 3-3). Relative wind The direction of the airstream inrelation to the airfoil (fig. 3-4). Angle of attack The angle between the chord lineand the relative wind (fig. 3-4).

AIRFLOW AROUND AN AIRFOIL
The generation of lift by an airfoil depends on theairfoil's being able to create a special airflow in theairstream. This airflow develops the lifting pressureover the airfoil surface. The effect is shown in figure3-5, which shows the relationship between lift andBernoulli's principle. As the relative wind strikes theleading edge of the airfoil, the flow of air is split. Aportion of the relative wind is deflected upward and aft,and the rest is deflected downward and aft. Since the upper surface of the airfoil has camber to it, the flowover its surface is disrupted. This disruption causes awavelike effect to the airflow. The lower surface of theairfoil is relatively flat. The airflow across its surfaceisn't disrupted. Lift is accomplished by this differencein the airflow across the airfoil.The shaded area of figure 3-5 shows a l ow-pressurearea on the airfoil's upper surface. This low-pressurearea is caused by the air that is disrupted by the camberof the airfoil, and it is the key t o lift. There is lesspressure on the top surface of the airfoil than there is onthe lower surface. The air pressure pushes upward o thelower surface. This difference in pressure causes theairfoil to rise. Now, you know that lift is developed by the difference between the air pressure on the upperand lower surfaces of the airfoil. As long as there is less pressure on the upper surface and more pressure onthe lower surface of an airfoil, an aircraft has lift. Lift isone of the forces affecting flight.

FORCES AFFECTING FLIGHTLEARNING OBJECTIVE:
Recognize thefour primary forces acting on an aircraft.

An aircraft in flight is in the center of a continuousbattle of forces. The conflict of these forces is the key t oall maneuvers performed in the air. There is nothingmysterious about these forces—they are definite andknown. The direction in which each acts can becalculated. The aircraft is designed to take advantage ofeach force. These forces are lift, weight, thrust, and drag.

LIFT
Lift is the force that acts in an upward direction tosupport the aircraft in the air. I t counteracts the effectsof weight. Lift must be greater than or equal to weight ifflight is to be sustained.

WEIGHT
Weight is the force of gravity acting downward onthe aircraft and everything in the aircraft, such as crew,fuel, and cargo.

THRUST
Thrust is the force developed by the aircraft'sengine. It acts in the forward direction. Thrust must begreater than or equal to the effects of drag for flight tobegin or to be sustained.

DRAG
Drag is the force that tends to hold an aircraft back.Drag is caused by the disruption of the airflow about thewings, fuselage (body), and all protruding objects onthe aircraft. Drag resists motion as it acts parallel and inthe opposite direction in relation to the relative wind.Figure 3-6 shows the direction in which each of theseforces acts in relation to an aircraft.Up to this point, you have learned the physical lawsof aerodynamics, airfoils, and the forces affectingflight. To fully understand flight, you must learn aboutthe rotational axes of an aircraft.

ROTATIONAL AXES LEARNING OBJECTIVE: Identify thethree axes of rotation and the terms relative to the aircraft's rotation about these axes.

LONGITUDINAL AXIS
The longitudinal axis is the pivot point about whichan aircraft rolls. The movement associated with roll isbest described as the movement of the wing tips (one upand the other down). Figure 3-7 shows this movement.This axis runs fore and aft through the length (nose totail) of the aircraft. This axis is parallel to the primarydirection of the aircraft. The primary direction of afixed-wing aircraft is always forward. Figure 3-8 showsthe longitudinal axis.

LATERAL AXIS
The lateral axis is the pivot point about which theaircraft pitches. Pitch can best be described as the upand down motion of the nose of the aircraft. Figure 3-7shows this movement. The pitch axis runs from the leftto the right of the aircraft (wing tip to wing tip). It isperpendicular to and intersects the roll axis. Figure 3-8shows the pitch axis and its relationship to the roll axis.

VERTICAL AXIS
The vertical axis runs from the top to the bottom ofan aircraft. It runs perpendicular to both the roll andpitch axes. The movement associated with this axis isyaw. Yaw is best described as the change in aircraftheading to the right or left of the primary direction of anaircraft. Figure 3-7 shows this movement. Assume youare walking from your work space to an aircraft located100 feet away. You are trying to walk there in a straightline but are unable to do so because there is a strongwind blowing you off course to your right. This movement to the right is yaw. The yaw axis is shown infigure 3-8.