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71 EAGLE RAF - AIR COMBAT TACTICS PAGE 7.

AIR PHYSICS

THE FOUR FORCES OF FLIGHT: When the four forces balance each other (Lift = Weight and Thrust = Drag) the airplane is said to be in a state of equilibrium. If any of the forces are unbalanced the aircraft will climb, descend, accelerate, or decelerate. The four forces are listed below, each with a description.

LIFT is created mainly by the wings. The air under the wing gets delayed, while the air above the wing increases with speed. Bernoulli's Theorem states that if a fluid increases in speed its pressure decreases. Air accentually is a fluid. The delayed airflow below the wing is much higher in pressure then the low pressure above, and gives us lift. Generally as airspeed increases, so does lift. As Angle of Attack (AOA), increases, the air below the wing becomes even more delayed, the air above increases in speed, and downwash increases, therefore total lift increases. As AOA increases to a certain point the Laminar or smooth airflow above the wing becomes more and more turbulent and the airplane starts to mush as there is little or no smooth air flowing over the control surfaces. This increases to a point which is called the Critical Angle of Attack. Lift is at its peak at this point, and further increase of AOA decreases lift as the airplane stalls. AOA is the angle in which the wing meets the Relative Wind which is parallel to the direction of flight (Figure 1.1). Lift opposes weight.

GRAVITY is the natural attraction of masses. Larger the mass greater the pull. The earths acceleration of gravity is 32.2ft/sec·. This means that a falling object will accelerate at a rate of 32.2ft per second, per second in a vacuum. Drag can slow this acceleration though, and the point where weight equals drag is called Terminal Velocity and the falling object remains at a constant speed. Gravity is perpendicular to the horizon. Gravity opposes Lift. See above.

THRUST is power from the engine. It acts through a line called the Thrust Datum and is controlled by the throttle and propeller controls. It opposes drag.

DRAG is the natural resistance of the air. As speed increases so does drag. There are two main forms of drag. Drag: rate of energy loss from the resistance of air. It gets stirred up, and heated slightly, and this energy disperses in the air until it becomes immeasurable. As an aeroplane's airspeed increases, so does Parasite Drag. Parasite Drag is created from parts other then the lifting surfaces (Wings, Ailerons etc.). It is reduced by streamlining, making surfaces smooth, Parasite Drag is usually minimized at slow speeds, and increases with speed. Induced Drag is the drag created by the wings. As you increase the angle in which the wing meets the airflow, called Angle Of Attack, lift increases (load factor or "G") but so does drag. Drag is said to be the price we pay for lift. In level flight, Induced Drag is minimized at high speeds, and increases as speed decreases. Altitude is more valued then speed, because of the sharp increase in Parasite Drag at high speeds, but with a manoeuvring aeroplane, both major forms of drag can be high. For example, and aeroplane at high speed pulling high G.

ENERGY. What is energy? Basically, to a fighter pilot, energy is life! Energy, for a fighter pilot is speed, altitude, and fuel. As an aeroplane climbs, it gains altitude, and if airspeed is constant, that energy is coming from Power: the rate of doing work or rate of energy addition to the system. As an aeroplane accelerates in level flight, it gains airspeed or energy and if it decelerates looses energy. Altitude and Airspeed are related. One can be transferred into the other. If you loose airspeed, you probably gain altitude; loose altitude you gain airspeed but total energy is the same. Controlling both altitude and airspeed is done using both the elevator, and throttle control, usually both together. We loose energy to drag.

Throughout the story of air fighting runs the quest for height, for the fighter on top had control of the air battle." Air Vice Marshal J. E. "Johnnie" Johnston, RAF

With this in mind, it goes to show why the fighter pilot valued speed and altitude. Altitude energy has the potential to be transferred into speed energy, and vice-a-versa. Beware of the Hun in the Sun!

STALL. Many people believe that an airplane only stalls at a certain speed, and that when it stalls the airplane falls out of the sky. This is far from the truth. An airplane can stall at any attitude and any speed, as long as the Critical Angle of Attack is exceeded. Angle of Attack (AOA) as talked about above, is the angle in which the wing meets the relative airflow. As AOA increases both lift and drag increase until drag exceeds lift and lift drops off. Any further increase in AOA, and the airplane becomes more stalled drag increases and lift decreases. The airplane does not suddenly fall out of the sky like many may believe.There are two types of airflow over a wing Laminar (smooth) and Turbulent. They are separated by a transition point or Separation Point. This Separation Point, moves forward as angle of attack increases giving us less and less Laminar airflow over the wing which is needed to produce lift. As the turbulent airflow moves forward, we get the control problems and buffeting most people associate with a stall because of the circular motion of Eddies. At slower and slower speeds at level flight, a greater AOA is needed to create enough lift to support the airplane. At a certain speed, called the Stalling speed or Vsl, the airplane in level flight will stall as the pilot increases AOA to maintain level flight. Most airplanes do not have AOA indicators so the only reference is the Airspeed Indicator (ASI) but one must remember that the airplane can stall at any speed as long as the Critical AOA is exceeded. This is very important to a fighter pilot because of the rapid control movements and maximum performance that is needed for certain manoeuvres. Stalling AOA is usually around 20° depending on aircraft design.As an airplane turns it offsets its lift to the side, using lift to turn in around the corner. Greater the bank, the more lift is required to maintain level flight and to kick the aircraft around the turn. Therefore the pilot increases AOA. A fighter pilot may even stall the aircraft in a turn to get maximum lift, and maximum turn performance. Since drag also increases, the airplane will slow down to a point, in a Sustained Turn.

know and use all the capabilities in your airplane. If you don't, sooner or later, some guy who does use them all will kick your ass!" Lieutenant Dave "Preacher" Pace, USN