Four Forces of Flight Lift Weight Thrust Drag Source NASA Glen Research Center Learning Technologies http://www.grc.nasa.gov/WWW/K-12/airplane/forces.html A force may be thought of as a push or pull in a specific direction. This slide shows the forces that act on an airplane in flight. The motion of the airplane through the air depends on the relative strength and direction of the forces shown above. If the forces are balanced, the aircraft cruises at constant velocity. If the forces are unbalanced, the aircraft accelerates in the direction of the largest force. Source: NASA Glen Research Center

Two of the four forces on an airplane Standards in this lesson: Physics-motion and forces-Newton’s laws When forces are balanced no acceleration occurs; thus an object continues to move at a constant speed or stays at rest (Newton's 1st Law). When one object exerts a force on a second object, the second object always exerts a force of equal magnitude and opposite direction (Newton's 3rd Law). The relationship between the universal law of gravitation and the effect of gravity on an object at the surface of the earth Lift and Drag Two of the four forces on an airplane General Concept: Just like in the first page of this manual, an airplane flies by balancing forces. If the trust of the airplane’s engines is greater than the force of the wind (drag force), the plane will go forward. Likewise, if the lifting force of the wings is greater than the weight of the plane (the force of gravity), the plane will rise in the air. Therefore, to make the airplane work, one must have sufficiently powerful engines to push the plane through the air, and have designed wings to lift the plane in the air. Once in the air, airplanes maneuver by moving control surfaces which provide torques about the center of mass of the airplane. These torques will rotate the aircraft in the appropriate direction.

Lift Lift is an aerodynamic force Lift must exceed weight for flight Generated by motion of aircraft through air Created by the effects of airflow past wing The top of an airplane wing is curved, so that air has to travel further to reach the trailing end of the wing. Therefore, the air on the underside of the plane is exerting more pressure, causing the plane to lift.

Weight Weight is not constant varies with passengers, cargo, fuel load decreases as fuel is consumed or payload off-loaded Direction is constant toward earth’s center May be assumed concentrated at the center of gravity

Drag An aerodynamic force Drag is also called air resistance. the force that pulls the plane backward. Two broad drag classifications Parasite drag: drag created by airplane design Induced drag: by-product of lift generation Acts through the center of pressure

Thrust Forward-acting force opposes drag Direction of thrust depends on design Propulsion systems produce thrust Equal to drag in straight, constant speed flight

Stability Center of Gravity Longitudinal Stability Lateral Stability The center of gravity is the average location of the weight of an object Longitudinal Stability The stability of an aircraft in the longitudinal, or pitching, plane under steady-flight conditions Lateral Stability Stability about the airplane's longitudinal axis, which extends form nose to tail, is called lateral stability Directional Stability Directional stability is stability of a moving body or vehicle about an axis which is perpendicular to its direction of motion.

How an Air Craft Moves 3 Axes of movement Roll: Ailerons Controls banking motions of aircraft Pitch: Elevator Controls angle of attack (up, down movement) Yaw: Rudder Moves plane laterally When yawing, outside wing has greater lift, causes turn