An aircraft experiences four major forces; thrust produced by the engine, drag produced by air resistance induced by the design, weight from mass and lift by the force produced by the wing. In straight and level flight, lift is equal to weight and thrust is equal to drag(Grc.nasa.gov, 2015)
The wings provide the majority of the lift force required to overcome the weight of the aircraft during all phases of flight. According to the first law, for the plane to climb, the magnitude of an external lift force must be greater than the force of weight.(Grc.nasa.gov, 2015) The primary portion of this ‘outside’ force of lift is produced by the wings, which are also used for fuel storage. The design of the wing is determined by the target mission of the aircraft; desired speed, size, range and service ceiling. As the aircraft travels through the air, the airflow over the wings produces a lift force. When the magnitude of lift exceeds the magnitude of weight, the aircraft climbs. (Pokap? Akoranga P?taiao, 2011). The engines propel the aircraft forward, allowing the wings to move it upward. Once the plane is in the sky the four forces balance, so the plane reaches terminal velocity and maintains its speed unless acted on by an outside force ( first law).
According to Newton’s second law, the force (F) required to accelerate an airplane in any direction is equal to the product of its mass (m) and the acceleration (a), ( F= ma) (Grc.nasa.gov, 2015) To produce lift the aircraft must have some acceleration, since they are proportional. Recent development of aircraft has seen a large emphasis placed on material selection and development. Historically, commercial aircraft were manufactured from aluminium and titanium alloys, but more recently, composite materials have been employed to reduce weight and increase efficiency. Since mass is constant, to increase the acceleration, force must be increased, so the lighter an aircraft the less force required to push it, a smaller force will require less fuel, allowing more efficient flight. Reinforced plastic (fibre reinforced polymer) has a high strength-to-weight ratio and does not corrode. It is clear that a knowledge of Newton’s second law has allowed aircraft designers to develop more efficient aircraft that will reduce the impact of the aviation industry on the environment. By making the aircraft and wings lighter, a lower force is required for flight, resulting in lower fuel burn, lower emissions, longer range and quieter engines.
Newton’s third law of motion is evidenced by the flight control surfaces and ancillary devices contained on the wing of the aircraft. For each action by the ancillary devices, there is an equal and opposite reaction. Consider the ailerons, when deflected above or below the wing chord, cause the aircraft to roll. By lowering the aileron into the airflow below the wing, it results in an increased lift force at the wing tip, deflecting the wing upwards. This creates a rolling moment and when used in conjunction with the rudder, allows the aircraft to make a turn. As the aircraft travels faster through the air, less deflection of the ailerons is required in order to produce an adequate rolling moment.
Similarly, modern aircraft employ leading and trailing edge flap and slat devices to alter the shape of the wing. This proves advantageous when flying at low speeds, descending and climbing. By creating more induced drag through altering the shape of the wing, more power is required to maintain the same speed and altitude. Looking at it the other way, if power remains constant, this results in the aircraft slowing down, if speed is held constant by the pilot, the aircraft will descend. Utilising Newton’s third law is how each and every aircraft lands today. Pilots utilise their ability to alter the characteristics of the wing to safely fly at low speed when approaching the runway to land.
Spoilers are found on the upper surface of the wing and are utilised to significantly reduce the lift force acting on it. By impeding the airflow on the upper surface of the wing, it reduces the lift, ensuring the weight of the aircraft is the dominant force acting in the vertical axis. By raising the spoilers, the opposite reaction is for lift to decrease and the aircraft to descend. This is useful for pilots who need to descend the aircraft quickly or to prevent the aircraft climbing when it has landed on the ground.