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Forces on an Aircraft

The Forces acting on an Aircraft Include lift, weight, thrust and Drag

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These Forces act on the aircraft during flight and change throughout the flight.



Lift:

Lift acts vertically through the centre of pressure on the aircraft wing. Lift acts and opposite to weight and allows the aircraft to remain inflight. If lift is reduced and weight is larger than the aircraft will go down. If lift is larger than weight the aircraft will climb. Lift is not always equal to weight but must be equal to maintain level flight. The aircraft does not need to have lift to fly either. Lift can come in the form of thrust. A vertical trust force equal to or greater then weight can allow the aircraft to accelerate and climb like a rocket or aerobatic aircraft that has a much higher thrust to weight ratio like a Extra 300 or Airforce Jet.

Weight:

Weight is the force of Mass*gravity and acts towards the earth at all times during flight. Weight is measured in kilograms or pounds and goes through the Centre Of Gravity of the aircraft. The weight Force on the aircraft changes through flight as the fuel is burnt by the engine or engines. Therefore the COG Centre of Gravity will also change through flight as well as the force of lift. The lift force will be smaller later into the flight then during the start of the flight as the force of the weight will be larger during the start of the flight and full fuel. The COG plays a large part on the aerodynamics and design of the aircraft and this will be discussed later. For example the COG affects the location of the undercarriage, stall speed, and stability of the aircraft.

Thrust:

Thrust is the force that propels the aircraft forward and is equal and opposite to drag, it acts parallel to the longitude axis of the aircraft. Thrust must be larger than drag to accelerate and vice-verse. Thrust is produced from the engines by the process of chemical to kinetic energy transfer from fuel being burnt by the engines. Thrust changes depending on the amount of power available and drag of the aircraft. Aircraft such as aerobatic planes and Military Jets have a high trust to drag ratio and therefore can fly vertically like a rocket.

Drag:

Drag opposes thrust and acts parallel to the relative airflow. Drag changes throughout flight as the aircraft is designed to have low drag in cruise and higher drag during landing to help slow down. The Drag curve shows how total drag is equal to induced drag and parasite drag. Induced drag is that created to lift the aircraft such as wingtip vortices (air slippage) and parasite drag is that from the skin and surface of the aircraft such as antenna and rivets. During flight induced drag reduces in cruise well parasite drag increases. The opposite is said when taking off or landing that Induced drag is large and parasite drag low. The point at which are equal is min-drag and will produce the best endurance for the aircraft.

Tail Plane force:

Aircraft are designed so if the power is not available or reduced, or a stall occurs they have a natural tendency to nose down. Since there is a nose down force it must be balanced out by the tailplane. That is why there are 5 forces acting on a aircraft in level flight. The 5th force being the tailplane producing a downward force with the elevator to allow the aircraft to fly straight and level. During flight the force of lift must equal weight and this is done by adding a force of lift with the tail plane.

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