Forces on an Aircraft

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Talking about the forces on an Aircraft, There are basically 4 forces that are assumed to be acting on an aircraft.

 

Forces on a plane

The forces acting on a plane are:

1. Lift
2. Weight
3. Thrust
4. Drag

Lift: It is the force that acts upwards on a plane, and is responsible helping the plane to gain high altitudes. It is produced as a result of air flowing over the wings and can be calculated by the help of Bernoulli's principle or theorem. The lift force is the key aerodynamic forces that is used to counter the weight of an aircraft and makes it possible to gain altitude.

Lift force is therefore dependent on the density of the air r, the airspeed V, the type of airfoil and on the wing’s area according to the formula below:

Lift Force = 0.5 x r x V² x Wing's Lift Coefficient x Wing Area

Where the Lift Force is in Newton, Wing Area in m² and the airspeed in m/s. The standard density of the air is 1.225kg/m³. The wing's lift coefficient is a dimensionless number that depends on the airfoil
type, the wings aspect ratio (AR), Reynolds Number(Re) and is proportional to the angle of attack (AoA) before reaching the stall angle.

Weight: It is a universal force of gravity that acts on any body and that pulls the plane towards the ground. The magnitude of the force depends on the mass of the aircraft as a whole and is distributed throughout the aircraft but can be assumed to be acting through a single point,known as the Centre of Gravity (C.G.).

Weight= mass x acceleration due to gravity

where the weight is in Newton (N) if the mass is in kg and the acceleration is given in m/s².

Thrust: It is the forces that propels the aircraft through the air. It is usually generated by the help of external engines and varies with the amount of engine power being used. Generally the force is applied in the direction opposite to the direction of the motion of the aircraft but considering the Newton's Third law (Every action has an equal and opposite reaction) , the direction of the force is assumed to be along the direction of the motion of the aircraft.

The magnitude of the thrust depends on many factors associated with the propulsion system used:

- type of engine
- number of engines
- throttle setting
- speed

The direction of the force depends on how the engines are attached to the aircraft. The glider, however, has no engine to generate thrust. It uses the potential energy difference from a higher altitude to a lower altitude to produce kinetic energy, which means velocity. Gliders are always descending relative to the air in which they are flying.

Drag: Any unnecessary force can be termed as drag and in case of an aircraft it can be called as fluid friction. Since the aircraft is moving in a fluid medium the fluid exerts some frictional force on different parts of the plane and this force inhibits the relative motion. Similar to friction, drag can be useful at times.

Drag can be classified according to its different sources namely :

1. Form Drag - The drag that depends on the shape of a body is known as Form Drag. When air flows over a surface its pressure and velocity which gives rise to drag and the drag produced is directly proportional to the surface perpendicular to the flow of air. 
2. Induced Drag - The drag that depends on the wing's generation of lift. An example of this Drag is the vortex created on an aircraft wingtip due to the spillage of air from the lower portion of the wings which tends to exert a force on the wingtips in downward direction. This phenomenon can be minimised by the use of sharklets or wingtips.
3. Interference Drag - When an aircraft moves through the air, air usually flows in the form of streamlines over the wings of the aircraft. If these streamlines get mixed to each other, they result in formation of a drag known as Intereference Drag. It accounts for the 5% to 10% of the total drag experienced by an aircraft.
4. Parasitic Drag - All that drag not associated with the production of lift is termed as Parasitic Drag.

In straight and-level , un-accelerated flight, lift is equal to the directly opposite weight and thrust is equal to and directly opposite drag. Notice that the arrows which represent the opposing forces are equal in length, but all four arrows are not the same length. This indicates that all four forces are not equal but that the opposing forces are equal to each other. 

The arrows which show the forces acting on an airplane are often called vectors. The magnitude of a vector is indicated by the arrow’s length, while the direction is shown by the arrow’s orientation. When two or more forces act on an object at the same time, they combine to create a resultant.