aerodynamic forces
FUNDAMENTAL FORCES
VERTICAL FORCES
LIFT
the positive vertical force which lifts the aircraft upwards. without lift, no aircraft would be able to rise. lift is created when high pressure air passes below the airfoil and low pressure air passes below. air moves from high to low pressure, making the airfoil (therefore the plane) lift. lift would not be able to occur without air passing over the airfoil at an optimal speed due to the thrust of the plane.
WEIGHT
the negative vertical force. its caused by gravity, therefore it is always present. weight is dependent on the mass of the aircraft (as weight = mass x gravity), therefore the less mass, the less weight.
HORIZONTAL FORCES
THRUST
the positive horizontal force which pushes the plane forward. it’s created by the engine (propeller or turbo fan). the engine creates thrust by pushing air forward, then, following newton’s third law (every action has an equal and opposite reaction), the air has a reaction by pushing the aircraft forward, creating thrust. aircrafts need to be at a specific thrust speed before the air pressure can create enough lift force.
DRAG
the negative horizontal force which slows down an aircraft. it is always present in motion. it should be minimised to save fuel and time.
EQUILIBRIUM
equilibrium - when forces in a dimension (i.e. the result force) equals 0
horizontal equilibrium - thrust = drag
vertical equilibrium - life = weight
complete equilibrium - vertical and horizontal equilibrium, therefore the aircraft is stationary, or constant velocity with no acceleration
CP AND CG
CP - centre of pressure / centre of lift. the point at which lifting forces act through. i.e., the point where the plane would balance if lifted by a string. the CP can move around during flight, but is roughly at the midpoint of the leading and trailing edges of the wing.
CG - centre of gravity. the point at which the aircraft’s weight acts through. i.e., the point where the plane would balance if placed on your finger, like a pivot
TDF - tail down force. the downwards force exerted by the horizontal stabiliser on the tailplane to resist the aircraft’s tendency to pitch down
WEIGHT DISTRIBUTION AND PITCH
a stably loaded aircraft will be made with the CG in front of the CP. this is to distribute the weight and to ensure the aircraft is dynamically stable (such as during stalls), otherwise the plane would constantly be trying to flip around so the CP stays behind the CG. however, when the CP is behind the CG, an aircraft will tend to pitch down due to the weight and lift. the stronger these forces are, the more an aircraft will pitch. to counteract this, the horizontal stabiliser exerts a downwards force on the tail, called tail down force (TDF), regulating the aircraft’s latitude. however, now there are 2 downwards forces and 1 upwards force, so the lift must equal the weight acting downwards from the CG as well as the TDF (lift = weight + TDF) to maintain equilibrium, therefore the lift force must be larger. we can increase lift force by increasing the angle of attack (AOA). the greater the downwards weight force at the CG (or the further forward the CG is), then the greater the pitch down tendency, therefore a greater TDF is needed for balance. this requires more lift, which is achieved by increasing AOA.
STALL
stall - rapid drop in lift
when an aircraft stalls, it loses its lift. without lift, equilibrium is lost, and the aircraft will rotate around its CG, pitching down and decreasing the AOA to break the stall. therefore, the pilot must pitch up to the critical AOA to ensure there is enough AOA to decrease so the aircraft does not continue pitching down and eventually fall. this is an example of the increased stability from having the CG in front of the CP.
DRAG
without forward CG, AOA has to be higher to conteract TDF, increasing AOA increases drag (specifically induced drag)
could theoretically fly at 0 AOA if u moved weight rearwards and had CG directly under CP, therefore no need for TDF and therefore less drag. this means sacrificing stability for aerodynamics. this means in a stall it will be harder to recover cuz there is no angle of attack to break the stall
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