Forces during flight

Forces acting on a plane

  • During flight there are four main forces on an aircraft:
    • Thrust: This is the force that moves the plane forwards.
      • The engine provides this force.
      • To move forward it most overcome the air resistance or drag.
    • Lift: This is the upwards force that keeps the plane in the air.
      • This force is created by the shape of the aerofoils (explained later)
      • To stay in the air, it must equal the weight force
      • It is always perpendicular to the direction of thrust.
    • Drag: This is the air resistance that occurs from thrust.
      • Newton's 3rd Law: For every force there is an equal and opposite reaction.
      • This is caused by the resistance of the air.
    • Weight: This is the result of gravity pulling the plane towards the ground
      • The force is always perpendicular to the horizontal.
  • In steady flight, that is if the plane maintains its current velocity, all the forces are in equilibrium.

Moments on a plane.

  • If the plane isn't in steady flight however, there are three moments that can be adjusted. One for each axes.
  • It should be noted that to adjust these, parts of the plane, known as control surfaces are flipped up or down.
  • Flaps are the only control surfaces that do not affect the moments, but rather control the speed of the aircraft by extending the flaps, increasing the camber of the wing and thus the drag.
    • It allows for decreased stalling speeds and allows the plane to fly slower
    • This is extremely useful in times when the pilot needs to decelerate, such as during lift off or landing.
  • This is whether the plane's nose faces upwards or downwards, or moving about on its lateral axis.
  • It determine's the angle of attack and the direction of thrust.
    • The angle of attack is the angle the plane's thrust makes with the horizontal.
  • It can be controlled by the horizontal stabilisers but mainly the elevators.
    • By moving the elevator up, it induces drag and reduces lift, which points it upwards. And vice versa
  • This is when it turns left or right about its Normal axis.
  • It is controlled by the rudder, but in use it's often applied with a roll.
    • If the rudder turns left, the plane turns left and vice versa.
  • This is when the plane turns on its Longitudinal axis.
  • It is controlled by the ailerons.
    • This involves lifting one aileron and dropping the other as shown: