Forces of Flight
There are forces which are experienced during flight.
- Thrust – The forward propulsion force produced by the propeller or aircraft’s propulsion system.
- Drag – The air resistance which opposes the thrust force.
- Lift – The upward force which is required for the plane to fly.
- Weight – The aircraft’s weight which is caused by the gravity and opposes the lift.
The lift force is generated by the wing. At a certain angle of attack, the shape of the wing’s airfoil creates a particular shape (refer to the picture below) in the airflow, a curved streamlines which generates pressure changes which produce the lift. Lower pressure on the wing’s upper surface and higher pressure on the lower surface causes this pressure difference, and therefore the lift force.
Notes: this hypothesis is taken from a research by University of Cambridge (25 Jan 2012).
The distribution of lift
The lift around the wing’s surface is not distributed evenly and it is explained by referring to the pressure distribution on airfoil. At a certain angle of attack, the top surface produces more lift than the bottom surface. The maximum lift, pulling and pushing forces combined, is located at the most curved surface; approximately 1/3 of chord length from the wing’s leading edge (front edge).
Pressure distribution on airfoil, picture by UK Open University. Link: http://firstflight.open.ac.uk/aerodynamics/index.html
Thrust and Drag
The Lift which is required for the flight, cannot be generated without the airflow on the wing surface. Thrust is the propulsion force which drives the aircraft forward, thus producing the airflow. One of the examples of the aircraft propulsion system is the propeller which is coupled with a rotating engine to spin the propeller to produce the thrust.
Drag is the thrust resistance and it should be reduced to a minimum amount to optimize the flight efficiency. The drag is generated from the shape of the aircraft. Landing gears, wing strut bar, the wing, tail, the fuselage, and other parts which experience the airflow should be designed to produce streamline airflow; starting by eliminating as many protruding external parts as possible. If it is the fundamental parts, remove them during flight, such as retracting landing gears during flight.
3 Planes of Movements
When the aircraft moves in one plane, it rotates about an axis. During flight, it can rotate about one, two or all 3 axes at once. There are 3 maneuvers on these axes:
- Maneuver about its longitudinal axis will cause the aircraft to pitch.
- Maneuver about its lateral axis will cause the aircraft to roll.
- Maneuver about its normal axis will cause the aircraft to yaw.
3 Basic Control Surfaces
The aircraft needs 3 basic control surfaces in order to conduct the 3 maneuvers as explained previously:
- The ailerons are for rolling maneuver. To roll right, the right aileron needs to move up and the left aileron moves down. This causes the right wing to go down and left wing to go up, thus rolling the aircraft to the right.
- The elevator is for pitching maneuver. To pitch up, the elevator needs to move up. This causes the aircraft’s tail to move down and aircraft’s nose to move up; increasing the flight’s altitude.
- The rudder is for yawing maneuver. To yaw right, the rudder needs to tilt to the right. This causes the aircraft’s tail to tilt to the left and the aircraft’s nose to tilt to the right.