An aircraft stays up because its wing deflects the oncoming air downward and the air pushes back on the wing with an equal and opposite force, which is lift. In steady level flight lift equals weight and the engine’s thrust equals drag, and the wing’s angle to the oncoming air, not its curved shape, is the main control of how much lift it makes. This describes how aircraft work; it is not flight training, and flying an untested home-built aircraft is among the most lethal activities in this reference.
Part of Aerodynamics and Flight
The physics common to every aircraft: four forces, one wing, and the relative wind they all depend on.
A Wing Works By Moving A Great Deal Of Air A Little Way Down
A wing moving through air leaves the air behind it moving slightly downward. The wing has to push that air down, and the air pushes the wing up with the same force. That reaction is lift. The pressure differences on the wing’s surfaces (Pressure Distribution) are the means by which the wing hands that force to the airframe, and the speed changes behind them are described by Bernoulli’s relation (Bernoulli’s Principle). The two descriptions are of the same flow, not rival theories.
The popular explanation, that air must travel faster over the longer top surface to meet the air below, is wrong, and the article on Bernoulli’s principle shows why. Nothing in this section depends on it.
Four Forces Balance In Steady Flight
- Lift acts at right angles to the oncoming air and is made by the wing.
- Weight acts toward the earth and is the aircraft’s mass times gravity.
- Thrust acts along the line of the propeller and is made by the engine.
- Drag acts against the direction of travel and is the price of moving through air.
When the aircraft neither climbs, descends nor changes speed, lift equals weight and thrust equals drag. Any imbalance accelerates the aircraft. The balance is explained in Four Forces of Flight, with the arithmetic.
Angle Of Attack Is The Quantity A Pilot Really Controls
The angle of attack is the angle between the wing’s chord line and the direction the air is coming from. More angle makes more lift, up to a point; past that the smooth flow over the top breaks away and lift falls, which is a stall (Stall Dynamics). A stall is a matter of angle, not of speed: the same wing can stall at a high speed in a hard turn. This is why a flat board can be made to fly and why a symmetrical wing flies upside down. The shape of the section (camber) shifts how much lift appears at a given angle; it does not replace the angle.
What This Section Covers, And What It Cannot Teach
The rest of the section follows the carrier (Aerodynamics and Flight): wing design, structure, propulsion, instruments and testing. Every quantity that decides whether an aircraft flies, such as a lift coefficient or an aerofoil’s ordinates, is a measured value that must come from published wind-tunnel data. Where this section cannot supply one it says so rather than printing a guess.
Common Mistakes
- Believing the equal-transit-time story — it leads to wings designed for camber alone and to pilots who think stalls belong to low speed only.
- Treating the four forces as separate — raising the nose changes lift, drag and the thrust direction at once.
- Treating a reference as a teacher — reading cannot give the judgement a first stall or a first landing demands.
Safety
A home-built aircraft from salvaged material is lethal
Structural failure in flight cannot be survived and is a matter of calculation, not impression. A pilot who has learned from a reference alone, flying an aircraft nobody has flown before, is attempting the most dangerous flight of the aircraft’s life. Read Flight Testing before building anything, not after.
What’s Next
- Learn next: Four Forces of Flight, the balance with arithmetic
- Combine with: Wing Design, where the lift is actually made