With this short article I would like to initiate new series: Fixed Wing 101 where I will describe some basic concepts connected with fixed wing airplanes that should help beginners to enter the hobby. Today: why airplanes fly and why, from time to time, they fall down from the sky…
Airplanes fly thanks to the lift. It is a force generated by wings (by the way, propeller thrust and wing lift are the same force. After all, propeller is a rotating wing) thanks to pressure difference. When air pressure below the wing is higher than above it, lift appears. To archive level flight, lift has to be big enough to counteract mass and gravity.
The most important condition for a wing to generate lift is: wing has to move though the air (or air has to move around the wing). If there is no movement, there is no lift.
When there is air movement, there are two factors responsible for lift:
Bernoulli’s principle states:
when gas or liquid is moving faster, it has lower pressure.
Simple. To obtain lower pressure above the wing, we have to make air move faster over there. This is why wing has a shape (airfoil) it has: top side (above chord) of a wing is longer than the lower side. Wing splits air into 2 stream. Upper one has longer way to travel than lower one, so it has to move faster. If it moves faster, it has lower pressure. If it has lower pressure, lift appears.