How airplanes fly

- basic principles of flight.

The basic principles of why and how airplanes fly apply to all planes, from the Wright Brothers' first machine Wright Flyer to a modern Stealth Bomber, and those principles are the same for radio control and full size airplanes alike.

Although the true physics of flying airplanes are quite complex, the whole subject can be simplified a bit - enough for us to get a fundamental understanding of what makes a plane fly, at least!

Aerodynamic forces.
 Essentially there are 4 aerodynamic forces that act on an airplane in flight; these are lift, drag, thrust and weight (i.e. gravity).

In simple terms, drag is the resistance of air molecules hitting the airplane (the backward force), thrust is the power of the airplane's engine (the forward force), lift is the upward force and weight is the downward force. So for airplanes to fly and stay airborne, the thrust must be greater than the drag and the lift must be greater than the weight (so as you can see, drag opposes thrust and lift opposes weight).

This is certainly the case when an airplane takes off or climbs. However, when it is in straight and level flight the opposing forces of lift and weight are balanced. During a descent, weight exceeds lift and to slow an airplane drag has to overcome thrust.
The picture below shows how these 4 forces act on an airplane in flight:

How airplanes fly - aerodynamic forces acting on a plane in flight
Thrust is generated by the airplane's engine (propeller or jet), weight is created by the natural force of gravity acting upon the airplane and drag comes from friction as the plane moves through air molecules. Drag is also a reaction to lift, and this lift must be generated by the airplane in flight. This is done by the wings of the airplane...

The generation of lift has been an argued theory in the past, but certain principles have been known about and agreed on for a long time now.

A cross section of a typical airplane wing will show the top surface to be more curved than the bottom surface. This shaped profile is called an 'airfoil' (or 'aerofoil') and the shape exists because it's long been proven (since the dawn of flight) that an airfoil generates significantly more lift than opposing drag i.e. it's very efficient at generating lift.

During flight air naturally flows over and beneath the wing and is deflected upwards over the top surface and downwards beneath the lower surface. Any difference in deflection causes a difference in air pressure ('pressure gradient') and because of the airfoil shape the pressure of the deflected air is lower above the airfoil than below it. As a result the wing is 'pushed' upwards by the higher pressure beneath or, you can argue, it is 'sucked' upwards by the lower pressure above.

One of the argued, but commonly discounted, theories of lift generation is related to Newton's 3rd Law of Action & Reaction, whereby the air being deflected downwards off the lower surface of the wing creates an opposite reaction, effectively pushing the wing upwards. This may well be the case but it's the pressure difference between both surfaces that is the commonly agreed factor of lift generation.

How air behaves over an airfoil
Above: the general movement of air over an airfoil.

The faster a wing moves through the air, so the actions are exaggerated and more lift is generated. Conversely, a slower moving wing generally creates less lift.
It's important to note, though, that different wing designs (airfoil and shape) generate lift more (and less) efficiently than other designs at different speeds, depending on what the plane has been designed for.

A direct reaction to lift is drag and this too increases with airspeed. So airfoils need to be designed in a way that maximises lift but minimises drag, in order to be as efficient as possible.

Angle of Attack and lift.
Another crucial factor of lift generation is the Angle of Attack - this is the pitch angle at which the wing sits in relation to the relative airflow around it (see pic further up this page).

As the Angle of Attack increases so more lift is generated, but only up to a point until the smooth airflow over the wing starts to break down and so the generation of lift cannot be sustained; this point is called the critical Angle of Attack. When the CAoA is reached the sudden loss of lift results in the wing stalling and the weight of the airplane cannot be supported any longer.
When a stall occurs a sudden loss of altitude is inevitable unless the pilot rectifies the situation immediately by decreasing the AoA and getting the wing to generate lift once again. Typically a stall recovery means simultaneously pushing the nose of the plane down and increasing power to gain airspeed.

The Angle of Attack should not be seen as a lesser important factor in lift generation than the airfoil shape of the wing, in fact the AoA is the single most important factor. For example, a flat-section wing can produce adequate amounts of lift all because of the Angle of Attack - the big difference is in the efficiency of the lift generation; flat wing sections carry a large penalty in terms of much higher drag, compared to an airfoil section where drag is substantially less.