Introduction to Aerodynamics

A model aircraft that is hanging still in air during strong winds may be subject to the same aerodynamic forces as a model aircraft that is flying fast during calm weather.

The aerodynamic forces depend much on the air density. For example, if a glider glides 25 meters from a given altitude during low atmospheric pressure, it may glide 40 meters during high pressure. The air density depends on the atmospheric pressure and on the air temperature.

The air density decreases with increasing of the air temperature and/or with decreasing of the atmospheric pressure. The air density increases with decreasing of the air temperature and/or with increasing of the atmospheric pressure.

A flying aircraft is subject to a pressure depending on the airspeed and the air density. This pressure increases exponentially with increasing of the airspeed. The aircraft's resistance to the airflow (drag) depends on the shape of the fuselage and flying surfaces. An aircraft that is intended to fly fast has a thinner and different wing profile than one that is intended to fly slower. That's why many aircraft change their wing profile's on landing approach by lowering the flaps located at the wings' trailing edge and the slats at the leading edge in order to keep a reasonable lifting force during the  much lower landing speed.

The wings' profile of a slower aircraft is usually asymmetric, this causes the airspeed on the wings upper side to be higher than the underside, which in turn makes the pressure on the upper side to be lower than the underside, thereby a lift force is created. The lift force of a symmetric profile, is based on the airspeed and on a positive angle of attack to on-coming flow.

The picture to the left shows the airflow through two wing profiles.

The uppermost profile has a lower angle of attack than the lowest one. When the air flows evenly through the surface is called a laminar flow. A too high angle of attack causes turbulence on the upper surface and dramatically increases the air resistance (drag) this may result in an abrupt loss of lift, which is known as stall.

Summarizing: The aircraft generates lift by moving through the air. The wings have airfoil shaped profiles that create a pressure difference between upper and lower wing surfaces, with a high pressure region underneath and a low pressure region on top. The lift produced will be proportional to the size of the wings, the square of airspeed, the density of the surrounding air and the wing's angle of attack to on-coming flow before reaching the stall angle.

How does a glider generate the velocity needed for flight? The simple answer is that a glider trades altitude for velocity. It trades the potential energy difference from a higher altitude to a lower altitude to produce kinetic energy, which means velocity. Gliders are always descending relative to the air in which they are flying.

How do gliders stay aloft for hours if they constantly descend? The gliders are designed to descend very slowly. If the pilot can locate a pocket of air that is rising faster than the glider is descending, the glider can actually gain altitude, increasing its potential energy.

Pockets of rising air are called updrafts. Updrafts are found when the wind blowing at a hill or mountain rises to climb over it. (However, there may be a downdraft on the other side!) Updrafts can also be found over dark land masses that absorb more heat from the sun than light land masses. The heat from the ground heats the surrounding air, which causes the air to rise. The rising pockets of hot air are called thermals.

Large gliding birds, such as owls and hawks, are often seen circling inside a thermal to gain altitude without flapping their wings. Gliders can do exactly the same thing.