A Beginners' Guide to Aerodynamics

a guest post by John LeGrand

Introduction

Aerodynamics is the study of forces and motion of objects through the air.

Basic knowledge of the aerodynamic principles is highly recommended before getting involved in building and/or flying model aircraft.

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 air density it may glide 40 meters during high density.

The air density depends on the atmospheric pressure and on the air temperature. The air density increases with decreasing of the air temperature and/or with increasing of the atmospheric pressure. The air density decreases with increasing of the air temperature and/or with decreasing 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 wings' profiles on landing approach by lowering the flaps located at the wings' trailing edge and the slats at the leading edge in order to keep enough lifting force during the much lower landing speed.

The wings' profile of an aircraft is usually asymmetric, which makes the pressure on the wings' upper side lower than the underside, causing the air on the wings upper side to accelerate downwards, thereby a lift force is created.

The air always flows away from areas of higher pressure toward areas of lower pressure, thus the air over the wing top accelerates as it enters the lower pressure region (where the air curves toward the wing), whereas the air under the wing slows down as it enters the higher pressure region. So, one may also say that the wings create lift by reacting against the air flow, driving it downwards, producing downwash. The top of the wing is often the major lift contributor, usually producing twice as much lift as the bottom of the wing.

The lift force of a symmetric profile is based on the airspeed and on a positive
angle of attack to the airflow, which makes the air react as it was asymmetric.

The following picture 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, which dramatically increases the air resistance (drag), this may cause the flow to separate from the upper surface resulting in an abrut reduction in lift, known as stall.

Summarising:
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.