BASIC AERODYNAMICS
GRADE 12 - AVI711
WEEK 2 – 2.1, 2.2, 2.3, 2.4
AIMS AND OBJECTIVES
 By the end of this lesson you should be able to understand the
following terms in relation to aerodynamics:
• Pressure.
• Temperature.
• Density.
• International Standard Atmosphere (ISA) tables.
GENERAL IDEA
The physics of the atmosphere is very important to
aviation. With altitude, we have a change in various
properties. Most importantly, pressure, density and
temperature.
Anything that is airborne is affected depending on its
altitude. The atmosphere of the earth is put into
categories again depending on altitude. See the figure.
PRESSURE - ATMOSPHERIC
We know pressure is simply Force per Area. In this
case we are talking about atmospheric pressure. It is
the same concept only now we are talking about the
pressure due to atmosphere or air.
Think of it as the force acting on you (per unit area)
because of all the air molecules surrounding you.
These molecules become less and less as we go higher
in altitude. From this we can then say that the
pressure decreases with altitude. A video on D2L can
help explain this.
TEMPERATURE
 The regions at which aircrafts cruise happen to be
much colder than at sea level. This can have huge
affects on aircrafts such as icing.
 Icing is where water particles freeze on the airplane
wings, increasing the aircrafts weight and causing a
stall.
 We measure temperature in Kelvin (K).
DENSITY
Air molecules have mass. Pretend we are
filling a box with air, the more air we can put
inside, the heavier it would be.
The amount you are able to fit into the box
tells you how much density it is capable of
having. Therefore, air density changes with
altitude (it decreases). This is because gravity is
weaker and there are less molecules pushing
down on the aircraft.
The SI unit for air density is kg/m^3.
AFFECTS OF THESE PROPERTIES ON AIRCRAFTS
1
2
 Take the equation for Lift, 𝐿 = 𝜌𝑉 2 𝑆𝐶𝐿 , where 𝜌 is the density of air. Therefore, the lower the
value of 𝜌, the higher our lift force (which is good).
 Unfortunately, air pressure and temperature change the density of the air. For example, the colder
the air then the higher the density as the molecules contract.
INTERNATIONAL STANDARD ATMOSPHERE (ISA)
Now that we know lift is dependent on the density, and
that density can be affected by temperature and pressure,
the pilot must take into account these differences.
Pretend we are going to fly from the UK in the winter, the
flight conditions and requirements will be different if we
were to fly from UAE in the summer. So we must come up
with a standard table with these properties at different
locations and altitudes. The pilots and engineers can refer to
this when calculating.
We call this table the International Standard Atmosphere.
This table informs us of the ambient (atmospheric)
temperature, pressure and even the speed of sound at
different altitudes.