The temperature lapse rate is assumed to be uniform at a rate of 2°C per 1000 ft (1.98°C) from mean sea level up to a height of 36 090 ft (11 000 m) above which the lapse rate becomes zero and the temperature remains constant at -56.5°C.

ICAO Standard Atmosphere

The Atmosphere 2

Atmosphere The 2

The kinetic energy of one cubic metre of air moving at a stated speed is given by the formula:

Kinetic Energy = ½ ρ V2 joules

where ρ is the local air density in kg/m3 and V is the speed in m/s

If this cubic metre of moving air is completely trapped and brought to rest by means of an open-ended tube the total energy will remain constant, but by being brought completely to rest the kinetic energy will become pressure energy which, for all practical purposes, is equal to:

Dynamic Pressure = ½ ρ V2 N/m2

Key Facts

A pilot needs to know how much dynamic pressure is available, but dynamic pressure cannot be measured on its own because static pressure will always be present. The sum of static and dynamic pressure, in this context, is known as ‘Total’ pressure.

(Dynamic + Static pressure can also be referred to as Stagnation or Pitot pressure).

Total Pressure = Static Pressure + Dynamic Pressure

This can be re-arranged to show that:

Total Pressure - Static Pressure = Dynamic Pressure

The significance of dynamic pressure to the understanding of Principles of Flight cannot be overemphasized.

Because dynamic pressure is dependent upon air density and the speed of the aircraft through the air, it is necessary for students to fully appreciate the factors which affect air density.

•Temperature - increasing temperature decreases air density. Changes in air density due to air temperature are significant during all phases of flight.

•Static pressure - decreasing static pressure decreases air density. Changes in air density due to static pressure are significant during all phases of flight.

•Humidity - increasing humidity decreases air density. (The reason increasing humidity decreases air density is that the density of water vapour is about 5/8 that of dry air). Humidity is most significant during take-off and landing.

Increasing altitude will decrease air density because the effect of decreasing static pressure is more dominant than decreasing temperature.

The Atmosphere 2

Atmosphere The 2

Measuring Dynamic Pressure

All aerodynamic forces acting on an aircraft are determined by dynamic pressure, so it is essential to have some means of measuring dynamic pressure and presenting that information to the pilot.

A sealed tube, open at the forward end, is located where it will collect air when the aircraft is moving. The pressure in the tube (pitot tube) is Dynamic + Static and, in this context, is called “Pitot” pressure (because the air is inside the pitot tube).

Some way of ‘removing’ the static pressure from the pitot pressure must be found. A hole (vent) in a surface parallel to the airflow will sense static pressure. Referring to the diagram below, if the pressure from the pitot tube is fed to one side of a diaphragm mounted in a sealed case, and static pressure is fed to the other side, the two static pressures will cancel each other and the diaphragm movement will be influenced only by changes in dynamic pressure.

Movement of the diaphragm moves a pointer over a scale so that changes in dynamic pressure can be observed by the flight crew. But the instrument is calibrated at ISA sea level density, so the instrument will only give a ‘true’ indication of the speed of the aircraft through the air when the air density is 1.225 kg/m3.

This is not a problem because the pilot needs an indication of dynamic pressure, and this is what the instrument provides. The instrument is made in such a way that it indicates the square root of the dynamic pressure in nautical miles per hour (knots) or statute miles per hour (mph). So, if this “Indicated Airspeed” is doubled, the speed of the aircraft through the air will also be doubled.