Dynamic Pressure
Dynamic pressure is a result of the relative movement of a body through the air and varies with air density. Holding your hand up out of a moving vehicle demonstrates this pressure. Together with static pressure it forms the total pressure exerted at some point. Especially airspeed indicators (ASI) rely on correct dynamic pressure measurements by the pitot tube and expresses these measurements into an indicated airspeed (IAS). As an aircraft climbs or descents we find that the air density changes significantly. The kinetic energy (KE) of 1 cubic metre of air with a specific density and moving at V meters per second is expressed as:

KE = 0,5 x Air Density x Speed squared

When converted to pressure, the dynamic pressure (q) can be written as:

q = 0,5 x Air Density x Speed squared

The dynamic pressure therefor greatly depends on:

  • As speed increases more molecules will impact per second and thus increases dynamic pressure. Remember that this is the relative velocity of the airplane and the air flow that matters. It doesn't matter whether the aircraft moves through the air or the air flows around it while on the ground. Either approach gives us the same answer.
  • The density of the air varies with altitude and meteorological conditions. Less dense air means less molecules striking per second and thus a lower dynamic pressure (without regard to airspeed)(figure 1.1).
Figure 1.1 - Increased air density, increased dynamic pressure

 

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