Right Seat – Flight Instructor’s Corner

Insights From Bernoulli

By  Bob Johnson, ATP, CFII, MEI

Early in our study of aerodynamics we learn that air particles, being fluid in nature, behave in accordance with Bernoulli’s equation as they flow around our aircraft’s surfaces. The Bernoulli equation (which is derived from Newton’s second law) is simply an expression for the conservation of energy in fluid flow, and for our purposes has two major components: dynamic pressure and static pressure. Their sum yields a total pressure which is constant along a given streamline ( the path of successive fluid particles in steady flow); and to conserve that constant total pressure, one form of pressure must increase at the expense of the other. For example, a higher dynamic pressure ( a product of air density and the square of the speed) results in a lower static pressure felt against the surface. This is the pressure interplay which ultimately manifests itself as the lifting force on the wings and other surfaces.

Now it should be realized that it is this same total pressure which is captured or sensed by the pitot system and transmitted to the inside of the airspeed indicator’s diaphragm. With the application of a static pressure source on the outside of that same diaphragm, we get a difference (or total pressure minus static pressure) yielding dynamic pressure (again, a function of density & speed squared), expressed as airspeed. Therefore, it should be apparent that at low air densities (i.e., high density altitude) there must be an increase of speed to compensate for the loss of air density to produce a given dynamic pressure essential for lift production. In pilot parlance, we say that a higher true airspeed (TAS) is required to to produce a given indicated airspeed (IAS); or conversely, a given IAS at high density altitude yields a greater TAS. This is one reason why airplanes need more runway to accelerate to the required IAS for take off when operating at high density altitude. In fact, on average you may estimate that your TAS increases approximately 2% per 1,000 feet for the same IAS; but the runway requirement increases at a much higher rate (consult your plane’s POH and a CFI familiar with your plane). For additional high density altitude operating considerations and techniques, I suggest reviewing the related article from Red Baron’s PIREP archives (July, 2010) on Red Baron’s website.  It should be noted too, that for the purposes of this discussion it is assumed that any instrument error or static system position error is negligible. That is, IAS equals CAS (calibrated airspeed).