User talk:Deeday-UK/Archive 1a

A common fallacy about static vs. dynamic pressure

Quote:

«If you take a pitot tube, static pressure is what you measure with port holes normal to the flow (such that the ram air does not enter them). It is called static to indicate that the ram air component (dynamic pressure half rho vsquared) does not come to play. [...] Thus, the static referers to the fact that the value does not include the ram air components».

Apart from the fact that the 'ram air' does not enter any port in the Pitot tube – given that inside the tube the air is not flowing – all the ports on a Pitot tube simply measure the pressure of the air, i.e. the force exerted by the air on the area of the hole. The only difference between a port that is flush along the tube and one that faces the flow is that the latter will create a stagnation point, and therefore will measure the so-called stagnation pressure. In all this, the dynamic pressure never comes to play, because it is a pressure only dimensionally, but it has nothing to do with any force exerted on any surface; it is simply defined as the product of density and square of velocity, full stop.

The fluid pressure anywhere in the flow field is often called static pressure simply because, looking at the Bernoulli equation, if the term that contains V is called dynamic, well... why not call the other one static, which makes the equation so elegant? Unfortunately, static pressure is first and foremost the pressure of the fluid (constant everywhere) when the fluid is at rest, and it is equal to the pressure of the same fluid, far away from any body, when the fluid is flowing (static pressure at rest = free-stream pressure when flowing). To use static pressure indiscriminately in the flow field creates ambiguity.

One last element: in the Pitot tube, why is the static port called so? Is it because it lies flush along the surface an the 'ram air' does not come to play? If that was true, we could drill a hole anywhere on the surface of the aircraft (e.g. on the underside of the wing; where the flow never separates) and use that as static port, so why no-one ever does that?. The reason of the name is that the static port is meant to measure the 'true' static pressure, i.e. the free-stream pressure, which, if Bernoulli is applicable, allows us to immediately work out the free-stream velocity, which is equal to the aircraft speed, which is what Pitot tube is for.
Now we have a problem: how do we measure the fluid pressure far ahead of the aircraft, using a static port fitted on the aircraft? Answer: we have to find those spots where the effects of the perturbation to the free-stream flow (pressure increase and subsequent drop), induced by the aircraft, are locally null, so that the local fluid pressure equals the static (free-stream) pressure. That typically happens slightly behind a stagnation point, which is why you'll find static ports in the front portion of Pitot tubes or fuselages.
Giuliopp (talk) 16:22, 13 January 2008 (UTC)

It is not uncommon to do as you say in order to measure the local flowfield. If you are looking for freestream conditions then you do as you speak of for a pitot-static tube. And the stream lines do go into and back out of a stagnated right handed cylinder. What you call static pressure is usually called free-stream static pressure. Static pressure is technical jargon and jargon means what the community chooses it to mean. It does no good to be right if nobody knows what you are talking about. We all know what we are talking about in the context of the conversation or airplanes would not work correctly. Quoting from a reagular dictionary is not satisfactory for jargon.Mangogirl2 (talk) 01:42, 16 January 2008 (UTC)