Re: True Airspeed Riddle? air is half as dense at around 22,
But the static port does not compensate for loss in pressure it is only necessary to provide a figure to subtract for the pitot pressure(total pressure) inorder for the airspeed indicator to read dynamic pressure.
An airspeed indicator displays dynamic pressure. Dynamic pressure cannot be measure directly like static (measured perpendicular to flow) and total pressure (ram air such as in the pitot tube.) Dynamic pressure can be thought of as kinetic energy. Bernoulli’s principle (static pressure +dynamic pressure= total pressure,) allows the inference of Dynamic pressure as the total pressure gathered from the pitot tube minus the static port pressure.
It is often unclearly explained that the static port is needed so the airspeed indicator will remain accurate as an aircraft experiences different barometric pressures. It is sometimes suggested that the static port corrects the airspeed indication to remain more accurate while an airplane climbs and cruises at higher altitudes that have lower static pressures. This erroneous understanding claims that if the static port did not compensate for the loss of pressure/density that the airspeed indicator would read a speed much lower than actual as the airplane reached higher altitudes. The problem with this explanation is that the airspeed indicator does read a lower than true airspeed as the airplane reaches higher altitudes, inspite of the static port. This is because the airspeed indicator is not giving a ratio between kenitic energy (dynamic pressure) and static pressure, but is only reading kinetic energy (mass*acceleration, where the mass is decreasing as a plane reaches less dense air) comparing it with nothing! It is the function of the static port to provide a pressure to subtract from the total pressure reading of the pitot tube inorder to obtain some kind of reading. It is because there is no good way to measure dynamic pressure/kinetic engergy (it can only be derived) and compare it to static pressure that the total pressure most be settled for in order to obtain some kind of reading. If an instrument could measure kenitic energy and compare to static pressure there would be no error of indicated airspeed at higher altitudes.
What is the static port for then? A static port allows for an airspeed Indicator to read zero when there is no airspeed at any barometric pressure (in Death Valley of up in the Himalayas.) If a trapped, set vacuum where used instead, like in a barometer, then based on barometric conditions, an airspeed indicator could indicate airspeed with no motion of air about the airplane at all. It is correct to say the static port provides accuracy to an airspeed indicator by providing a static pressure baseline, just be careful in how you think about it!
