How Does A Pitot Static System ACTUALLY Work?

[aerospacepilot]

I am majoring in Aerospace Engineering, and I know all about the math behind a pitot static system. Basically static pressure + dynamic pressure = total pressure. Static pressure is measured from the static source, and the total pressure is measured at the tip of the pitot tube. Dynamic pressure = .5*density*velocity^2, and the density for low speed airplanes is just the standard sea level value. So by rearraging the equations, you can come up with an equation for equivalent airspeed.

But that wasn't good enough on my IFR checkride oral. The examiner wanted to know HOW the pitot-tube and static source ACTUALLY measured the pressure, how the difference (P_total - P_static) is determined, and how that values is placed on the airspeed indicator. Does anyone know how this mechanical system works?

Also, for the engineers, are equivalent airpseed and indicated airspeed the same thing??
Thanks.

[tgrayson]

Originally Posted by aerospacepilot

Also, for the engineers, are equivalent airpseed and indicated airspeed the same thing??

No. You need to correct IAS to CAS, which will be *close* to EAS. For low airspeed/low altitude aircraft, the error is very small. For faster airplanes, you need to apply another correction, called either the "compressibility correction" or the "scale altitude correction" to get EAS.

As for the airspeed indicator, the schematic I use just has a chamber vented on one side the total pressure and the other to static pressure. The net pressure difference moves some gearing attached to the needle.

[Milesar]

Was he referring to the instrument itself? The airspeed indicator (where the differences are measuered) contains a diaphram where the pitot source connects, the static souce is connected to the airspeed indicator outside of the diaphram. (insert drawing here)

The static source allows the ambient air to circulate around the diapham that moves with the different amount of dynamic pressure (this is connected to the dial by mechanical linkages). If you think about some of the errors that present themselves when certain ports are covered, such as the static source, it can help understand how the airpseed indicator gets its readings.

Hope this helps a bit.

[aibal]

The ASI has Total Pressure fed to it (Ram air from the pitot tube) and Static Pressure(The static port).

Total Pressure is a posative pressure. Static Pressure is a negative pressure. Total Pressure - Static Pressure = Velocity Pressure(Dynamic)

The Diaphram inside the ASI is therefore effected by Velocity Pressure(Same as Dynamic Pressure).

The Diaphram is linked to gears which move the ASI needle. The ASI Scale is displayed in MPH or Knots. The Calculation that is used when determining this is the square root of Velocity pressure * 4005 = Feet per minute(Velocity), which can be converted to MPH or Knots.