Pitot-Static Tubes

[stephen2688]

Can someone explain why a pitot-static tube would be used instead of a conventional pitot tube?

The conventional pitot tube measures total pressure and sends this to the ASI, the static pressure from the static ports on the side of the aircraft is then fed to the ASI and whatever the static pressure is, it then offsets this (cancels) what the static pressure is when it came through the front of the pitot tube to give us dynamic pressure.

The pitot-static tube has the holes in the side of the tube 90 degrees to the airflow to obviously measure static pressure I presume then to calculate the dynamic pressure? Is this the case? If so what's the point of the static ports on the side of the aircraft for the ASI if the static pressure is known from the pitot-static tube?

[john_tullamarine]

For the pitot-static system, proper, I can't bring to mind any aircraft which uses both a combined head and fuselage static ports - there should be no reason to do so.

The pitot bit is comparatively straightforward and, providing that dimensions are reasonable, errors generally aren't a major concern.

The static bit, on the other hand, can present significant difficulties to the designers to get within acceptable error boundaries. As to whether the designer goes for a combined mount or fuselage statics is up to the designer's assessment of which is the better way to achieve the design standards requirements.

If you are looking at an aircraft with both a combined head and fuselage statics, my guess would be that the fuselage statics are for some other purpose than the pressure instrument panel ?

[tdracer]

What John said. The placement of Pitot tubes is pretty simple since once you're out of the boundary layer you get a good total pressure. Static pressure is far messier. You need an area of without any meaningful local acceleration of the air (such as the curvature of the nose) - ideally a long, constant diameter body section - otherwise there is distortion of the static pressure measurement due to the acceleration (or deceleration) of the airflow over a curved surface.
The 767 used pitot statics, while the concurrently developed 757 had pitot tubes with body mounted statics. Go figure...