piclist 2000\05\11\171653a >
Thread: Square root (was: Pressure to MPH) [OT]
www.piclist.com/techref/index.htm?key=pressure+mph+OT%5D
BY : Don Hyde email (remove spam text)

The air (or water) inside the pitot tube and its associated plumbing doesn't
move (relative to the tube and the vehicle it's attached to).  This makes it
relatively insensitive to a few drops of moisture etc., though if you look
at airplanes, the pitot tube usually turns up as it goes into the plane, so
that moisture will not travel inside so easily.

The pressure inside is generated because the air entering the opening (it's
pointed forward) is being accelerated(or decelerated) to the speed of the
tube (and vehicle).  So all it takes is a tube pointed in the right
direction.  It's actually not particularly sensitive to the shape, though it
needs to be located away from other objects that distort the wind field.  As
someone else pointed out, it is the static port that is hard to get right,
since Bernoulli effect will tend to generate a vacuum, and locating it and
shaping it just right to avoid that is pretty tricky.

Since the pressure is proportional to the square root of speed, they tend
not to be very sensitive at low speeds.  In a typical light plane, the
airspeed indicator is pretty accurate and responsive at 100 MPH or more, but
at 50, (which is more important since the stall speed is usually around
there), they are seldom accurate to better than 10% or so.  On  most light
planes, the airspeed indicator stays rock steady on 0 until at least 30 MPH
or so.

The little stick sticking up is not often used because it creates more drag,
and is even worse at low speeds because it's proportional to the cube root
or something like that.  It also tends to sort of average over its length,
much of which is inside the boundary layer and is therefor sampling air
that's being dragged along with the vehicle, which will make it read low.

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