Re: External inspection

Date:         13 Feb 99 02:26:07 
From:         Matthew Willshee <mjw44@cam.ac.uk>
Organization: University of Cambridge, England
References:   1 2 3 4 5
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JF Mezei wrote:

> I do not understand. If the static pressure trapped inside the system
> were enough to show a V1 speed on the pilot's gauges, shouldn't pilots
> have noticed that the gauges were reading V1 while the plane was iddle?

Assuming incompressible flow, Bernoulli's equation tells us that:

p + 0.5 * rho * V * V = p0

p is static pressure - "barometric pressure"
rho is density
V is speed
p0 is stagnation or total pressure - the pressure you would measure in
the air if you slowed it to a stop.

0.5 * rho * V * V is called the dynamic pressure.  It is the difference
between static and total pressure due to the motion of the air.

A pitot tube has two parts
  - a forward pointing tube brings the air to rest (relative to the
plane) and hence the pressure in it is total pressure p0
  - a hole pointing at right angles to the direction of travel measures
the static pressure p.

Subtract p from p0 and you get dynamic pressure 0.5 * rho * V * V - from
which you can extract speed (given density which is a function of
altitude and temperature, hence IAS versus TAS discussions elsewhere in
the group).

So now we tape over the static ports - obviously while the plane is on
the ground!.  We fix our reading of static pressure to sea level
atmospheric pressure.  Any change in this before takeoff will be
negligible so the static ports read the correct pressure on takeoff.
There is no airspeed indication problem.  At altitude though, our
trapped pressure is too high so we calculate dynamic pressure too low
and underestimate airspeed.  Obviously, it would be much easier to spot
taped over dynamic ports, as this value pressure changes as we
accelerate.

Air is not incompressible, so there are some inaccuracies in using
Bernoulli but the argument will be the same in principle with
compressible flow.

--
Matthew Willshee                             E-Mail: mjw44@cam.ac.uk
Churchill College, Cambridge, CB3 0DS              : 96mjw@eng.cam.ac.uk