Low cost air speed indicator.

     Not compensated for temperature or altitude.

     Useful in the back seat of a tandem aircraft without
     a rear instrument panel.

     It is a simple water manometer 20 inches high.

     Paul Lamar

   will it work as well with antifreeze liquid? i am thinking of wintertime...
   Yvon Cournoyer

   It will work but it may be off slightly as antifreeze is slightly
   less dense than water. It can be calibrated or recalculated
   once one determines the exact density of anti freeze.
   The water calculations are based on 65 pounds per cubic foot.

   Paul Lamar

 My reference for ethylene glycol gives a weight of 69.5 lbs/cu ft.
 Propylene glycol would be safer and is 64.675 lbs/cu ft. A 50% mix
 with H2O would be a negligible difference.

 Dale Davies

 50 50 would be half way between 69.5 and 65 :)

 Paul Lamar


  I was thinking about the propylene mix.

Dale Davies

Most use ethylene.

Paul Lamar


Not as cheap as a water manometer but perhaps a bit more practical.

This is a low pressure gage from McMaster Carr for $57.

Most real air speed indicators are well over $200.
It is 15 inches of water max. You can also get 30 inches of
water however the resolution down where you need it on landing approaches
is not as good. 100 MPH is 4.86 inches of water. If you wish
you can draw a new calibration card in MPH or Knots. Here is a conversion
chart.

The main drawback is the reading is relative to cockpit pressure
and not necessarily static pressure.

The second one is 0 to 1 psi. It is differential pressure (relative to
static) but it is harder to read accurately as the needle only moves
through about 90 degrees and a bit more money at $77. These are probably made by Magna Helix and I have extensive experience with these fine products.

I have an electronic  digital air speed indicator but I am not comfortable with that. A failed airspeed indicator is one device that can kill you quick.
Both of these could be used as backups if you have the room on the panel for
them.

Paul Lamar