How to Test Your
Airspeed Indicator
BY FRED KEIP, EAA 93236, EAA HOMEBUILT AIRCRAFT COUNCIL MEMBER
GLASS TUBE ON PLASTIC
STICK
TAPE OVER
DRAIN
I FOUND THE FOLLOWING article in the October
1987 issue of the Canard Pusher, the
Long-EZ/VariEze newsletter published by
Burt Rutan. It was written by Vern Vawter
and describes a simple method to calibrate
your pitot system and airspeed indicator.
One instrument in my airplane that has
been a source of constant irritation is the
airspeed indicator. For some reason mine
always reads too low, and my friends, at least
during hangar flying sessions, say that their
airplanes are always faster than mine.
On the verge of an inferiority complex, I
decided to do some investigating which
revealed that airspeed indicators are based
on well-known physical law, and that it is
feasible for owners to check and calibrate
their own aircraft’s speedometer.
Before I relate the principles of airspeed
theory, based on Bernoulli’s principle, let’s
get right into how simple it is to make an
instrument called a manometer, which is
easily put together with a little this and that
found at most hardware stores.
BLOW HOLE
WHEN DISTANCE
BETWEEN A & B
IS 9. 64 INCHES,
AIRSPEED SHOULD
READ 140 MPH.
EQUIPMENT REQUIREMENTS:
1. Approximately 10 feet of clear plastic tubing, preferably 1/4-inch inside diameter
2. A board, 30 inches long, suitable for
mounting the plastic tubing in a “U” shape
3. A small tee fitting
4. A yardstick
5. A few ounces of water with a little bit of
food coloring to aid visibility, and a small
quantity of detergent as a wetting agent
TESTING PROCEDURES:
1. Assemble the manometer as shown in
the illustration.
