From: AJBrent@aol.com
To: x-plane@me.umn.edu
Subject: Airspeed Refresher Training
Date: Fri, 19 Sep 1997 03:28:53 -0400 (EDT)

Excerpts from the book:  An Invitation to Fly--Basics for the Private Pilot.

The airspeed indicator registers the total pressure from the pitot head and
subtracts from it the static pressure supplied from the static ports.  This
remainder is called dynamic pressure, which is the measure of the airplane's
forward speed.  This speed is displayed on the instrument's face on a
graduated scale called indicated airspeed (IAS).  Remember that this value
represents the airplane's speed through the air, not necessarily it's speed
across the ground.  Why?  Once it is airborne, the airplane becomes part of
the local mass of air.  If the mass of air is moving (that is, if the wind is
blowing), the airplane will move with the air.  While this is an important
consideration during takeoffs and landings (when the airplane is making the
transition between flight and ground operations) and for navigation (the
moving airmass can carry the plane off course, like a ship in ocean
currents), it means very little to the pilot in terms of normal flight
dynamics.  The airplane flies because of the speed of the relative wind, and
this is what the airspeed indicator measures, not ground speed.

Types of Airspeed:
--Indicated Airspeed.  This is the direct reading of airspeed taken from the
face of the instrument, uncorrected for air density, positional errors due to
the pitot head installation, or internal instrument error.
--Calibrated Airspeed (CAS) is the indicated airspeed corrected for minor
installation and pitot head position error and mechanical losses within the
instrument itself.  The manufacturer or instrument repair shop provides these
values on a cockpit reference card or in the Pilot's Operating Handbook.
[In X-Plane, I assume we are provided a perfect airspeed instrument so that
IAS and CAS are the same.  CAS is not simulated.]
--Equivalent Airspeed is calibrated airspeed corrected for the
compressibility effects of high-speed flight.  Normally this is not relevant
to private pilot flight planning.  [And is not simulated in X-Plane as of
ver. 3.4.  Equivalent airspeed is also the same as IAS in X-Plane.]
--True Airspeed is equivalent airspeed (or calibrated airspeed if
compressibility effects are negligible) [IAS in X-Plane] corrected for the
effects of less dense air at higher altitudes.  For most light airplanes,
true airspeed and calibrated airspeed are very close at sea level, but they
can diverge rapidly after the airplane climbs several thousand feet.  Since
true airspeed reflects the actual physical rate at which the aircraft is
moving through the air, it is of key importance in air navigation.

You can easily recall the sequence of airspeed corrections leading to true
airspeed by memorizing the acronym ICE-T, the first letters of the four
airspeeds presented above.  [Indicated, calibrated, and equivalent airspeeds
are all the same in X-Plane.  So, it's just IT.]  Equivalent airspeed is
important only on high-performance, turbine-powered airplanes.  True
airspeed, however, must be determined before wind correction angle or ground
speed can be computed for any airplane.  To make quick, accurate computations
of wind correction angle, time, distance, ground speed, and true airspeed,
you will need either a flight computer, a kind of circular slide rule, or an
electronic flight calculator, a pocket calculator constructed with special
keys and reference programs for air navigation problems.  To determine true
airspeed using the flight computer, you must know the following:  pressure
altitude, which may be read from the altimeter in flight with 29.92 set in
the Kollsman window; temerature in degrees Celsius, which may be read in
flight from the OAT gauge [must be converted from Fahrenheit in X-Plane]; and
indicated airspeed, which may be read from the airspeed indicator in flight.
-------
I've tried it on X-Plane using a circular, slide-rule type flight computer
while flying a Beech B99 and the F-86 at various speeds and altitudes; and it
works!  My calculated TAS matched X-Plane's displayed TAS to within 2 knots
every time.

Andy Schroeder
ajbrent@aol.com