70 lines
4.1 KiB
Text
70 lines
4.1 KiB
Text
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From: AJBrent@aol.com
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To: x-plane@me.umn.edu
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Subject: Airspeed Refresher Training
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Date: Fri, 19 Sep 1997 03:28:53 -0400 (EDT)
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Excerpts from the book: An Invitation to Fly--Basics for the Private Pilot.
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The airspeed indicator registers the total pressure from the pitot head and
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subtracts from it the static pressure supplied from the static ports. This
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remainder is called dynamic pressure, which is the measure of the airplane's
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forward speed. This speed is displayed on the instrument's face on a
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graduated scale called indicated airspeed (IAS). Remember that this value
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represents the airplane's speed through the air, not necessarily it's speed
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across the ground. Why? Once it is airborne, the airplane becomes part of
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the local mass of air. If the mass of air is moving (that is, if the wind is
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blowing), the airplane will move with the air. While this is an important
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consideration during takeoffs and landings (when the airplane is making the
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transition between flight and ground operations) and for navigation (the
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moving airmass can carry the plane off course, like a ship in ocean
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currents), it means very little to the pilot in terms of normal flight
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dynamics. The airplane flies because of the speed of the relative wind, and
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this is what the airspeed indicator measures, not ground speed.
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Types of Airspeed:
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--Indicated Airspeed. This is the direct reading of airspeed taken from the
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face of the instrument, uncorrected for air density, positional errors due to
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the pitot head installation, or internal instrument error.
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--Calibrated Airspeed (CAS) is the indicated airspeed corrected for minor
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installation and pitot head position error and mechanical losses within the
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instrument itself. The manufacturer or instrument repair shop provides these
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values on a cockpit reference card or in the Pilot's Operating Handbook.
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[In X-Plane, I assume we are provided a perfect airspeed instrument so that
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IAS and CAS are the same. CAS is not simulated.]
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--Equivalent Airspeed is calibrated airspeed corrected for the
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compressibility effects of high-speed flight. Normally this is not relevant
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to private pilot flight planning. [And is not simulated in X-Plane as of
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ver. 3.4. Equivalent airspeed is also the same as IAS in X-Plane.]
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--True Airspeed is equivalent airspeed (or calibrated airspeed if
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compressibility effects are negligible) [IAS in X-Plane] corrected for the
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effects of less dense air at higher altitudes. For most light airplanes,
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true airspeed and calibrated airspeed are very close at sea level, but they
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can diverge rapidly after the airplane climbs several thousand feet. Since
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true airspeed reflects the actual physical rate at which the aircraft is
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moving through the air, it is of key importance in air navigation.
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You can easily recall the sequence of airspeed corrections leading to true
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airspeed by memorizing the acronym ICE-T, the first letters of the four
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airspeeds presented above. [Indicated, calibrated, and equivalent airspeeds
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are all the same in X-Plane. So, it's just IT.] Equivalent airspeed is
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important only on high-performance, turbine-powered airplanes. True
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airspeed, however, must be determined before wind correction angle or ground
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speed can be computed for any airplane. To make quick, accurate computations
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of wind correction angle, time, distance, ground speed, and true airspeed,
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you will need either a flight computer, a kind of circular slide rule, or an
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electronic flight calculator, a pocket calculator constructed with special
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keys and reference programs for air navigation problems. To determine true
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airspeed using the flight computer, you must know the following: pressure
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altitude, which may be read from the altimeter in flight with 29.92 set in
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the Kollsman window; temerature in degrees Celsius, which may be read in
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flight from the OAT gauge [must be converted from Fahrenheit in X-Plane]; and
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indicated airspeed, which may be read from the airspeed indicator in flight.
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-------
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I've tried it on X-Plane using a circular, slide-rule type flight computer
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while flying a Beech B99 and the F-86 at various speeds and altitudes; and it
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works! My calculated TAS matched X-Plane's displayed TAS to within 2 knots
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every time.
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Andy Schroeder
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ajbrent@aol.com
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