Wednesday, February 02, 2005

Speed Trap

No, I didn't get a ticket today. I've never had one. I almost always drive within ten percent of the speed limit, and aerial observation is a good way to discover the speed traps before I drive home.

I'm reading Flight Theory and Aerodynamics: A Practical Guide for Operational Safety, as I promised to, and I'm looking at the section on airspeed measurement. The way it explains IAS/CAS/EAS/TAS is somewhat confusing.

Under the heading Indicated Airspeed it mentions position error and notes that "it will be included in the instrument error chart." Then the paragraph ends. The next paragraph, under the heading Calibrated Airspeed, reads, "Calibrated Airspeed (CAS) is obtained when the necessary corrections have been made to the IAS. In fast, high-altitude aircraft, the air entering the pitot tube is subject to a ram effect, which causes the diaphragm to be deflected too far. The resulting airspeed is too high and must be corrected."

The above makes it sound as if a correction for the ram effect is made to the IAS to derive the CAS, when in fact the IAS is only corrected for position error to get CAS. This one-error-ahead situation continues through to true airspeed. To prevent confusion, I would place each paragraph heading before the error it corrects for, or make it clearer when the description switches to a new sort of error.

For anyone who is confused without even having read the chapter:

IAS = whatever the instrument says
CAS = IAS corrected for aircraft-specific position error, found in the POH
EAS = CAS corrected for ram effect (compressibility) using a chart
TAS = EAS corrected for non-standard air density
For aircraft below 10,000' and 200 kts, EAS can be considered equivalent to CAS.

I still haven't found a way to explain the concept of true airspeed that works for all audiences. Equivalent airspeed represents the rate of passage of your aircraft past the air molecules that happen to be there. Those air molecules will be more closely or more loosely packed, depending on the pressure and temperature, so their rate of passage is not the true airspeed, the speed of the aircraft through the air mass itself. People don't always grasp the concept of "movement through the air mass itself." Density is a function of pressure and temperature so the TAS is determined by correcting EAS for temperature and pressure. Maybe simplest is best, as this article from an inflight magazine illustrates.

2 comments:

aluwings said...

The flight attendant entered the flight deck one evening, and innocently asked, "How fast are we going? A passenger wants to know..." I must have been bored or feeling philosophical, because the answer went something like... "Well, this indicator says were going 250 knots due west, but that means it's more like 480 knots. But there's a 80 knot wind blowing against us, so we're really going west at about 400 knots... then again, the earth is rotating from west to east - at this latitude that's probably about 1,000 miles per hour, so we're really travelling east at about 600 knots... then again as the earth is revolving around the sun, I guess we're heading towards the belt of Orion at this time of year at some horrific speed, then there's the motion of the solar system and the galaxy... and of course the universe is still expanding ... (somewhere early on in that rather complete answer, I may have heard the cockpit door slamming behind me...). Go figure. Ask a question then not even listen to the answer...

majroj said...

Go back a step and tell them about how a pitot tube works. We sod crawlers think in terms of an odometer, not equating the ram pressure in the tube with how fast the tip is being pushed through the air.
When your pax's (formerly referred to as "SOB's", or "souls on board") ask about how fast, give them ground speed. I'm sure you have a ground-pointing Doppler just to answer that question, no?