October 2012

Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.

Issue link: http://experimenter.epubxp.com/i/84816

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Page 45 of 47

Fl ight Testing Techniques ibility. This word "compressibility" is often associated with high- speed, near-sonic flight, but in this application it has to do with the air pressure in the pitot system. In a nutshell, at faster speeds and higher altitudes, the static pressure sensed in the pitot system (remember, the pitot sys- tem senses total pressure or static plus dynamic pressure) is not the true static pressure. The sensed static pressure is higher due to this compressibility effect, so the total pressure in the pitot system is artificially high. This causes the airspeed indicator to show a faster speed than the air- plane is actually flying. The good news for most of us is we usually don't fly fast enough or high enough to worry about correcting for this error. Table 1 shows the corrections that would have to be applied to a sampling of calibrated airspeeds at different altitudes. Un- less you fly faster than 200 knots calibrated airspeed higher than 10,000 feet pressure altitude, you can probably safely ignore this correction. Note: The manufacturer calibrates the airspeed indicator to read correctly under standard day, sea level conditions, so there is no calibrated-to-equivalent correction necessary when flying under these conditions at any speed. Equivalent and True Airspeed The higher you fly, the less dense the air is. This decrease in air den- sity affects the pressure sensed by the pitot system, and therefore, the reading on your airspeed indica- tor. Say you fly your airplane at 100 knots equivalent airspeed at sea level. The pressure in your pitot system causes your airspeed indicator to read 100 knots. (Let's assume the corrections for indica- tor, position, and compressibility errors are zero for simplicity.) If 46 NO. 2/OCTOBER 2012 you fly your airplane at 100 knots equivalent airspeed at 10,000 feet, the less dense air means a lower sensed pressure in the pitot sys- tem, and that results in a lower airspeed reading on your airspeed indicator. Alternatively, if you fly your plane at 10,000 feet with an airspeed indicator reading of 100 knots, your true airspeed will be faster. T e sensed static pressure is higher due to this compressibility eff ect, so the total pressure in the pitot system is artifi cially high. You may be familiar with airspeed indicators that have true airspeed indicating capability. Rotating a temperature scale to align outside air temperature with your pres- sure altitude on a pressure altitude scale also rotates a true airspeed scale behind the indicating needle, allowing you to read true airspeed directly along with observed air- speed. This simple device works because aligning the outside air temperature and pressure altitude scales compensates for density altitude. Density altitude is pressure altitude corrected for temperature. True airspeed is equivalent airspeed corrected for density altitude. True Airspeed and Ground Speed Ground speed is true airspeed cor- rected for wind. This wind correction is learned by every private pilot and used by every pilot every time we fl y. Although ground speed has nothing to do with your airplane's airspeed indicating system, it completes our look at the fl ight speed picture. As Figure 1 shows, what you read on your airspeed indicator is observed airspeed. Correct the observed airspeed for internal gauge errors, and you get indicat- ed airspeed. Correct the indicated airspeed for installation/position errors to get calibrated airspeed. Account for high-speed and /or high-altitude flying to find equivalent airspeed. Correct equivalent airspeed for density altitude to find true airspeed. Apply wind corrections to your true airspeed to determine ground speed. If all these different airspeed corrections sound intimidating, take heart. If your flying habits or airplane limitations keep you below the equivalent airspeed correction altitudes and airspeeds, you'll need just two tests. The manometer bench test will account for any errors in the gauge, and an airspeed calibration flight test will take care of any installa- tion errors. Okay, we've laid the groundwork with this airspeed primer for next month's topic – airspeed calibra- tion. We'll take a look at a few flight-test methods you can use to identify your airplane's position er- ror corrections. Questions about flight testing? E-mail experimenter@eaa.org; please put "Flight Testing" in the subject line. Ed Kolano, EAA 336809, is a former Marine who's been flying since 1975 and testing airplanes since 1985. He considers himself extremely fortunate to have performed flight tests in a variety of airplanes ranging from ultralights to 787s.

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