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
Fl ight Testing Techniques Table 1 the published calibrated airspeeds. You use calibrated airspeed in your true airspeed calculations, so the airplane manufacturer wants to ensure you know what you read on the airspeed indicator may have to be adjusted before you perform any planning. We're differentiating between observed airspeed and indicated airspeed because the airspeed in- dicator itself may not be complete- ly accurate. FAA airworthiness standards for small airplanes call for minimum instrument calibration error. This is the error inherent in the gauge itself. Indicated airspeed is observed airspeed corrected for airspeed indicator internal errors. You can have your indicator bench- tested to determine any errors between what the airspeed indica- tor reads and what it should read based on the pitot and static pres- sures applied to it during the bench test. You've probably seen articles explaining how to do this with a simple water manometer used to apply air pressure to the airspeed indicator. The applied pressures correspond to appropriate air- speed readings. The technician compares the appropriate indi- cated airspeeds with the readings observed on your airspeed indica- 44 NO. 2/OCTOBER 2012 tor. Once you know these errors, you can correct what you read on the airspeed indicator (observed airspeed) to what it should read (indicated airspeed). Indicated airspeed is observed airspeed corrected for airspeed in- dicator internal errors. Note: Some texts refer to what you read on the gauge (what we're calling ob- You've probably seen articles explaining how to do this with a simple water manometer used to apply air pressure to the airspeed indicator. served airspeed) as indicated air- speed and the airspeed corrected for indicator internal errors (what we're calling indicated airspeed) as true indicated airspeed. Too much of an opportunity to confuse it with true airspeed, so we'll stick with observed airspeed. Indicated and Calibrated Airspeed The observed-to-indicated bench test is accomplished by applying specific pitot and static pressures to the respective fittings on the back of your airspeed indicator. These same fittings are used to connect your airspeed indicator to your airplane's pitot and static lines that go to your airplane's pitot tube and static ports. These lines route the air pressure sensed at the pitot tube and static sources to your airspeed indicator. Unfortu- nately, the pressures sensed at the pitot tube and static sources are not always the real ambient pressures, and an in-flight calibra- tion is necessary to account for these errors. Although it's a common belief that the errors are mainly due to the pi- tot tube not being oriented directly into the relative wind, for example, during slow flight or high angle of attack flight, most of the problem comes from the static side of the system. Static ports are generally along the side of the fuselage or pitot tube. The object is for these ports to be exposed to the ambi- ent air pressure without allowing any ram pressure to enter. Ram pressure is pressure caused when air is forced into the port by virtue of the airplane's forward speed. It's the pitot tube's job to sense that ram pressure along with the