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/90184
So it is with airspeed. If you don't start from a known place, everything else based on that place is suspect. Now it's time to take your airplane flying to determine whether its airspeed indicator is telling the truth or whether it's lying. Last month we described the various airspeeds pilots deal with: • Observed corrected for instrument error yields indicated. • Indicated corrected for installation error yields calibrated. • Calibrated corrected for compressibility yields equivalent. • Equivalent corrected for density altitude yields true. • True corrected for wind yields ground speed. Okay, let's get practical. Your airspeed indicator is already installed and plumbed, so you don't need to worry about the instrument error. Most homebuilt airplanes don't fly fast enough or high enough to worry about the compressibility effects. This makes your air- speed calibration easier because you can assume your calibrated and equivalent airspeeds are the same. The practical information you want to know is the calibrat- ed airspeed that corresponds to the observed airspeed you read on your airspeed indicator, and that's what this test will tell you. There are a couple of reasons you'll want to know the relationship between calibrated airspeed and what you read on your airspeed indicator. One is so you can calculate true airspeed for cross-country planning and know what you should read on your airspeed indica- tor to achieve that true airspeed. Another is that most airspeed limits, like maximum flap extension speed, are usually stated in calibrated airspeed. There are several acceptable methods for airspeed calibration: They range from exotic laser tracking to trailing bomb usage to pacer airplane formation flying to the fairly math-intensive tower flyby to the simple ground course. And then there are several methods that use GPS, and we'll address those in future Experi- menter issues. Ground Course The ground course method, sometimes called speed course, is straightforward. You simply time how long it takes to fly a known distance. You determine your ground speed by dividing the distance flown by the time it took to fly it. Then you apply a correction for air density because you probably won't fly your test at sea level on a standard day, and you get your calibrated airspeed. Now compare this airspeed with what you Consider selecting your course so that you can eas- ily see your airplane's shadow on the ground. You can get a much more accurate time hack by noting when your shadow passes the checkpoints, or better yet, a straight-line ground feature perpendicular to your course that passes through your checkpoints. Another advantage to using your shadow is you can fly much higher—a couple of hundred feet—and still get an ac- curate time hack at the checkpoints. EAA EXPERIMENTER 51 saw on your airspeed indicator during your test run, and you'll know what your airplane's calibrated air- speed is when your airspeed indicator reads the value it did during the test. By flying reciprocal headings (not tracks) for each test and averaging the ground speeds, you eliminate the wind effects. Repeat the process for the range of airspeeds your plane is capable of flying, and you can create a table or plot of calibrated versus observed airspeed. Repeat the entire process for each different landing gear and flap configuration to get ad- ditional applicable plots or tables. There are a couple of reasons you'll want to know the relationship between calibrated airspeed and what you read on your airspeed indicator. There are a few practical rules for the ground course method. First, you'll need a ground course with some special features. It should be essentially flat, because you may be flying very low. The lower you fly, the easier it will be to accurately time your start and end points passage. Do not fly lower than about two wing- spans to ensure you'll remain out of ground effect. Flatness is also warranted because each run must be flown at a constant airspeed, and you'll want to avoid climbing or descending. Terrain features should be consistent to avoid any- thing that could cause a variation in airspeed or alti- tude like land/water shorelines that can generate ther- mal activity or abrupt drop-offs with associated up/ down drafts. Clearly identifiable start and end points will be necessary. You don't want to be searching for that special tree among many when flying in this risky environment. Plan your course so your checkpoints are to the side of your track. This will make it easier to "hack" your time as the leading edge of your wingtip passes the checkpoint.