Experimenter

JAN 2013

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.

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Table Look-Up Method The table in Figure 2 lists pressure altitudes and their corresponding pressures. Use the pressure (P) that corresponds with the pressure altitude during your test run along with the VT (ground speed in Figure 1) and OAT to calculate VC with the following equation. P is in pounds/square foot as presented in Figure 2, OAT is in degrees centigrade, VG and VC are in knots, and 0.369 includes the necessary standard sea level values of pressure and temperature to simplify the equation. Using data from our example grid and Figure 2, the equation looks like this. like Figure 1. Of course, your table probably won't have two VC columns, unless you decided to use both methods. Now it's time to turn your data grid into something more pilot friendly. A Picture's Worth 103 Words Remember, the reason for going through all this trouble is to produce a reference that will show you what your calibrated airspeed is when your airspeed indicator reads any observed airspeed. Your data grid provides that correlation for several speeds, but a plot of VC versus VO will show you that information for every airspeed. Get out a piece of graph paper and draw horizontal and vertical axes. Label the horizontal axis "Observed Airspeed" and the vertical axis "Calibrated Airspeed." Next, plot the Notice the VC (table) values are slightly different from the V C (eqn) values in Figure 1. This discrepancy is caused by the interpolation of P from the table in Figure 2 for the example calculation. The VC (eqn) in Figure 1 was calculated using a standard atmosphere table, which is more accurate than simple interpolation of Figure 2. When you consider that 0.01 knot represents a difference of about 1 foot per minute or less than three airplane lengths after an hour of flying, it's probably not worth worrying about. Equation-Only Method If you don't have access to standard atmosphere tables, dislike interpolating, or would prefer to do the entire data reduction with your calculator, here's how. Again, VG and VC are in knots, and OAT is in degrees centigrade. PA is in feet, just as you recorded it on your data grid. The numerical values account for the necessary standard sea level values of pressure, temperature, density, temperature lapse rate, and a bunch of other constants to simplify the equation. Plugging in the data from our example grid, we have By comparing the calculated values of VC (table) and VC (eqn) in Figure 1, you can see either way works fine. Repeat the entire data reduction procedure for your remaining test data pairs, and your data grid should look EAA Experimenter 47

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