Experimenter

September 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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airspeed calibration information (from our airspeed calibration flight tests), we converted the observed airspeed to indicated airspeed and then to calibrated airspeed. Then we used one of those painfully long equations to convert calibrated airspeed to true airspeed, but you can perform this calculation using a flight computer, conversion chart, that same long equation, or any method you like—the answers will be the same. We omitted the conversion from calibrated airspeed to equivalent airspeed, because the testing was conducted at a slow enough airspeed, and our test altitudes were low enough to disregard this conversion. Generally speaking, you can assume equivalent and true airspeeds are the same when flying slower than approximately 200 knots calibrated airspeed and below approximately 10,000 feet pressure altitude. points and fill in the VX for the altitudes between the tested altitudes. We did this, and the result is Figure 5. Notice that we plotted VX in observed airspeed instead of true airspeed in Figure 5. While true airspeed was needed for the graphical solution to work, we used observed airspeed here, because this is what we'll see on the airspeed indicator—much handier. We simply converted the VX in true airspeed to calibrated airspeed, then used the results of our airspeed calibration flight testing to come up with the corresponding observed airspeed, then plotted the points. We've shown the line extrapolated to zero density altitude, but as we cautioned last month, this is a bit of a stretch. Another sawtooth climb test through a 1,500-foot We now have the true airspeed and climb rate for each sawtooth climb. Figure 2 shows the relationship between these two speeds and the associated climb angle, γ. Notice that the longer the rate of climb arrow in relation to the true airspeed arrow, the steeper the flight path angle will be. Another way to say this is the steepest flight path angle occurs when the ratio of climb rate to true airspeed is the maximum attainable. During the VY data reduction, we created a plot of climb rate versus observed airspeed. This time we plotted climb rate versus true airspeed. Using observed or calibrated airspeed for this plot won't work; it must be true airspeed, as shown in Figure 3. Notice also that the origin of both axes in Figure 3 is zero and the scales are linear (i.e., same distance between 20 and 40 knots as between 80 and 100 knots). These are more must-haves for this method to work. We then faired a curve through the data points in Figure 3, then removed the individual data points to come up with Figure 4. Fairing a curve fills in the missing data and makes the next step easier. We drew a straight line from the origin (zero airspeed, zero climb rate) to the curve so it just touches the curve without passing through it. This tangent line from the origin to the curve touches the curve directly above the VX (in true airspeed) and directly to the right of the VX climb rate. This tangent-to-thecurve relationship provides the most vertical speed for the least flight path speed, maximizing the climb-rate-totrue-airspeed ratio, or VX. Okay, so we now have VX for 3,800 feet density altitude. If we were to perform the same graphical solution for our climb data from 6,700 and 9,600 feet, we'd have VX for three different altitudes. Plotting each of these VX speeds versus density altitude, we could fair a curve through the EAA Experimenter 41

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