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/134623
F li g h t Te s t in g Te c hn i q u e s tude adjustments even if the airspeed remained within tolerance. It's a lot easier to make sense of the data after the flight if you have good notes rather than trying to remember on which of the dozen test points that anomaly occurred. Having plenty of other points that do fall on the curve allows you to disregard an outlier, or regard it with less confidence than the others when fitting the curve. The flight path angle is a negative number because we were descending during the 85-knot test speed as shown by the negative numbers in the Alt Chg and ROC columns of the worksheet. Enter the flight path angle in the FPA column. The Plot With the worksheet complete, you can now construct the flight path stability plot. Figure 3 shows the plot of airspeed versus flight path angle from our example data. Notice we used observed airspeed despite the fact that true airspeed was used for the flight path angle calculation. We did this because observed airspeed is what you see in the cockpit, and that's more useful to you on final approach than calibrated or true airspeed. After plotting the individual data points, fair a smooth curve among the points. See how the curve fills in for the missing data? You also can see that testing at a fairly evenly spaced set of airspeeds makes for an easier curve fit and helps show any outliers. In our example, the pilot wandered outside the airspeed tolerance during the 66-knot test. Making a note of deviations like this immediately following that run can help explain a point that doesn't fall on the curve. While airspeed control and timing deviations are obvious "noteworthy" events, don't discount remarking if a run just didn't feel right. Maybe you felt you made too many pitch atti- Remember, it's the character or shape of the curve that shows you how sensitive the flight path angle is to airspeed deviations. For example, let's say you normally fly the final approach at 75 knots but use 70 knots for short field approaches. That puts you at the peak of the curve where any airspeed deviation ultimately results in a steeper flight path angle. Any flight path angle corrections in this case will require a power adjustment. Good to know, huh? We started the test with the airplane in level flight at the final approach speed of 75 knots. We did this for convenience to remain within the 500-foot altitude band and to minimize power changes. Had we started at 75 knots on a 3-degree flight path angle, the numbers would be different, but the shape of the curve would be the same. We'd also have had to add power to climb back into the test altitude block between tests, increasing the likelihood of contaminating the data by not having the power identical from run to run. For our example airplane, we could simply slide the curve down until the 75-knot point is even with, say, the 3-degree flight path angle. This will still show the flight path angle sensitivity to airspeed deviations and now the resulting flight path angle for a variety of airspeed deviations after you're established on that 3-degree approach. Finally, remember that this curve only applies to the tested configuration. If you sometimes use half flaps, test that way as well. Same goes for other configurations that can affect performance such as open cowl flaps (although not likely) or flying your Cub with the hatch open. That's it. For the past three months we talked about why flight path stability is important, how it affects the way you fly your airplane on final approach, how to perform the flight test, and how to turn that flight test data into your airplane's flight path stability curve. Next month we will tackle a different subject. 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. Figure 3 40 Vol.2 N o.6 / June 2013