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/142883
F li g h t Te s t in g Te c hn i q u e s On the way back down through the test block, note the OAT at the midpoint of the block if you didn't record it during the climb. You'll need OAT to calculate density altitude later. The midpoint OAT should remain the same throughout your testing for each block. If you notice the OAT is changing during subsequent test runs, you can adjust the bottom and top of your test block (but keep its height the same) so the new midpoint with the new OAT is the same density altitude as the old midpoint with the old OAT. If this is starting to sound complicated, don't worry. The entire sawtooth series usually can be accomplished quickly enough to avoid changing OAT concerns. Stabilize your plane for the next climb test run at a different airspeed, and repeat the sawtooth procedure until you've completed the climb test for every test airspeed. By the Numbers 1. Load your airplane for the climb test weight and balance. 2. Take off and set 29.92 in the altimeter. 3. Establish the climb test condition before climbing through the bottom of the test altitude block. Engine, pitch attitude, airspeed, and trim should be stabilized. 4. Record the start and finish altitudes and the time it takes to climb through the altitude block. 5. Descend with idle power below the bottom of the test block, recording OAT at the block midpoint. 6. Repeat Steps 3 through 5 for all planned test airspeeds. 7. Land (don't forget to reset your altimeter), review your data for reasonableness, reload your airplane, and repeat the sawtooth climb test profile for the next test altitude block. Garbage In/Garbage Out: Quality Data Counts Good results from your sawtooth climb tests require solid preparation, precise flying, and good observations. The more diligent you are with these, the better your results will be. Here are a few guidelines. Airspeed control. Some of these results are very sensitive to airspeed variations. Traditional airspeed tolerance for this test is ±1 knot. This may sound unrealistic, but it can be accomplished by any pilot with a little practice and a diligent trim effort. 40 Vol.2 N o.7 / July 2013 Smoothness counts. Keeping your airspeed ±1 knot at the expense of large or abrupt flight control deflections will contaminate your data. Every time you move a flight control surface, you change the airplane's drag. The bigger and faster the surface movement, the greater the drag change. Some small adjustments are expected; and it's okay to make these adjustments within the test altitude block, but make them smoothly. Turbulence. Perform the test early in the morning or just before dusk to avoid the thermal turbulence of midday. It only takes one bump to invalidate your airspeed or control deflection tolerance in a small airplane. Avoid flying above terrain that may produce thermal variations or up/down drafts. You shouldn't be anywhere near clouds. Wind. A dead-calm day is best but not very realistic. Fly your climbs perpendicular to the wind to avoid any shear effects or transient airspeed indications. If you want to be even more diligent, perform each climb twice, the second immediately after the first in the opposite direction. This will further minimize any wind effects and help keep you in the same "piece of sky" for your data recording. Pitch attitude reference. Perform the test on a clear day with a distinct horizon, and use the horizon to maintain the proper pitch attitude for the climb. A grease pencil mark on the windscreen or side window may help you detect and correct tiny pitch changes before the airspeed is affected. Steam gauge artificial horizons are too coarse, and glass panel attitude indicators could have you chasing that last pixel when it may not be necessary. Straight climb. Perform each test on a constant heading. Turns while timing your climb will affect your climb performance data. Knock it off early. If you find you're almost set up as you enter the test block, it's likely you'll exceed some tolerance during the test. Rather than forcing the test run and collecting questionable data or abandoning the test halfway through the run, recognize the unstable setup, reduce power, descend, and set up again. This will save time and minimize weight changes due to fuel burned. Altitude block height. Accurately timing through a 100-foot altitude block will be difficult in an airplane with a 2,000-fpm climb rate. On the other hand, remaining within tolerance through a 1,000-foot block in an