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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42 Vol.3 No.3 / March 2014 FLIGHT TESTING TECHNIQUES you'll know the slow end of the band is 116 knots. If it doesn't drop, the end of the band is somewhere between 112 knots and 115 knots. Continue this bracketing technique until you fi nd the slow end of the trim-speed band. Once you know the speed of the slow end of the trim- speed band, repeat the entire procedure at airspeeds faster than the original 120-knot trim speed. Using only the control stick, accelerate to a few knots faster than 120. Do not retrim or adjust the engine or propeller controls. Stabilize at the new, faster airspeed, and relax your push on the stick. If the nose rises, you're outside the band. If the nose does not move relative to the horizon, you are still inside the trim-speed band, so accelerate a few more knots and try again. Use the same bracketing technique and careful observation as you did for the slower airspeeds until you determine the fast end of the trim-speed band. Let's say the fast end of the band was 124 knots and the slow end was 114 knots. This means your airplane has a 10-knot trim-speed band under this fl ight condition. It will maintain any airspeed within that 10 knots hands-free and without re- trimming. Knowing this can ease your future frustration when you're having dif culty trimming to an exact speed. One way to deal with a 10-knot trim-speed band during normal fl ying is to make those fi nal tiny airspeed adjustments by nudging the stick forward or aft a tiny bit, and let the control system friction hold the elevator at its post-nudge defl ection. This is often a lot easier than chasing an exact target airspeed by retrimming. An even easier solution is to just accept fl ying a couple of knots faster or slower than your target cruise speed. We identifi ed the 10-knot trim-speed band as 114 to 124 knots in our example, but the band will not always be a 6-knot- slow/4-knot-fast situation. It is a band, so it could lie anywhere around the 120-knot "trim" speed. The next time you establish a 120-knot cruise speed, the band could be 112 to 122 knots, 119 to 129 knots, or even 120 to 130 knots. You can never be sure where you are within the trim-speed band unless you test it. Naturally you are not going to perform this laborious test every time you level of , and it is not necessary. Just know that your airplane has a 10-knot trim-speed band during cruise fl ight around 120 knots. You should repeat the entire test with your airplane set up for the landing pattern because the trim-speed band could be dif erent than it is when cruising. Another important reason to perform this test for the landing condition is that we tend to fl y much more precise airspeeds around the pattern, particularly on fi nal approach. A large trim-speed band can mask any stick force cues to airspeed deviations, making it easier to drift of your desired fi nal approach speed as more and more attention is directed outside the cockpit and less time is spent scanning your airspeed indicator. Knowing you have a small trim-speed band allows you to trim for your fi nal approach airspeed and concentrate on glideslope and line-up. Whenever you notice you're holding back- or forward-stick, you'll know you're fl ying slower or fast- er than the speed you trimmed for, assuming there's no change in confi guration or power settings. All pilots use tactile cues like this whether they realize it or not, and this of -speed stick force cue is valuable feedback on fi nal approach as progressive- ly more of your attention is directed outside the cockpit. If your airplane has a large trim-speed band, you'll know that a diligent airspeed indicator scan is absolutely essential for tight airspeed control on fi nal approach. BY THE NUMBERS 1. Establish straight-and-level fl ight with the airplane trimmed for hands-of fl ight at a steady airspeed. 2. Decelerate a few knots using only the control stick. Do not retrim or adjust engine or propeller controls. 3. Stabilize at the new airspeed. 4. Note the relationship between your plane's nose (or canopy bow or some fi xed airplane reference) and the real horizon. 5. Relax your pull on the stick while observing your plane's nose and the horizon. 6. If the nose does not drop relative to the horizon, you are inside the band. 7. Repeat the process, starting at Step 2. 8. If the nose drops relative to the horizon, you are outside the band. 9. Using only the control stick, establish a new steady air- speed between this speed and the last speed at which the nose did not drop. Do not retrim or adjust engine or propel- ler controls. 10. Repeat the process, starting at Step 3. 11. Continue this bracketing technique until the slow end of the trim-speed band is determined. 12. Using only forward stick, establish a steady airspeed a few knots faster than the original trim speed from Step 1. Do not retrim or adjust engine or propeller controls. 13. Repeat Steps 3 through 8, substituting the word "push" for "pull," "rise" for "drop," and "fast" for "slow." 14. After determining the slow and fast end speeds of the trim- speed band, subtract the smaller number from the larger number—the dif erence is the trim-speed band. You don't have to rush out to test your airplane if you've never had a problem trimming your airplane or tightly control- ling its airspeed. On the other hand, if you have experienced these symptoms, maybe it's not your fault. It's worth a check during your next $100 hamburger jaunt. This month we explained how to determine your airplane's trim-speed band. We also discussed how the size of the trim- speed band can af ect airspeed control, frustration, and safety on fi nal approach. Now we're ready to delve into longitudinal static stability testing, and that will be next month's topic. Ed Kolano, EAA 336809, is a former Marine who's been fl ying since 1975 and testing airplanes since 1985. He considers himself extreme- ly fortunate to have performed fl ight tests in a variety of airplanes ranging from ultralights to 787s. E A A E X P _ M a r 1 4 . i n d d 4 2 EAAEXP_Mar14.indd 42 3 / 3 / 1 4 1 0 : 3 5 A M 3/3/14 10:35 AM