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/271000
EAA Experimenter 41 FLIGHT TESTING TECHNIQUES HAVE YOU EVER BEEN frustrated with your airspeed control on fi nal approach? How about taking several minutes to get your plane trimmed for that exact cruising speed? Well, it might not be your fault. It might just be your plane's trim-speed band that's causing your frustration. The trim-speed band is the range of air speeds an airplane will maintain with no force exerted on the control stick. This phenomenon is usually caused by friction in the longitudinal control system that prevents the elevator from returning to its pre-defl ected position. Held at its new defl ection by friction, the elevator now commands a slightly dif erent wing angle of attack, which results in a slightly dif erent airspeed. Not all airplanes have a trim-speed band, but you won't know until you check for it. If you've never been frustrated while trying to trim your airplane for an exact airspeed, your plane may not have a trim- speed band. If you have experienced dif culty nailing an exact speed or found after you thought your plane was trimmed for one speed that it seemed happy to fl y a few knots faster or slower, there may be a trim-speed band. It's worth checking out. Here's how. THE TEST PROCEDURE Start by establishing a straight-and-level fl ight condition with the airplane trimmed for hands-of fl ight. It's not necessary to be absolutely level; a slight climb or descent is acceptable, but a steady airspeed is essential. Note your observed airspeed (what you read on your airspeed indicator). Let's say this "trim speed" is 120 knots. Now slow down a couple of knots using only back-stick. Do not retrim or adjust the engine or propeller controls. Take your time to make sure you are stabilized at a steady speed. Let's say you've stabilized at 117 knots. You'll probably be climbing slightly, and that's okay. Slowly release your pull force on the stick while watching your airplane's nose and the horizon. If the airplane's nose drops when you release the back-stick force, you are outside the trim-speed band. If the plane's nose remains at the same place relative to the horizon, you are still inside the trim-speed band. Do not rely on the artifi cial horizon, altimeter, vertical speed indicator (VSI), or the airspeed indicator for this test. The nose movement relative to the real horizon is a much fi ner gauge. If the nose moves down, it will occur immediately upon relaxation of the back-stick force, but it will probably not pitch nose-down quickly. You'll have to be meticulous to observe this sometimes subtle pitch change. Any head motion can give the appearance of the nose moving relative to the horizon, so make sure your head is perfectly still when you relax the back-stick. It's easy to become narrowly focused during this test, so don't forget your other pilot obligations, especially seeing and avoid- ing other aircraft. If the airplane's nose did drop when you released your pull on the stick, you know that 117 knots is outside the trim-speed band. Again using only the control stick, stabilize at 118 or 119 knots and repeat the test. Let's say you stabilized at 118 knots, and the nose dropped again when you relaxed your pull. Repeat the test at 119 knots. If the airplane's nose did not drop when you relaxed your pull at 117 knots, slow down a few more knots, and repeat the test. Repeat the test at progressively slower speeds until the nose drops. Let's say the nose did not drop at 117 knots, so you tested again at 112 knots where it did drop. You now know the slow end of the band is somewhere between 112 knots and 117 knots. You might try 115 knots next. If the nose drops here, Trim-Speed Band How to fi nd it for your airplane BY ED KOL ANO E A A E X P _ M a r 1 4 . i n d d 4 1 EAAEXP_Mar14.indd 41 3 / 3 / 1 4 1 0 : 3 4 A M 3/3/14 10:34 AM