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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36 Vol.3 No.5 / May 2014 FLIGHT TESTING TECHNIQUES did during the trim-speed band test) until you notice the plane's nose begin to lower. You'll have to watch carefully because the nose drop will be subtle. Any head move- ment that changes the relationship between your eyes, the plane's nose, and the horizon can appear to be a pitch attitude change, so make sure you hold your head steady for this check. As soon as you notice the nose drop, note how much pull force you're exerting and reapply a little more back-stick to prevent the airplane from accelerating. The pull force you apply here should not be more than the original 5 pounds you were holding when you fi rst stabilized at 110 knots, because the idea is to stay within the friction band and maintain the 110 knots. If the airspeed increases when the nose drops, sim- ply re-establish the 110-knot condition without retrimming or adjusting engine or propeller controls. Let's say your pull on the stick is 2 pounds when you notice the nose drop. So far, you've established the low end of the control system friction band. Now repeat the process in the opposite direction. Slowly increase your pull force on the stick until you notice the nose start to rise. Note the force you're exerting on the stick, then relax your pull back to the original 5 pounds. Let's say the pull force is 6 pounds when you notice the nose rise. The total friction in your longitudinal control system for this flight condition is 4 pounds (6 – 2 = 4). You must perform the test in both directions because you don't know where your original 5-pound pull was within the friction band when you first stabilized at 110 knots. In this example, our airplane will maintain 110 knots with any stick pull between 2 pounds and 6 pounds. Only by subtracting the low force from the high force can you determine the friction. Although spot-checking the friction is a good idea, de- termining the friction band for the entire range of tested airspeeds is not a test you would likely perform unless you were looking for answers to fl ying task problems. For ex- ample, maybe you have a dif cult time maintaining your fi nal approach airspeed within 5 knots. Sometimes you nail it, and sometimes you think you have it nailed only to discover you're 5 knots of a few seconds later. Control system friction could be the cause, but you won't know for sure until you perform this test. Figure 2 shows a static stability curve that includes fric- tion. This level of static stability documentation is probably unnecessary for your fl ight test program, but the curve il- lustrates the ef ect of control system friction. The green curve represents the high end of the friction band, and the blue curve represents the low end. We've also included the original 5-pound pull force as a single data point to show where it lies within the friction band. In our example airplane, you could exert any pull force on the stick between 2 and 6 pounds, and not change your airspeed once established at 110 knots. This is because the friction prevents the elevator from moving until you either decrease your pull to less than 2 pounds or increase it to more than 6 pounds. For this example, we've presented the 4-pound friction band as constant, depicted by the green and blue lines parallel to each other. That is, the friction remains the same 4 pounds at all tested airspeeds between 97 knots and 142 knots. There is no rule that says the friction must be constant, but it usually is. Notice the curve does not extend into the region between 114 knots and 124 knots. This 10-knot spread represents our example airplane's trim-speed band that we determined two articles ago. Remember, the defi nition of the trim-speed band is a range of airspeeds the airplane can maintain hands- free. No stick pull or push force is needed within the band to maintain airspeed there, but you could exert a variety of stick forces (up to the breakout force) within the band with no re- sultant airspeed change. This month we are interested in the airplane's static stability outside its trim-speed band, so stick forces are not shown within the trim-speed band. BY THE NUMBERS 1. Trim for hands-free, level flight at the desired flight condition. 2. Using only back-stick, decelerate and establish a stabi- lized condition a few knots slower than the slow-speed end of the trim-speed band. Do not retrim or adjust engine or propeller controls. 3. Check for friction. a. Carefully watch your airplane's nose and the horizon as you slowly relax your pull force on the stick. b. When the plane's nose begins to lower, note your pull force and reapply some back-stick to prevent the airplane from accelerating. c. Slowly increase your pull force on the stick. d. When the plane's nose begins to rise, note your pull force and slightly relax your back-stick pull to prevent the airplane from decelerating. e. Determine the friction band by subtracting the stick force in Step 3b from the force in Step 3d. 4. Using only forward-stick, accelerate and establish a stabi- lized condition a few knots faster than the fast-speed end of the trim-speed band. Do not retrim or adjust engine or pro- peller controls. 5. Check for friction. a. Carefully watch your airplane's nose and the horizon as you slowly increase your push force on the stick. b. When the plane's nose begins to lower, note your push force and slightly relax some forward-stick to prevent the airplane from accelerating. c. Slowly relax your push force on the stick. d. When the plane's nose begins to rise, note your push force and reapply your forward-stick push to prevent the airplane from decelerating. e. Determine the friction band by subtracting the stick force in Step 5d from the force in Step 5b. E A A E X P _ M a y 1 4 . i n d d 3 6 EAAEXP_May14.indd 36 5 / 5 / 1 4 3 : 2 0 P M 5/5/14 3:20 PM