Stability & Friction
OCTOBER'S "TEST PILOT" INhold that airspeed. There's troduced non-maneuverno need to measure this ing static longitudinal staforce because you're just bility. In November we going to ensure that it inexplained how control syscreases w i t h your deviatem friction can create a tions from the trimmed airtrim speed band, a range Static longitudinal stability flight test technique speed. We'll explain how to of airspeeds your airplane deal with control system will m a i n t a i n hands-off, friction a little later. and gave the step-by-step After you get a feel for flight test technique for determining flight with the airplane trimmed for the pull force it takes to fly 110your airplane's trim speed band. hands-off flight. knots, slow to and stabilize at 100 This month we'll discuss the flight You need not be absolutely level knots. At this speed the pull force test procedure for assessing your (a slight climb or descent is accept- should be greater than it was at 110 plane's static longitudinal stability at able), but a steady, hands-free airknots. You can c o n t i n u e this airspeeds slower and faster than the speed is essential. Note your ob- process u n t i l you reach the stall edges of its trim speed band. This served airspeed (what your airspeed warning speed, but that's probably test has two levels of detail. First is a indicator reads). Let's say this "trim taking the test a little too far for the simple check of whether your aircruise flight condition. For cruise speed" is 120 knots. plane has positive, neutral, or negaNormally your first test would be flight the idea is to learn how much tive static longitudinal stability. This to determine your airplane's trim stick force you need to fly slower or may be all you really need to know if speed band, using the technique ex- faster than your trim speed. Operathe friction in your airplane's longi- plained last month. In that example, tionally, this would be a temporary tudinal control system is negligible. we found a trim speed band from situation because you'd trim for the Next is the test that determines connew airspeed. 114 knots to 124 knots. Under landing pattern conditions trol system friction and how it afFrom our 120-knot trim speed, fects your plane's handling qualities. slow down to, say, 110 knots using the motivation for this testing is difback-stick only. Make sure the speed ferent. Here, you want to assess your Basic Approach you choose is slower than the slow plane's stick-force cues of an airspeed You'll assess your airplane's static end of your trim speed band. Do not change so you can use them as tactile stability by noting how much pull or re-trim or adjust the engine or profeedback, another notification of an push force it takes to fly slower or peller controls. inadvertent airspeed deviation. It's a good idea to perform this test all the After you've stabilized the airfaster, respectively, than the airspeed for which the plane is trimmed. Start plane at a steady 110 knots, note way down to the stall warning airby establishing straight and level how much pull force it takes to speed—at a safe altitude of course. 96
When you've mapped the range of speeds slower than the slow end of the trim speed band, repeat the process for airspeeds faster than the band's fast end. Apply forward-stick to accelerate, but don't touch the trim or engine/prop controls. In our example a i r p l a n e you'd probably stabilize at 130 knots and note how much push force it takes to maintain that speed. Then add a little more forward-stick and stabilize at 140 knots. Repeat this until you map the airspeeds faster than the trim speed. Remember, you're not expanding your airplane's flight envelope during this test. Do not exceed V N E or any other airspeed in the flight envelope you have not already cleared. In the landing condition, your fastest test airspeed should not exceed any speed l i m i t a t i o n of extended landing gear or flaps. N a t u r a l l y , your a l t i t u d e w i l l change during these tests, and that's o k a y — w i t h i n reason. When you approach 1,000 feet above or below the safe altitude where you initially trimmed for hands-off flight, simply climb or descend toward the original altitude and resume your testing. It's important not to re-trim or adjust the engine and propeller controls during this repositioning maneuver. We used 10-knot increments in our target-speed examples, but stabilizing a couple of knots faster or slower than the target speed is okay. What's important is getting a feel for the stick-force requirements when flying off-trim airspeeds. Predictability is a good thing, so the stick force should increase smoothly as the airspeed deviations increase. There should be enough stick force change to be obvious to you that an airspeed deviation occurred, but not so much force t h a t i n t e n t i o n a l offspeed flight is difficult. Figure 1 shows a simplified static stability curve. Remember, the stick force corresponding to a particular airspeed is the force required to maintain that speed; it is not the
force needed to change from another speed to that speed.
Friction Performing the basic stability assessm e n t w i l l be tougher if your airplane's longitudinal control system has a lot of friction. This friction increases the stick force you must apply to move the elevator. And it de-
creases the stick force required to hold the elevator in its new position because the friction opposes the natural tendancy of the elevator to return to its pre-deflected position. Spot-checking for f r i c t i o n is a good idea while performing your
first test. Using our example, 110 knots is our first target airspeed data point. Stabilize at this speed by applying back-stick to decelerate from the 120-knot t r i m p o i n t . As the speed slows to 110 knots you'll probably vary the stick pull, searching for the pitch attitude that stabilizes the airplane at 110 knots. At this point you're holding some back-stick, 5 pounds, let's say. To check for friction, slowly decrease your pull force (just like you did during the trim speed band test) until you notice the plane's nose begin to lower. You have to watch carefully because the nose drop will be subtle. Moving your head changes the relat i o n s h i p between your eyes, the plane's nose, and the horizon and can appear to be a pitch attitude change, so hold your head steady for this check. As soon as you see the nose drop, note how much pull force you're exerting and reapply a little back-stick to keep the airplane from accelerating. The pull force you apply here should not be more than the original 5 pounds you applied when you
first stabilized at 110 knots. The idea is to stay within the friction band and maintain the 110 knots. If the airspeed increases when the nose drops, re-establish 110 knots without r e - t r i m m i n g or a d j u s t i n g engine/propeller controls. 97
Let's say you're applying a 2pound pull on the stick when you see 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 pull on the stick until you see the nose start to rise. Note the force you're exerting on the stick and then relax your pull back to the original 5 pounds. Let's say the pull force is 6 pounds when you notice
trouble maintaining your final approach airspeed w i t h i n 5 knots. Sometimes you nail it, and sometimes you think you have it nailed, only to discover a few seconds later that you're 5 knots off. 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 friction. This level of static stability documenta-
the nose rise. The total friction in your longitudinal control system for this flight condition is 4 pounds (6 -
tion is probably unnecessary for your flight test program, but the curve illustrates the effect of control system friction. The green curve represents the friction band's high end, and the blue curve represents the low end. To show where it lies within the friction band we've included the original 5-pound pull force as a single data point. In our example airplane, once established at 110 knots you could exert any stick pull force between 2 and 6 pounds and not change the airspeed. This happens because friction keeps the elevator from moving until you either decrease your pull to less than 2 pounds or increase it to more than 6 pounds. In this example the 4-pound friction band is constant, depicted by
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 and 6 pounds. Only by subtracting the low force from the high force can you determine the friction. Spot-checking the friction is a good idea, but determining the friction band for the entire range of tested airspeeds is not a test you'd likely perform, unless you were looking for answers to flying task problems. For example, maybe you have 98
the parallel green and blue lines. This means the friction is the same 4 pounds at all tested airspeeds between 97 knots and 142 knots. No rule says the friction must be constant, but it usually is. Notice that the curve doesn't extend into the region between 114 and 124 knots. This 10-knot spread is our example airplane's trim speed band, which we determined in November's "Test Pilot." Remember, the d e f i n i t i o n of the t r i m 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 resulting 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 back-stick only, decelerate and establish a stabilized speed a few knots slower t h a n the slowspeed end of the trim speed band. Do not re-trim or adjust engine/propeller controls. 3. Check for friction. 3a. Carefully watch the relationship between the horizon and your airplane's nose as you slowly relax your pull force on the stick. 3b. When you see the nose start to drop, note your pull force and reapply some back-stick to keep the airplane from accelerating. 3c. Slowly increase your pull force on the stick. 3d. When you see the plane's nose start to rise, note your pull force and slightly relax your stick pull to keep the airplane from decelerating. 3e. Determine the friction band by subtracting the stick force in step 3b from the force in step 3d. 4. Using forward-stick only, accel99
erate to and establish a speed a few knots faster than the fast end of the trim speed band. Do not re-trim or adjust engine/propeller controls. 5. Check for friction. 5a. Carefully watch the relationship between the horizon and your airplane's nose as you slowly increase your push force on the stick. 5b. When see the plane's nose start to drop, note your push force and release some forward-stick to keep the airplane from accelerating. 5c. Slowly relax your push force on the stick. 5d. When you see the plane's nose start to rise, note your push force and re-apply the forward-stick push to keep the airplane from decelerating. 5e. Determine the friction band by subtracting the stick force in step 5d from the force in step 5b. 6. Repeat steps 2 through 5, targeting a test airspeed a few knots slower and faster, respectively, than your previous test airspeed until you've mapped the entire airspeed range of interest. Note: Alternating the slow and fast tests keeps your airplane at approximately the same altitude for all test points. If you were interested only in how the stick force increases as you fly farther from the trimmed airspeed, it would be more illustrative to fly progressively slower or faster test airspeeds in order. For example, 110, 105, 100, 95, 90, 130, 135, 140, 145. 100
Measuring control system friction for every test airspeed is up to you, but here's a guideline. If the nose moves after the slightest change in stick push or pull, the difference between the two forces is small and the friction is probably insignificant. In this case, you might limit your testing to the "Basic Approach." If you have to relax and increase your stick force substantially, the airplane probably has a significant friction band. You could measure the friction, but you probably already have the answer to your airspeed control difficulty question. Center of gravity (CG) can drastically affect the shape of your airplane's static stability curve. The farther aft the CG, the flatter the static stability curve, meaning it takes less stick force to fly "off-speed" airspeeds. As the CG moves progressively aft, it will eventually reach the "neutral point." When the CG is at the neutral point the airplane will maintain any airspeed hands-free, i.e., n e u t r a l static stability. With enough friction in an airplane's longitudinal control system, the plane can appear to have neutral static stability, even with the CG well forward of the neutral point. Because you're looking for tiny pitch attitude changes during this test, calm air is essential. If it's bumpy, save the test for another day. Early morning is usually the best time to find calm air.
Those same tiny pitch changes can only be accurately detected with an outside reference. The artificial horizon, a l t i m e t e r , and vertical speed indicator are too coarse. The real horizon is the best reference. You can try using a distant cloud, but clouds move, and the closer you are to your external reference, the greater the likelihood of error. W h e n m a p p i n g t h i s test's airspeed range, it's not necessary to stabilize on an exact target airspeed. Your goal is to feel how the stick force increases as you fly farther from your trimmed airspeed. l;or our example, if the pilot had stabilized at 108 or 112 knots instead of the targeted 110 knots, it would have been okay. If we were being rigorous about this test, we'd be fairing a curve through all the data points anyway. When collecting data to create a curve, high-quality test data with a consistent airspeed spread between test points is more important than testing at the exact target airspeed. The same logic applies to our test. We're essentially constructing that curve in our heads as we feel how the stick force requirement changes at the different airspeeds. This month we explained how to perform static longitudinal stability testing for airplanes with negligible control system friction and for planes with enough friction to degrade its handling qualities. We also incorporated the results of our trim speed band test discussion from last month. Next time we'll present a few scenarios that show how an airplane's trim speed band, static stability, and control system friction help or hinder you during normal piloting tasks. Thanks for reading "Test Pilot." Send your comments and suggestions to Test Pilot, EAA Publications, P.O. Box 3086, Oshkosh, Wl 54903-3086 or to editoriaK&eaa.orx with TEST PILOT as the subject of your e-mail. ^S«P 101