MAY'S "TEST PILOT" SHOWED the versatility and increased safety of using angle of attack (AOA) instead

the power set. (The power setting doesn't matter in this example, only that it does not change.) Point E in of airspeed indications for Figure 1 depicts this condicertain flight conditions. tion for m a n y airplanes, We concentrated on two and in our example Point E i m p o r t a n t cases—maxirepresents a 75-knot apmum engine-out glide and proach speed down a 2-delanding pattern final apgree glide path. proach. For the glide we Leaving the power alone, showed that a single AOA yields able assumption. It would be the if you pull the stick back a bit and your a i r p l a n e ' s m a x i m u m glide case during a forced landing follow- fly slightly slower than 75 knots, range. ing an engine failure. And it makes your flight-path angle becomes shalWe also touted the advantage of sense from the standpoint of the sta- lower or less negative. For example, AOA over airspeed as a f i n a l ap- bilized approach we all work so hard if you held enough back-stick to proach speed reference, but what if to establish—an approach where the maintain 70 knots, your flight path your airplane doesn't have an AOA pilot is not continually correcting would be about 1/2 degree below indicator? How do you know you're judgment errors with avgas. horizontal f l i g h t , as depicted by flying the proper final approach airAssuming constant power allows Point D. Fly slower and (assuming the unspeed? More importantly, do you us to legitimately explore how know what happens to your vertical changing just one variable (airspeed) changed power is sufficient) your flight path when you deviate from affects one other variable (vertical flight-path angle becomes shallower, flight path). We'll explain the effect reaching level f l i g h t at 66 knots the proper speed? A simple flight test answers all power changes have on flight-path (Point C). Slower still, and you'll be three questions. The data reduction stability later; for now the constant at Point B, and in a slight climb. So far your plane is behaving as is straightforward. The math is easy. power assumption will make the iniwe'd expect because it's been flying And the results for your a i r p l a n e tial explanation easier. on the front side of the flight-path may surprise you. Don't think it's Once Around the Curve stability curve, where all airspeeds are worth it? Read on. Flight-path stability describes how Let's start with your typical final ap- faster than the airspeed for the curve's changes in airspeed affect your air- proach: You're on glide slope at the peak at Point B. When operating on plane's vertical flight path (or flight recommended approach speed with the front side of the flight-path stability curve, pull the stick path or glide path or back to fly slower and descent angle). Genershallow your f l i g h t ally it applies to the fipath, and push the nal approach f l i g h t stick f o r w a r d to fly condition where your faster and steepen your wings are level and flight path. you're controlling your Airspeeds to the left glide slope (or descent of Point B are on the angle) by a d j u s t i n g back side of the curve, power and airspeed. where the slower you For now let's asfly the steeper your desume power is conscent angle or f l i g h t stant during your final path is, unless you add approach. This is not power. Here's w h y . an entirely unreason103

Suppose you establish your airplane on final approach at Point A, about 47 knots. Notice that your flightpath angle is the same as if you were flying at Point E, or 75 knots. Remember, the power setting is unchanged, and all the points in Figure 1 represent a single power setting. If you're not aware that you're flying on the back side at Point A and you want to make your flight-path angle shallower, you might pull the stick back and slow down a few knots. I n i t i a l l y , your f l i g h t path would become shallower because you've increased the wing's lift by pulling the stick back. In effect, you'd be trading airspeed for altitude. This result is only a balloon effect. With the increased lift comes increased induced drag. As your airplane stabilizes at its new, slower airspeed, with no change in power the increased drag results in a steeper descent angle than you had at the Point A airspeed. This is the insidious nature of the back side. At this point you have two options for achieving a shallower flight-path angle: You can lower the nose and accept a temporarily steeper descent angle (same balloon effect in reverse) until the speed increases above the Point A value. At your new, faster airspeed, the descent angle is shallower. One problem with this nose-lowering option is that it goes against pilot intuition, especially on final approach, where you don't have a lot of altitude to trade for airspeed. That leaves 1O4

is probably the best idea at this point. Returning to our constant power assumption, we made it so we could see how changing one variable—airspeed—affects the vertical f l i g h t path. If you add power, it shifts the entire flight-path stability curve in Figure 1 upward. Every speed slower than Point B would still be on the back side, and every speed faster would still be on the front side, but the corresponding descent angles would all be shallower. Add enough power, and you might even climb at the Point A airspeed. Reducing power shifts the curve downward. Pull enough power, and Point B would be a descent, just like it would be in an engine-out situation. In short, power changes move the curve up or down, but they don't significantly affect the curve's shape. Shape Matters

your only other option—add power. Either way, completing the approach after wandering the back side would be a salvage effort, and a go-around

Airplanes don't share the same shape flightpath stability curve. Figure 2 shows the curves for two airplanes that have a 75-knot final approach speed. If the pilots of these airplanes pushed their respective sticks forward to establ i s h an 83-knot approach speed, they'd realize markedly different changes in their flightpath angles. Airspeed deviations in Airplane Y result in large changes in descent angle, making airspeed control a significantly more critical task than in Airplane X. Because small airspeed variations in Airplane Y result in comparatively large flightpath changes, its pilot must diligently control airspeed to remain on

you'd also be traveling forward slower. It will take longer to come down, but you won't travel as far as you would have at the faster speed. Let's take this scenario into the cockpit to see why it's an insidious perception deception. You're established on short final at 65 knots, but you see that you're not going to reach the runway. In an effort to stretch your glide, you nudge the stick back just a bit. As the plane's nose comes up, there's a reassuring balloon effect. Even after the balloon, the VSI needle settles onto a smaller descent rate as you continue flying at 55 knots. the desired glide slope. On the other behaved airplanes. The curve in Fig- From the information available to you hand, Pilot Y can make minor, short- ure 3 is smooth and predictable, in the cockpit, it looks like you've term glide slope corrections using too—unless you decelerate more solved the problem, and the VSI has just the stick and not have to adjust, than about 5 knots from the desired stabilized at a lower descent rate. then reset, the throttle. 75-knot approach speed. What you can't see is that your acAirplane X's descent angle is not Notice how sharply Figure 3's de- tions have steepened the airplane's as sensitive to airspeed variations, scent angle increases as the airspeed flight-path angle by 1/2 degree. and to make an effective flight path drops below about 70 knots. Trying Not in your airplane, you say? correction, Pilot X would have to de- to stretch a glide in this airplane can The curves in Figure 4 belong to a viate a lot farther from the proper result in the bottom falling out too Cessna 172. approach speed. The flatness of Air- close to the ground to recover, even It's the flight path angle that deplane X's curve can lead to sloppy with power. termines whether you'll reach the There's another insidious percep- runway or clear the trees into that airspeed control because Pilot X can maintain a near-proper glide slope at tion deception with some airplanes. forced-landing field. Unless your airAirplanes don't come with vertical- plane has a descent angle indicator, a number of different airspeeds. If Pilot X doesn't notice—and flight-path-angle indicators. They you need to understand your aircorrect—an airspeed deviation, he have airspeed and vertical speed in- plane's descent angle sensitivity to or she will likely see one of two out- dicators, and you adjust these to es- airspeed changes, i.e., its flight-path stability characteristics. comes. Airplane X will land hard be- tablish the desired descent angle. Some final approaches give you This month we introduced the cause it doesn't have enough airspeed left for a proper flare. Or it approximate glide slope indications flight-path stability concept, comwill float down the runway in the through VASI and PAPI systems, but pared different flight-path stability flare, dissipating its excess airspeed. at most VFR airports, pilots must vi- characteristics, played a couple of In A i r p l a n e X, you'd most likely sually assess their descent angle. We what-if games, and showed how difmake flight-path corrections by ad- do this by mentally combining the ferent flight-path stability characterjusting power rather than by chang- picture through the windscreen with istics determine your safety margins our flight instruments, but this in- and how your plane's flight-path staing airspeed. bility can affect the way you fly. The reality is most pilots don't fly formation can be deceptive. Figure 4 is a composite plot that Next month we'll present flight-test one-handed approaches. We manipulate the throttle and the stick to shows flight path and descent rate techniques and follow that up with control the vertical flight path. versus airspeed. Notice that slowing the data reduction that will enable Flight-path stability curves indicate from 62 knots to 55 knots results in you to create flight-path stability how much of each you'll have to use a slower descent rate but a steeper curves for your airplane. Please keep those comments and descent angle. This apparent contrato make glide slope adjustments. diction occurs because the change in suggestions coming to Test Pilot, Back Side or Dark Side? true airspeed has a bigger effect on EAA Publications, P.O. Box 3086, Because they are smooth and pre- the flight path than it does on the Oshkosh, WI 54903-3086 or edito dictable, the flight-path stability descent rate. Look at it this way. [email protected] with TEST PILOT as the ££•> curves in Figure 1 and 2 imply well- You'd be coming down slower, but subject of your e-mail. 105

IN THE JUNE "TEST PILOT" WE introduced flight path stability, or how airspeed changes affect y o u r a i r plane's vertical flight path. We described the intuitive expectations when flying on the front side of the flight-path stability curve—and the perception deception danger of the back side. We completed our discussion by looking at different flightpath stability curve shapes and how they influence the way we control glidepath during final approach. Now it's time to explain how you can test your airplane to determine its flight path stability characteristics. This f l i g h t test procedure is straightforward, but it can be demanding because you'll have to keep your airspeed within just one or two knots of the test speeds to acquire meaningful data. Please don't assume this is beyond your skill level. Absolutely smooth air is essential, and all you may need is a little practice. (And we'll give some helpful hints to help you nail that airspeed.) The basic test philosophy is simple. We want to know how changing airspeed on final approach affects your airplane's descent angle. Your airplane doesn't have a descent angle indicator, but, as we mentioned last month, your airplane's true airspeed and descent rate determine its vertical flight path angle (g, pronounced "gamma," Figure 1). We'll use this relationship to determine your plane's descent angle for every tested airspeed. From 118

this information, we'll create the flight-path stability curve. Flight-path stability information is most useful for final approach, so your plane's test configuration

should match this condition regarding the position of landing gear, flaps, cowl flaps, or any other external (drag-affecting) changes. You can perform this test in any configuration you like, but the results will apply only to that configuration. Throughout the test we're going to leave the power controls (throttle, prop, and mixture) alone. (If you normally fly power-off approaches, you can test that way, but it will take longer as you'll soon read.) Setting the controls for sufficient power for level flight at your typical final approach airspeed is a good place to start. The power the engine-propeller combination delivers depends on airspeed, so the power won't really be constant throughout the test, even with constant throttle, prop, and mixture settings. The effect this has on the shape of your flight path stability curve should be negligible. Here's the basic idea. You're going to record airspeed and descent rate at several airspeeds. How many test points you record is up to you, but more data will give you better results. You'll want to map the range of airspeeds you could conceivably fly on final approach. For example, if your plane stalls at 60 knots and its m a x i m u m gear-down speed is 100 knots, you m i g h t t a r g e t eight air-

speeds at 5-knot increments between 65 and 100. At a safe altitude establish level flight and trim tor hands-off flight. Note the observed airspeed (what you read on your airspeed indicator) and outside air temperature (OAT) because you'll use these to calculate your true airspeed after the flight. You'll also need your pressure altitude for this calculation, so set your altimeter to 29.92. Let's say your level flight speed is 80 knots. Using only back stick, slow down a few knots. Your target speed is 75 knots, but you can accept a couple of knots faster or slower. You don't have to be exactly on speed here because you're going to record data over a range of airspeeds and draw a curve to fill in the airspeeds you don't test. Just make sure the spacing between test points, i.e., the difference between the airspeeds where you record the data, is reasonably consistent. The flight-test data card in Figure 2 shows reasonably consistent test point intervals. At this new, slower airspeed you'll probably be climbing. That's okay. Stabilize your airplane at the new airspeed. Your plane will be stabilized at the test condition when its airspeed needle is rock steady, your pitch attitude is unwavering, and your pull on the stick is constant. Naturally, none of these things will happen as you slow down, but once you arrive at the test point airspeed, they must be stabilized. Technically, you should not retrim if your horizontal tail uses a movable trim device like a tab or stabilizer. If the trim mechanism is an internal spring/friction system, retrimming is okay. When you're sure you're stabilized, record your observed airspeed and OAT and t i m e your altitude change. Don't record the vertical speed indicator value for the altitude change. The VSI is too coarse for this test. Timing the altitude change will give you more accurate, refined data. Your airplane's performance will dictate how long your timing should 119

be. It should be long enough to have confidence in your data and short enough for you to m a i n t a i n the

rock-steady test-point flight condition. Generally, time for 30 seconds or an altitude change of 500 feet, whichever occurs first. (You can record the VSI reading to corroborate your timing.)

After you've recorded the data re-

lax your pull on the stick and take a

break. Then apply a push to the stick and establish your next target speed, which in our example would be 85 knots. You'll most likely be descending now, which is good, because it

will bring you back toward your initial level flight altitude. Perform the same test at this new, faster airspeed.

Take another break, and then perform the 70-knot test point, and so on until you have data spread at approximately 5-knot increments from 65 to 100 knots.

That's all there is to it.

By the Numbers

1. Establish a level flight condition with the airplane trimmed for

hands-free flight; set the altimeter to 29.92. . •

2. Record the observed airspeed and OAT. 3. Using only the control stick/yoke, establish a new airspeed a few knots slower than the speed recorded in Step 2. 4. When absolutely steady, begin timing, noting the altitude when timing begins. 5. Record the new airspeed and OAT. 6. Time for 30 seconds or 500 feet of altitude change. 7. Record the a l t i t u d e passing when timing is complete. Record the elapsed time. 8. Using only the control stick/yoke, establish a new airspeed a few knots faster than the speed recorded in Step 2. 9. Repeat Steps 4 through 7. 10. C o n t i n u e this a l t e r n a t i n g slower/faster process u n t i l all 120

planned airspeed test points are accomplished. 11. Reset your altimeter to the local setting.

Hints

Is he crazy? How am I supposed to maintain an off-trim airspeed within one knot for 30 seconds! Calm air is essential. The slightest turbulence can upset your airplane enough to cause bad data. Even if the airspeed indication doesn't change, the fact that your plane was just shoved up or down can ruin the test point. If you catch a rogue gust, just restart the test a f t e r you re-establish the steady flight condition. Wiggling control surfaces also contaminates data. You can maintain a constant airspeed while rapidly moving the stick fore and aft, but the resulting tail wagging creates drag that can affect your results. Don't chase the airspeed needle with your airplane's nose because the airplane will never really stabilize at the desired speed. Similarly, using an artificial horizon to maintain your pitch a t t i t u d e will likely lead to frustration because it may not provide the pitch attitude change discrimination necessary for steady, constant-airspeed flight. Use the real horizon to hold the required pitch attitude. It lets you see tiny pitch changes long before any instrument will indicate a change in the position of the airplane's nose. There are a couple of gotchas, however. If you move your head, you'll change the visual relationship between your plane's nose and the horizon. One way to avoid this is to put your head against the headrest and move just your eyes to look in different directions. If you don't have a headrest, you can put a grease pencil mark on the windscreen so it lines up with the top of the cowling. As long as the mark and the cowling top line up, your eyes will remain in the same spot. This test presents some challeng121

ing flying. Any pilot can do it, but not indefinitely. As soon as you're sure the airplane has stabilized on the test-point flight condition, begin timing. While you're waiting for that 30 seconds to pass, note the airspeed, OAT, and anything else—like VSI—you want to record. Note the OAT in the middle of the altitude change. If you save your timing for last, you'll have to maintain that demanding flight condition longer. So how do you write down all this stuff while concentrating on flying with your head glued to the headrest? Don't. Make a m e n t a l note of airspeed, OAT, start altitude, and end altitude during each test, and then write them down when you finish testing that speed. Or you can use either a small tape recorder plugged into your intercom or a copilot to record the numbers as you call them out. (No co-pilots unless you've completed the FAA-mandated fly-off time!) Flying with a co-pilot can be a good idea because he or she might detect an airspeed variation or ex-

122

cessive stick activity you might not notice when concentrating on flying the test point. Your scribe can also keep a rough plot of airspeed versus descent rate as a quality check on the data. Any data point that appears to fall far from the emerging curve is suspect. With this near-realtime analysis, you could re-fly the suspect points immediately rather than discovering them after your test flight. Your attention will be focused on the horizon while your co-pilot performs see and avoid duties. If you don't have a co-pilot, you may want to perform clearing turns prior to

each test.

Perform the tests in an order that keeps you near your original level flight altitude. Follow a climbing test point with a descending test point. You may have to perform two descending or climbing test points in a row to remain near the desired altitude. That's okay. You could return to your original altitude between tests, but this takes time and may force you to adjust power. Re-

maining within plus or minus 1,000

feet of your starting altitude should keep your data consistent. It's okay to abort a test and re-fly it for any reason. After flying a few test points, you'll know when you nailed it and when you didn't. Making some kind of quality remark for every test point on your data card can help explain a data point that doesn't f a l l on the curve. You can also use this metric when fitting the curve to your data points. Draw the curve right through the high-quality points. If the not-so-high-quality points don't fall on the curve, you'll already have a good idea why. Avoid changing the power settings between tests. It's nearly impossible to re-establish the exact power s e t t i n g s a f t e r you've changed them. While the particular power setting is not significant, the fact that it remains constant

for all the tests is. Finally, remember flight test's safety mantra—aviate, navigate, communicate, evaluate. Fly your

plane first. Airspace boundaries, collision avoidance, engine temperatures, and many other considera-

t i o n s have h i g h e r priority t h a n getting these data.

Next month we'll massage the flight test data in Figure 2 a little and construct the flight-path stability curve. Tell us what you'd like to read about in "Test Pilot." The address is Test Pilot, EAA Publications, P.O. Box 3086, Oshkosh, WI 54903-3086 or [email protected] with TEST PILOT as the subject of your e-mail. ij&> 123

Stick & Rudder

Test Pilot IT'S THE MIDDLE OF THE FLYING determine t r u e airspeed, season, and if you've alyou need density altitude ready flown to one of the and calibrated airspeed. To bigger sport aviation gathdetermine density altitude, erings like Sun 'n Fun or you need pressure altitude AirVenture, you've probaand OAT, and you already bly experienced the chalhave these in your raw test lenge of those tightly sedata. quenced landings. You What to do with all those numbers you gathered Fill in the Blanks know the k i n d — b e h i n d the slower airplane and Altitude Change—DurED KOLANO ahead of the twin—all of ing your f l i g h t test you you on the same final approach norThe flight-path stability data grid noted the pressure altitude when mally occupied by just one airplane provided last month contained ob- you started timing (PA1) and when at a time. served airspeed (read directly from you ended timing (PA2). Determine The controllers wanted the planes your airspeed indicator), start and t h e a l t i t u d e change d u r i n g your to maintain constant intervals, and finish altitudes and the elapsed time timed climbs or descents by simply to do that you tried to fly the same required to travel between them, subtracting PA1 from PA2 for each airspeed as the other guys. You made outside air temperature (OAT), and a test event. Enter the altitude change your best guess at how fast the plane place for optional remarks. Figure 1 in the Alt Chg column of your workahead of and behind you were flying shows our data reduction worksheet, sheet. Let's use the 85-knot test point on final approach. Maybe it was no which includes the data from last as an example. problem, and maybe it was. It de- month's test card data grid and the pended, in part, on your plane's calculated numbers we'll explain Alt Chg = PA2-PA1 flight-path stability characteristics. this month. Alt Chg = 2150-2200 = -50 feet July's "Test Pilot" explained how What we really want is your airto perform flight-path stability test- plane's flight-path stability curve, Rate of Climb—Once you've deing. We provided a few helpful hints which is a plot of airspeed versus the termined the altitude change for for steady flying and data gathering vertical flight-path angle. The data each test event, calculate the average and reminded you that recording reduction g r i d is just a tool t h a t climb rate for each event by dividing flight-test numbers is a much lower helps organize the calculations that the altitude change by the time it priority than f l y i n g safely. This t u r n raw test data into the flight- took for that change to occur. We multiplied by 60 to convert month we'll cover what to do with path angle. To see why, let's work backward. To determine the flight- the ROC f r o m feet/second to all those safely acquired numbers. path angle, you need true airspeed feet/minute. Enter this value in the Rgure 1, Test Card and the vertical flight-path rate. To ROC column.

Flight-Path Stability Data Reduction

j&udttts&^&uift&fc

75

PA2 Alt Chg Time (ft) (ft)(sec) NA 2,500 2,500 0

70

2,500 2,490

-10

30

81 2,400 2,375

-25

30

66

2,300 2,250

-50

30

85

2,200 2,150

-50

30

OAS (kts)

PA1 (ft)

61 2,100

2,025

-75

30

92

1,900

1,775

-125

30

99

1,600 1,350

-250

30

108

AUGUST 2001

•••^^••iM

ROC OAT (ft/min) (deg C) 8 0 8 -20 8 -50 8 -100 -100 8 -150 8 9 -250 9 -500

Avg PA Avg DA (ft) (ft) 2256 2500

2495

2250

2388

2117

2275

1978

2175

1854

2063

1715

1838

1556

1475

1108

TAS (kts) 78 72 84 68 87 63 94 101

FPA (deg) 0.00

Remarks

-0.16 -0.34 -0.83 -0.65

Low confidence

-1.36

Stall warning

-1.50 -2.81

^^H

ROC =

AltChg

-50 feet seconds feet ROC=_____x60___ = -100 30 seconds

minute

minute

Average Pressure Altitude—We'll take a little license here to simplify the math by using an average altitude for the remainder of the calculations. We can do this hecause last month we limited the altitude change to a maximum of 500 feet in the test procedure. Our sample data show a maximum altitude change of only half that, and the consistency in the OAT throughout the tests also validates this shortcut. Average pressure altitude is simply the halfway altitude during each test. Perform t h i s c a l c u l a t i o n t o r each test, and enter the average pressure altitude in the Avg PA column.

flight computer away just yet. You'll need it to determine the true airspeed for each test point based on the average density altitude and your airplane's calibrated airspeed. Your worksheet should have one, but you'll notice there is no calibrated airspeed column in Figure 1. Calibrated airspeed depends on your particular airspeed indicator error and your particular airplane's pitot-static system installation. Two RV-4s, for example, could have iden-

tical readings on their airspeed indicators but different calibrated airspeeds. You can only determine your airplane's airspeed calibration by performing un (lirsfjeeil-culihnition test flight. (Sec the Jnmifiry through March 2001 "Test Pilot" ml,innis.) We list only observed airspeed (OAS) for simplicity on our example data reduction worksheet, but you must use your airplane's calibrated airspeed for this calculation. For our

example, we'll assume the observed

PA1 + PA2 Avg PA = ————————

Avg PA =

2200 + 2150

= 2175 feet

Average Density Altitude—You'll need to use your flight computer or density altitude chart to determine the density altitude from the average pressure altitude and the OAT. For our example 85-knot test point with its average pressure altitude of 2,175 feet and an OAT of 8°C, the average density altitude is 1,854 feet as indicated in the Avg DA column. True Airspeed—Don't put your Flight Test Data OAS = Observed airspeed PA1 = Start timing pressure altitude PA2 = End timing pressure altitude Time = Time for altitude change OAT = Outside air temperature Post-Right Calculated Values Alt Chg = Timed altitude change ROC = Calculated rate of climb Avg PA = Average pressure altitude Avg DA = Average density altitude TAS = True airspeed t

FPA = Vertical flight path angle

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airspeed and calibrated airspeed are identical. For our example airplane's 85-knot test point, our calibrated airspeed of 85 knots and density altitude of 1,854 feet produces a true airspeed of 87 knots. Perform your true airspeed calculation and enter the result in the TAS column. Flight-Path Angle—Figure 2 shows how vertical rate and true airspeed determine an airplane's vertical flightpath angle, Y (Greek letter RQC sin = gamma). Basic trigonometry exV J.Q presses their relationship: ROC is listed in the data reduction worksheet in feet per minute, and TAS is in knots or nautical miles per hour. We'll convert TAS to feet per minute by multiplying it by 6,076 feet feet ROC per nautical mile minute and dividing it by SIDY =. . , 6076 , 60 m i n u t e s per „ nautical miles nautical mile TAS——.———— x hour. hour minutes 60. We now have ROC and TAS in feet per minute. If ROC 60 you prefer to work siny = in statute miles i-TAS 6076 per hour, simply 60 -100 = -0.0114 smy = s u b s t i t u t e 5,280 87 "6076" for 6,076 in the equation. Now that we know the units are correct, let's simplify the equation and plug in the ROC and TAS from our 85knot test point. This formula tells us what the sine of the flight-path angle is, but we want to know the angle itself. You can use a trig table or an inexpensive scientific calculator to find Y. ,

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The riot's the Thing Once you've calculated the FPA, you can now construct the flightpath stability plot for your airplane. Figure 3 shows the flight-path stability plot for our example airplane's data. We plotted the FPA and OAS for each test point. You can plot the FPA versus calibrated and true airspeed if you like, but OAS is what you read on your airspeed indicator and is therefore a more useful correlation to you in the cockpit. After plotting the OAS/FPA points on your flight-path stability chart, fair a smooth curve among the plotted points. The idea of the curve is to fill in your plane's flight-path stability performance between the actual test points. The curve might not pass exactly through each of your test points, but its character is important. The curve's character, or shape, tells you just how sensitive your plane's FPA is to airspeed deviations on final approach. For example, let's say before this test you've been flying your final approaches in our example plane at 75 knots. An a i r speed deviation of 5 knots faster or slower would result in a 1/4-degree steeper FPA—that's one dot on an ILS approach. There are several other revelations worth mentioning about this particular flight-path stability plot. Notice how the 75-knot approach speed you've been flying is at the curve's peak. This means any airspeed deviation results in a steeper FPA, and that means a shorter range. Attempting to stretch a glide in this plane would most likely cause a false reassurance with the shortlived balloon effect (June's "Test Pilot") followed by the bottom dropping out at the slower airspeed. Sport Aviation

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If t h i s were your airplane, to give yourself a littie front-side margin you m i g h t consider a d j u s t -1.5H ing your ap-2.0 H proach speed to 80 knots. With an 80-knot approach speed you could make minor glidepath-shallowing a d j u s t m e n t s with a little back stick without having to adjust the throttle, and this little extra speed margin might make for a more predictable flare. Notice that the 66-knot OAS test point appears to be farther from the curve than the others. We faired this curve this way intentionally because of a low-confidence test card remark noted immediately after the test. You should make such a remark for a test run that satisfied the technical criteria of airspeed control, timing, etc., but didn't "feel" as good as the others. Perhaps you made too many pitch attitude adjustments while timing or experienced a slight gust halfway through the test. Qualitative events like these may not show up in the test numbers, but they can help explain a point that does not fit the curve as well as the others. Another test card remark is the stall warning comment for the 61knot test point. If you performed your tests at a typical l a n d i n g weight, you might want to annotate your plot with this stall warning speed. How the stall warning affected your test is also noteworthy. If the warning form was airframe buffeting, you might want to repeat the test a couple of knots faster than the b u f f e t speed to remove any doubt about the buffet contaminating your data. Now that you've done all t h i s work, there's one more crucial point to make. This curve applies only to the airplane configuration you've tested. If you land using a variety of 112

AUGUST 2001

Observed Airspeed (knots) 110

F1gure3

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flap settings, you'll have to perform the test with the flaps at each deflection to create a curve for each configuration. Always having the landing gear down for these tests is a safe bet, but don't overlook other curvealtering configurations like sliding open your canopy in the landing pattern. It's unlikely that the cowl flap position would affect your airplane's flight-path stability curve, but—theoretically—it could. Whether it does depends on your airplane. This month we transferred raw flight test data from our hypothetical data cards to a flight-path stability data-reduction worksheet. We massaged the raw data into the data we needed to produce the flightpath stability curve, and then plotted the curve. In our example, plotting the curve revealed a couple of things we might not have discovered otherwise. And we now know just how sensitive our airplane is to final approach airspeed deviations. So next time you join the beehive of airplanes lining up for landing at that big show, you'll know what a few knots faster or slower will do to your flight-path angle. Next month we'll recap reader feedback from the February and March "Test Pilot," in w h i c h we asked for experiences using GPS as an airspeed calibration tool. What else would you like to see addressed in "Test Pilot?" The address is Test Pilot, EAA Publications, P.O. Box 3086, Oshkosh, WI 54903-3086 or [email protected] with TEST PILOT as the subject of your e-mail.