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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

;;..- • ' ; ; •

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.