Stick & Rudder

Test Pilot IN THE JANUARY "TEST PILOT" If you expect accurate we reviewed some level results from your flight flight performance basics, tests, you need to use showed how knowing specific numbers, and your airplane's level flight we'll use specific numfuel flow at various airbers in all "Test Pilot" speeds determines the Drawing your airplane's power curve articles. Unless specifiplane's maximum range cally stated otherwise, all and endurance, and exflight altitudes are presED KOLANO plained that you could sure altitude (h p ). Your determine the airspeeds altimeter gives that altiat which these essential perform- warning to maximum level flight tude when it's set to 29.92. You need ance characteristics occur. Now it's speed. You don't have to test every to know the pressure altitude and time to discuss how to fly the level possible airspeed because you'll be the outside air temperature (OAT) so flight tests that will give you these constructing a plot similar to Figure you can compute density altitude things, and what data you'll need to 1. With enough data points, you can (h d —remember it as "height denrecord during your test flights. draw a curve to fill in the missing sity"). Density altitude is all your data between tested airspeeds. Once engine cares about when it's trying The key to all flight testing is safety! Before you start the performance you have this curve, it's just a matter to produce power. Airspeeds can be confusing. All flight tests you should have made of a few simple calculations to deterseveral flights to "clear" the airplane's mine all the level flight performance pilots know that the speed you read envelope. For example, in the tests you'll need to optimize the flying of directly from your airspeed indicator is indicated airspeed, but not many we'll discuss you'll be gathering data your plane. at your airplane's maximum and minpilots have heard of true indicated airspeed, which accounts for gauge imum level flight speeds, so you must or instrument error. Let's avoid this know that you can fly the plane safely confusion by calling the airspeed at these performance envelope edges. you observe the observed airspeed In other words, don't even think (V 0 ). We'll address these terms in about recording performance data more detail when we get to the poston your first flights. Concentrate on flight calculations. While V0 is clearing the flight envelope by enwhat's important to you in the cocksuring that your airplane has pit, you'll need true airspeed (VT) for acceptable stability, controllability, range calculations, and you'll use hd and handling characteristics. Airspeed to determine VT.

Range & Endurance Testing

Level Simplicity The idea behind level flight testing is simple. You'll fly your airplane at a constant altitude and change power settings. This gives you a different airspeed for each power setting. When you change the power setting with the throttle, you're also adjusting the fuel flow. By repeating this procedure over the range of airspeeds, you'll see there's only one power setting, or fuel flow, that sustains level flight at every particular airspeed from stall 98

FEBRUARY 2000

Figure 1

Front Side & Back Side The Right Numbers Pilots deal with all kinds of numbers like airspeed and altitude all the time, and we often assume to know exactly what the numbers mean. Ads for airplanes often cite airspeed numbers, but what airspeed? True airspeed? Calibrated? Indicated? Most ads don't say, but if you misinterpret a high-altitude true airspeed for a sea-level indicated airspeed, you're going to be disappointed.

Every power curve has two sides and Figure 2 shows the power-required or fuel-flow-required curve divided into its front and back sides. The area to the right of the bottom of the curve is the front side, and to the left it's the back side. The distinction is mentioned here because the test technique is different depending on which side of the power curve you're flying. Before we put the whole thing together in a step-

by-step procedure, let's discuss the two techniques. Let's say you're flying straight and level at 140 knots (Point A). If you advance the throttle, the engine produces more power. If you maintain level f l i g h t , the airplane accelerates until it reaches a faster airspeed where the higher power produced equals the higher power required at Point B. Had you retarded the throttle at Point A, the engine would have produced less power than required for level flight at the Point A airspeed. If you maintain a constant altitude, the airplane will decelerate until the lower power produced equals the lower power required for the slower airspeed at Point C. During testing on the front side of the curve you start from a stabilized level flight condition (Point A) and record some data we'll talk about later. "Stabilized" means engine power is set; you're not climbing, descending, or turning; and your airspeed is constant. Then you pull the throttle back a little. At high power settings you can make larger power reductions, but you'll want to use smaller power reductions as the airplane gets slower. Your engine is now producing less power than the airplane needs to maintain the original airspeed, and it decelerates. If you maintain level flight, eventually the plane will stop decelerating and settle on a new sta-

bilized airspeed (Point C). You've returned to stabilized flight when the airspeed changes (decay) is less than 2 knots in one minute. You record the new data, retard the throttle a little more, and repeat the process. Eventually you'll reach the airspeed that corresponds to the bottom of the power- or fuel-flowrequired curve (Point D). If you pull

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the throttle back at this point, again your engine produces less power than needed, and the airplane decelerates. Only this time, as it slows the power required to maintain a constant altitude increases. In other words, it takes more power to fly slower when operating on the back side of the power curve. The slower you attempt to fly, the larger the

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Test Pilot power deficit. On the back side of the power- or fuel-flow-required

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speed a specific power setting gives you, you see how much power it

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Let's say you've determined that 110 knots is the bottom of the curve,

aka the bucket. You want data at 95 knots. From your stabilized level flight condition at 110 knots, note the rpm (let's say it's 1800) and then retard the throttle a little. As the airspeed approaches your 95-knot target speed, advance the throttle to a setting slightly higher than it was at the bucket airspeed. You know it takes more power to fly slower than the bucket airspeed; you just don't know how much. Let's pick 2000 rpm for this example. Carefully maintain the target 95 knots with back stick or forward stick—whatever it takes to fly the target airspeed using only the elevator. It's okay to trim. If you guessed right, 2000 rpm holds the airplane in level flight at 95 knots. Most test pilots, including the pros, don't often guess right, so you'll probably be in a slight climb or descent. If so, note the climb or descent rate. Let's say you're climbing at 200 fpm. A climb means the engine is producing too much power for level, 95 knot flight. Reduce the power a little. For this example, try 1900 rpm. Again, fly the target 95 knots until everything stabilizes. Let's say you're now in a 100 fpm descent. You now know the correct power setting for level flight is somewhere between 1900 and 2000 rpm. A quick mental calculation: The 100 rpm difference between 1900 and 2000 resulted in a 300 fpm difference in altitude change rate (200 up plus 100 down). You want to add enough power to overcome the 100 fpm descent rate or 1/3 of the 300 fpm altitude change rate. So add 1/3 the rpm difference—33 rpm. Okay, 33 rpm is a finer adjustment than you can probably read on your tach, so set the throttle to as close as you can get to 1930 rpm and continue to maintain the target 95 knots. If the altitude changes less than 50 feet in one minute after you stabilize at 1930 rpm and 95 knots,

Okay, Okay, Okay... You talked us into it. you are now ready to record the data. If you are still in a tiny climb or descent, just tweak the throttle a tiny bit until you nail the stabilized 95-knot, level flight condition. Testing on the back side is harder work than front-side testing, but both require patience. Avoid the temptation to rush to the conclusion that you've achieved a stabilized condition. For the front side, time the full minute before concluding that the airspeed is slowing less than the maximum-allowable 2 knots per minute. Don't try to save time by assuming that an airspeed change of less than 1/2 knot in 15 seconds is the same thing. Likewise for the back side. Time the full minute before concluding that your altitude has not changed more than 50 feet. Don't use the vertical speed indicator (VSI) for these tests; time the altimeter. The FAA says you have to fly off a specific amount of test time before you can venture abroad in your new airplane, so why not use it to get really accurate data?

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How precisely must you fly? For front-side tests be careful not to allow the airplane to climb or descend once the airspeed change nears 2 knots per minute. Even a slight climb or descent can affect the deceleration here, so it's important to really nail the altitude. When the airplane first starts to decelerate it might begin a small climb or descent. This does not affect the accuracy of your data if you maintain a constant altitude within a couple of hundred feet of the target altitude when you perform the 2knot-per-minute check. Back-side testing precision is similar, but airspeed is the parameter you really want to hold constant. Again, as long as you remain within a couple of hundred feet of the test altitude, your data should be fine. If you find yourself more than a couple of hundred feet from the test

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Test Pilot altitude, you'll have to adjust. Note the rpm, then maneuver the airplane back to the test altitude, and re-establish the last rpm setting and airspeed and resume the test. For both front- and back-side testing it's more important to be stabilized than it is to be exactly at the test altitude. Abrupt flight control movements contaminate your data. Small, smooth control inputs work best. Once you begin timing, if the airplane is stabilized you shouldn't need to make any control inputs. If this level of precision sounds too difficult, don't be discouraged. Practice the techniques while you're out flying for fun. When you feel ready, fly the test flight. Turbulence will make your flight test efforts frustrating. Early morning is usually the best time to find smooth air. You can also help yourself by not flying over geographic features

that tend to produce turbulence. For example, you're more likely to encounter turbulence flying near ridges or over water-land shorelines.

The idea behind level flight testing is simple. You'll fly your airplane at a constant altitude and change power settings. By the Numbers—Front Side

1. At a safe altitude, set the altimeter to 29.92 (and remember to

reset it before landing). 2. From slightly above the test altitude, make a shallow dive to achieve the maximum level flight airspeed (Vmax) plus 10 knots in level flight with full throttle at the test altitude. The reason for the shallow dive is to ensure your plane is flying just faster than its V max . Power required exceeds power available, and the airplane decelerates until it stabilizes at its V max . This technique is more efficient than allowing the airplane to accelerate in level flight to its maximum speed. 3. Maintain altitude and allow the plane to decelerate from the "dive speed." 4. When the deceleration is less than 2 knots in one minute, record the pressure altitude (hp—what the altimeter says when set to 29.92), the observed airspeed (V0), fuel flow, OAT, rpm, fuel weight, and remarks.

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FEBRUARY 2000

You won't use rpm in your calculations, but you may find it a convenient way to set power using

less than 50 feet of altitude change

knowing your airplane's weight when these data are gathered. Practically speaking, most aircraft fuel gauges are not nearly accurate enough. You can

in one minute at the target airspeed. 7. Record hp, V0, fuel flow, OAT, rpm, fuel weight, and remarks. Repeat steps 1 through 7 at progressively slower airspeeds. When finished, reset your altimeter to the local setting. If you plan your testing to finish near your land-

weight during the test by estimating fuel used based on fuel flow or recording how much fuel it takes to refill the tank after landing.

descent, minimizing the fuel burned during the return. This can help you determine a more accurate tested airplane weight.

your new performance charts. Fuel weight is included as a means of

determine your airplane's average

5. Reduce power.

Repeat Steps 3 through 5 until the airplane no longer maintains level flight. The last data point completed prior to the power reduction that cannot maintain level flight is at or near the bucket of the power- or fuel-flowrequired curve. Continue testing using the back-side technique.

By the Numbers—Back Side

1. Reduce the power slightly from the setting used for your slowest front-side data point—which should

be at or near the bucket—to allow the airplane to decelerate. 2. As the airplane approaches the new, slower target airspeed, set the power slightly higher than the setting used for your slowest front-side

data point.

3. Maintain the target airspeed,

which should be a few knots slower than your slowest front-side data point airspeed. Note the stabilized rate of climb or descent and the

rpm.

If climbing, reduce power

slightly. If descending, add a little bit of power. 4. Continue to maintain the target airspeed, and note the new stabilized rate of climb or descent and the new rpm. 5. Interpolate rates of climb/descent and power settings from Steps 3 and 4, and set the approximate rpm needed for level flight at the target airspeed. 6. Adjust power slightly if necessary to achieve stabilized flight with

ing airport, you can perform an idle

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3. Switch to the test tank, and begin timing. 4. Maintain the stabilized flight condition long enough to burn enough fuel so you can measure how much was burned when you refuel. 5. Record hp, V0, OAT, rpm, and remarks a few times during your test run, so you can calculate the average OAT andV0. 6. Stop timing and switch to the admin tank before descending. Record the elapsed time, and don't forget to reset your altimeter to the local setting. 7. After landing, refill the test tank and record how much fuel you burned. The fuel flow for this data point is fuel burned from the test tank divided by the time it took to burn it during the test run. 8. Refill the admin tank so you can calculate your airplane's weight during the test. Repeat Steps 1 through 8 on subsequent flights. Each flight produces a single data point, so vary the test airspeeds on each flight to map your airplane's level flight airspeed envelope. There are a few more inconveniences to address when using the two-tank method. Remember, your engine and your true airspeed calculations depend on density altitude. During your first test flight, you'll establish the cruise condition at some pressure altitude. You'll have recorded OAT, so you can calculate the density altitude during the test. On subsequent flights, you should fly the test at the same density altitude as the first flight. That means you'll have to bring a density altitude chart or flight computer along on subsequent flights so you can select the appropriate pressure altitude based on OAT to achieve the same density altitude as the first flight. The more consistent you keep your airplane's weight during each twotank test flight, the better your results will be. If you intend to fly with someone, take that person along on every test flight. We'll address weight considerations in detail when we discuss the postflight calculations.

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Final Thoughts

Safety first. Period. When making that shallow dive for the maximum level flight air-

speed—never exceed your airplane's VNE! Don't inadvertently stall your airplane trying for that slowest

back-side airspeed. You won't be cruising your airplane at these extremes anyway, so getting close is good enough. Remember, the goal of this testing is to produce useful range and endurance figures. Unless you have a personal curiosity, you don't even have to test at the maximum level flight speed or anywhere near stall speed. You can use the back-side test technique for front-side data points, but you can't use the front-side technique on the back side. Lean your fuel flow during the tests as you intend to lean during normal cross-country flying. If you like to fly rich-of-peak, test that way. Lean-of-peak? Test that way. Range and endurance are all about how quickly fuel is consumed, and leaning techniques play an important role in fuel consumption. Next month we'll explain what to do with the data you recorded during your front- and back-side testing. Starting with the raw numbers, we'll convert pressure altitude and temperature to density altitude, and convert observed airspeed to true airspeed. Then we'll plot the data and show how to find your airplane's maximum range airspeed and how far it can go along with its maximum endurance airspeed, as well as how long it can remain in the air. Finally, we'll discuss the effects of different weights, altitudes, and configurations. Until then, please send along your comments, questions, and suggestions for future topics to Test Pilot, EAA Publications. P.O. Box 3086, Oshkosh, WI 54903-3086. Address email comments to [email protected], and make TEST PILOT the subject of your message.—Ed •

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