Test Pilot: Airspeed Accuracy

Repeat this process for the range of airspeeds your plane is ca- pable of flying, and you can create a table or plot of calibrated versus ob- served airspeed.
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L AST MONTH “T EST P ILOT ” DIS (nose-to-tail) axis. cussed an airplane’s various You’ve seen these ports on airspeeds, starting with obthe sides of the fuselage or served airspeed, which is the pitot tube, and, obviously, what we pilots read on the a sideslipping airplane airspeed indicator. Applying Calibrating your airplane’s airspeed system changes the perpendicular rethe correction for the instrulationship—and pressure— ment (or gauge) error to the the static system senses. In a ED KOLANO observed airspeed gives us insideslip, the upwind port dicated airspeed, and correctsenses some ram air, the ing indicated airspeed for position instrument error is reasonable be- downwind port senses a lower preserror gives us calibrated airspeed. cause you want the practical infor- sure, and both cause erroneous airCorrecting the errors caused by mation, how the calibrated airspeed speed readings. (Some planes have the high-altitude effects of com- corresponds to the observed airspeed static ports on both sides of the fusepressibility and flying at a fast air- you read on your airspeed indicator. lage to help balance the ram and low speed gives us an airplane’s equivaKnowing the relationship be- pressures to minimize the errors.) lent airspeed, and applying the tween observed and calibrated airBesides sideslips, anything that correction for density altitude yields speed enables you to accurately cal- influences the slipstream to change the true airspeed. And to determine culate true airspeed, which is speed or direction (like the propeller, our ground speed, we apply a wind important for cross-country (and wing, door hinges, rivet heads, etc.) correction to our true airspeed. fuel requirement) planning, and to can cause false static pressure sensOf all the airspeeds, only one do know what you should read on the ing. And this includes changing you determine through a flight test, airspeed indicator to achieve that your flying speed. The only way to and this month we’ll explain in de- true airspeed. You’ll also use this ob- know how these changes affect your tail one technique for calibrating served-calibrated correlation for air- airspeed reading is a flight test to calyour installed airspeed indicating speed limits such as the maximum ibrate your installed system. system and mention a few other flap extension speed, which usually You can calibrate your airspeed techniques. Because most homebuilt is given as a calibrated airspeed. system using several acceptable airplanes don’t fly high or fast As discussed in January, the methods that range from exotic laser enough to worry about the effects of pitot/static system plumbing causes tracking or flying formation with a compressibility, you can assume the error between indicated and cali- pace airplane (but not a pace plane your calibrated and equivalent air- brated airspeeds. Variations in the calibrated with another airplane that speeds are the same. static pressure circuit account for was calibrated with…) to a fairly If all you’re interested in is corre- most of the installation or position math-intensive tower flyby or the lating your airplane’s observed air- errors because it’s supposed to sense simple ground course. speed with its calibrated airspeed, only ambient air pressure, which is you can also eliminate the gauge why static ports are usually perpen- Ground Course correction. Not worrying about the dicular to the airplane’s longitudinal The tried and true ground course

Airspeed Accuracy

Sport Aviation


Test Pilot method is straightforward. You time how long it takes to fly a known distance, use the time to determine your ground speed, and then correct the speed for air density because you probably won’t fly your test at sea level on a standard day. The result is your calibrated airspeed for the observed airspeed you flew dur-

ing the test. To eliminate the effect of wind, fly a reciprocal heading for each test and average the ground speeds (Figure 1). Repeat this process for the range of airspeeds your plane is capable of flying, and you can create a table or plot of calibrated versus observed airspeed. Repeat the entire

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process for each different landing gear and flap configuration to get additional applicable plots or tables. There are a few ground rules for the ground course method. You’ll need a ground course with some special features. It should be essentially flat because you may be flying at a low altitude, where it’s easier to accurately time your start and finish point passage. To remain out of ground effect, plan to fly lower than two wingspans above the ground. A flat course also helps you maintain a constant airspeed—an essential test requirement—and avoid a climb or descent. Terrain features should be consistent to avoid anything that could cause a variation in airspeed or altitude, like a shoreline or abrupt dropoff. Your course should have clearly identifiable start and finish points. You don’t want to be searching for that special tree in the forest flying in this risky environment. Plan your course so your checkpoints are to the side of your track. This will make it easier to “hack” your time as the leading edge of your wingtip passes the checkpoint. Consider selecting a course where you can easily see your airplane’s shadow. You can get a much more accurate time hack by noting when your shadow passes the checkpoints, or better yet, a straight-line ground feature perpendicular to your course that passes through your checkpoints. Another advantage to using your shadow is that you can fly much higher—a couple hundred feet—and still get an accurate time hack at the checkpoints. Smooth air is essential for good data. Usually early morning is the best time for calm conditions, and the sun’s low position ensures that your shadow won’t be under you. The FAA recommends less than 10 knots of wind for this test, but I’d stick with less than 5 knots with no gusts. The faster the wind speed, the less accurate your data will be. Lim-

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JULY 2003

1.5 nm iting test day wind to 5 ence, you may want to perWind knots ensures even direct form the entire test profile at 030/5 crosswind effects on your maximum and minimum data will be less than 0.5 weights. knots. Calm is best. The FAA recommends testRun 1 Run 2 The location of your airing several speeds between 1.3 plane’s center of gravity VS1 and maximum level flight speed. V S1 is your airplane’s shouldn’t affect your data, stall speed in the tested configbut the airplane’s gross uration. The 1.3 factor is there weight can. The heavier for safety. Remember, you’ll be the airplane, the higher low and slow, and that means angle of attack it must fly Heading 270 Heading 090 you won’t have a lot of opto generate the lift equal Time = 35.4 sec Time = 36.6 sec Gnd Spd = 152.5 kt Gnd Spd = 147.5 kt tions should something go to the weight. Because wrong. higher angles of attack cre147.5 + 152.5 Select a ground course ate stronger upwash and Avg Gnd Spd = = 150 kt whose length is compatible downwash around the 2 with your airplane’s speed wing, which can affect the Figure 1 range. FAA Advisory Circular pressure the static port 23-8A, “Flight Test Guide for senses, weight can have an influence in your calibration. To spot-check several airspeeds at a Certification of Part 23 Airplanes,” check this, perform the entire test near-minimum weight. If your com- recommends a 5-mile course for airprofile at a heavy weight, and then parison yields a significant differ- speeds faster than 250 knots and a 1-

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Sport Aviation


Test Pilot mile course for airspeeds slower than 100 knots. For an airplane with a test speed range of 65 to 150 knots, a 1.5-mile course is probably a good choice. Course length is up to you, but longer courses require very demanding flying for longer periods, and shorter courses can mean larger air-

speed errors if your timing is off. For example, a 1-second timing error on a 1-mile course flown at 150 knots produces a 6-knot airspeed error. That same 1-second timing error on a 5-mile course causes an airspeed error of less than 2 knots. Finally, remember to use nautical miles when computing your speed

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in knots. Mixing and matching nautical and statute miles and speeds gives some interesting errors.

Test Procedure Do not make your first pass over the ground test course during your airplane’s first test flight. Preview the course the day before to become familiar with it. Survey the course from a safe altitude, say, 1,500 feet AGL, making sure that you have plenty of turnaround room at both ends. Look for your checkpoints, obstacles, and other disqualifying features like wires, farm animals, etc. Then make a run in both directions at a comfortable cruise airspeed at a lower altitude. Make a few more passes at progressively lower altitudes, until you are satisfied the course is suitable and you are comfortable flying it at the low test altitude. As part of your test planning, determine the correct MSL altitude that will put your plane at the desired safe AGL altitude, based on the course’s known elevation. After you’ve established the desired test altitude, set your altimeter to 29.92 to read the pressure altitude you’ll need to calculate density altitude after your test flight. The barometric pressure can change between your weather brief and arrival at the course, so keep your eyes on the terrain as you approach your test altitude. If it looks like you’re too low, raise your test altitude to a more comfortable height. Steady is essential. Have the airplane stabilized in the test configuration at the test airspeed in level flight at the test altitude on the correct heading with the power set before passing the start checkpoint. Record your configuration, pressure altitude, observed airspeed, outside air temperature (OAT), and run direction before you pass the start checkpoint. Include altitude, airspeed, and OAT in your scan during the run, and update your recorded

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JULY 2003

data, if necessary, after the test run. These parameters should not change, and recording them when established for the test run but before the start checkpoint frees you to concentrate on steady flying with a diligent outside scan. (Figure 2 shows a sample data card.) This is a risky flight environment, so stack the safety deck in your favor. Rather than writing your data on a kneeboard, consider using a micro tape recorder (properly secured with wires safely routed) or transmitting your data to someone on the ground to record. If you’ve accomplished your FAArequired flyoff (and you have an extra seat), a copilot can record the data you call out over the intercom. A second crew member can provide another set of eyes for monitoring altitude, keeping an eye on the engine instruments, and watching for birds. Now that I’ve suggested an


on-board human data recorder, let me discourage this idea. Test flights can be risky, and the minimum flight crew essential to the flight test or safety is prudent. You and he or she must decide whether the benefits outweigh the risks. Begin your timing as you pass the start checkpoint. Call “hack” into your recorder or transmit or say it over the intercom for your copilot to operate the stopwatch. Your hands should remain on the stick and throttle. Maintain your altitude throughout the test run. Because you were already established “on condition” before the run began, there should be no need to make power or trim adjustments. If the airspeed changes during the run, scratch that run and try it again. Use the horizon as a pitch attitude indicator to avoid chasing an artificial horizon, vertical speed indicator, or any other flight





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instrument. Keep your eyes outside the cockpit. Passing your end checkpoint, stop your timing. Make a qualitative assessment of the run you just performed. If the airspeed varied or you made aggressive control inputs or the heading wandered, consider not counting that run, and start again with the reciprocal heading. Give yourself plenty of room to turn around for the reciprocal heading run. There’s no need to remain at the low test altitude during this repositioning. Leave yourself enough room to get established on condition before beginning the second run. With enough turnaround room, you should be able to accomplish the turn and setup without changing power or trim if you fly smoothly. Repeat the process on the reciprocal heading. When you are satisfied with the quality of two reciprocal runs, set up for the next test airspeed


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Sport Aviation


Test Pilot and repeat the test run pairs until you’ve mapped the airspeed range.

By the Numbers 1. Stabilize the airplane in level flight at the test airspeed in the test configuration in level flight at the test altitude on the correct heading with the power set. 2. Set your altimeter to 29.92. 3. Record your configuration, pressure altitude, observed airspeed, and OAT. Note your power setting to help you quickly re-establish your airspeed for the reciprocal heading run in case you have to make a power adjustment during the turnaround. 4. Begin timing as you pass the start checkpoint. 5. Fly a constant heading, constant airspeed, and constant altitude test run. Note any non-steady parameters, including OAT and engine power settings. 6. Stop timing as you pass the end checkpoint. 7. Decide whether the quality of your run is satisfactory. If not, repeat the run. If it is satisfactory, turn around and perform Steps 1 (Step 2 is already done.) through 6 flying the reciprocal heading. 8. Repeat Steps 1 through 7 for every test airspeed. 9. Reset your altimeter to the local setting before returning for landing. 10. Repeat the entire process for every configuration of interest. 11. Repeat the process at a different weight, spot-checking a few airspeeds for post-flight comparison.

Low but Not Low Risk Yes, I’ve harped on the risky nature of this test, and here’s more. The airplane is perfectly capable of flying these test profiles. It doesn’t care whether it’s two wingspans above the ground or a few thousand feet. The risk is situational. Even a slight distraction can result in ground contact at these test altitudes. 138

JULY 2003

Date Weight CG Course Length

Gear Position

Flaps Position

Speed Course Data Card Excerpt

Elapsed Observed Time (sec) Airspeed (kts)

Pressure Altitude (ft)

OAT (° C )

Figure 2 For example, an airplane flying 50 feet AGL at 150 knots will hit the ground in less than 6 seconds if the flight path is just 2 degrees below horizontal. Stay heads-up out there, and maintain a good external scan. Some of your runs will be low, slow, and dirty. Should it lose power, your airplane won’t have a lot of excess energy (speed) you can convert into altitude (and time aloft). Consider this when selecting your ground course. Perfectly flat and clear is ideal, and you can’t do any better than a long, off-duty runway (make sure you have the airport/control tower’s permission and cooperation). You should have plenty of clear surface both paved and unpaved here. An additional advantage of an airport test site is that the airport survey map tells you exactly how long your run is. For a non-airport test site, check local government records to determine your exact test course length. Regardless of your test site, you should have a plan if things go wrong. If the engine stops, you won’t have time to contemplate your actions. Base your plan on the topography, your airplane’s capabilities, and the nature of the emergency. For example, you’d probably handle an engine stoppage different

from a bird strike to the wing. Mentally rehearse your actions for every conceivable emergency. Low and slow is not the only flight configuration for concern. Low and fast means bad things happen faster. If your electric trim suddenly decides to run away nosedown, faster airspeed means less time for you to react. Finally, fly legally. Federal Aviation Regulation 91.119 gives the minimum safe altitudes above people and buildings (but you should not be testing above them anyway). When flying over sparsely populated terrain, you must remain at least 500 feet from any person, vessel, vehicle, or structure. No matter where you fly, even over barren terrain, FAR 91.119 says that if the engine quits, the minimum safe altitude is one that allows an emergency landing without undue hazard to persons or property on the surface. Next month we’ll cover the data reduction and convert all those test run timings into true airspeed and calibrated airspeed, and then we’ll correlate them to the observed airspeed readings. Editor’s Note: While test pilot Ed Kolano is on sabbatical, we’ll be reprinting some of his most popular columns from the past three years.