stick & rudder flight advisor Is Your Airspeed Indicator Honest? An accuracy how-to H.C. SKIP SMITH One of the most important gauges on the panel is the airspeed indicator. This is particularly true if you are doing any kind of performance measurement. Aerodynamic forces, such as lift and drag, are proportional to the square of the velocity, so that any error in the indication is multiplied. Hence, it is necessary to determine this error before proceeding to other items of performance. Flight testing to determine performance is required in Phase I of homebuilt certification, and is often advisable for used production airplanes, particularly if major modifications are made. First of all, let s talk about various airspeed values. What you see on the instrument is called indicated airspeed, or IAS. What you should see if there were no errors in the system is called calibrated airspeed, or CAS. The airspeed indicator actually measures dynamic pressure, which involves density, but it reads out values of velocity. It, therefore, must Figure 1 use a constant value of density, and the value used is standard sea level density. Of course, the density varies from day to day, and more so with altitude. Calibrated airspeed, then, must be corrected for actual density to yield what we term true airspeed, or TAS. This variation is not considered an error, but simply a necessary adjustment because of the method of measurement. Before you start writing letters, I should point out there is another value called equivalent airspeed (EAS), which is CAS corrected for compressibility. Compressibility occurs only when the airplane approaches, or exceeds, the speed of sound, but there are some noticeable effects above about 250 to 300 knots. True airspeed, strictly speaking, is EAS corrected for density. To simplify the discussion, I will not consider this correction, because most of us fly aircraft well below this speed. The normal procedure for us little guys, then, is to read IAS, correct it to CAS, and then correct that value to TAS, which is the actual speed of the aircraft through the air. It is the first correction, IAS to CAS, that must be determined by some kind of flight test, and what we will consider in this discussion. There are a number of methods that have been devised over the years for this purpose. They all involve determining the true airspeed from flight tests, converting that to calibrated, and then comparing it to the indicated value. Some of these methods, such as tower fly-by, radar tracking, and beacon marking, involve sophisticated equipment and facilities. One method that does not, and that has been used by individuals and smaller aircraft industries, is the speed-course method. The speed-course method involves choosing two parallel landmarks, such as roads or power lines, and determining the exact distance between them, as depicted in Figure 88 MAY 2006
1. The airplane is flown at low altitude across this course at a constant indicated value, and timed. The distance divided by the time will yield the groundspeed, which is also the true airspeed if there is no wind. Usually, though, there is some wind, so a flight in the opposite direction is necessary. If the wind is perpendicular to the landmarks, the groundspeed into What you see on the instrument is called indicated airspeed, or IAS. What you should see if there were no errors in the system is called calibrated airspeed, or CAS. the wind is true airspeed minus wind speed, or TAS-WS. Flight in the opposite direction will give groundspeed equal to TAS + WS. If the two values are averaged, that is, added and divided by two, the wind speed will cancel out and the TAS will result. An advantage of this method is that it can be used even if the wind is not a direct head wind/tail wind, as also shown in Figure 1. As long as the airplane heading is kept perpendicular to the landmarks, the TAS vector will be into, or away from, the wind component in that direction. A crosswind component will cause the airplane to drift, ending up at point C instead of B, but this is of no consequence to the problem. The important thing to remember is to point the airplane exactly perpendicular to the landmarks, and let it drift. You are not trying to navigate to a certain location here. Determining the orientation of the landmarks is necessary, and it is now obvious why a line must be selected, rather than just a point. In recent years, though, a marvelous device called GPS has appeared. EAA Sport Aviation 89
flight advisor Figure 2 In addition to greatly simplifying our navigation, it can be used for a number of other purposes, and one of these is airspeed calibration. Even the simplest handheld GPS will provide groundspeed directly, and quite accurately, too. Thus, it is not necessary to do any timing and calculating to get groundspeed. You still have to deal with wind, though. Note in Figure 1 that the airplane actually follows a longer path between A and C than from A to B. Since flight along the A-C path, that with a crosswind, arrives at the same time as it would going from A to B with no crosswind, it would have to be moving faster. It is flight along this track that the GPS reads to calculate groundspeed. It would, therefore, not give the groundspeed for the flight between the landmarks close, with most wind conditions, but not exactly. A number of methods have been devised to negate the wind factor. One of these involves taking groundspeed readings in three directions at 90 degrees to each other. Another requires four directions. The results are then inserted into an equation that yields true airspeed. Although the details of the derivation of the equation involved is somewhat complex for the average pilot, the result is not too hard to use to calculate TAS. One of the problems with such methods is that they are time-consuming. More important, though, the more readings you take, the more chance there is for error in holding the exact heading and airspeed. Another method, which I favor, involves only two headings for each airspeed value, one into the wind and one downwind. The trick here is to determine, fairly accurately, the wind direction. This task can be accomplished by first consulting the winds aloft forecast. Such information may not be entirely accurate, but it is a good starting point. Before beginning the tests, fly into the wind as forecast, by use of the heading indicator. Then note the track of the airplane on the GPS. If you are directly into the wind, the two values should agree, and remain so. If not, make a small heading correction, and try again. A few attempts should result in a pretty good determination of how the wind is blowing. Now you can begin the actual test. Fly into the wind at a specific IAS value, holding heading and airspeed as accurately as possible, and observe the groundspeed readout. When it settles out to a fairly constant reading, record it. Then go to another IAS reading, and do the same thing. Repeat this procedure for the entire range of airspeeds normally flown. When completed, turn 180 degrees, and repeat the entire procedure in the opposite direction. Don t forget to also record the indicated values. Usually, about every 5 knots or miles per hour of IAS will give a pretty good correction chart. The upwind and downwind values for the same indicated value are then averaged, just as in the speed-course method, by adding and dividing by two, to give TAS. What you now have are TAS values for respective IAS values, but what you want is a comparison of IAS with CAS. Hence, the true airspeed has to be converted to calibrated. Various methods can be used for this conversion, but they all require the input of temperature and pressure altitude, so be sure to record these values. Temperature can be obtained from the outside air temperature (OAT) gauge, and pressure altitude is indicated when the altimeter is set to 29.92, so be sure to change to that setting before performing the tests. Don t forget to reset for actual conditions before landing. The simplest way to convert to calibrated values is by use of an E6B computer or whiz wheel, if anybody still uses these. Once temperature and pressure altitude are lined up, the calibrated values can be read on the inside disk for true values on the outer edge of the wheel. A much more accurate method is to perform the same procedure with an electronic flight calculator, often referred to as an electronic E6B. These devices actually multiply the TAS value by the square root of the density ratio, the ratio of density at the test altitude to standard sea level density. You could also perform this calculation by determining density altitude (again, from temperature and pressure altitude), and then looking up the density ratio for that altitude in standard altitude tables. If you are adept at using spreadsheet programs, such as Excel, the exact equation for the multiplier is ((519*((1-0.00000689*PALT)^5.256))/(T+460))^0.5 where PALT is the pressure altitude input in feet, and T is the temperature input (from OAT) in degrees Fahrenheit. The entire speed run should be done at the 90 MAY 2006
same altitude, so this calculation only needs to be done once for all airspeed values. Multiplying TAS by this factor gives the CAS, which can then be compared to the indicated values. Figure 2 shows these various calculations in a spreadsheet program, with the last column (CAS) resulting to compare to the first (IAS). For actual use in flight, you would probably want to list just these two columns. They could also be plotted out in a graph, as in Figure 3. Spreadsheet programs are a big help, but a hand-calculated chart could also be constructed fairly easily. Whichever Figure 3 version you prefer would then be used to correct from IAS to CAS in future flight tests. One other aspect of these tests should be addressed. There are two main types of airspeed errors: gauge error and position error. Gauge error is the inherent error in the indicator itself, plus the installation system. It would be the same for all flight conditions. Position error, on the other hand, will vary with angle of attack, and, hence, with weight. Position error is the error in the position of the pressure probes, primarily the static, with respect to the airstream. The tests as described would correct for the overall error, but would apply only to the weight tested. To be more accurate, you should probably run the whole process at various gross weights. Light air- EAA Sport Aviation 91
planes, however, don t have that great a range of weights, so often the weight variation is negligible. Production aircraft normally list a correction in the pilot s operating handbook (POH) for maximum gross weight. I prefer to run the tests at the weight at which I most frequently fly. In my case, that is with just one aboard my four-place airplane. If you want a bit more One other aspect of these tests should be addressed. There are two main types of airspeed errors: gauge error and position error. Gauge error is the inherent error in the indicator itself, plus the installation system. It would be the same for all flight conditions. accuracy, though, you could make a chart for the maximum gross weight, and also for the minimum flyable weight. However you choose to display the calibration data, the result should provide a simple method to convert indicated values to calibrated airspeed. This correction is the first step in obtaining true airspeed, which is needed for further flight tests. SHARE YOUR FLIGHT ADVISOR STORY E-Mail your FA success stories to editorial@eaa.org, with FA as the subject. 92 MAY 2006