Last February I was a competitor in the first Great Hawaiian Air Race (see " Racing the Trade Winds," page 87). So that any airplane could compete fairly against all others, it was necessary to determine a handicap speed for each of the participating aircraft. The winner of the race was the pilot who exceeded his handicap speed by the greatest number of knots.
Determining how fast an airplane can really fly is not as easy as it might seem. The performance data shown in the pilot's operating handbook cannot be used because aircraft of the same make and model invariably fly slower or faster than advertised, depending on rigging, actual power output, condition, and the airframe manufacturer's optimism. Nor can airspeed indicators be used to compute true airspeed because they can be woefully in error (sometimes much more than many realize). Although the airspeed calibration chart in the pilot's operating handbook corrects for positioning errors of the pitot-static sources, it does not correct for instrument error. As an airspeed indicator ages, it tends to become less accurate. Many pilots have only an approximate idea of how fast their aircraft move through the air.
Five years ago ("Proficient Pilot: Tracking true," June 1994 Pilot) I described in this column how GPS can be used to determine the true airspeed of an airplane. The procedure involves placing the aircraft in a shallow-banked turn and then keeping a sharp eye on groundspeed as shown on the GPS. (A handheld unit is just as good as one that is panel-mounted.) When groundspeed reaches either a minimum or maximum value, the airplane is heading either directly into or away from the wind. The pilot should note this heading, roll out of the turn, reestablish that heading, and allow the airplane to stabilize in cruise flight. Then record the groundspeed.
The next step is to make a one-eighty turn, again allow airspeed to stabilize, and then note the groundspeed while flying in this, the opposite direction. Averaging the downwind and upwind groundspeeds yields true airspeed.
This, however, wasn't accurate enough for the race committee, which uses a procedure that initially seemed to me like smoke and mirrors. The first step involved flying on any cardinal heading while in level flight with a wide-open throttle at a specified altitude. Someone designated by the committee sat in the right seat and jotted down the observed GPS groundspeed every 10 seconds or so for about two minutes. He also ensured that the pilot didn't do anything to intentionally slow the airplane, such as mistrimming, leaving the cowl flaps open, allowing altitude to wander, and so forth. (It is in a pilot's best interest to have as low a handicap speed as possible, because this would improve his ability to win the race.)
After the 12 groundspeed readings had been recorded, the pilot was instructed to turn 90 degrees right to the next cardinal heading, at which time 12 additional groundspeeds were noted during a stabilized two-minute leg. Finally, the pilot made another 90-degree right turn, and 12 more groundspeeds were noted.
When on the ground, the 12 groundspeeds noted during each leg were added together, and the result was divided by 12. This averages the groundspeeds observed during each leg and cancels almost the entire effect of speed variations that normally occur during any given flight segment. Some fancy mathematics were then applied to the three groundspeeds to determine true airspeed. It seemed like black magic. I could not understand how airspeed can be determined without knowing wind velocity. Drift was different on each leg, but this did not seem to matter. Finally, however, I began to understand that this was similar to lessons learned about simultaneous equations during high school algebra. But fear not. You won't need to dust off a math book to solve this problem.
Craig Cox, a pilot and computer programmer, has a Web site on the Internet ( www.reacomp.com) that makes it fun and easy to determine true airspeed. After entering the site, click on "True Airspeed." A small diagram (an applet) will appear. Then simply enter the average groundspeeds for each of three legs. Not only will the true airspeed (rounded off to the nearest knot) magically appear, but so will the wind velocity encountered during your flight. As a bonus, a small diagram of the tracks made good also appears. (Any masochist adept at plotting wind triangles can solve the problem with vectors, but setting up the diagram requires some imagination.)
The accuracy of the airspeed indicator can be determined by noting 12 indicated airspeeds during each of the three legs. At the end of the flight, add all 36 indicated speeds together and divide the result by 36 to obtain the average indicated airspeed. Then apply any correction for calibrated airspeed and use your aviation computer to determine true airspeed using the pressure altitude and outside air temperature experienced during the flight. Then compare the results. I did this recently using three different airplanes and found that the three airspeed indicators were in error by two, five, and 11 knots, respectively.
This procedure is also an excellent way to determine the effect of speed modifications planned for your airplane. Perform it before and after installation to see if you got your money's worth.
