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Calibration Step 1, Part Deux

As we get ready for beta test, I thought it might be beneficial to write up a "how to" to help with developing aircraft curves--i.e., how to do a "hard calibration" the old-fashioned way...we have a plan to automate this process, and figuring out if that's in the art of the doable is going to be the #1 objective during beta testing.

Here's an edited video of a calibration sortie that I recently flew.

Absolute Alpha, True Airspeed, and Static Source Pressure Flight Test Protocol

The GPS “horseshoe” method of flight test can be utilized to derive basic aerodynamic data for the airplane helpful in calibrating pitot/static and angle of attack systems. This method consists of flying three or four legs at a constant pressure altitude and power setting. GPS-derived ground track and ground speed are recorded for each leg in addition to engine power, OAT, fuel on board (FOB) and IAS parameters for each run. Because actual gross weight is required for post flight data calculations, this method requires current weight and balance data as well as a fuel computer on board to accurately determine FOB for each data point. It is also necessary to be able to accurately determine engine power (manufacturer’s chart, test data, etc.), so the airplane must be equipped with a manifold pressure sensor. These tests are initially conducted at a pressure altitude of 6000-8000 feet.

Each GPS ground track must be separated by 90 degrees or more. A standard “horseshoe” track may consist of four legs flown in cardinal directions or three legs flown as portion of a “box” pattern or an equilateral triangle. It is generally recommended that a four leg “box” pattern be flown, since that allows the pilot one extra data point in the event there is a problem on one of the legs, however, the three-leg pattern is the most efficient to allow a maximum number of trim shots in minimum time. The pilot should fill out the run card or dictate parameters into a recording device. As a minimum, a time hack for each leg should be recorded so run parameters can be pulled from recorded data post flight from an EFIS (if equipped and desired). If equipped, use of auto pilot is recommended. When operating at speeds below L/DMAX, it may be necessary to hand fly (auto pilot logic is can be challenged on the back side of the power-required curve).

For the purpose of test, each horseshoe pattern (whether three or four legs) is considered a “run.” Each leg is a “trim shot” where GPS ground track and speed are recorded. A minimum of 3 runs in each flap configuration are required (flaps up, half flaps and full flaps), however 4 or 5 runs are desired for a high-fidelity output. If flaps are extended, then VFE becomes the maximum speed for that run. Speed points should be on the front and back side of the power required curve. E.g., VMAX/high-speed cruise, 55% cruise, Caron’s Speed, L/Dmax, Vmin pwr req, and stall plus 5. Additional speeds and power settings may be flown at the pilot’s discretion. More data points are better than not enough.

Monitor engine cooling during test. Some aircraft may suffer low cooling airflow at low airspeeds and high-power settings causing CHT and/or oil temperature to approach limits. It may be necessary to alternate high and low speed runs to accommodate cooling requirements. If the airplane is equipped with reflexed flaps, then “flaps up” runs should be conducted with flaps in the reflexed position. For testing with flaps down, half and full flap data points are required. For an airplane with 30 degrees of flap travel, half flaps would be 15 degrees. If a digital protractor is available, it is recommended that actual full flap be measured and noted; and half flap calculated from that result.

Set and/or reference zulu time on aircraft clock, EFIS, GPS or watch for easy reference during flight. Climb to altitude (6000-8000’ pressure altitude). Set 29.92 into reference altimeter to fly pressure altitude. Establish desired magnetic heading (cardinal directions work well, and GPS ground track will take care of itself—don’t over think it). Alternatively, if using an auto pilot that allows setting a ground track, that may be used in lieu of the mag heading technique. Set flaps (if deployed, do not exceed VFE), set desired power, trim, engage auto pilot (if equipped/desired). Allow aircraft to stabilize on track and ground speed. Note time, OAT, manifold pressure, RPM, fuel on board, GPS ground track, GPS ground speed and indicated airspeed on run card. Maintain identical parameters (IAS/altitude/pitch/power settings) for each leg/trim shot. Note for Van’s RV-types with non-slotted flaps: Vfe for half flaps or less is 10 MPH greater than published Vfe.

Generic Run Card

Here's a link to downloadable generic run card in Word format for recording flight test data. Taking along a brave flight test engineer to record data or rigging a video camera is very helpful for this drill!

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