Flight Testing - Calibrating the Air Speed Indicator (ASI)

 
  First off, let me clarify that I'm not actually calibrating the ASI, but rather quantifying the amount of error displayed across the entire range of indicated airspeeds. The calibration will come in the form of a chart that will be placed in the aircraft Pilot Operating Handbook (POH). This chart will allow you to convert the Indicated Airspeed (IAS) as read directly off the ASI into Calibrated Airspeed (CAS) that accounts for instrument and pitot-static system errors.

ASI errors can creep in due to a variety of reasons, including obstructions or leaks in the pitot/static plumbing, improper placement of the pitot tube or static source, or simply a sloppy ASI gage. Troubleshooting these errors is notoriously difficult. But calibrating your ASI is quick and easy using a handheld GPS. The GPS calibration method involves flying at a constant indicated airspeed (say 60 mph indicated) at three different headings. One of the difficulties in using GPS speeds to calculate airspeed is the effect of wind. By flying three different headings and mathematically crunching the geometry, we can precisely determine both the speed and direction of the wind, and then subtract those wind effects to give us the actual airspeed of the airplane. The spreadsheet makes this effortless and automatically displays the calibration graphs.

Here is how the flight test procedure is conducted:

  1. Gather the necessary supplies. You will need a handheld GPS, set to display heading in degrees and speed in mph, plus a note card to record your test data.
  2. Select a time when the air is free of turbulence. Steady wind is OK, but gusty conditions and lots of bumps will give you poor data.
  3. Pick a starting altitude and airspeed. It is important to maintain these as precisely as possible.
  4. Fly the first test heading and allow the aircraft to become stabilized before recording the indicated airspeed (V indicated), GPS speed (Vg 1) and GPS track/direction (Track 1).
  5. Repeat step 4 for two additional directions (any two will serve just fine) and record GPS results. Be sure to maintain a constant altitude and indicated airspeed throughout the test.
  6. Enter all the test data into the spreadsheet. For the best results we need to gather data across the entire speed range. I like to start at approx 1.3 Vso and record data for speeds in 10 mph increments up to full power cruise speed. At a minimum, collect data near stall speed and at the typical cruise speed.

All of the number crunching is done for you in the form of an Excel spreadsheet. To use the spreadsheet, you simply fly the test procedure and fill in the data in the yellow shaded cells, and the program does the rest.

Additionally, the spreadsheet will graph the results if you repeat the test at several different airspeeds. The graph will serve as a handy reference tool where you can use any indicated airspeed for your plane and find the resulting calibrated airspeed off the graph. The second graph presents the calibration in a slightly different form, where each indicated airspeed corresponds to a given amount (positive or negative) of ASI error.

Let's say you performed three test runs at different airspeeds (i.e. close to your stall speed, cruise speed, and somewhere in between), plugged all that into the spreadsheet, and now had the following graph.

It is a simple matter to find CAS. Say your ASI is reading 110 mph, you find 110 on the bottom axis (IAS), follow it up to the plotted line, and over to CAS on the vertical axis. Doing this, you determine that the actual calibrated airspeed is 108 mph. Using the alternate graph, 110 mph IAS results in +2 mph of ASI error.

Now that we have a calibration curve, what is the next step? What deductions can we make based on the results? First off, you'll have to decide if the performance of your ASI is adequate As-Is, or if changes need to be made. If the percentage error is typically less than 10%, I would find that generally acceptable. In the case of my data, my ASI is fairly accurate at cruise speeds (approx 5% error), but as much as 15% slow at near-stall speeds. This result is typical for many airplanes; the ASI reads slower-than-actual near the stall speed. Indeed, flight testing showed my ASI was reading in the 30's at stall, which I know isn't correct (it should be in the 40's).

Let's examine what's going on here. First off, the ASI is simply a differential pressure gauge, with the pressure difference (delta P) converted to read in MPH (or knots, or kph, or whatever). One end of the pressure gauge (ASI) picks up ram pressure (via the pitot) while the other end picks up ambient (static) pressure. Simply put, the higher the delta P, the higher the indicated airspeed. The first deduction is this: if we have a lower-than-actual indicated airspeed near the stall, we must have a lower-than-actual delta P. The only two ways we can get a lower-than-actual delta P is 1) too low ram pressure, or 2) too high static pressure. Too low ram pressure is normally caused by misalignment of the pitot tube with the incoming airstream (which is worst at the high angle of attack near the stall). Too high static pressure can be caused by interference with the bottom of the wing (ie the large high-pressure that develops under the wing starts to expand and influence the static port pressure), or perhaps the static port itself is becoming aligned with the incoming airstream and picking up some ram pressure. This is even more probable if you use a two-tube pitot-static probe with the static hole located only along the bottom of the probe. I modified my static probe to include several holes in different locations to reduce this possibility.

Having looked at the cause of my ASI error near the stall, there really isn't much I can do about it. I can't significantly change the alignment of the pitot probe. It's already ready very accurate at cruise speeds, and that's a pretty good compromise on alignment. My static port has several (cross-drilled) holes to eliminate picking up any ram pressure, so there's not much more I can do there other than relocate the port entirely. There may be some techniques to modify the static port that I can try, but lots of experimentation is needed. About the best I can do for now is note the IAS and CAS calibration charts in the POH and go with that.

ASI Calibration Spreadsheet (Excel)

 

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Updated: 07 Jan 14