Marc Zeitlin
Exalted Grand Poobah
I posted this to the COZY and COBA (Canard Owners and Builders Association) mailing lists, but I thought it might be of interest here as well for those approaching, in, or even well past Phase I testing.
Airspeed calibration is a large part of the Phase I and 14 CFR Part 91.319(b) compliance test periods. The test cards for performing calibration runs are shown in Kevin Walsh's excellent Phase 1 Test Protocol document, available here:
4th bullet point down. He doesn't include the spreadsheet used for crunching the numbers gathered in the testing, though, but I'll get to that later.
I won't go into all the reasons that have been discussed ad-infinitum as to why one wants to calibrate their airspeed indication - I suppose if you don't care whether your instrumentation is correct, then there's no need, but if you haven't done it and if it isn't documented, then you have no idea if your instrumentation is correct or not, and it could be argued that you're not in compliance with the regulations. Don't expect EFIS TAS or winds to mean much to you either if you haven't calibrated your airspeed (or your magnetometer, for winds).
On the way home from the Kanab canard Fly-In in September, we flew in loose formation with Cedric Gould in COZY MKIV N117CG (with Deanie and me in COZY MKIV N83MZ). We were never more than a couple hundred yards away from each other for a bit over 2 hours. Coms with Cedric revealed that his Garmin G3X was indicating exactly the same Ground Speed, per the GPS, as my Dynon Skyview (which is to be expected, since we hardly moved with respect to one another for >2 hours).
I was indicating ~142 - 144 KIAS, with a TAS of about 166 KIAS at the altitude at which we were both flying - this is pretty much what I've seen at the weight, CG and altitude that we were at for the whole 2K hours of flight time in the plane. Cedric, however, was indicating 10 - 12 kts. faster in both IAS and TAS. Obviously, one or both of us were wrong - we both could not be right (sans some sort of chronosynclastic infundibulum). We needed (well, wanted) to resolve the discrepancy.
Having recovered from my surgery in late October, last Sunday morning we both headed out to the airport and flew out to the Antelope Valley at 8,000 ft PA (barometric pressure set to 29.92 in-Hg). We used the techniques in the above mentioned document to fly four leg calibration runs at IASs varying from 65 KIAS to 145 KIAS. Here's what the data input for the spreadsheet we used looked like at each IAS, this one for the 85 KIAS run:
While not required to be orthogonal, best accuracy is obtained that way, so we flew tracks of 0, 90, 180 and 270 degrees. The bright yellow boxes are pilot input. In this example, you can see the four legs flown, with varying GS's due to wind. The spreadsheet calculates the wind velocity and direction as well as the True Airspeed, then takes the averages of the four versions of the calculations to get a lower error input for the CAS calculations. The equivalent Calibrated Airspeed for the average value of TAS is shown to the right, and the CAS - IAS Error (the whole point of the calibration) is shown all the way on the right.
We can see that in this case, while I was indicating 85 KIAS, my CAS was 90 kts, for an error of about 5 knots - I was actually going about 5 kts faster than I was indicating.
Now, if we take all the data collected between 65 and 145 KIAS and graph it as well as the error, we get:
Showing the CAS value vs. IAS, as well as the error in IAS at each IAS. We can see that for MY plane, the lowest error is at high speeds, meaning that in cruise, my CAS is within a knot or two of the IAS and TAS and winds (assuming magnetometer accuracy) will therefore be pretty accurate. At low speeds, however - in the pattern or at approach speeds - I'm actually about 4 - 8 kts faster than indicated (and a bit more than that in TAS, depending upon altitude and temperature) and so the TAS and winds may be pretty inaccurate. The curve fit line through the error data smooths out the jitter, which actually isn't too bad - this was a mildly bumpy day, and the data is still fairly clean. On a VERY smooth day, I'd expect even cleaner data.
Now, since my plane's IAS/CAS relationship at higher speeds is pretty damn accurate (within a knot), the error that Cedric and I saw on the way back from Kanab SEEMS to point to something in his plane - Cedric's in the process of reducing his data and then will attempt to determine what's going on that makes him think that he's got the fastest parallel valve 180 HP O-360 COZY MKIV on this (or any other) planet
.
Let me know if you have any questions, comments or suggestions about calibration issues or this testing.
Airspeed calibration is a large part of the Phase I and 14 CFR Part 91.319(b) compliance test periods. The test cards for performing calibration runs are shown in Kevin Walsh's excellent Phase 1 Test Protocol document, available here:
4th bullet point down. He doesn't include the spreadsheet used for crunching the numbers gathered in the testing, though, but I'll get to that later.
I won't go into all the reasons that have been discussed ad-infinitum as to why one wants to calibrate their airspeed indication - I suppose if you don't care whether your instrumentation is correct, then there's no need, but if you haven't done it and if it isn't documented, then you have no idea if your instrumentation is correct or not, and it could be argued that you're not in compliance with the regulations. Don't expect EFIS TAS or winds to mean much to you either if you haven't calibrated your airspeed (or your magnetometer, for winds).
On the way home from the Kanab canard Fly-In in September, we flew in loose formation with Cedric Gould in COZY MKIV N117CG (with Deanie and me in COZY MKIV N83MZ). We were never more than a couple hundred yards away from each other for a bit over 2 hours. Coms with Cedric revealed that his Garmin G3X was indicating exactly the same Ground Speed, per the GPS, as my Dynon Skyview (which is to be expected, since we hardly moved with respect to one another for >2 hours).
I was indicating ~142 - 144 KIAS, with a TAS of about 166 KIAS at the altitude at which we were both flying - this is pretty much what I've seen at the weight, CG and altitude that we were at for the whole 2K hours of flight time in the plane. Cedric, however, was indicating 10 - 12 kts. faster in both IAS and TAS. Obviously, one or both of us were wrong - we both could not be right (sans some sort of chronosynclastic infundibulum). We needed (well, wanted) to resolve the discrepancy.
Having recovered from my surgery in late October, last Sunday morning we both headed out to the airport and flew out to the Antelope Valley at 8,000 ft PA (barometric pressure set to 29.92 in-Hg). We used the techniques in the above mentioned document to fly four leg calibration runs at IASs varying from 65 KIAS to 145 KIAS. Here's what the data input for the spreadsheet we used looked like at each IAS, this one for the 85 KIAS run:
While not required to be orthogonal, best accuracy is obtained that way, so we flew tracks of 0, 90, 180 and 270 degrees. The bright yellow boxes are pilot input. In this example, you can see the four legs flown, with varying GS's due to wind. The spreadsheet calculates the wind velocity and direction as well as the True Airspeed, then takes the averages of the four versions of the calculations to get a lower error input for the CAS calculations. The equivalent Calibrated Airspeed for the average value of TAS is shown to the right, and the CAS - IAS Error (the whole point of the calibration) is shown all the way on the right.
We can see that in this case, while I was indicating 85 KIAS, my CAS was 90 kts, for an error of about 5 knots - I was actually going about 5 kts faster than I was indicating.
Now, if we take all the data collected between 65 and 145 KIAS and graph it as well as the error, we get:
Showing the CAS value vs. IAS, as well as the error in IAS at each IAS. We can see that for MY plane, the lowest error is at high speeds, meaning that in cruise, my CAS is within a knot or two of the IAS and TAS and winds (assuming magnetometer accuracy) will therefore be pretty accurate. At low speeds, however - in the pattern or at approach speeds - I'm actually about 4 - 8 kts faster than indicated (and a bit more than that in TAS, depending upon altitude and temperature) and so the TAS and winds may be pretty inaccurate. The curve fit line through the error data smooths out the jitter, which actually isn't too bad - this was a mildly bumpy day, and the data is still fairly clean. On a VERY smooth day, I'd expect even cleaner data.
Now, since my plane's IAS/CAS relationship at higher speeds is pretty damn accurate (within a knot), the error that Cedric and I saw on the way back from Kanab SEEMS to point to something in his plane - Cedric's in the process of reducing his data and then will attempt to determine what's going on that makes him think that he's got the fastest parallel valve 180 HP O-360 COZY MKIV on this (or any other) planet
.Let me know if you have any questions, comments or suggestions about calibration issues or this testing.
