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Airspeed calibrations
Hi All
I'm scratching my head over something that has me befuddled. On my return trip from Sun Valley, ID to Livermore, CA I was at 12,500' and showing 7.2gph fuel burn, 136kts indicated and 169kts true on my GRT Horizon EFIS. Ground speed showed 148kts with an outside air temp of 44F. After I said "woo-hoo" to myself I very quickly said "better verify fuel burn and airspeed when I get home." I refueled at home an the totalizer was dead on, so the fuel burn portion of the data was correct. The next day I did both a wind triangle and a 4 leg gps airspeed calibration run and ran them through the various spreadsheets available. Results: my "indicated" true airspeed (true airspeed shown on the EFIS, which I think is simply calculated from the indicated airspeed on the EFIS, using the air/data info about temp and pressure) is about 12 kts higher than the calibrated/calculated speeds from my testing. This result passes the sniff test of being right in the ball part of what I was expecting/hoping for when I had a finished airplane, on that sort of fuel burn. The part that has me befuddled is that means that my indicated airspeed is off by a considerable amount, even though it has passed the pitot static check with flying colors. The EFIS specifically prevents you from adjusting the indicated airspeed so that it will agree with backup instrumentation, but you CAN adjust the True Airspeed shown by doing a true airspeed calibration procedure. So a couple of questions: 1. After doing the true airspeed calibration will the True Airspeed shown on the EFIS actually be Calibrated Airspeed? 2. I thought true airspeed was basically indicated airspeed with a calculated adjustment for temperature and pressure. Does this mean that my indicated is really far off and can't be corrected, even though it passed an IFR standards pitot/static check? I have a Gretz heated pitot tube, with the static vents integrated into the rear top and bottom of the heated tube. It would be a challenge to change the static vent situation, but my assumption has been that the geometry was well worked out. Thanks for the help Jeremy Constant |
If you passed the IFR cert and you haven't opened the pitot/static system I would say the issue almost has to be static error, especially given your static setup. Are you sure your altitude is indicating appropriately?
There are a couple of methods for testing this. Some have recommended doing a high-speed pass down a runway to see how off your altitude is. I would go and fly loose formation with someone else and compare numbers. |
Alt. check
Set your alt to 29.92 and at some alt ask ATC to give you a read back on you transponder alt.
Kent |
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Using this calculator...
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It shows 170 kts True Airspeed for the 136 kts indicated... at 12,000 ft and 44F and 29.92 alt setting. The EFIS is correct - the GPS just showed you had a 22 kt or so headwind component on that leg...:) If your alt. setting was 30.10 on that day, it calculates out to 169 kts... Sounds like your instrumentation is correct to me.... |
Some help with some of it
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To answer question 1, no. CAS is what your IAS would read if everything were perfect. TAS is faster if the air is thinner, for which see your E6B or my stuff linked below. To find out your CAS using GPS, see my spreadsheets, linked below; there is a page just for that. You are already on the right track. You should calibrate your GRT's TAS per their instructions to begin with. However, the 3-way method from NTPS is better and there is a way to manually correct the calibration on the GRT. This can be tedious, but you should only have to do it once. To address question 2, the instrument and pitot-static system can be perfect on the ground (mine is) and still be off quite a bit in the air (mine is off as much as 5-6 kts. KIAS). The most obvious culprit is something about the placement of the static. There are other possibilities, too. As a reality check, mine will to 160 KTAS at 8,000' Density Alt. on 8 gph. This is pretty accurate and you can benchmark yours against it. Mine is not the fastest nor the most efficient 7A by any means. My spreadsheet does not go to 12,500, so I used my E6b. At 12500', 169 true is 139 calibrated/indicated. At 8000', 160 true is 142 calibrated/indicated. This suggests that my benchmark and your observed data are very similar in IAS/CAS which means that the drag in both cases would be very similar. If you can do that on 7.2 gph you are doing very well. However, using the GRT you have to go by the "calibrated" TAS and either have a correction card for IAS or figure it later (if you care). TAS is how fast you are going. CAS is what the airplane feels as drag, IAS is whatever it is. If I have not totally confused you, then I hope this is helpful. |
The Pitot/Static check tells nothing about your airspeed accuracy JJ. It only sows that your static system is tight, that the altimeter is correct for a given pressure, and (if you ask, and they check it), that your airspeed indicator shows the right speed for a given pressure differential between static and Pitot. It does not tell you if your airspeed is going to be accurate in flight, because that depends on the proper position and design of your static ports (and to a much lesser extent, your Pitot placement - it is REALLY hard to mess up pitot....).
If you put your static ports where Van's said to put them, and you used their design, and you don't have any leaks, you should be pretty close. If either of the first two are not true, then you might or might not be spot on. And a leak should have been caught by the guy who did your Pitot/static cert. All that said, I concur with Gil that your TAS sounds pretty good for your flight conditions - maybe even just a tad high, but we can give you the benefit of the doubt that you built a clean, fast airplane! ;) Paul |
Humm...
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Kent |
Let's start with the simple stuff first.
Maybe your static system has developed a leak since the pitot-static check. It happens. It has happened to me, twice. You can do a quick and dirty static system leak check as follows:
If the static system has no leak, and your flight test results are repeatable, then it is almost certain that you have static system position error. In that case, tell us how your static system compares to Van's design (location of static ports, "Pop" rivet static port, etc). Guys who have flush static port often report errors about the magnitude you have reported. Note that if this is static system position error, it is also affecting the altimeter. You would have been flying roughly 225 ft lower than the indicated altitude at the case you described. You might have some ASI instrument error. I've seen reports of some unservicable ASIs that had more than 10 kt of error. A serviceable EFIS should have quite low instrument error, but there is a chance that your EFIS is not serviceable. Keep this possibility in mind if nothing else pans out. |
lots to chew on
Thanks everyone. I'll start chasing this down again, once the temperatures dip below 3 digits:eek: Yes...I'm a wimp!
I REALLY hope it's not a static position error, but if it is, I'll bite the bullet, cap off the static source on the pitot tube and install the pop rivets and static lines in the fuselage at the stations established by Van. I know about not monkeying with the static ports, but mine were already integrated into the Gretz heated pitot tube. I'll try to figure out how to put suction on the static sources to do the quick and dirty test, but they are on the rounded pitot tube at the back, top and bottom. I can just picture it now...me on the ground under the wing, sucking on the back of the pitot tube, friends walk in, take a look and without saying a word, quietly back out...:D The EFIS ASI agrees with the steam gauge ASI and the altimeter agrees with the altitude encoder on the Garmin 330, when I set the altimeter to 29.92 as well as agreeing with the field elevation when set to local altimeter setting. I'll do the TAS calibration, do more 3 way NTPS tests to establish exactly how badly I suck at flight testing and see where to go from there...thanks for the reality check numbers and all the suggestions. Jeremy |
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