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Accuracy of the GRT EIS airspeed indicator
- Thread starter j_omega
- Start date
Good
I just had an "official" test done by an A&P on my field of my altimeters airspeed indicators, both analog and GRT, and it was very accurate. The GRT does have a calibration which I played with, but didn't quite understand. Didn't seem to be needed or make much difference.
For example, here are my readings for 200kts from the GRT:
We hesitated longer here than at other speeds to confirm no leaks, so I have more readings at this speed.
His calibrated machine was not digital, so I think that the tolerances of a few knots are about as accurate as you could even read on his gauges.
I just had an "official" test done by an A&P on my field of my altimeters airspeed indicators, both analog and GRT, and it was very accurate. The GRT does have a calibration which I played with, but didn't quite understand. Didn't seem to be needed or make much difference.
For example, here are my readings for 200kts from the GRT:
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199.2
199.6
199.8
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199.7We hesitated longer here than at other speeds to confirm no leaks, so I have more readings at this speed.
His calibrated machine was not digital, so I think that the tolerances of a few knots are about as accurate as you could even read on his gauges.
I'm particularly interested with what you came up with at the lower end of the range closer to stall speeds. This is where I would expect to see the larger measurement errors.
These low speed errors are mostly due to the pitot tube no longer pointing straight into the relative wind. You can buy an expensive pitot mount on gimbals and fins that always points into the wind. Or, most GRT efis units will calculate/show angle of attack, which is actually more useful at low speeds (I prefer the aural warning tones).
low speed indications
The numbers I have in my excel for lower speeds will probably not be that useful, we gradually increased "speed" and called it out as it passed different speeds, not stopping to let the logs write down a stabilized speed. The speeds shown by the GRT at these lower speeds were within the tolerances they have for this test, which I believe is 2.0kts up to 150kts, 2.5kts from 150kts to 180kts, and then 3.0kts up to 280kts. I only tested up to 235kts.
rzbill
Well Known Member
Why would you expect GRTs electronic pressure differential sensor to be less accurate than other measurement means such as a mechanical ASI? Yes there could be error at low speed but as noted that is usually the pressure sampling system (static port or less likely pitot) rather than the differential sensor so all ASI indicators would have the same error.
Because according to the datasheet, the uncalibrated accuracy of the pressure sensor they use is +/- 7 PSI across the entire range. The equation for pressure vs airspeed is non-linear and is more sensitive to changes at lower speeds.
https://www.nxp.com/docs/en/data-sheet/MPX5010.pdf
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That must be a typo. 7 psi is a huge error for IAS.
Yeah, sorry, that's supposed to be 0.07 PSI (0.5 kPa). That still correlates to 10s of knots on the lower end of the scale. I guess GRT has just calibrated the error down to a reasonable level. I was just curious how accurately they had calibrated it...
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rzbill
Well Known Member
Yeah, sorry, that's supposed to be 0.07 PSI (0.5 kPa). That still correlates to 10s of knots on the lower end of the scale. I guess GRT has just calibrated the error down to a reasonable level. I was just curious how accurately they had calibrated it...
Fair enough. I see the reason for inquiring. Thanks.
