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how far off was your 7's air speed?

KayS

Well Known Member
Hi All,

i think most have done a GPS speed box run to determine their pitot/static error. let's omit the the instrument error, as this is different for every instrument. but for a standard RV7 with the static ports and pitot tube in the locations the manual tells you... how far off was your calibrated ASI over the speed range? was it indicating more or less than actual TAS?

Kay
 
STATIC ERROR

G'day Kay,

KIAS was within 1Kt of Van's published stall speeds for 1400 & 1800lb.

GPS box indicated (best fit) linear difference from stall to 4KIAS slow at 140 KIAS cruise.

Heated Dynon pitot. Standard Vans rivet static ports as per plans.

Cruise performance at WOT, 2400RPM, LOP, seems consistent with flight plan, Dynon TAS/winds/groundspeeds, and other RV-7 data posted here.

RV-7, IO-360M1, 2 x Pmag, Hartzell CS blended.

Regards,

Bob
 
Using the Van's recommended static ports will give quite accurate airspeed reading, within 1 to 2kt through out the speed range. Flush static ports will give around a 10kt error. Inaccurate airspeed reading are usually caused by leaks in the pitot piping or static ports not accurately measuring the free stream pressure. Indicted airspeed will usually be lower than TAS, look up how to calculate TAS from IAS/CAS. The difference depends on your altitude and the OAT (difference in local air density compared to the standard atmosphere).

Pete
 
TAS Calculation

http://www.edwilliams.org/avform.htm. Was shared by Grands Rapid Technology support staff. I wanted to know how TAS was calculated with EIS4000 and Sport SX EFIS.

You can get real close by subtracting MSL from cruise altitude, multiply IAS by 2% for each 1000’

EXAMPLE 6750’ cruise altitude, surface below 750MSL ..... 6000’ above surface MSL. 146 KTS IAS X 1.12 =163KTS
 
Hi All,

i think most have done a GPS speed box run to determine their pitot/static error. let's omit the the instrument error, as this is different for every instrument. but for a standard RV7 with the static ports and pitot tube in the locations the manual tells you... how far off was your calibrated ASI over the speed range? was it indicating more or less than actual TAS?

Kay

One key detail required to accurately answer your question is what actual static ports were installed in the standard location.

This one point has had major error induction in many RV's over the years when static ports other than the kit recommended ones were used.
 
I plotted from 110/120/140/160 KTAS against GPS - multiple triangles and although it wandered a little with speed, was 1.4 kts low relative to GPS.

Vans rivet port, late 7 plan location.

It is sensitive though. My ports when viewed with a head magnifier had little burrs around the center. The rivet puller anvil hole was a little larger than the center hole in the pull rivet. I sanded lightly with #1000 sandpaper and it changed reduced the curve about .5 kts to it's current value. The paint shop masked the port rivet and I rechecked the calibration curve after paint to find it virtually identical.
 
My airspeed indication is slightly pessimistic, by about 1-2 kts, but that's within experimental error. Standard Van's setup with the rivet static ports.
 
my first try GPS speed run resulted in around minus 5 kt. that means my air speed indicator is a little bit pessimistic.

i have some fancy static ports installed that protrude 2 or 3/32. i don't recall the supplier.

when looking at your pop rivet reports, it seems to me that the standard super-simple el cheapo Van's solution is the best one. as so often, looks like Van's engineering did in fact invest more thinking into something than i did.
 
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Still 6 or 8 knots slow, after replumbing all static and pitot lines and drawing a vacuum on all lines to assure "NO LEAKS". Using Vans screw on static ports.
 
Hi All,

i think most have done a GPS speed box run to determine their pitot/static error. let's omit the the instrument error, as this is different for every instrument. but for a standard RV7 with the static ports and pitot tube in the locations the manual tells you... how far off was your calibrated ASI over the speed range? was it indicating more or less than actual TAS?

Kay

If your using the standard GA nylon pitot and static pluming, be sure and use teflon tape on the threads of the nylon fitting that screws into an aluminum housing such as a steam gauge or Garmin AHRS. Otherwise the those nylon to alum attach points will leak.
 
Are the ADC boxes not able to take calibration data to account for type and placement of static ports? It seems like a few GPS box runs and you'd have a decent curve fit that you could program into the ADC. Are they really set it and forget it at the factory with no way to account for aircraft specific calibrations?
 
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Are the ADC boxes not able to take calibration data to account for type and placement of static ports? It seems like a few GPS box runs and you'd have a decent curve fit that you could program into the ADC. Are they really set it and forget it at the factory with no way to account for aircraft specific calibrations?

Depends on the brand EFIS - the MGL I have in my littlest airplane has a calibration curve offset and scale factor you can adjust for ASI - it allowed me to tune the airspeed using GPS four-course runs without having to actually screw around with static ports. And for the purists, yes - I also adjusted altitude error out using another curve adjust.

I have never looked at my other EFIS’s to see if they have adjustments - I haven’t needed them on any of the RV’s with stock static ports, and the Tundra seems close enough for bush work....

Paul
 
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