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Airspeed Calibration??

Randy Erwin

Active Member
Is there a method for calibrating the airspeed indicator following installation? I mean, once the plane is flying, how do you know if the indicated airspeed is correct? :confused: My instructor and I were doing some touch and goes this morning in my newly acquired RV-8, and we found that it was stalling (at altitude) well up into the white band of the ASI - like around 65 mph - and over the runway it would stop flying at around 70-75. This is with about 25-30 gal of fuel aboard and about 400+ pounds of pilots. The aircraft weighs 1088.

After getting a pitot-static check accomplished, what else can be done to verify that the ASI is giving us accurate information? It appears to us that it is reading a bit high.
 
search archives, and ...

Kevin Horton has done a good job of writing up the calibration procedure on his website, and he wrote a couple of articles for kitplane magazine, I think.

But if you search the archives here, you will find lots of good info.
 
Instrument vs System

For calibrating the instrument, Kevin Horton's method is it. I recommend doing that first. I just looked at Kevin's links and there is more there than I remembered. Tons of links to excellent stuff.

However, for calibrating the system, you need more. By system I mean that a perfectly good instrument can give you very wrong readings when installed in a flying airplane.

For calibrating the system, please look at any of the spreadsheets in the left-hand box on this page:
http://home.cogeco.ca/~n17hh/Models/models.html
There you will find a method and pre-programmed calculations to use your GPS, altimeter and OAT to get a precise result of your airplane's Calibrated Air Speed (CAS) correction. Look at the third tab, the one labled "IAS-CAS". A perfect IAS would be the CAS.

Please feel free to use this thread or PM or email if you want any help with it.
 
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Thanks very much to all of you for your responses. It will take me a bit to digest all of the material. Rest assurred that I will report back here with my findings.
 
If I read your question correctly, you are interested in knowing the airspeed system errors at the stall. That is very difficult, unless you mount a flight test boom with swiveling pitot static tube well ahead of the wing. If you use the method I describe on my web site, and in a Kitplanes article, then you can only get down to perhaps 10% above stall speed. But, if you determine the errors at a number of speeds from 110% of stall speed upwards, you may see a trend that you can extrapolate to stall speed to get a best guess.

I did some experimenting a couple of years ago on a technique to determine airspeed system errors at the stall using GPS data, but the results were a bit inconsistent. The original testing was on a C182. I'll try again with my RV-8 someday, and if I can come up with a technique that gives repeatable results I'll publish it.
 
Is there a method for calibrating the airspeed indicator following installation? I mean, once the plane is flying, how do you know if the indicated airspeed is correct? :confused: My instructor and I were doing some touch and goes this morning in my newly acquired RV-8, and we found that it was stalling (at altitude) well up into the white band of the ASI - like around 65 mph - and over the runway it would stop flying at around 70-75. This is with about 25-30 gal of fuel aboard and about 400+ pounds of pilots. The aircraft weighs 1088.

After getting a pitot-static check accomplished, what else can be done to verify that the ASI is giving us accurate information? It appears to us that it is reading a bit high.


Go to

http://www.reacomp.com/true_airspeed/index.html

and follow the instructions.

Set up the discovered TAS on an E6B computer and work backward to arrive at Calibrated Air Speed. Compare it to your indicated speed at the time TAS was established and you have the error factor.

There is a way to check IAS on the ground with a test instrument. I had it done years ago. A mech friend simply dialed in 100 knots on his meter and compared it to what the indicator read.
 
Data point for static port correction and performance data.

RV7A with flush mount static port, riveted in normal location per drawings.

I did not want to drill out the 4 rivets on each static port and then have to find a helper to rivet new protruding ports in place, so...

I cut a washer in half, (not proper terminology here but they fit AN3 size bolts and are the thick ones...), and super glued them in front of each static port.

Prior to installing the washers the TAS on my Dynon Skyview was reading 7 knots slow, verified with same test method.

With the washers in place, it is now reading about 2 knots fast.

This was verified with 3 way GPS runs, (actually 4 but the spread sheet I used had 3) at 8000' with the data crunched on the NTPS spread sheet.

Data taken at 8000' 23" MAP, 2400RPMs, using auto pilot on 4 cardinal headings with time to stabilize at each heading. RV7A with all the fairings on, no paint but fiberglass has rough primer on it. OAT @ 48F

Heading TAS GPS GND SPD
270 172 169
360 170 174
090 172 170

From this data the spread sheet gave me Vtrue of 169.6 knots
My average TAS read out from those same 3 headings was 171.3.

Seeing the airspeed reading more accurately and higher makes me feel better about the speeds I am seeing, but they still seem a bit low when compared to Van's published performance numbers for the 7A.

The Dynon was showing 67% power but that may not be accurate.

Balls to the wall at 8000' and 2650 RPMs, (I did this run after the test but did not do 3 or 4 way run and crunch data for full power run) 22.9 MAP ( EMS showing 76% Pwr) with TAS reading at 177 knots, or 203.5MPH. (Need a little adjustment to get those last 50 RPMS on WW RV200)

My engine is a new IO-375 running dual Pmags and forward facing cold air induction, 8:1 compression, rated at 195 HP. With this setup it seems that I am coming up a bit short, (maybe those last 50 RPMs and a paint job will bring it in?). Van's numbers for Solo Wt, 8000', 180HP = 208 MPH, with 200HP = 215 MPH. It seems like I should be seeing speed between 208 MPH and 215MPH. I was running ROP, close to best power for this test.

To those of you that like to verify this kind of thing, did I do this right?
Am I correct in thinking I should be seeing a little more speed with my setup?

Anyway, for those considering placement of a half washer in front of flush static ports to correct TAS readout, I think this gives a pretty good data point to go by, (the washers gave me about 9 Knots TAS reading difference). Plus, it is so easy to do. I might mess around with smoothing out the washer edges a bit to dial in TAS even closer...

Randall in Sedona
 
Thanks Randy for the firsthand data. My TAS indicates about 5kts too slow, so I'll need some in front like you. Am thinking I'll start with a thin -L washer and go from there.

Just regular old super glue? I want to makes sure they stay on since indicated approach and stall speeds may change too. Could find myself surprised if one fell off in flight. :eek:

Also, how far away is the inside radius of the washer from the static hole? I wonder how much of a difference that might make?
 
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I just kind of centered the partial washer over the hole.

I used gel superglue after roughing the area slightly with scotch brite followed by an alcohol wipe. I have not really checked how well they are held on yet, but will test soon as I remove a bit of metal from them:)

Randall in Sedona
 
For testing purposes you can just use layers of electrical tape to create a dam. Be sure to test each change at a variety of airpseeds. A dam in front of the static port can create a little vacuum in the system (as opposed to just neutralizing pressure) and you might find variation at different speeds.

I probably burnt $500 in avgas fooling with the static ports on my Rocket - I eventually got them to within a knot however, and a slow speed flight over the runway at 50' or so shows reasonable altitude - another variable you want to check.
 
Paper hole reinforcements cut in half can be added in front or behind ports in layers as well. Once a proper "thickness" has been determined by flight test, a metal washer of similar dimension can be glued in place, if desired.

This is a variation on the tape technique mentioned above.

If you have a steam ASI, an in-expensive manometer can be used to accurately determine correction values.

Don't forget to check the static system for leaks, as well.

Cheers,

Vac
 
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FWIW: I used gorilla tape when I worked on my static port error. After the 1st flight test I added a 2nd piece of tape, doubling the thickness. After the 2nd flight test I added a third piece of tape so it was 3 times thicker. That wound up being too thick so I went back to 2 pieces thick .

In my case the correct thickness is twice the thickness of gorilla tape. With this information I ground down a piece of aluminum to that thickness and permanently bonded it to my static source. Now the error is less than 1kt from slow flight to max speed. Your results may vary.

:cool:
 
I have a SafeAir static port. My Dynon seems to read about 6-7 kts TAS too slow at high speed when compared to what the 3-way GPS spreadsheet says it should be, tapering down to little to no error at stall speed. I've confirmed this with multiple flight tests. Have also confirmed on a couple occasions that I don't have any static leaks.

Yesterday I put a half stack of binder hole reinforcers about the thickness of a thin washer just forward of the static port like what is shown here...

dmyb7n.jpg


I flew again, but no significant change in the indicated airspeed compared to what the spreadsheet said it should be. Still 6-7 kts low on the TAS indication. Should I have put them closer to the actual hole in the static port rather than simply around the portion of the port that protrudes from the skin?

Thanks for any thoughts.
 
Should I have put them closer to the actual hole in the static port rather than simply around the portion of the port that protrudes from the skin?

Yes.

Either that or build up the dam until it exceeds the depth of the protrusion.

Assuming your problem is you are getting a bit of pressure into the static port (at higher speeds) based on its location/configuration then you need to change the airflow around this port. The dam you have looks like it isn't high enough to reach the depth of the port, so airflow changes with that would be minimal.

May or may not be work 2 cents.
 
Pitot side test

I like the I Fly Ez test as a starting point. Checks the pitot side of thing for leaks and the function of the airspeed indicator. one question, how is the manometer connected to the pitot? If your looking for leaks in the pitot system wouldn't the connection to the pitot be a good place for a leak?
 
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