Round airspeed differs from Dynon Skyview tape

[ron sterba]

Got One round gauge and it's airspeed. My Dynon airspeed tape differs by six miles per hour. Your thought on fixing or finding a solution to match. Don't know where to start but here at VAF. Thanks Doug!

Ron in Oregon
RV9A Flying 2016

[mike newall]

Use a manometer attached to the pitot to check both gauges. It is a standard check for certificated airplanes on annual checks. A known amount of pressure should give a known airspeed. You can then determine which is in error. If it is the Dynon, I do believe there is adjustment available in the setup - you can fiddle with most of the parameters. If it is the round gauge, throw the Falcon ASI away and buy a UMA

Be careful and get the correct calibration instrument, we are talking very small pressures here

[Vac]

Hi Ron!

The manometer for checking a mechanical ASI is a great idea, however you need to be aware of a couple of things. First, it's possible to damage any ASI or the transducers in your Dynon if you over-pressurize them. Also, on the Dynon website, there is a white paper that details the proper way to perform a pitot/static check without damaging the instruments. In all likelihood, the transducers in the EFIS are spot on. If your ASI was certified (bench checked) at some point, instrument error may be recorded in a log book or on a form 8130.

Chances are pretty good, that it's a combination of static source pressure, installation and mechanical error that's producing the difference in readings. If you look at Kevin Horton's website (he's a regular poster here on VAF), he has an excellent discussion regarding static source pressure error.

It takes proper flight test, data reduction and ground testing to get a pitot/static system fully dialed in and to account for differences such as the one you are seeing in airspeed. Do know that both of your airspeed indications are accurate ratio displays insofar as displayed stall speed (regardless of the accuracy of the actual airspeed number), is always displayed stall speed (for a given gross weight) and any ratio based on that number (Vref, ONSPEED, etc.) is going to be fairly reliable. Another thing to note is that different portions of the speed envelope may be effected differently by error--i.e., it may be more pronounced at slow speed/high AOA.

Also note that the 24-month "pitot/static" check you have performed at an avionics shop does not check for installation or static source pressure (indicated airspeed) error although it's a great way to check for leaks in your system.

Proper pitot/static checking and getting a static system properly "dialed in" can be a bit of work, especially when mixing mechanical with electronic sensors. I've been through this integrated the back-up mechanical ASI display with my EFIS in my RV-4.

Also, 43-13 has some good, basic techniques using surgical tubing as a pressure source that can be helpful diagnosing basic pitot/static problems. Here's a good link on how to build an inexpensive manometer if you choose to go that route: http://www.iflyez.com/manometer.shtml I built one and have found it to be quite accurate. I modified mine and use a small blood pressure bulb with a relief valve for pressure input so I can make very fine changes and "hold" the pressure with the valve to leak check the pitot system. It can be used with an EFIS if you follow the Dynon guidance and don't over pressurize, but as I said, I think you'll find the "instrument" error in an electronic transducer to be negligible.

There is lots of great discussion on this board about airspeed accuracy! You might want to search the forums for discussion on "airspeed calibration, static source pressure error, GPS speed runs, static" and "pitot" as key words. If you've got a system that's built in accordance with Van's plans (including the static port configuration), you are likely already in the ball park and one of your indications (ASI or speed tape) is closer to ground truth, but you won't know until you test. If you are interested in discussing test techniques, drop me a PM or e-mail and I'd be happy to help you out.

Cheers,

Vac

[noelf]

I have a steam gauge and a Dynon 10A. The Dynon unit is more accurate than the steam gauge, but for me the steam gauge is easier to read. I can see the relative position of the needle with peripheral vision but with the Dynon unit, it provides a number that I need to focus on and read.

During my Phase I testing, I plotted the reading of one vs the other over the speed range of stall-to-Vmax and found that the DeltaV was not consistent. At stall, the difference was about 4 knots. At the high end, about 9 knots.

[Michael Henning]

Make sure the the unit is set to mph or kts to match steam gauge.

[Carl Froehlich]

The manometer will tell you truth.

The standard analog airspeed instrument can be calibrated if it is not accurate. Take it apart and you will see a beam and some set screws that come into play at different speeds. Using your manometer you can adjust the screws to calibrate. It takes a few iterations to get it just right.

While you have the airspeed instrument out, check it for static leaks as well. While the pitot line goes to a sealed bellows, the static line goes to the can. I've found the face glass to be a source of static leaks. If this is the case, careful application of black RTV on the outside of the glass to can seal solves the problem.

Carl

[ron sterba]

Gentleman! I presented a chapter in phase one of my 9A which I have never explored or had any inkling in my 43 years of renting Cessnas. I'm open to explore and I feel you presented the mechanical discrepancies which presents another phase of learning. I am going to go up flying today and I'll start with the simplest test and that would be ( GPS ) Dynon,IPad viruses dial. The dial is new. On past 10 hours on the plane I have kept my airspeed up on the dial since it shows the faster speed than the Dynon. I thank you all for your direction. I'll be reposting as my tests become fruitful or not in this phase one process. Up to now it's be pretty fun.

Thanks
Ron in Oregon
RV9A

[springer]

[quote=mike newall;1130284 If it is the round gauge, throw the Falcon ASI away and buy a UMA [/QUOTE]

I agree. I had a 10 kts difference between a Dynon 180 and a Falcon ASI. Replaced with a UMA and was able to reduce the spread to 5 kts.

[John Owen]

Do your instruments share the same static source? I had a discrepancy in airspeed indication as well and thought that both instruments had the same static but the ARHS in the back were plumbed to a standard fuselage static source and the other instrument (G-5) was hooked to a Piper-type pitot static under the wing. I ended up plumbing the rear static source forward and now they are exactly the same. Actually, first, I opened the panel system to ambient and it was a lot closer proving to me that there had to be a problem with the static. Mine was a little more interesting yet as it was also driving my autopilot crazy coming from the Piper static.

This may not be your problem but at least demonstrates what a seemingly minor difference can cause in airspeed indications (and autopilot sensitivity in my case).

I hope that helps some.
John

[rvbuilder2002]

Ron,
You made no mention of whether the two instruments are connected to the same pitot and static sources.

If they are, then the simple answer is that one of them is wrong. Doing the GPS speed boxes should show you which it is.

BTW, if you are using the higher one as primary, to be safer for approach speeds. That is actually backwards from what you would want to do.
If it is higher because it reads artificially high, then you are actually at a slower speed than what you think you are.
If you want to have the most safety margin, use your normal airspeeds with reference to the indicator that reads the lower value.