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-   -   Low air speed (https://vansairforce.net/community/showthread.php?t=80254)

JohnJacobsen 12-20-2011 06:16 PM

Low air speed
 
I could use a little help on this one. My air speed has been reading low since first flight. I am indicating a stall of 35 knots, the airspeed is consistantly off thru the whole speed range. Rv-7 equiped with Dynon D-180, Dyon pitot, safe air static.
Pitot static tested ok, air damns in front of static no effect, static vented to cabin slight change, new static and pitot lines no effect, drilled out safe air ports and installed rivet (like vans ports) no effect, added washer under static port (rivet) no effect, different pitot tube no effect.
Have not compared to another airspeed indicator as I only have one. Seems to me that the D-180 is off, but Dynon tells me that since it passed the pitot static test that it was probably ok.
If anyone has any suggestions for me I would appreciate it.

N941WR 12-20-2011 06:24 PM

I would go back and check your entire pitot system again.

Mine was reading low, and like you, my stall was at 35 MPH. I replaced the entire pitot and AoA lines, from the D100 back to the Dynon pitot tube. Found a pretty good leak at the connection to the pitot tube. Problem solved.

JohnJacobsen 12-20-2011 06:29 PM

I've already replaced the lines, both pitot and static. As well as passing a pitot static test with a Barfield pitot static tester.

Tom Martin 12-20-2011 06:38 PM

set your altimeter to field elevation and then do a low pass (100) over the runway and note the altimeter setting. You will have to eyeball the 100 feet but if the altimeter is reading something much different then the 100 feet plus field elevation then you have a static error.

lostpilot28 12-20-2011 07:35 PM

Already replaced the lines and checked the static system for leaks? My next guess would be the instrument. Fortunately for you, Dynon has (I think) the ability to calibrate your airspeed and altimeter.

LettersFromFlyoverCountry 12-20-2011 07:46 PM

Any other RVers in the area with a similar Dynon instrument you could swap out for a quick check?

AX-O 12-20-2011 07:48 PM

Did you try the recalibration function on the Dynon?

JohnJacobsen 12-20-2011 07:49 PM

Nobody I know that has a D-180. Looking for a steam gage Airspeed to hook up for comparison.

JohnJacobsen 12-20-2011 07:51 PM

Yeah tried recalibration

NDrv8r 12-20-2011 07:53 PM

Quote:

Originally Posted by JohnJacobsen (Post 607946)
Have not compared to another airspeed indicator as I only have one. Seems to me that the D-180 is off, but Dynon tells me that since it passed the pitot static test that it was probably ok.
If anyone has any suggestions for me I would appreciate it.

airpeed calibration is not part of the static system test, however the Barfield test box is capable of airpeed testing. my skyview and D6 were spot on.

you might consider using a homemade water manometer to check your airpeed and plumbing. search for other threads on airspeed, you'll find more info.

LifeofReiley 12-20-2011 07:57 PM

Quote:

Originally Posted by JohnJacobsen (Post 607979)
Nobody I know that has a D-180. Looking for a steam gage Airspeed to hook up for comparison.

Stop by the airport, we can locate one you can borrow for testing. :) I'll email my phone #.

Tony Kirk 12-20-2011 08:57 PM

Manometer Article
 
One of my fellow chapter members, John McAvoy, published an article in Sport Aviation about calibrating you airspeed system using a Manometer.

Here is the link to the article and materials on his webpage http://www.iflyez.com/manometer.shtml.

Yes, his favorite building material is different than ours, but I don't hold it against him. ;)

Vac 12-21-2011 09:42 AM

John,

Take a look at this thread: http://www.vansairforce.com/communit...=static+source .

What you're discribing might be a static source pressure error. There's some other good info on this phenomenon (provided by Kevin Horton) in the flight test section. You'll still need manometer data, but if that shows the Dynon display to be accurate, then you'll need some flight test data. It generally takes some effort to get indicated airspeed and altitude "dialed in."

If you have any questions, you can drop a PM or e-mail.

Cheers,

Vac

jjconstant 12-21-2011 10:20 AM

Is your static source in the same location and using the same pop rivet head as Van supplies? Both location and shape have a strong effect on indicated airspeed. Low indicated speed implies that you may be getting pressurized air in your static source. Deploying flaps can change your static pressures at the low end if you use differing static source locations and shapes.

All best

Jeremy

Bavafa 12-21-2011 10:30 AM

@ 35K, it seems like a big discrepancies

Is the airspeed off by a consistent amount or varies significantly?

You seem to have tested all but the instrument itself, so I would suggest to try to take that also out of the equation before messing more with the pitot/static system.

Of course, this is based on the assumption that the location of Pitot and static are on the suggested location by VANs. A quick check to make sure your pitot is not bent in a wrong angle, though doubtful since you have been checking every thing else.

Good luck

WingsOnWheels 12-21-2011 12:30 PM

I'm not exactly close by, but I have a new airspeed indicator sitting in the box at home. If you make it up to Dallas, you are welcome to borrow it for testing. Just e-mail me if you are interested.

Walt 12-21-2011 05:34 PM

If you get up this way to pick up the airspeed indicator in the above post, stop by and I'll throw my pitot static box on it and check it.

JohnJacobsen 12-22-2011 04:40 AM

The static is pretty close to Vans location. About half an inch forward. The air speed stays consistantly off thru the speed range.

Thanks for the offer to borrow an AS indicator in Dallas. I will see what I can get done down here first. :)

I have access to a pitot static test set down here.

Thank you everyone for all the suggestions and offers. Hope to have this figured out soon. This is a great community. :)

NickAir 12-22-2011 05:03 AM

Let us know what you find out. Instrument or static system.

Brantel 12-22-2011 08:00 AM

I went thru this as well.

Take a look at this thread:

http://www.vansairforce.com/communit...ad.php?t=56387

I think like post 64 is were I found the fix....

Slagergren 12-22-2011 01:15 PM

Low airspeed problem
 
I had/ possibly still have a similar problem with my D180/ RV7 airspeed inidicated lower by 6-10 mph than what should have been. I have Dynon's older pitot/ AOA probe which has much larger vent opening and possibly different plumbing inside. I ended up piling up the paint just in front of the stock Van's static ports to get the speed corrected at least at my crusie speed however its not a linear correction. I was going to suggesat trying one of Dynon's new AOA/Pitot probes and I see you tried a different probe. Also I built my own water manometer and was able to bench check somewhat the D180 was calibrated. I posted some pictures of the whole probe on this site about 3 months ago.

Steve

Slagergren 12-22-2011 01:22 PM

D180 static calibatrion
 
The D180 has a speed calibration which is as I understand it a simple zeroing of the pressure sensor used on the static side only. There is no calibration to actually add some correction factor to the ram air pressure sensor.

Steve

JohnJacobsen 12-22-2011 04:31 PM

Brantel
That was one of the many posts I had read :) . Tried it, had no effect :(

JohnJacobsen 01-15-2012 01:10 PM

Update
 
Well I flew with another airspeed indicator installed along with Dynon. Airspeed is the same on both, so its not the Dynon.
Guess I will go back and look at static ports and pitot tube placement. :confused:

schristo@mac.com 01-15-2012 01:59 PM

you have a leak...
 
Sure sounds like you have a leak in the pitot system...

NickAir 01-15-2012 03:03 PM

Try checking your system from the pitot. If you don't have the tools for this you can have a shop do a simple static system test which will confirm if a leak exists. The leak can then be traced and repaired.

I did this once and found I had two problems:
1) A slight leak at the hose on Pitot connection.
2) A bad ASI.

Frustrating I know, although you are almost to a resolution. Hang in there.

RV10inOz 01-15-2012 03:33 PM

Static error is a most likely cause, many of us are victims here.

But.........nick now you think it is correct, but I have seen one guy who had the AOA tube and pitot tube crossed over. The both sorta work but the AoA calibration never works so that was how I found his problem.

Triple check you have the right tube in the right port.

NickAir 01-15-2012 03:44 PM

Quite possible that plumbing is misapplied. Good point.

JohnJacobsen 01-15-2012 04:06 PM

AOA
 
I have checked that AOA and pitot are in the correct ports. But believe me I will double check that :p .
Almost positive it isnt a leak. I had Barfield pitot static tester hooked to it with spot on AS and Alt and no leaks. But will check it again.
Thinking of adding another washer under the static.

P.S. Thanks for all the suggestions. :)

N941WR 01-15-2012 05:20 PM

Quote:

Originally Posted by JohnJacobsen (Post 617887)
I have checked that AOA and pitot are in the correct ports. But believe me I will double check that :p .
Almost positive it isnt a leak. I had Barfield pitot static tester hooked to it with spot on AS and Alt and no leaks. But will check it again.
Thinking of adding another washer under the static.

P.S. Thanks for all the suggestions. :)

I went through the same thing earlier this year and my issue was a leak at the pitot tube connection.

Crossing the AoA and AS lines will give you a zero airspeed indication. Been there, done that.

I have a steam gauge AS indicator along with my Dynon and they both indicated the same AS. When I tried flying with either the round gauge or the Dynon, the airspeed indications were the same as when both were connected.

I wound up replacing both the AoA and airspeed lines from the pitot tube to the instrument panel. It was a pain but that was when I found the leak at the pitot. The black lines were replaced with AN fittings.

RV10inOz 01-15-2012 07:01 PM

This was a rough experiment to solve my -9 knot speed error, it started out with glueing a rivet head over the ports, this gave a +6 knot error, I started filing an edge and in the end thought about some Cessna and other certified ports that had a fence around the front.

I glued on a piece of Aluminium TIG welding wire (Aluminum for you guys who speak funny :D) in the shape of an arc, went and flew it, error sub 2 knots :)

Then I wondered should I do something more "professional" and risk the errors again or leave it how it is.............550 hours later.......... yep she is still there! :o



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