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Airspeed indicator fail (again)

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carlisle

Joined: 20 Jun 2009
Posts: 72
Location: Sioux Falls, SD

Posted: Thu Apr 08, 2010 5:05 pm Post subject: Airspeed indicator fail (again)

Finished spring inspection and cleanup on my Model II.  Took it up for first flight and the ASI reads 40 mph too low.  This is the second time it's failed.  First one was replaced last year and had only 150 hrs on it.  New one has less than 25.



Wonder if the extreme cold here in Sioux Falls has anything to do with it?  Plane is hangared.



Chris

Tom Jones

Joined: 12 Mar 2006
Posts: 752
Location: Ellensburg, WA

Posted: Fri Apr 09, 2010 6:46 am Post subject: Re: Airspeed indicator fail (again)

 	  carlisle wrote: 	
		  Finished spring inspection and cleanup on my Model II.  Took it up for first flight and the ASI reads 40 mph too low.  This is the second time it's failed.  First one was replaced last year and had only 150 hrs on it.  New one has less than 25.



Wonder if the extreme cold here in Sioux Falls has anything to do with it?  Plane is hangared.



Chris 	




Maybe a leaking pitot line?  To check it slip a length of about 10 inches of surgical tubing over the pitot tube.  Slowly roll the tubing up until the airspeed indicator reads about 60 mph or so.  Hold it there and the needle should remain steady for at least a minute.



Do you have a static port installed.  Maybe a bug or water in the static line?



If the static side of the ASI ever sees pressure higher than the pitot side that can damage the ASI.

_________________
Tom Jones
Classic IV
503 Rotax, 72 inch Two blade Warp
Ellensburg, WA

mikeperkins

Joined: 22 May 2007
Posts: 123

Posted: Fri Apr 09, 2010 7:54 am Post subject: Re: Airspeed indicator fail (again)

Hi Chris, 



I agree with everything Tom said about examining your pilot/static system before you replace your ASI. I've never heard of ASIs being damaged by cold temperatures, but I suppose it's possible. Mine sat outside for two years in Illinois and then moved into a hanger, surviving year after year without any problems. 

Some years ago, I made a water-column manometer calibrated in MPH for testing my ASI. I'd suspected that my original "Denny Aerocraft" ASI with the little fox on it wasn't reading right. After testing it on the manometer, I began joking with people that its scale was calibrated in Dennys per Hour: 10 MPH high on the high end and 5 MPH low on the low end. I'm sure it was that way from the day I installed it. Soon after finding this out, I bought a fast-acting 0-100 helicopter ASI, a purchase that I've never regretted. It responds to gusts immediately and is quite accurate. 

There's not a lot to making a manometer, but it took me an entire afternoon. It would not be hard for me to box it up and mail it to you as a loaner. Let me know. 

Mike

carlisle

Joined: 20 Jun 2009
Posts: 72
Location: Sioux Falls, SD

Posted: Fri Apr 09, 2010 10:35 am Post subject: Re: Airspeed indicator fail (again)

Thanks guys.  Feel kind of silly.  Started tracing back the pitot line and found the 3/8 in hole I'd drilled in it when drilling out the pop rivets for the windshield that I replaced last week.  Oops.  Embarassing.



Mike.  I built one of those manometers last year when I was messing around with my prior broken ASI.  It probably wasn't the most accurate example in the world but taught me a lot about the sensitivity and potential fragility of these instruments.  Rather than re-assemble that this year, I tested this one against a known accurate one with a simpler method.  Attach a 6 in or so piece of 1/8 in Tygon tubing or similar to the nipple on the back of the instrument and a 1cc insulin syringe to the other.  Inject a little air, usually about 1cc, and see what it reads.  If the two instruments are close, say withing 5mph of each other, it's probably ok.  As a measure of how sensitive the thing is, when you plug the syringe onto the tubing, the pressure created just by that minuscule volume change gives you about 10 mph so you have to subtract that from the final reading after injecting.  1cc of air is about 80 mph using that system.



Chris

mikeperkins

Joined: 22 May 2007
Posts: 123

Posted: Fri Apr 09, 2010 11:44 am Post subject: Re: Airspeed indicator fail (again)

Well, Chris, at least you didn't make up some big story. Glad you found it. . . . Duct tape? (smile)

Pat Reilly

Joined: 06 Aug 2009
Posts: 345

Posted: Fri Apr 09, 2010 2:37 pm Post subject: Airspeed indicator fail (again)

Mike, How did you build your manometer and calibrate and use it with your pitot system? 

  

 Pat Reilly

 Mod 3 482 Rebuild

 Rockford, IL

  

 



 

 On Fri, Apr 9, 2010 at 10:55 AM, mikeperkins <michael.perkins(at)rauland.com (michael.perkins(at)rauland.com)> wrote:

  	  Quote: 	
		  --> Kitfox-List message posted by: "mikeperkins" <michael.perkins(at)rauland.com (michael.perkins(at)rauland.com)>

 

Hi Chris,



I agree with everything Tom said about examining your pilot/static system before you replace your ASI. I've never heard of ASIs being damaged by cold temperatures, but I suppose it's possible. Mine sat outside for two years in Illinois and then moved into a hanger, surviving year after year without any problems.

 Some years ago, I made a water-column manometer calibrated in MPH for testing my ASI. I'd suspected that my original "Denny Aerocraft" ASI with the little fox on it wasn't reading right. After testing it on the manometer, I began joking with people that its scale was calibrated in Dennys per Hour: 10 MPH high on the high end and 5 MPH low on the low end. I'm sure it was that way from the day I installed it. Soon after finding this out, I bought a fast-acting 0-100 helicopter ASI, a purchase that I've never regretted. It responds to gusts immediately and is quite accurate.

 There's not a lot to making a manometer, but it took me an entire afternoon. It would not be hard for me to box it up and mail it to you as a loaner. Let me know.

Mike





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-- 

Pat Reilly

Mod 3 582 Rebuild

Rockford,IL

  [quote][b]

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Sat Apr 10, 2010 5:50 am Post subject: Airspeed indicator fail (again)

Disconnect your pitot line at the ASI and blow a bit of N2 backward through

it.  What are bets you will find some six or eight legged varmint inside the

tube.  No N2?  Then try filtered shop air.  If that doesn't work check for a

leak somewhere in the pitot line again a little shop air will make finding

the leak easy.



Noel



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_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

mikeperkins

Joined: 22 May 2007
Posts: 123

Posted: Sun Apr 11, 2010 6:09 am Post subject: Re: Airspeed indicator fail (again)

Pat, 



I've torn my house apart looking for a page from an ancient Kitplanes magazine that I used to make my manometer. I think Kitplanes allows archival searches, and I'll go look there in the next couple of days. 



It's just two water columns in U-shaped tubing that measures pressure that you add by rolling up some surgical hose connected to one of the two columns. Tinting the water with a little dye helps a lot. I chose to take the ASI out of the airplane because I wanted to compare two ASIs at the same time. But a perfectly-sealed pitot system won't leakdown, so it should be possible to test it on the airplane, too. 



When I find it, I'll attach a copy of the Kitplanes article in a posting. In the mean time, you can review Jim Weir's article on the same topic, but his article is not as easy to follow: http://www.rstengineering.com/rst/articles/KP89JUL.pdf



Neither of the articles express the following point very well: the given water displacement is the DIFFERENCE between the two columns of water, not the distance above where the column was when it was at rest. In other words, when you mark the manometer from a fixed 0 point, you have to divide the given displacement numbers by two (because the water goes up in one column and equally down in the other column). 



Mike

pwmac(at)sisna.com
Guest

Posted: Sun Apr 11, 2010 6:54 am Post subject: Airspeed indicator fail (again)

The water manometer method has been debunked over and over.  It will 

not yield accurate result unless you are a sea level on a standard 

atmospheric day. Last time I challenged the EAA article . The EAA 

tech person agreed . A waste of time to build a manometer. They even 

sent me a link that proved what I thought was always the case. Keep 

searching on Google and you will find the link. Just send your meter 

to a meter company and get it calibrated. Any thermodynamics 

engineering grad should be able to generate the formulas.

Paul

========================



At 08:09 AM 4/11/2010, you wrote:

 	  Quote: 	
		  



Pat,



I've torn my house apart looking for a page from an ancient 

Kitplanes magazine that I used to make my manometer. I think 

Kitplanes allows archival searches, and I'll go look there in the 

next couple of days.



It's just two water columns in U-shaped tubing that measures 

pressure that you add by rolling up some surgical hose connected to 

one of the two columns. Tinting the water with a little dye helps a 

lot. I chose to take the ASI out of the airplane because I wanted to 

compare two ASIs at the same time. But a perfectly-sealed pitot 

system won't leakdown, so it should be possible to test it on the 

airplane, too.



When I find it, I'll attach a copy of the Kitplanes article in a 

posting. In the mean time, you can review Jim Weir's article on the 

same topic, but his article is not as easy to follow: 

http://www.rstengineering.com/rst/articles/KP89JUL.pdf



Neither of the articles express the following point very well: the 

given water displacement is the DIFFERENCE between the two columns 

of water, not the distance above where the column was when it was at 

rest. In other words, when you mark the manometer from a fixed 0 

point, you have to divide the given displacement numbers by two 

(because the water goes up in one column and equally down in the 

other column).



Mike

mikeperkins

Joined: 22 May 2007
Posts: 123

Posted: Sun Apr 11, 2010 9:33 am Post subject: Re: Airspeed indicator fail (again)

Thanks for bringing that up, Paul. As a TC, I enjoy refining what I know and spreading correct information. 



You're quite correct about not being able to use a fixed scale on a water manometer. But a water manometer can indeed yield accurate results if density altitude equations are applied. The equations are the same ones used to find TAS.



Here's a website that will do all the math for you. Just enter the manometer reading (in inches) plus the existing conditions (temp, baro, altitude, dew pt); it instantly gives you the true airspeed. http://chrusion.com/BJ7/ASICalc.html



Or you can premark the manometer scale. Just enter the airspeeds you want to measure; it gives you the manometer markings in inches. . . . . And if you scroll down from the calculation section, there are some simplified plans. 



I wouldn't want to mess with an ASI in an attempt to calibrate it. But I also think that checking an ASI for accuracy can be a part of being "Experimental." 



Thanks again, Paul.

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