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Pitot testing

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recapen(at)earthlink.net
Guest

Posted: Tue Dec 02, 2008 11:31 am Post subject: Pitot testing

Folks,



I have been doing my Pitot testing.  I have built a manometer per the EAA chapter 1000 instructions and using the MicroEncoder chart and Kevin Horton's spreadsheet.



Here are some results:



At lower airspeed numbers, the manometer readings are exact.  As the airspeed numbers rise, the manometer readings deviate from the numbers on the spreadsheet that I have (thanks Kevin Horton) to the low side - meaning the manometer reading is lower than specified for a given airspeed indication...also meaning that the airspeed would indicate an amount higher than actual for the correct manometer reading.  All are within the 3% or 5kts specified by FAR 23.1323.



Also, I am experiencing a very slow bleed-down - on the order of 4 to 7 minutes to lose 100kts of airspeed at 0035'MSL.  The only way I found this was walking away to record my numbers in the computer after I was done - when I came back it was lower.....did not see it between tests while I was doing it though.



I've looked through FAR 23.1323 - Airspeed indicating system.  It does not have anything in it regarding a leak tolerance...unlike the FAR 23.1325 - Static pressure system, which allows 100fpm (at) 1000' AGL indicated.  What do folks use for a leakdown tolerance?  Or is this good enough to not matter?



Thanks,

Ralph Capen

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khorton01(at)rogers.com
Guest

Posted: Tue Dec 02, 2008 3:17 pm Post subject: Pitot testing

On 2-Dec-08, at 14:29 , Ralph E. Capen wrote:



 	  Quote: 	
		   

 <recapen(at)earthlink.net>



 I have been doing my Pitot testing.  I have built a manometer per  

 the EAA chapter 1000 instructions and using the MicroEncoder chart  

 and Kevin Horton's spreadsheet.



 Here are some results:



 At lower airspeed numbers, the manometer readings are exact.  As the  

 airspeed numbers rise, the manometer readings deviate from the  

 numbers on the spreadsheet that I have (thanks Kevin Horton) to the  

 low side - meaning the manometer reading is lower than specified for  

 a given airspeed indication...also meaning that the airspeed would  

 indicate an amount higher than actual for the correct manometer  

 reading.  All are within the 3% or 5kts specified by FAR 23.1323.



 Also, I am experiencing a very slow bleed-down - on the order of 4  

 to 7 minutes to lose 100kts of airspeed at 0035'MSL.  The only way I  

 found this was walking away to record my numbers in the computer  

 after I was done - when I came back it was lower.....did not see it  

 between tests while I was doing it though.



 I've looked through FAR 23.1323 - Airspeed indicating system.  It  

 does not have anything in it regarding a leak tolerance...unlike the  

 FAR 23.1325 - Static pressure system, which allows 100fpm (at) 1000'  

 AGL indicated.  What do folks use for a leakdown tolerance?  Or is  

 this good enough to not matter?


	


If a pitot system has no leaks, there is no air moving in it if you  

are at a constant airspeed.  The pressure will be the same everywhere  

in the pitot system - i.e. the pressure at the entrance to the ASI  

will be the same as the pressure at the pitot tube.  If there is a  

leak in the system, air must continually flow from the pitot tube to  

the leak to replenish the air lost through the leak.  Air will only  

flow if there is a pressure differential to push it, so the pressure  

at the leak has to be lower than the pressure at the pitot tube.  But,  

if the leak is small, the speed at which the air will be flowing will  

be small, and the pressure differential required to push the air at  

that rate will be small.  Thus the effect of a very small leak is  

negligible.  The closer a leak is to the pitot entrance, the less  

significant it is, as the pressure loss is proportional to the length  

of tube that has air flowing it it.  No air is flowing in the part of  

the system between the leak and the ASI, so no further pressure loss  

is occurring there.



A leak becomes a problem if it is big enough so there is an  

appreciable pressure loss required to push the air through the system  

from the pitot tube to the leak.



Your leak is very small, so it probably isn't causing an accuracy  

problem now.  But, it is a sign of a flaw in your system somewhere,  

and I would wonder whether it will further degrade such that someday  

it becomes a problem.  If you don't find and fix it, I would do some  

sort of periodic check to ensure that it hasn't gotten worse.



--

Kevin Horton         RV-8 (Grounded)

Ottawa, Canada

http://www.kilohotel.com/rv8

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chasb(at)satx.rr.com
Guest

Posted: Wed Dec 03, 2008 8:52 am Post subject: Pitot testing

I have not done manometer tests on my flying RV-6A, but I have done leak down tests on both the static and pitot system. No leaks in either system.

I have done airspeed calibration runs in the air using GPS ground speed while flying North, South, East and West and averaging the results. I run the speed tests at various altitudes and temperatures and calculated the True Airspeed in each case. All the test runs had similar results. At slow speeds (below 80 KTAS) my airspeed indicator is accurate to within one knot. At moderate speeds (120-130 KTAS) my airspeed reads four to five knots fast. At high speeds, (165 KTAS) the error grows to about seven knots. I have one of Van's airspeed indicators and it has no adjustment capability.

I find it interesting that Ralph's manometer tests also show a higher than actual airspeed indication at high speeds. I am at a loss as to why this happens and wonder if others have noticed similar errors. 

Is this typical of Van's airspeed instruments? An acquaintance with an RV-9 and a Dynon system says his airspeed indications are very accurate at all speeds and altitudes. 

Inquiring minds want to know.

Charlie Brame

RV-6A  N11CB

San Antonio

-----------------------------------------------------------------





Time: 11:31:46 AM PST US

From: "Ralph E. Capen" <recapen(at)earthlink.net (recapen(at)earthlink.net)>

Subject: Pitot testing

Folks,



I have been doing my Pitot testing.  I have built a manometer per the EAA chapter

1000 instructions and using the MicroEncoder chart and Kevin Horton's spreadsheet.



Here are some results:



At lower airspeed numbers, the manometer readings are exact.  As the airspeed numbers

rise, the manometer readings deviate from the numbers on the spreadsheet

that I have (thanks Kevin Horton) to the low side - meaning the manometer reading

is lower than specified for a given airspeed indication...also meaning that

the airspeed would indicate an amount higher than actual for the correct manometer

reading.  All are within the 3% or 5kts specified by FAR 23.1323.



Also, I am experiencing a very slow bleed-down - on the order of 4 to 7 minutes

to lose 100kts of airspeed at 0035'MSL.  The only way I found this was walking

away to record my numbers in the computer after I was done - when I came back

it was lower.....did not see it between tests while I was doing it though.



I've looked through FAR 23.1323 - Airspeed indicating system.  It does not have

anything in it regarding a leak tolerance...unlike the FAR 23.1325 - Static pressure

system, which allows 100fpm (at) 1000' AGL indicated.  What do folks use

for a leakdown tolerance?  Or is this good enough to not matter?



Thanks,

Ralph Capen

 [quote][b]

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rv9jim(at)juno.com
Guest

Posted: Wed Dec 03, 2008 9:19 am Post subject: Pitot testing

Chuck,

        I have a single scale air speed indicator.  I originally had one

of Van's multi reading a/s indicator but found it had to much information

on the dial.  So, since I do all things in Knots and Nautical miles, I

bought an indicator that reads only in Knots.  I don't care to convert

back and forth with speeds.   Keep it simple.  My EFIS gives me TAS, GS,

and IAS (all in Knots).  Almost more than I need to know.   My testing of

the system for pressure and static required me to "glue" all the fittings

that are not directly required for instrument removal.  I used "Goop"

adhesive available at the aviation section at the Home despot.  Great

stuff.  It also holds my static line in places in the cockpit where it

droops a bit.  It also holds my wiring under my "arm rest" on the copilot

side for the ELT panel indicator. 

Jim

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khorton01(at)rogers.com
Guest

Posted: Wed Dec 03, 2008 2:08 pm Post subject: Pitot testing

The average of runs in four cardinal directions is only equal to the  

TAS if the wind is calm.  The error is small if the wind speed is low,  

but the error can be large if the wind speed is large.  If you are  

going to use this method, pay attention to how much the ground speed  

varies on the four runs.  If the four ground speeds are not too  

different, then the winds are low, and it is safe to use the average  

of the four runs.  At 150 kt, TAS, if the difference between the  

highest and lowest ground is less than 25 kt, then the average of the  

four ground speeds should be within a half kt of the TAS.  If the  

difference between the highest and lowest ground speeds is 35 kt, the  

error could be as high as 1 kt.  If the ground speeds are quite  

different, then it would be more accurate to use a method that  

accounts for the effect of wind, such as the four leg option of the  

NTPS spreadsheet.



http://www.ntps.edu/Files/GPS%20PEC.XLS



Kevin Horton

On 3-Dec-08, at 11:50 , Charles Brame wrote:



 	  Quote: 	
		   I have not done manometer tests on my flying RV-6A, but I have done  

 leak down tests on both the static and pitot system. No leaks in  

 either system.



 I have done airspeed calibration runs in the air using GPS ground  

 speed while flying North, South, East and West and averaging the  

 results. I run the speed tests at various altitudes and temperatures  

 and calculated the True Airspeed in each case. All the test runs had  

 similar results. At slow speeds (below 80 KTAS) my airspeed  

 indicator is accurate to within one knot. At moderate speeds  

 (120-130 KTAS) my airspeed reads four to five knots fast. At high  

 speeds, (165 KTAS) the error grows to about seven knots. I have one  

 of Van's airspeed indicators and it has no adjustment capability.



 I find it interesting that Ralph's manometer tests also show a  

 higher than actual airspeed indication at high speeds. I am at a  

 loss as to why this happens and wonder if others have noticed  

 similar errors.



 Is this typical of Van's airspeed instruments? An acquaintance with  

 an RV-9 and a Dynon system says his airspeed indications are very  

 accurate at all speeds and altitudes.



 Inquiring minds want to know.



 Charlie Brame

 RV-6A  N11CB

 San Antonio



 -----------------------------------------------------------------

 Time: 11:31:46 AM PST US

 From: "Ralph E. Capen" <recapen(at)earthlink.net>

 Subject: Pitot testing

 Folks,



 I have been doing my Pitot testing.  I have built a manometer per  

 the EAA chapter

 1000 instructions and using the MicroEncoder chart and Kevin  

 Horton's spreadsheet.



 Here are some results:



 At lower airspeed numbers, the manometer readings are exact.  As the  

 airspeed numbers

 rise, the manometer readings deviate from the numbers on the  

 spreadsheet

 that I have (thanks Kevin Horton) to the low side - meaning the  

 manometer reading

 is lower than specified for a given airspeed indication...also  

 meaning that

 the airspeed would indicate an amount higher than actual for the  

 correct manometer

 reading.  All are within the 3% or 5kts specified by FAR 23.1323.



 Also, I am experiencing a very slow bleed-down - on the order of 4  

 to 7 minutes

 to lose 100kts of airspeed at 0035'MSL.  The only way I found this  

 was walking

 away to record my numbers in the computer after I was done - when I  

 came back

 it was lower.....did not see it between tests while I was doing it  

 though.



 I've looked through FAR 23.1323 - Airspeed indicating system.  It  

 does not have

 anything in it regarding a leak tolerance...unlike the FAR 23.1325 -  

 Static pressure

 system, which allows 100fpm (at) 1000' AGL indicated.  What do folks use

 for a leakdown tolerance?  Or is this good enough to not matter?



 Thanks,

 Ralph Capen

- The Matronics RV-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?RV-List

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