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Static Port Tuning

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Larlaeb

Joined: 02 Apr 2013
Posts: 75
Location: League City, Texas United States

Posted: Thu Apr 16, 2015 11:49 am Post subject: Static Port Tuning

I was a little surprised that during the first flight after replacing my nose cone that my MKIII would only do about 50 knots indicated.   I really didn't think that modifying the nose shape should hurt me that much.   Checking with GPS told me I was correct and something was off with my airspeed indication.  After messing around with it a bit and trying several things it became clear that I had changed the pressure around the static port by modifying the nose.  I was about to change the position or put in another port altogether when I came across this article.  This will probably be old news to some of you but it was interesting to me.  Particularly the part on 'tuning' the static port using a rubber 'O' ring.  I tried it and it works great.  Took a couple of tries to get it positioned right but I now go just as fast (unfortunately not much if any faster) than I used to.



The link is here:



http://www.ch601.org/resources/pitot-static/pitot.htm



The tuning portion is quoted below and assumes a tube type static port with holes drilled in the sides.



"If your airspeed reading is slow, it is because the static port is in a slight high pressure zone. This can be corrected by slipping a small "O" ring over the end of the static tube ahead of the tiny drilled holes (vents, that is).



Moving the "O" ring aft on the tube (in very small increments) will increase the airspeed - on the gauge (not for the airplane, amigo . . . sorry). Conversely, moving the "O" ring forward, away from the ports, will decrease the indicated airspeed in much the same manner. Very small changes in the "O" ring position make a noticeable difference in the indications.



What happens is that you are attempting to lower the static pressure "felt" by the static ports with the "O" ring interrupting the airflow, thereby causing a slight lowering in air pressure behind (downstream) the ring.



The closer the ring is to the ports, the lower the induced pressure.



If on the other hand the static-tube port happens to be located in an area of low pressure, moving the ring to a position behind the ports should provide a slight increase in the pressure felt by the static ports with results opposite to that described above.



Because a rubber "O" ring will deteriorate, it should be replaced with a metal ring having identical dimensions after the proper location is determined. A dab of paint or epoxy should hold it in place."



Thanks,

Allan

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lcottrell

Joined: 29 May 2006
Posts: 1494
Location: Jordan Valley, Or

Posted: Thu Apr 16, 2015 12:14 pm Post subject: Static Port Tuning

Your tuning is very interesting. Another thing that you might do is to cut your pitot tube at about a 45 degree angle. That matched my speed to the actual speed reading, it also keeps it from being blanked out by angle of attack.Larry

On Thu, Apr 16, 2015 at 1:49 PM, Larlaeb <larlaeb(at)gmail.com (larlaeb(at)gmail.com)> wrote:

 	  Quote: 	
		  --> Kolb-List message posted by: "Larlaeb" <larlaeb(at)gmail.com (larlaeb(at)gmail.com)>

 

 I was a little surprised that during the first flight after replacing my nose cone that my MKIII would only do about 50 knots indicated.   I really didn't think that modifying the nose shape should hurt me that much.   Checking with GPS told me I was correct and something was off with my airspeed indication.  After messing around with it a bit and trying several things it became clear that I had changed the pressure around the static port by modifying the nose.  I was about to change the position or put in another port altogether when I came across this article.  This will probably be old news to some of you but it was interesting to me.  Particularly the part on 'tuning' the static port using a rubber 'O' ring.  I tried it and it works great.  Took a couple of tries to get it positioned right but I now go just as fast (unfortunately not much if any faster) than I used to.

 

 The link is here:

 

 http://www.ch601.org/resources/pitot-static/pitot.htm

 

 The tuning portion is quoted below and assumes a tube type static port with holes drilled in the sides.

 

 "If your airspeed reading is slow, it is because the static port is in a slight high pressure zone. This can be corrected by slipping a small "O" ring over the end of the static tube ahead of the tiny drilled holes (vents, that is).

 

 Moving the "O" ring aft on the tube (in very small increments) will increase the airspeed - on the gauge (not for the airplane, amigo . . . sorry). Conversely, moving the "O" ring forward, away from the ports, will decrease the indicated airspeed in much the same manner. Very small changes in the "O" ring position make a noticeable difference in the indications.

 

 What happens is that you are attempting to lower the static pressure "felt" by the static ports with the "O" ring interrupting the airflow, thereby causing a slight lowering in air pressure behind (downstream) the ring.

 

 The closer the ring is to the ports, the lower the induced pressure.

 

 If on the other hand the static-tube port happens to be located in an area of low pressure, moving the ring to a position behind the ports should provide a slight increase in the pressure felt by the static ports with results opposite to that described above.

 

 Because a rubber "O" ring will deteriorate, it should be replaced with a metal ring having identical dimensions after the proper location is determined. A dab of paint or epoxy should hold it in place."

 

 Thanks,

 Allan

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=440927#440927

 

 

 

 

 

 

 

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Larlaeb

Joined: 02 Apr 2013
Posts: 75
Location: League City, Texas United States

Posted: Thu Apr 16, 2015 12:20 pm Post subject: Re: Static Port Tuning

Putting the angle on the front of the pitot tube is something I had already read about from you and others on the forum and I've had that in place for a good while now.  Good stuff.

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rowedenny

Joined: 09 Mar 2008
Posts: 338
Location: Western PA

Posted: Thu Apr 16, 2015 5:06 pm Post subject: Static Port Tuning

I did a similar thing in wrapping black tape around the static tube just aft of the static holes to create a higher pressure zone over the static port. Do not archive



Dennis "Skid" Rowe

Mk3, Rotax 670, 

Leechburg, PA



 	  Quote: 	
		   On Apr 16, 2015, at 3:49 PM, Larlaeb <larlaeb(at)gmail.com> wrote:

 

 

 

 I was a little surprised that during the first flight after replacing my nose cone that my MKIII would only do about 50 knots indicated.   I really didn't think that modifying the nose shape should hurt me that much.   Checking with GPS told me I was correct and something was off with my airspeed indication.  After messing around with it a bit and trying several things it became clear that I had changed the pressure around the static port by modifying the nose.  I was about to change the position or put in another port altogether when I came across this article.  This will probably be old news to some of you but it was interesting to me.  Particularly the part on 'tuning' the static port using a rubber 'O' ring.  I tried it and it works great.  Took a couple of tries to get it positioned right but I now go just as fast (unfortunately not much if any faster) than I used to.

 

 The link is here:

 

 http://www.ch601.org/resources/pitot-static/pitot.htm

 

 The tuning portion is quoted below and assumes a tube type static port with holes drilled in the sides.

 

 "If your airspeed reading is slow, it is because the static port is in a slight high pressure zone. This can be corrected by slipping a small "O" ring over the end of the static tube ahead of the tiny drilled holes (vents, that is).

 

 Moving the "O" ring aft on the tube (in very small increments) will increase the airspeed - on the gauge (not for the airplane, amigo . . . sorry). Conversely, moving the "O" ring forward, away from the ports, will decrease the indicated airspeed in much the same manner. Very small changes in the "O" ring position make a noticeable difference in the indications.

 

 What happens is that you are attempting to lower the static pressure "felt" by the static ports with the "O" ring interrupting the airflow, thereby causing a slight lowering in air pressure behind (downstream) the ring.

 

 The closer the ring is to the ports, the lower the induced pressure.

 

 If on the other hand the static-tube port happens to be located in an area of low pressure, moving the ring to a position behind the ports should provide a slight increase in the pressure felt by the static ports with results opposite to that described above.

 

 Because a rubber "O" ring will deteriorate, it should be replaced with a metal ring having identical dimensions after the proper location is determined. A dab of paint or epoxy should hold it in place."

 

 Thanks,

 Allan

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=440927#440927

- The Matronics Kolb-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:

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