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Airspeed Indicator Adjustments?
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RDavidson



Joined: 16 Dec 2010
Posts: 127
Location: Pueblo, Colorado

PostPosted: Mon Oct 22, 2012 9:05 pm    Post subject: Airspeed Indicator Adjustments? Reply with quote

Guys,

I flew on a guys wing and determined that my airspeed indicators are off by about 20 mph. I also checked it with a GPS. Also, I put one new, out of the box AI and it is 20 mph off too. I have replaced the hose for the pitot tube as well.
The pitot tube is home made by the original owner.
So, what are my options now?
Can I calibrate the AI to the home made pitot tube?
Or do I need to try to modify the pitot tube?

Any suggestions would be greatly appreciated!

Thanks,
Ron
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Don Adamson



Joined: 08 Dec 2005
Posts: 243
Location: Lonoke, AR

PostPosted: Tue Oct 23, 2012 4:35 am    Post subject: Reply with quote

Ron,
Are the two airspeed indicators static ports connected and where are you getting your static air from?
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Don Adamson
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bmcj



Joined: 05 Dec 2005
Posts: 810
Location: California, Fresno

PostPosted: Tue Oct 23, 2012 8:12 am    Post subject: Reply with quote

It's possible that it may be a pitot system error (pitot tube placement or leakage on the high pressure side), but I also agree with Don that static source problems are often the cause of airspeed indication errors.

Does it read 20 MPH too high or too low? Is the error proportional to the speed you are flying? In other words, if you cut your speed by 25%, does the error also decrease by 25%?

Bruce Smile
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Bruce

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RDavidson



Joined: 16 Dec 2010
Posts: 127
Location: Pueblo, Colorado

PostPosted: Tue Oct 23, 2012 3:01 pm    Post subject: 20 to fast Reply with quote

It is 20 mph faster than actual.
The original setup had the pitot tube and static port combined, a tube within a tube. I disconnected the static line from the pitot tube and ran static ports out the left and right sides of the front cockpit.

All of this was done to try to correct the same problem I have now, reading 20 mph too fast.

The pitot tube is located on the top left wing just outside of the I strut. It seems to parallel the AoA. I will try to post a picture.

Thanks,
Ron
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Dan Benkert



Joined: 02 Dec 2005
Posts: 76
Location: Rapid City, SD

PostPosted: Tue Oct 23, 2012 7:06 pm    Post subject: ASI Reply with quote

I had similar issues and found a simple manometer made from plastic tubing very useful to find leaks, etc.
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RDavidson



Joined: 16 Dec 2010
Posts: 127
Location: Pueblo, Colorado

PostPosted: Tue Oct 23, 2012 9:11 pm    Post subject: Leaks Reply with quote

Dan,

Was it a leak causing your problems?

I tested my pitot line and it did not leak.
I'm really stumped!

Ron
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Dan Benkert



Joined: 02 Dec 2005
Posts: 76
Location: Rapid City, SD

PostPosted: Wed Oct 24, 2012 7:17 am    Post subject: ASI Reply with quote

Found 2 small pitot leaks. also found a static line connection error. Both A/S inds now read 125 in cruise. That's lower than before I had them O'Hauled, but reasonable and low end is by the book at stall speed. I'm not convinced acro entry speeds and red line are on the money yet. Need to calibrate the manometer to determine that.
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RDavidson



Joined: 16 Dec 2010
Posts: 127
Location: Pueblo, Colorado

PostPosted: Wed Oct 24, 2012 11:55 pm    Post subject: ? Reply with quote

I had a connection error too, and thought that my gauge calibration could have been distorted, but I fixed the connection and installed a different, freshly calibrated AI and it was still 20mph fast!

I'm not so sure that my AIs are working well at all below 70 mph. To be honest, I don't look at them when landing...I use my GPS ground speed and add the headwind component plus a couple of miles an hour for the insurance company.

Should I look at buying a reliable pitot tube? Any suggestions on installation on an already covered wing? Or should I save this for a project when I'm recovering my wings?

Any suggestions on which pitot tube to buy?
How about where to place it on the wing?

Thanks,
Ron
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Wild-Bill



Joined: 29 Apr 2012
Posts: 8
Location: left coast

PostPosted: Thu Oct 25, 2012 11:03 am    Post subject: Reply with quote

Ron,
Just a couple of questions, where is your pitot located and have you noticed any errors in your altimeter? One of two things is happening, the pitot is, for some reason, being over pressurized or your static ports are pulling a vacuum. While it is not good practice you may want to open the static on the airspeed indicator to the inside of the cockpit, if it still reads too high then you can zero in on the pitot as being at fault.
Bill
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RDavidson



Joined: 16 Dec 2010
Posts: 127
Location: Pueblo, Colorado

PostPosted: Fri Oct 26, 2012 2:48 pm    Post subject: Bill Reply with quote

Bill,

Thanks for the idea.

I tried it today and there was not a change in airspeed reading...still 20mph fast.

I've also checked the altimeters and they are accurate. I checked them while flying on somebody's wing and also with a watch with an altimeter and they were spot on.

I'm guessing that points back at the pitot tube. Mine is located between the top left wing tip and where the I strut connects to the top wing. It sticks out of the leading edge about 6 inches. I forgot to take a pic.

Thanks,
Ron
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bmcj



Joined: 05 Dec 2005
Posts: 810
Location: California, Fresno

PostPosted: Fri Oct 26, 2012 3:16 pm    Post subject: Reply with quote

Repeating my earlier question... How does the error vary across the speed range? Is it linear (proportional to your actual speed), or 20 mph high no matter what your speed is, or...??? Basically, how much does it vary as you speed up or slow down? Is it zero at zero? Do you notice changes with throttle inputs? Is there any chance the plumbing for the vacuum system could be tied into the static system?

Forgive me for asking questions that you have likely already considered, but maybe we'll get lucky.
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Bruce

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RDavidson



Joined: 16 Dec 2010
Posts: 127
Location: Pueblo, Colorado

PostPosted: Sat Oct 27, 2012 2:54 pm    Post subject: Pitot Reply with quote

Bruce,

No worries, I have changed so many things that it took me a while to figure out my airspeed indicators were off.

It is hard for me to tell what the AI is doing below 80mph (Indicated) because I'm kinda of busy looking outside at that point. But from 130mph to 80mph it is exactly 20mph fast. It is at 0mph on the ground and doesn't seem to be effected with throttle movement on the ground or in the air.

Originally, both static and pitot lines were hooked to a homemade pitot tube that consisted of a 3/16 or so inch dia tube with a smaller tube inside. The outside tube has several slits cut in the side.
I replaced the pitot line with new tubing of same size and type. I disconnected the static line from the pitot and added two static ports on the fuselage instead, with all new tubing as well.

I installed a new AI in the rear cockpit and it reads the same as the one it replaced, as well as the AI in the front cockpit, 20mph fast.

The original problem that led me to chasing this was that I noticed my AI was not even close to the GPS ground speed, and my altimeters were incorrect. The altimeters corrected themselves when I added the static ports, but airspeed was still 20mph fast.
Note: the original AI setup did not have the static lines hooked to them, but I have tried with them on and off and still see 20mph fast either way.

After writing all of this, I'm wondering if the slits in the side of the pitot tube were done as an attempt to calibrate the AIs? Does anyone know what the slits are for?

I'm going to drive out to the hangar and take a pic.


Thanks,
Ron
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RDavidson



Joined: 16 Dec 2010
Posts: 127
Location: Pueblo, Colorado

PostPosted: Sat Oct 27, 2012 4:54 pm    Post subject: pictures Reply with quote

Guys,
Here are some pics of my pitot tube setup. there are 6 slits and three holes in the outer tube. The outer tube actually shoulds the pitot and static tubes that enter the outer tube at the leading edge of the wing.

What are the slits and holes for?



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Wild-Bill



Joined: 29 Apr 2012
Posts: 8
Location: left coast

PostPosted: Sat Oct 27, 2012 8:20 pm    Post subject: Reply with quote

Ron,
The pitot that you have appears to be a modified Pt-19 or Army Stearman pitot static assembly. The round holes are drain holes for the pitot side and usually face down (make sure that they are clean). The sloted holes are the static ports. On some Pt-19's you will see a ring in front of the static ports, this is because they tend to read low. Looking at what you have my only guess is that the pitot is too close to the leading edge. I say this because the pitot static assembly on a PT-19 extends out from the leading edge about eighteen inches.
Bill
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RDavidson



Joined: 16 Dec 2010
Posts: 127
Location: Pueblo, Colorado

PostPosted: Sat Oct 27, 2012 8:49 pm    Post subject: Too short... Reply with quote

Bill,

Thanks for that info...I'll look around and see if I can find an example of a good PT-19/Stearman pitot tubes.
Maybe I need to extend the outer tube? I think I could replace the outer tube...

Thanks Bill

Ron
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