ASI overreading

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goldie
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ASI overreading

Postby goldie » Wed Dec 03, 2014 7:55 am

Hi
I recently bought a Bantam and the ASI is overeading by about 10 knots
Is there a way to reset/ calibrate, could the plastic tubing from the pitot
be causing the problem if it is perished?? The tubing appears OK and the pitot
appears good. Any ideas?

Thanks
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Re: ASI overreading

Postby nicow » Wed Dec 03, 2014 9:31 am

What tipe ASI is it?
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Re: ASI overreading

Postby goldie » Wed Dec 03, 2014 11:04 am

Hi Nicow,
Possibly a VDO............ all the other instruments are marked VDO
except the ASI and altimeter.

Tx,
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Re: ASI overreading

Postby KFA » Wed Dec 03, 2014 6:37 pm

It's probably a Falcon or UMA. The most common errors of the ASI system is Instrument, Position and Static. The latter 2 are most probably your problem and I guess the last one your biggest one. The Bantam does not have a static system the last time I checked. They normally just leave the static port at the back of the ASI open, the propwash then creates a pressure drop in the cabin (wind over the windscreen creates a vacuum behind it) and that causes the ASI to over read. The solution would be to experiment with installing a static line. This is a science in it's own.
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Re: ASI overreading

Postby goldie » Thu Dec 04, 2014 4:35 pm

Thanks for your advise Stephan,will check.

Rgds,
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Re: ASI overreading

Postby Baitbird » Sat Dec 06, 2014 7:55 pm

Might be stupid.
But check if your ASI is in Naitical Miles ( knots) or Statute miles.

I made the mistake when over time comparing my ASI with GPS and the 2 were set to different speeds. :oops: :oops:

And It took me while to realise this :roll: :roll:
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Re: ASI overreading

Postby KFA » Sat Dec 06, 2014 8:05 pm

Just remember that your TAS goes up by approx 2%/1000ft. So if you are in Gauteng your ASI will indicate about 10-12% less than TAS. In zero wind conditions your TAS should equal your groundspeed, your groundspeed will not be equal to your indicated speed unless you are at the coast.
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Re: ASI overreading

Postby lion » Sat Dec 06, 2014 9:21 pm

KFA wrote:Just remember that your TAS goes up by approx 2%/1000ft. So if you are in Gauteng your ASI will indicate about 10-12% less than TAS. In zero wing conditions your TAS should equal your groundspeed, your groundspeed will not be equal to your indicated speed unless you are at the coast.
You are gonna be going pretty fast in a 'zero wing' condition :lol:

Seriously though...I have been experimenting (and reading what I could find on the internet) with a static on my gyro where the ASI has always over-read by 10- 15 MPH for the reasons posted above - ie. no static and a negative pressure build up behind the panel.

I took a copper tube, drilled four holes and blocked the front of the tube. The 'secret' I finally discovered is to place an O ring in front or behind the four holes to create a high or low pressure which will 'adjust' your airspeed indication. Be aware of your altitude above sea level. I am at sea level which makes it easier. A 1mm or less movement of the O ring makes quite a difference to the ASI reading.

O ring in front of the holes for ASI (creates lower pressure) over reading and behind (creates higher pressure)for ASI under reading.

It took around four flights to find the correct position for the O ring. My ASI is now accurate to approximately 2-3 MPH at sea level.

A small collar around the tube would be a better long term solution as I expect the O ring will deteriorate over time.
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Static with rubber O ring.jpg
Last edited by lion on Mon Dec 08, 2014 6:54 am, edited 2 times in total.
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Re: ASI overreading

Postby KFA » Sat Dec 06, 2014 9:29 pm

:wink: Saw the mistake just before you posted....Corrected
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Re: ASI overreading

Postby Duck Rogers » Sat Dec 13, 2014 9:15 pm

KFA wrote:...... The solution would be to experiment with installing a static line. This is a science in it's own.
It's more like black magic than science vhpy
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Re: ASI overreading

Postby KFA » Sat Dec 13, 2014 9:26 pm

Our Test Pilot told me that the position of the static port is determined by taking a hand full of darts and throwing them at a picture of the plane.....then pick one for the static position :wink:
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Re: ASI overreading

Postby Jean Crous » Wed Dec 17, 2014 8:13 pm

KFA wrote:Our Test Pilot told me that the position of the static port is determined by taking a hand full of darts and throwing them at a picture of the plane.....then pick one for the static position :wink:
:lol: :lol: :lol: :lol: :lol: :lol: :lol: Mense !!!!!
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Re: ASI overreading

Postby Induna » Sat Dec 20, 2014 11:58 am

I had exactly the same problem due to no static vent outside the cain. I installed a static system and... MAGIC! Everything OK since then...

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