ASI overreading

[goldie]

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

[nicow]

What tipe ASI is it?

[goldie]

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

Tx,

[KFA]

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.

[goldie]

Thanks for your advise Stephan,will check.

Rgds,

[Baitbird]

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.

And It took me while to realise this

[KFA]

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.

[lion]

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

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.

[KFA]

Saw the mistake just before you posted....Corrected

[Duck Rogers]

...... 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

[KFA]

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

[Jean Crous]

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

Mense !
J

[Induna]

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...

Jacques