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Old April 25th 04, 11:24 PM
Martin Gregorie
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On Fri, 23 Apr 2004 09:45:33 +1000, Mike Borgelt
wrote:

On Wed, 21 Apr 2004 11:57:40 +0100, Martin Gregorie
wrote:

On Wed, 21 Apr 2004 10:02:05 +1000, Mike Borgelt
wrote:

Generally pitot and static probes/ports are much more sensitive to
sideslip than are good TE probes.(Good TE probe = Irving type two hole
probe )

Mike, just a quick question:

My ASW-20 is fitted with a fin mounted Irving-type TE probe except
that it only has a single hole in place of the usual two holes. What
advantages would there be to swapping it for a two-hole version?



Martin,

While I have seen wind tunnel data that suggests that the suction
around the rear 180 deg of the tube is constant there do seem to be
variations and the two hole probe may be less sensitive to sideslip.

Also:

Many years ago now just after the Irving probe design became available
I built one in 1/4" tube and one in 3/16" tube and a Nicks(single hole
type)in 3/16" as per Oran Nicks drawings. I then flew my Mini Nimbus
with a second ASI connected to probe and pitot. I checked the two
instruments against each other and applied static system corrections
to the readings of the normal ASI and reckoned I could get down to an
error band of +/- 4%.

I had had a feeling that the Nicks tube was under compensating and the
test showed it to be short of suction by 20% and a little more at low
speeds.

The Irving probe made per Irving was within my error band and the
3/16" Irving was a little short on suction getting worse at low speeds
leading me to conclude that there may be a Reynolds number effect for
the low airspeeds and smaller diameter tubes.

Interestingly Nicks recommended the hole 2 x tube diameters from the
end and Irving said 1.5

Current Irving type tubes have settled on about 1.67 as the hole
distance from the end and the holes are closer together than on the
original Irving design and 6mm or 1/4" tube is used.

Mike Borgelt


Thanks, Mike. That's exactly what I wanted to know.

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