AA Butterfly versus CNv LCD wind calculation

The triple probe on the Stemme extends to about 3 feet in front of the
nose. That should be in pretty much undisturbed air.

On 9/26/2016 12:20 PM, Tango Eight wrote:
On Monday, September 26, 2016 at 1:05:36 PM UTC-4, wrote:
Mechanical varios don't play well with modern pressure transducer type varios and should not share TE sources with them.

Evan Ludeman / T8
A practice many seem to ignore. Do you think there is any advantage to using a fin mounted Prandtl tube versus fuselage pitot/static ports? Do you prefer mechanical or electronic TE? How do you "plumb" your CNvXC and B400?
CNv works great on electronic compensation *provided* the pitot/static system is essentially perfect. The only practical, reliable way to get that is with a triple probe set up on the fin. Don't share the static with mechanical instruments.

I have a triple probe set up plus fuselage static ports.

I plumb all three triple probe sources to CNv, the TE also goes to B400. ASI uses triple probe pitot and fuselage static. CNv works well on the probe (an old venturi style probe) but even better on the pitot/static.

best,
Evan

--
Dan, 5J

It's not undisturbed air. On my ex Stemmes , serial 11-18, the nose TERRY was severely under compensatedevelopment due to the bow wave. I increased the TE "washer" size to 3/4" IIRC and that helped a lot. Other Stemmed owners followed suit and agreed. Stemmes did not come stock with fin mounted probes then, some owners got them when the issue became known.

In the past I flew an old Stemme S10V for many hours. Loved it, but vario was very undercompensated.
Would love you to expand on the "washer" mod. I just can't figure what you mean.

thanks!

Aldo Cernezzi




Il giorno martedì 27 settembre 2016 10:13:43 UTC+2, bumper ha scritto:
It's not undisturbed air. On my ex Stemmes , serial 11-18, the nose TERRY was severely under compensatedevelopment due to the bow wave. I increased the TE "washer" size to 3/4" IIRC and that helped a lot.

On Tuesday, September 27, 2016 at 6:52:44 AM UTC-7, wrote:
In the past I flew an old Stemme S10V for many hours. Loved it, but vario was very undercompensated.
Would love you to expand on the "washer" mod. I just can't figure what you mean.

thanks!

Aldo Cernezzi


Also (and Dan),

As mentioned, the Stemme TE probe is adversely affected by the nose bow wave. You can see this with the classic stick pull push as the vario needle follows showing lift or sink - "stick thermal".

The TE probe is the "top hat" style with the brim of the hat creating a low pressure area directly behind the probe - the negative pressure is reduced by the bow wave. Increasing the top hat diameter compensates (mostly) for the bow wave and dramatically reduces stick thermal indication.

To increase diameter, I used a 3/4" OD nylon washer with an ID to closely match the horizontal barrel of probe. The washer is cut from OD to ID at an angle to allow the washer to be "threaded" onto the probe just ahead of he top hat flange - thus effectively increasing it's diameter and further reducing the pressure behind the probe in flight.

I did a bunch of ground testing too, using a leaf blower and various means to smooth the airflow to the probe while using a manometer to monitor pressure in an attempt to increase compensation further still. Larger washer did not work. Going to a cone shape instead of a washer to further smooth flow also made no improvement. Best I could come up with was that washer . . . and it works!

IIRC, I found the washers at West Marine years ago, I'm sure they can be found on-line.

bumper

Capito!
thanks!
aldo

The TE probe is the "top hat" style with the brim of the hat creating a low pressure area directly behind the probe - the negative pressure is reduced by the bow wave. Increasing the top hat diameter compensates (mostly) for the bow wave and dramatically reduces stick thermal indication.

Interesting, would you mind sending me more detailed information? In the
mean time, I'll be sure my CNv is electronically compensated and see how
that works.

On 9/27/2016 2:13 AM, bumper wrote:
It's not undisturbed air. On my ex Stemmes , serial 11-18, the nose TERRY was severely under compensatedevelopment due to the bow wave. I increased the TE "washer" size to 3/4" IIRC and that helped a lot. Other Stemmed owners followed suit and agreed. Stemmes did not come stock with fin mounted probes then, some owners got them when the issue became known.

--
Dan, 5J