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