Bruce Hoult wrote:
In article ,
Shawn sdotcurry@bresnananotherdotnet wrote:


wrote:

All this talk of masses,forces,accelerations,AOA changes etc is
irrelevent. Its simply a change in the apparent wind caused by the
introduction of a new vector (the thermal or sink).

Lets start with a simple example. The glider is just a point fixed in
free space. Introduce a horizontal wind of say X kmh. The glider's ASI
would register X kmh. Now move the airmass vertically (up or down -
doesnt matter) by Y kmh. The glider's ASI will show an *increase* in
speed equal to the vector addition of the X and Y components.

Now since a real glider actually flies down a slight hill this changes
the relative angles of the vectors. The thermal (or sink) is still
vertically oriented (for simplicity) but the glider's vector is tilted.
I never can remember how to set up the vector triangle so I wont try
and describe it here. But the end result is that lift causes a
proportionaly larger increase in ASI. Sink is interesting - for small
sink the ASI drops but for large sink the ASI increases. The anomaly is
dependent on the gradient of the hill.

Check one of my earlier posts in this thread for the math. A 10 kt
thermal will change the IAS of a 38:1 glider by about 1/4 kt. Something
else is going on.


Your calculation took into account only the fact that the glider is
going slightly downhill, so the vertical gust increases the airsped in
the direction the glider is travelling very slightly. He's talking
about something else -- basically that your airspeed indicator doesnt'
in fact measure the speed of the glider in a direction parallel to the
fuselage centerline. It in fact registers *any* airflow that comes
more-or-less from the front, even if it is at a reasonable angle to the
fuselage centerline. Airspeed indicators are designed that way on
purpose so that changes in AOA or small slip angles don't cause the
airspeed indicator to read differently.

I can see that as a possibility too. Now I've read three plausible
hypotheses. Anyone have any data?

Shawn