How do you explain why the A/S increases on thermal entry?

This seems like a lot of effort to explain how the vertical component of
a thermal may be seen as a horizontal component on your ASI. Isn't it
simpler just to assume that the thermal *has* a horizontal component,
and that's what your ASI reports, momentarily, as you enter it? In
every thermal I've actually seen (i.e. every dust-devil), the apparent
horizontal component of the air movement is at least as great as the
vertical component.

Of course, if this hypothesis were true, you'd be as likely to see your
ASI drop as rise, as you enter...

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.

Suppose you're flying at 50 knots in a glider with infinite L/D and hit
a 10 knot thermal. After a second or two the glider will have
accelerated upwards and come to equilibrium with the thermal, but the
instantaneous effect is that the total wind is now a little stronger --
sqrt(50^2 + 10^2) = 50.9902 knots -- at an angle to the fuselage
centerline of arctan(10/50) or about 10 degres.

If you have less than infinite L/D then the increase will be a little
more.