Sharing a thermal

Bill Daniels wrote:

"Eric Greenwell" wrote in message
...


Yikes! I don't know what you are flying, but it takes my ASH 26 about 27
seconds to make one turn (50 kts IAS, 8000' msl, 8.2 lb/sq ft wing
loading). That's measured from a flight trace. My ASW 20 was a little
quicker, flying at 7.5 lb/sq ft, but nothing like 14 seconds.


You might re-check your bank angle and your static port position error. 27
seconds seems too long.

Using Frank's calculator, 55 knots @ 35 degrees gives about the right
diameter and time. I don't know the density altitude that day, but the
speed sounds a little low. The IAS was likely 50 knots, where I usually
thermal when empty.

Looking at a contest flight, the thermal I measured then plugged into
the calculator gave 65 knots @ 40 degrees to get the right diameter and
time (27 seconds). That was a 9800' msl on a warm summer day, so the
speed seems about right; also, I was flying with water.

Next time I thermal, which might be 5 months from now, I'll have to
concentrate on bank angle more and redo the measurements. I generally
find myself flying at steeper bank angles than most pilots, except at
contests with good pilots, where it's about the same. Maybe the
instrument mounting holes on our panels aren't quite at 45 degrees...

But, what I was getting around to saying is your comment -

The average 45 degree bank thermalling turn takes about
13 - 14 seconds

may be true for you, but not most pilots, based on my observations in
sport and contest flying, where 22+ seconds is typical. Maybe you meant
your average turn?


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Eric Greenwell
Washington State
USA