On May 20, 7:47*pm, Ramy wrote:

This is theoratically correct, but I found out more often than not
that the lift right below cloudbase (keeping FAR clearance of course)
is stronger and more widespread than couple of thousands below, which
allows much faster speed without loosing altitude.

Ramy

Okay, I broke out the polar and did some simple math for a "typical"
scenario.

Take a cloud street that is 16 miles long with 4 knot average thermal
strength and 10 knot peak thermal strength.

Pilot A pulls back to 70 knots and climbs in the average lift (2.4
knot net climb rate). It takes him 14 minutes to reach the end of the
street and he has gained 3,360 feet.

Pilot B climbs in the strong core, taking 30 seconds to center and
achieving a 8.4 knot average climb rate thereafter (note the higher
sink rate for circling flight). After 5 minutes he has climbed 4,250
feet. He then cruises at 110 knots for 16 miles, giving up 890 feet in
the process. Both pilots arrive at the end of the cloud street at the
same time and altitude.

Conclusion: stopping to circle in a thermal weaker than 10 knots puts
you behind the pilot who climbs straight ahead.

Taking Ramy's point about stronger lift closer to cloudbase into
account, let's assume you find an average 5 knots after a circling
climb instead of 4 knots for climbing straight ahead. In this case you
need a minimum 8.3 knot thermal before stopping to circle makes sense.
If you assume 6 versus 4 knots average lift cruising closer to
cloudbase then you only need a 6.6 knot thermal - but that starts to
feel like a pretty strong thermal strength gradient with altitude.

This analysis doesn't take into account a slight true airspeed
advantage for the pilot who climbs first due to his higher average
altitude.

Overall, I think this confirms that it pays to avoid circling under a
cloudstreet for anything but the very strongest lift. This is even
more true if you are heading into an upwind turnpoint.

9B