Losing time in Cloud-Streets?

On May 26, 2:16*pm, Tim Taylor wrote:
On May 26, 1:09*pm, Tim Taylor wrote:





On May 22, 10:02*am, Andy wrote:

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

Andy,

Ok, I tried to take your scenario and put some numbers to it. *I used
the polar for an LS-4 (no DG 300 numbers handy). *I Assumed 4 knots of
lift along the 16 mile street and a 10 knot thermal at the end. *The
glider starts at 3000 feet below the clouds and ends at cloud base at
the end of the street (similar to Reichmann). *I added 20 seconds of
centering penalty and a 20% higher sink rate while thermalling.

Don't know if the table will get scrambled on most viewers but here is
the data. *It shows that flying near MC speeds is optimum as long as
you can find the strong thermal ahead. *If you expect to find a strong
thermal ahead don't slow down to minimum sink speeds. *I think I will
use a MC speed of about (expected climb from thermal - average street
strength) as a good compromise MC setting to fly. *In this case 10 - 4
= MC setting of 6 to fly.

MC * * *miles/hr * * * *time minutes * *Average speed
* * * * 45 * * *14.99 * 64.05
* * * * 50 * * *14.53 * 66.06
* * * * 55 * * *14.14 * 67.89
0 * * * 60 * * *13.80 * 69.56
1 * * * 68 * * *13.34 * 71.96
2 * * * 76 * * *13.18 * 72.84
3 * * * 84 * * *12.62 * 76.05
4 * * * 90 * * *12.33 * 77.86
5 * * * 94 * * *12.18 * 78.80
6 * * * 98 * * *12.07 * 79.55
7 * * * 104 * * 11.95 * 80.35
8 * * * 112 * * 11.87 * 80.85
9 * * * 118 * * 11.87 * 80.88
10 * * *124 * * 11.90 * 80.66

TT

Sorry found a sign error in my calculations. Here are the corrected
values:

MC * * *miles/hr * * * *time minutes * *Average speed
* * * * 45 * * *14.99 * 64.05
* * * * 50 * * *14.53 * 66.06
* * * * 55 * * *14.14 * 67.89
0 * * * 60 * * *13.80 * 69.56
1 * * * 68 * * *13.34 * 71.96
2 * * * 76 * * *13.25 * 72.46
3 * * * 84 * * *12.90 * 74.43
4 * * * 90 * * *12.75 * 75.27
5 * * * 94 * * *12.70 * 75.57
6 * * * 98 * * *12.69 * 75.68
7 * * * 104 * * 12.71 * 75.54
8 * * * 112 * * 12.82 * 74.88
9 * * * 118 * * 12.95 * 74.11
10 * * *124 * * 13.12 * 73.16

Nice job Tim. At what speed do you get to top of lift before you
reach the 10-knotter?

The curves are pretty flat on the slow end down to just above best L/D
speed (say Mc=1). Also notice that once you get to 3 knots on the
McCready there is only 13 seconds of difference between the best and
worst times up to Mc=9. That means you MUST center a thermal on the
first circle to make it worthwhile to stop. Your 20% higher sink for
circling flight is about right for 25-30 degrees of bank - at 45
degrees the sink rate is around 70% higher, so the thermals would have
to be fat AND easy to core. More typically I find that around 1/3 of
the time when I turn in a thermal I wish I hadn't and another 1/3 of
the time it takes me a few turns to center it - that's on a good day.
I don't know if that nets out to a 20 second expected loss or not.

I'm normally pickier about stopping to circle at the beginning of the
street versus when I'm getting towards the end - though I am now
reconsidering this view. Higher TAS with altitude and likely stronger
lift closer to cloudbase would argue for climbing up early rather than
late.

My -27 is a little better on the run than my old LS-4 which I think
biases the optimal a bit more to straight ahead climbing over
circling. My flying style has certainly migrated in that direction.

9B