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

"Eric Greenwell" wrote in message
...

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?


Nope, I meant my standard thermalling turn in the Nimbus is 14 seconds, 45
degrees and 45 MPH. If the core is small, I'll tighten up to a 60 degree
bank and 48mph. Ballast will, of course, bump those numbers up a little.

Eric, I think your turns are way too shallow and fast to make use of the
strong cores.

Bill Daniels

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?


Nope, I meant my standard thermalling turn in the Nimbus is 14 seconds, 45
degrees and 45 MPH. If the core is small, I'll tighten up to a 60 degree
bank and 48mph. Ballast will, of course, bump those numbers up a little.

Eric, I think your turns are way too shallow and fast to make use of the
strong cores.

Bill Daniels

Eric &Bill
If a stall speed of a glider is 36 kt at a wing loading of 6 lb/sq/ft in a
45 deg.
it will be about 44 kt and at a 60 deg bank it will 50kt.
With a loading of 9lb/sq/ft the stall speed will be 44kt at a 45 deg bank it
will be 53kt and at a 60 deg bank it will be 61 kt.
I do not know what your respective wing loading are but both of you could be
right. It is more beneficial to turn tighter with a lighter wing loading ,
then it is
with a heavier wing loading.
Regards
Udo

Udo Rumpf wrote:


Eric, I think your turns are way too shallow and fast to make use of the
strong cores.

Bill Daniels

You might be right, but I'm not routinely outclimbed by other pilots
wherever I fly, even by US World team members, so experience suggests
I'm not too far off the mark. I do try tighter banking occasionally, and
sometimes it helps, but it usually not, so the 40 degree bank is what I
normally end up using.

Perhaps you are circling more tightly than needed, your thermals are
much smaller than the ones I encounter, or as Udo suggests, it's the
wing loading difference. Or all items...

It would be interesting to look at the circling times of good contest
pilots using contest flight files, or perhaps the OLC files. Circle time
(our primary parameter in the discussion) and circle diameter can be
obtained directly from the flight trace, the true airspeed can be
determined by noting the highest and lowest ground speeds during the
circle and taking the average. These can be plugged into Frank's web
calculator to get the bank angle.


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Eric Greenwell
Washington State
USA

Eric Greenwell wrote:

Udo Rumpf wrote:


Eric, I think your turns are way too shallow and fast to make use of the
strong cores.

Bill Daniels

Ooops! The above was written by Bill, not Udo, as I accidentally replied
to the wrong posting.

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Eric Greenwell
Washington State
USA

OK, this is going to get good.

Eric thinks wide fast turns work best for him. I think 45 degree turns are
best on average with steeper turns useful in small cores. If fly as slow as
I can without risking a stall.

Lets have a poll. Tell us how you thermal.

Bill Daniels

Bill,
I try to fly, if the conditions warrant it, a 45 deg. bank
my wing loading is 8.3 lb/ft/sq at that bank my turn will be 22 seconds.
My speed will be 52 kt. indicated.
Udo
PS. I will check some of my contest recordings to make sure my memory is
still ok. Also I open up the turn when more lift is indicated and tighten
when
less lift. I use the Borgelt and I react to the trend, that is when the
needle or sound just start coming out of the bottom or falling off. Works
for me like a charm.
Udo

"Bill Daniels" wrote in message
news:2bidd.499463$8_6.239366@attbi_s04...
OK, this is going to get good.

Eric thinks wide fast turns work best for him. I think 45 degree turns
are
best on average with steeper turns useful in small cores. If fly as slow
as
I can without risking a stall.

Lets have a poll. Tell us how you thermal.

Bill Daniels

Bill Daniels wrote:
OK, this is going to get good.

Eric thinks wide fast turns work best for him.

"Fast" is relative: at 8.2 lb/sq ft, going a slower is
counterproductive. I can slow down to 45 knots from the usual 50, but
the glider isn't very steady, feels "draggy", and it doesn't climb any
better, even in very smooth thermals (this measurement done when
circling with other gliders). In anything but very smooth thermals, the
50-52 knots is needed to have decent control, anyway.

I think 45 degree turns are
best on average with steeper turns useful in small cores. If fly as slow as
I can without risking a stall.

This may be appropriate for the Nimbus 2, but not for the ASH 26, where
the stall is noticeably lower than minimum sink. Waibel told me once
that he considers this a safety feature.


Lets have a poll. Tell us how you thermal.

And what, and where. These are likely important reasons for your bank
angle and speed preferences.


Bill Daniels



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Eric Greenwell
Washington State
USA

"Bill Daniels" wrote in message
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Lets have a poll. Tell us how you thermal.

Bill Daniels


I do whatever it takes to make the averager number bigger! What that might
be depends on whether I am at Truckee or Ely or Williams; whether the lift
is turbulent or smooth, big or small core, alone or with other gliders, with
or without water, bugs on the wings, etc. The correct bank angle and speed
might change from one side of the circle to the other and is changing,
experimenting, trying to iterate the best possible lift.

When I see another glider climbing better in another part of the thermal, I
just go there!

It really matters that the climb rate is as high as possible in each climb
taken. When I fly with others in our DuoDiscus I am amazed at how carefully
many pilots follow the speed director (which is on a 30 second averager) in
cruise, but climb sloppily and lazily, accepting whatever comes and not
actively searching for the best thermals and using them to the max.
Maximizing climb and avoiding sink are the best ways to cover miles (or
km.). Interthermal glide speeds are relatively unimportant.

Of course, most of this is just experience -- many hours of flying and
thermalling -- and better learned in thermals than on Usenet...
-Bob Korves
5H DuoDiscus
5K LAK-17a