Thermal Lift Dynamics

[[email protected]]

I've been flying my 12m Silent-IN all by mysel up in Northern Maine.
I'm finding conditions are usually marginal. 5kts is the best I've
ever seen on the very best day. Typically I see 2-3kts and real
scratchy at that.

I often run into a kind of lift (particullarly up near the cloud decks)
where the lifts comes on gradually, then crashendo's and crashes behind
it. It doesn't feel like a typical thermal. It feels more like some
kind of mechanical turbulance or a sheer wall.

In other regions I've flown (Eastern Washington State) lift blew the
caps off vario and it wanted to shooved you right through bottom of the
cloud. Here (Northern Maine) it's just so weak I can barely make it to
the decks, then this "sheer lift" forms and I can't stay in lift long
enough to make good progress.

A plausable technical description of what I've observed would really
help me visualize the phenom and maybe fly it better.

Thanks,

Bruce Meacham

[Adrian Jansen]

wrote:
> I've been flying my 12m Silent-IN all by mysel up in Northern Maine.
> I'm finding conditions are usually marginal. 5kts is the best I've
> ever seen on the very best day. Typically I see 2-3kts and real
> scratchy at that.
>
> I often run into a kind of lift (particullarly up near the cloud decks)
> where the lifts comes on gradually, then crashendo's and crashes behind
> it. It doesn't feel like a typical thermal. It feels more like some
> kind of mechanical turbulance or a sheer wall.
>
> In other regions I've flown (Eastern Washington State) lift blew the
> caps off vario and it wanted to shooved you right through bottom of the
> cloud. Here (Northern Maine) it's just so weak I can barely make it to
> the decks, then this "sheer lift" forms and I can't stay in lift long
> enough to make good progress.
>
> A plausable technical description of what I've observed would really
> help me visualize the phenom and maybe fly it better.
>
> Thanks,
>
> Bruce Meacham
>
Read Mike Borgelt's article:

http://www.borgeltinstruments.com/Gusts.html


--
Regards,

Adrian Jansen adrianjansen at internode dot on dot net
Design Engineer J & K Micro Systems
Microcomputer solutions for industrial control
Note reply address is invalid, convert address above to machine form.

[phil collin]

wrote:
> I can definately identify with the horizontal turbulence phenomina that
> this discusses. But what I'm talking about is not the same. I've
> approached this stable at min-sink and identified lift with the
> altemeter. Then the vario rises quickly for about 2 second, then the
> bottom drops out and I'm in heavy sink
>
> I'm wondering if the shape/behavior of a thermal changes as it matures
> and approaches a cloud (aka tops out)?
>
> -Bruce
>
>> Read Mike Borgelt's article:
>>
>> http://www.borgeltinstruments.com/Gusts.html
>>
>>
>> --
>> Regards,
>>
>> Adrian Jansen
>
sounds like wave, have you tried exploring straight lines, across the
wind etc?

[[email protected]]

Bruce,

Absolutely, practically all characteristics of a thermal change rapidly
as it approaches the boundary layer, which is not always the same as
cloud base. (I'm not an expert on this subject, but I believe you could
call it a mere coincidence if the top of the lift is the same as
cloudbase.)

We have many days out west where the top of the lift will be several
thousand feet below cloudbase, but when there's enough moisture in the
air and the dewpoint is low enough, cloudbase will be below the top of
the lift, in which case you have to be most careful not go get sucked
into the clouds.

A well known German pilot whose name is escaping me at the moment (not
Reichmann) wrote several articles on thermal characteristics, one of
them is in the "Going farther and faster" books that you can purchase
from the Australian gliding federation. (Other RASers can hopefully
reply with those details.) The books are highly recommended for
newbies, and the stuff on thermals is quite informative.

~ted/2NO

ps the GFA web page where you can buy the books is here:
http://www.gfa.org.au/sales/index.php#books. If Google mangles that
address, just Google for their web page, it's easy to find.

[[email protected]]

Doh! Ingo Renner, of course. Somebody please tell me his secret for
centering thermals in a single turn!

[Bill Daniels]

"phil collin" > wrote in message
...
> wrote:
>> I can definately identify with the horizontal turbulence phenomina that
>> this discusses. But what I'm talking about is not the same. I've
>> approached this stable at min-sink and identified lift with the
>> altemeter. Then the vario rises quickly for about 2 second, then the
>> bottom drops out and I'm in heavy sink
>>

The shear zone near the edges of thermals will create horizontal vorticies.
If you fly through one of these shear vortices at right angles, and the
vortex is rotating the top away from your flight path, you will experience
what you described.

First, you will experience smoothly increasing lift, then as you pass
through the vortex core, a sharp transition to strong sink which will
smoothly decrease. These are commonly encountered when working small,
strong thermal cores since you are spending a lot of time in the shear zone.
A low boyancy/shear ratio can also create them. Just keep circling and
watch your averager.

Bill Daniels

[Adrian Jansen]

wrote:
> I can definately identify with the horizontal turbulence phenomina that
> this discusses. But what I'm talking about is not the same. I've
> approached this stable at min-sink and identified lift with the
> altemeter. Then the vario rises quickly for about 2 second, then the
> bottom drops out and I'm in heavy sink
>
> I'm wondering if the shape/behavior of a thermal changes as it matures
> and approaches a cloud (aka tops out)?
>
> -Bruce
>
>
>>Read Mike Borgelt's article:
>>
>>http://www.borgeltinstruments.com/Gusts.html
>>
>>
>>--
>>Regards,
>>
>>Adrian Jansen
>
>
Yes, as others have pointed out, this has been well explored. Certainly
you can imagine that the vertical airstream has to become horizontal
near the thermal top, else you still go up. And so you get horizontal
wind gusts, leading to the vario effects. In fact if you are flying
outward from the centre, you get a tail wind, which corresponds to heavy
sink on the vario. And of course you still get patches of vertical
motion as well, so the altimeter registers too. Things are never simple.

--
Regards,

Adrian Jansen adrianjansen at internode dot on dot net
Design Engineer J & K Micro Systems
Microcomputer solutions for industrial control
Note reply address is invalid, convert address above to machine form.

[Doug Haluza]

Are you sure you are circling tight enough? It may be that the thermal
cores are very narrow, and you are not getting into them. You may have
experienced larger thermals out west, and could circle in them without
banking that much.

Try making a small right angle bend out of solid wire, and mount it up
on the panel so it shows a 45 degree angle like a letter-v (you may
have seen something similar on an aerobatic airplane). Use it as a
reference with the horizon to see if you really are banking at 45
degrees in a tight thermal. You may be surprised to learn that what you
thought was 45 degrees was only 30 or so. It's a very common problem.

Other things to check are your vario TE compensation, and tubing leaks.
It may also be instructive to carry a logger and analyze the flight
traces with a flight ananlysis program like SeeYou or StrePla.

wrote:
> I've been flying my 12m Silent-IN all by mysel up in Northern Maine.
> I'm finding conditions are usually marginal. 5kts is the best I've
> ever seen on the very best day. Typically I see 2-3kts and real
> scratchy at that.
>
> I often run into a kind of lift (particullarly up near the cloud decks)
> where the lifts comes on gradually, then crashendo's and crashes behind
> it. It doesn't feel like a typical thermal. It feels more like some
> kind of mechanical turbulance or a sheer wall.
>
> In other regions I've flown (Eastern Washington State) lift blew the
> caps off vario and it wanted to shooved you right through bottom of the
> cloud. Here (Northern Maine) it's just so weak I can barely make it to
> the decks, then this "sheer lift" forms and I can't stay in lift long
> enough to make good progress.
>
> A plausable technical description of what I've observed would really
> help me visualize the phenom and maybe fly it better.
>
> Thanks,
>
> Bruce Meacham

[Stephen]

"Doug Haluza" > wrote in message
oups.com...

> Try making a small right angle bend out of solid wire, and mount it up
> on the panel so it shows a 45 degree angle like a letter-v (you may
> have seen something similar on an aerobatic airplane). Use it as a
> reference with the horizon to see if you really are banking at 45
> degrees in a tight thermal. You may be surprised to learn that what you
> thought was 45 degrees was only 30 or so. It's a very common problem.
>


In a glider with a reasonably supine seating position I use the instrument
mounting screws as a 45 degree reference. It's not so easy to do in a glider
where you're sitting up more because the instruments aren't so near the
horizon.

Stephen

[[email protected]]

YES! This is it! The thermal flattenning makes good sense. It may be
some kind of wave.. Nearest mountain to me is occationally up-wind, but
75 miles away , Mt. Katahdin (5,100'). This was a very windy day
(30kts at altitude) and they where from the mountains... but that
phenomina was too weak to ride 1-2kts coming in, 5kt peak, then boom
-8kts :(

As for the circle tightness... 12m Silent aint no ballasted speed
deamon. It's a light (290kg gross) ship with a 33kt stall speed and
1.36kt min-sink. It's really a Sunday thermal machine. It's also a
self launch and I have it all to myself, so I've been flying quite a
bit lately. I have a bit over 15 soaring hours just this spring (35hrs
TT-G). I've gotten pretty good at keeping it up on marginal days. I
commonly ride the stall buffet (Vs+10kts) in a 60 degree bank satisfied
with 1kt average lift just to stay up a little longer. I did 6.1hrs
last week on a day that the vario never saw anything over 4kts and
typically 1-2kts.

This kind of weather sucks for x-country cause there's rarely enough
strong lift to know I'll get home. On stronger days I have done some
"near" x-country... Headed to two nearby airport 15miles away in a
right triangle from home. But those where really just "circle-up and
slide-on-home deals", very few real risks. Highest climb: 7,000' from
a 1,500' shut-down. But I had to really milk the last 1,500'.

For a newby... I think I'm holding my own. Thank you all for the good
info. I read it all and I'll look for the book.

-Bruce

Bill Daniels wrote:
> "phil collin" > wrote in message
> ...
> > wrote:
> >> I can definately identify with the horizontal turbulence phenomina that
> >> this discusses. But what I'm talking about is not the same. I've
> >> approached this stable at min-sink and identified lift with the
> >> altemeter. Then the vario rises quickly for about 2 second, then the
> >> bottom drops out and I'm in heavy sink
> >>
>
> The shear zone near the edges of thermals will create horizontal vorticies.
> If you fly through one of these shear vortices at right angles, and the
> vortex is rotating the top away from your flight path, you will experience
> what you described.
>
> First, you will experience smoothly increasing lift, then as you pass
> through the vortex core, a sharp transition to strong sink which will
> smoothly decrease. These are commonly encountered when working small,
> strong thermal cores since you are spending a lot of time in the shear zone.
> A low boyancy/shear ratio can also create them. Just keep circling and
> watch your averager.
>
> Bill Daniels

[[email protected]]

Hey Eric,

I flew out of EPH once about 6 years ago... Oh man, you have a
beutiful thing there! Dust devils and 10kt elevator lift that lasts
all day long. [sighh]

My conditions are very similar to the western half of the state (in
summer). I use the bank that's required to stay in the thermal. I
haven't found a thermal I can stay in with anything under a 45d bank.
If it's tight and peaky (which it usually is) I use more. I
continously move around the thermal re-coring and checking listening...
I think I'm getting "good" at it, but it's hard to tell without another
glider in the thermal for comparison.

Some day I'll move back somewhere with good conditions... but for now,
I'm taking what the good lord is giving and I'm happy to do it.
Besides anyone can stay up all day when conditions are good... It's a
good learning experiance to do it when the conditions are marginal.

If there's one thing I've learned... Thermals are absolutely
unpredictable. One minute their everywhere, next minute poof all gone!
Wait it out a few thousand feet, everywhere all over again. I've
definately notices patterns of stagnation. I imagine it's related
either to cloud shaddowing and or re-triggering.

-Bruce

Eric Greenwell wrote:
> wrote:
>
> > I've gotten pretty good at keeping it up on marginal days. I
> > commonly ride the stall buffet (Vs+10kts) in a 60 degree bank satisfied
> > with 1kt average lift just to stay up a little longer.
>
> I've not flown in your area, but may I suggest that this technique might
> lead to seeing only 1 knot climbs?
>
> Try flying at 35-45 degrees at your level flight minimum sink speed:
> it's a lot easier to control the glider and feel the thermal, and the
> sink rate is less. Weak thermals are usually plenty big for a glider
> that flies as slowly as yours does without the need to bank so steeply.
>
> In addition, a lot of gliders have a relatively high sink rate if you
> fly near the stall buffet, thus the recommendation for "cruise minimun
> sinnk", but check your manual for the proper thermalling speeds.
>
> --
> Change "netto" to "net" to email me directly
>
> Eric Greenwell - Washington State, USA
>
> www.motorglider.org - Download "A Guide to Self-launching Sailplane
> Operation"

[[email protected]]

Bruce,

Wait until you read Renner's article on thermal life cycles and
convenction patterns throughout the day. He writes that thermals will
be more frequent and less strong early in the day, but as the day
unfolds and the lift patterens develop and consolidate, the thermals
become more powerful but farther apart, so by the time that happens you
want to be high already.

Of all the aspects of gliding and x/c soaring I've studdied to date,
nothing challenges (and frustrates) me more than weather and thermals.
The pilots who know how to do their own research and follow the
conditions in real time have a distinct advantage. I envy them!

If you ever get a chance, attend one of Carl Herold's camps at Ely. The
weather stuff alone is worth it!

~tuno

[Eric Greenwell]

wrote:

> I've gotten pretty good at keeping it up on marginal days. I
> commonly ride the stall buffet (Vs+10kts) in a 60 degree bank satisfied
> with 1kt average lift just to stay up a little longer.

I've not flown in your area, but may I suggest that this technique might
lead to seeing only 1 knot climbs?

Try flying at 35-45 degrees at your level flight minimum sink speed:
it's a lot easier to control the glider and feel the thermal, and the
sink rate is less. Weak thermals are usually plenty big for a glider
that flies as slowly as yours does without the need to bank so steeply.

In addition, a lot of gliders have a relatively high sink rate if you
fly near the stall buffet, thus the recommendation for "cruise minimun
sinnk", but check your manual for the proper thermalling speeds.

--
Change "netto" to "net" to email me directly

Eric Greenwell - Washington State, USA

www.motorglider.org - Download "A Guide to Self-launching Sailplane
Operation"

[Mike Schumann]

Where are you flying out of?

Mike Schumann

> wrote in message
oups.com...
> Hey Eric,
>
> I flew out of EPH once about 6 years ago... Oh man, you have a
> beutiful thing there! Dust devils and 10kt elevator lift that lasts
> all day long. [sighh]
>
> My conditions are very similar to the western half of the state (in
> summer). I use the bank that's required to stay in the thermal. I
> haven't found a thermal I can stay in with anything under a 45d bank.
> If it's tight and peaky (which it usually is) I use more. I
> continously move around the thermal re-coring and checking listening...
> I think I'm getting "good" at it, but it's hard to tell without another
> glider in the thermal for comparison.
>
> Some day I'll move back somewhere with good conditions... but for now,
> I'm taking what the good lord is giving and I'm happy to do it.
> Besides anyone can stay up all day when conditions are good... It's a
> good learning experiance to do it when the conditions are marginal.
>
> If there's one thing I've learned... Thermals are absolutely
> unpredictable. One minute their everywhere, next minute poof all gone!
> Wait it out a few thousand feet, everywhere all over again. I've
> definately notices patterns of stagnation. I imagine it's related
> either to cloud shaddowing and or re-triggering.
>
> -Bruce
>
> Eric Greenwell wrote:
>> wrote:
>>
>> > I've gotten pretty good at keeping it up on marginal days. I
>> > commonly ride the stall buffet (Vs+10kts) in a 60 degree bank satisfied
>> > with 1kt average lift just to stay up a little longer.
>>
>> I've not flown in your area, but may I suggest that this technique might
>> lead to seeing only 1 knot climbs?
>>
>> Try flying at 35-45 degrees at your level flight minimum sink speed:
>> it's a lot easier to control the glider and feel the thermal, and the
>> sink rate is less. Weak thermals are usually plenty big for a glider
>> that flies as slowly as yours does without the need to bank so steeply.
>>
>> In addition, a lot of gliders have a relatively high sink rate if you
>> fly near the stall buffet, thus the recommendation for "cruise minimun
>> sinnk", but check your manual for the proper thermalling speeds.
>>
>> --
>> Change "netto" to "net" to email me directly
>>
>> Eric Greenwell - Washington State, USA
>>
>> www.motorglider.org - Download "A Guide to Self-launching Sailplane
>> Operation"
>

[[email protected]]

Mike,

I'm flying out of KCAR - Caribou, Maine

http://www.airnav.com/airport/KCAR

-Bruce

Mike Schumann wrote:
> Where are you flying out of?
>
> Mike Schumann