Thermal Lift Dynamics

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

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.

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?

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 he
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.

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

"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

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.

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

"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

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