|Jan 30, 2006, 04:52 PM|
Stupid Question: Slermalling
Sorry for the stupid question, but this is how I learn here at RCGroups -- LOTS of really smart people here
I was doing a bit of slermalling in last weekend's flights and I was amazed at how *technical* this kind of flying can be, depending on conditions.
As I mentioned in the above thread, Del Valle's West-facing valley had it all that day -- big lift, decent slope lift, pitiful slope lift, and BIG SINK!
I was speaking with a buddy that competes on the TD stage, and he was telling me when he flies in TD meets (in their flat-field environment off a winch launch), he's not familiar with big sink situations. He's stated that he has flown through sink, but not strong situations where you worry that the plane might go down.
I'm either making more out of this than there actually is (maybe my friend does fly in the kind of conditions I experience, but I'm so new at it so it seems scary) or maybe there is a difference on the slope.
Does anyone know if there is a marked difference in sink as found on the slope vs. as found in a flat field?
Happy Flying ;-)
|Jan 30, 2006, 05:05 PM|
Thats my experience too. I find the trick is to accelerate away,out of the sink, and I usually will run at 45 degrees away from the slope while I look for the next themal coming up the valley. Anyway, works for me.
I've only flown Del Valle twice but thats some technical air.
|Jan 30, 2006, 05:19 PM|
What goes up, must come down.............. a thermal looks just like a water fountain. Up in the middle, spills off and down the sides. That's why when you're looking for a thermal, the first indicator you found one is your wingtip goes down. That tells which direction it's in, for the most part. Which ever wingtip goes down, turn that way.
It's not as noticable nor predictable as the rotor coming off a slope but if you get up high enough in a big thermal, there's enough sink on the downwind side to get your attention and them some.
Not common but I've had standard and unlimited class TD ships flipped over coming out of a thermal. Looks like your flying thorugh a huge rotor while doing a roll.
Fly through a dust devil with a floater........you'll see
|Jan 30, 2006, 05:28 PM|
I always thought that you were supposed to turn towards the wing tip that rocks "up", due to the fact that the rising tip was closer to the center of the thermal.
Now I'm confused....
And JV, yeah, there are some gnarly thermal/sink cycles some slope sites....
Mammoth was a great example!
|Jan 30, 2006, 05:46 PM|
We were doing the same thing Sunday. Flying inland slope and when the wind would die off we would float up. WIERD!! Then Falling fast in just as big a sink cycle.
I got my weasel so high that orientation started to become impossible, so I would shove the stick into the corner and spin to earth. Though I must admit to diving out of a thermal after losing orientation, I wiggled the stick a very little to re-aquire perspective. Because it was coming at me I pushed the stick forward for more and more down elevator to pull out inverted, and do three rolls to show off to the kids on the hill.
I like slermalling
|Jan 30, 2006, 05:56 PM|
It helps to several planes ranging out in different directions.
We will often punch WAY out, or down w/o choice, looking for the next thermal. This can be a bit unnerving but very rewarding.
|Jan 30, 2006, 06:01 PM|
The sink is the same, if anything it's actually not as bad as it is on a flat field since we still usually have a bit of slope lift. The BIG difference is that we are usually not flying 118" TD planes with lifty airfoils and 7 ounce wing loadings. So when we do hit a bit of sink our stuff usually drops like a stone.
There's always lift/sink cycles on inland slopes. The main time you notice them is when the wind is light and the slope lift is weak. When the wind is up and there is good lift it's not much of a factor but you will still get those few minutes of great lift when the thermal blows through.
The real test is when your plane starts falling in the sink cycle. At that point you know there is a big thermal out in front of the slope sucking up the wind. Do you have the cajonees to go out and find it?? Most guys don't!
|Jan 30, 2006, 06:08 PM|
the retrieval hike down and back up is very rewarding!
it is fun if you catch some lift just before you give up and make it back to the top!
|Jan 30, 2006, 06:08 PM|
|Jan 30, 2006, 06:41 PM|
Slermalling is a constant part of our flying in Colorado.
Even in the dead of winter. Even with 30-50mph winds, there's always
some sort of convective activity.
Think of a thermal as a big 3d vortex. Goes up in the center quickly, down
around the sides slowly (usually the down area is covers more area so
moves slower), and at the bottom of the thermal near the ground
the air is moving horizontally from below the areas of sink toward
the central area of lift from all sides equally. The sink is actually useful
because it forces warm air that would have been sitting still, to move
toward the direction of least resistance (ala the base of the thermal).
And this whole thing drifts over the ground near the speed of the
Here's an image I made up a long time ago to show this
Red is rising, or warm inflow feeders along the ground, blue is
sink and green shows the effect of the inflow on prevailing winds.
Out on flat ground, this idealized model isn't too far from reality much
of the time. When your wind flags shift directions it indicates
the flow of air in at the base toward the center of the thermal.
Average the direction of a couple different flags and you get
a pretty good idea of exactly where the thermal is located. If you happen
to be directly downwind of the thermal then the inflow at the base
of the thermal is opposite the direction of the prevailing wind so
the wind speed you feel drops off and you know to fly directly upwind.
Ok, so here's how things change when thermals get near slopes.
Firstly, since we depend on the background wind to generate slope lift,
if there happens to be a thermal directly up wind, pulling air toward it
then it reduces or in some cases, completely stops the wind you
need. Combine that with the fact that the inflow is is being pushed
downward anywhere outside the central "core" of the thermal, plus
the fact that we typically fly much higher wing loading gliders than the
thermal guys, and what you see is your slope plane falling out of the sky,
The best thing to do in the situation where you find yourself
in that big sink behind the thermal is to put the nose down, and punch
straight out from the slope as fast as you can. Thermal guys know
that the faster you fly through sink, the faster you'll get out of it.
Try to milk it along the edge of the slope and you'll end up hiking.
The difference between slope pilots and slermal pilots is that slope
pilots hike every time a big thermal shows up out front, and slermal
pilots end up higher than ever.
Other things that thermals do near slopes.
1. They stall out and stop moving as fast as the wind.
This is because a thermal wants to feed from all directions equally but
when it drifts into a slope the downwind/uphill side is harder to
feed from, while the upwind/downhill side is easier to feed from
because warm air would rather go up than down, and because the
upwind side typically draws from a larger area of warm air. This means
that after running from bad sink near the slope into a thermal in front of it,
it's not always best to just circle and drift like the flatland folks are taught
to do. It's often better to drive out further or while circling, maintain
your distance from the hill. The region of maximum lift is moving slower
or even stalled, while the wind actually blows through it (just bend upwards
as it passes through the core).
A couple graphics to try to show the slope thermal a little bit
This one was more to show why the thermal can't feed from
the uphill/downwind side as easily. Oh and I remember now, it was also
to show why thermals can actually lean *upwind* when they
run into a slope, because the wind at the base is already moving
faster due to slope compression, and the thermal only feeds on the
upwind side so it develops on the front, and decays on the back.
And 3d view:
Which shows the background wind completely stalled behind
the thermal combined with sink, and the large inflow from the upwind/downhill
2. Thermals glue themselves to ridges. This is just an extension of 1.
Because a thermal prefers to feed from downhill, the base of the thermal will
wander about in such a way that dowhill feeders are maximized. That
means if there's a ridge that runs vertically up the slope, then thermals
will tend to collect and become amplified over it. Corners are good too,
and the top of the mountain you're standing on will be an attractive
place for a thermal to hang as well. You may notice that cumulous clouds
will sit directly above hills and mountain tops. Despite what it looks
like the clouds are not stationary, but the lifty region that generates them is.
On Zion, we've got a north "point" and a south ridge (some powerlines
run up the ridge), so whenever it gets sinky directly in front of us, we
usually fan out and drive straight out from the mountain and if that doesn't
work, we go for either the north point or the powerlines, and have
a very high success rate for finding booming thermals. Some days
they follow the ridge up every 5-10 minutes like clockwork. The
paragliders use the same tactics. Sometimes we follow their lead,
sometimes they follow ours.
A graphic to show this:
It's a bit more subtle but if you look closely you can see there's
a ridge running vertically up the slope and the thermal is feeding from
both sides of it.
And another which is actually a fair model of the lower SE bowl on
the south side of Green Mountain.
What I was trying to show here was something about the way
the resident thermals change direction. They'll drift straight toward
the hill, but as soon as they find one of the diagonally running
ridges, they change direction and follow first the ridge, then the
corner of the mountain, and end up centered directly over the top
of the mountain for several minutes before the sink finally flushes out
all the feeders. When the feeders start drawing in cold air, that
shuts off the thermal and there's no longer lift generated at
the bottom. It becomes a detatched bubble of rising air.
It's not uncommon here to see slope gliders 500-1000 feet away from
the mountain. A few people have made saves from as far as 1/2 mile
out, and nearly 800 feet down below our feet.
|Jan 30, 2006, 06:50 PM|
BTW, the sink is not always the same.
Jeff points out that the lift often last longer the sink lasts longer. They
effectively suck up more of the available warm air, so the thermals tend to
last longer, with larger areas of sink between them, and because they
are fed with more total energy, the lift rate is higher, but the average sink
rate also must be higher.
Also, we tend to catch thermals that are much more developed
than they are near the ground. If I'm flying from flat ground
then I'm catching weak thermals as low as about 50-100 feet and working
them a 1000-1500 over at which point if I find myself in a real hat sucker
I'll think about getting the glider back down. If I'm standing at the top of
a 1000 foot slope, then that little thermal I'd have caught from
flatland has already had 1000 feet to develop as it drifts downwind.
The lift is bigger, the sink is bigger. We're seeing them on a whole
different scale. If I were a 1000 or 5000 feet higher still I'd be flying a
paraglider or full scale glider seeing coorespondingly higher lift and sink rates.
|Jan 30, 2006, 07:01 PM|
|Jan 30, 2006, 07:04 PM|
Oh and another little detail that's useful.
If you find yourself in a long series of thermal lift and sink cycles,
remember where the last one or two were located. See the first
illustration. See how the thermals are spaced. Let's say the closest
thermal passes by me to the left and I work it for as long as I can, then it
blows over the back of the mountain and is gone. I take my
altitude come screaming down, do some huge aerobatics, then realize
that I"m in pretty big sink after the passing of the thermal
(The wind blows harder after a thermal passes, but it's all
downward moving air, so not useful. I often see paraglider pilots
launch into this windy sink. Very frustrating to watch.).
I need to find another thermal, but I should not expect to find it directly
upwind of the last thermal. The greatest amount of sink will be upwind of it,
especially while on the slope. I cut sideways or diagonally upwind. If
the last thermal passed me on the left, then I expect to find the next
closest somewhere upwind to my right. This is true with
flatland thermals too.
At Zion it means we and the paragliders often alternate between
the flying the north point, and powerline ridge, back and forth, all day long,
with periods of "normal" slope flying halfway in between them.
That's just what inland slermal flying is all about.
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