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8.2.16.1 So how do I find a thermal?




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This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.

8.2.16.1 So how do I find a thermal?

From: Mike Dennett

"Anyone interested in learning about thermals will find some
good reading in a copy of Dave Thornburg's "The Old Buzzard's
Soaring Book". It describes how model fliers can predict the
patterns of thermal activity, and contains lots of easily read
material on the behavior of air in general.

http://www.fatlion.com/sailplanes/releases.html

is one source I found that lists them for sale.

Your description of the wind picking up again would indicate
to me that the lift is just downwind. The best seat of the
pants indicator I have found is the sudden warm, calm period
(like you mentioned) that may be first accompanied by a gentle
flow of air in the opposite direction of the prevailing breeze
depending on its strength. That air is headed upwind to the
core of the thermal and then upwards. Good time to toss that
hand launch glider right about then. Or push the button. I
imagine air headed radially inwards towards the core from all
directions, like a great vacuum cleaner nozzle has descended
from the sky and is traveling with the prevailing wind
direction."

Another article I found is at
http://www.f4bscale.worldonline.co.uk/Thermals.htm

Peter Alway writes in his unique but colorful style: "Bumbling
into Thermals: How to make your rocket go up after the
parachute opens

It's all connected. The rolling of noodles as your soup comes
to a boil, the puffy clouds on a hot summer's day, the fine
granulation on the sun's visible surface, the slow grinding of
continents, and the miracle of a model rocket drifting up and
away, never to be seen again. All are examples of convection,
a process that transports heat from the bottom of a fluid
upward. One sort of convection, called a thermal, is the key
to success in model rocket duration competition.

The NAR sanctions dozens of model rocket meets every
year. Each meet offers an assortment of events: spot landing,
altitude, craftsmanship, and duration. Duration events are
popular because the participants can score them objectively
with nothing more than a stopwatch (Actually two watches and
two timers to operate them for each flight to improve accuracy
and to provide backup in case of a blunder). A contestant in a
duration event is usually worried about three factors: Weight
of the model, aerodynamics of the model, and air. Obviously, a
lighter model will fall more slowly. Aerodynamics can be as
subtle as a glider's airfoil, or as simple as a parachute's
diameter. Only after the modeler builds and preps the model
does he worry about air.

The holy grail of duration flying is catching a
thermal. Thermal is a flier's term for the rising, warm air of
a convection cell. You may have seen a convection cell in a
pot of soup heating up. As the stove heats the bottom of the
pot, the soup at the bottom warms up and expands, becoming
less dense than the cold soup above. The heavier cold soup
falls to the bottom, and the hot soup at the bottom rises. The
soup at the top loses heat to the air, cools, and becomes more
dense, the soup at the bottom is heated and becomes less
dense, so the process repeats. In fact the process continues
steadily; the soup constantly turns over. Depending on the
details of the pot and the stove there will be a continuous
upwelling of soup in the center of the pot, and a continuous
sink at the edges (with my gas stove and a small pot, I
actually get an upwelling at the edges and a sink in the
middle, but the process is the same).

Even though the sunlight that heats the Earth's atmosphere
comes from above, it warms the air near the ground from
below. The sunlight passes through clear air unhindered and
heats the ground. The warm earth then transmits it's heat to
the air. Warm air near the ground heats up, expands, and
becomes less dense than the cool air above. The cool air
sinks, pushing the warm air up, and creates a convection cycle
just like the one in the soup pot. But without noodles, so you
can't see it. Your task as a duration modeler is to inject
your rocket into the thermal, the warm, rising air in the
convection cell.

Not all thermals are created equal. Some are stationary,
hanging over dark asphalt roads, parking lots, or
roofs. Others roll along with the wind. Some are weak, some
are so strong they become 'dust devils' that can pull the hat
off your head. Some are tiny, some grow so high that you can
see their tops in the form of puffy cumulus clouds. Thermals
tend to be weak at dawn, and grow stronger and more frequent
on a sunny afternoon.

You have probably felt thermals without realizing it. If
you've been out on a hot day, you have probably felt the
relief of a cool breeze out of nowhere. That's the down draft
of a convection cell, the 'sink' of a thermal. That's exactly
what you don't want to launch into. But what do you want to
fly into? How can you detect a thermal?

I confess that I'm not the world's best thermal spotter, but I
have techniques that are better than flipping a coin, and I
have seen some tricks that others use. They are a mix of
rendering thermals visible and detecting them by feel.

Some folks use a pole with a ribbon. They attach a 10-20 foot
length of audio cassette tape to the end of a 10-foot bamboo
pole. As the tape blows in the wind, they will watch for a
moment where the wind lifts the tape above the end of the
pole. The updraft is a thermal, and that is their signal to
launch. I'm not sure that most thermals have really formed at
just 10 feet up, but then again, the ribbon-on-a-stick folks
tend to beat me in competition.

Some people prefer a bubble machine to put some 'noodles' in
the soup of air that surrounds us. The principle is the
same'rising bubbles indicate rising air.

Other modelers build little weather stations' a sensitive
electronic thermometer connected to a computer that records
the pattern of rising and dropping temperatures. When the high
is 5 degrees over the low, you've got something, and you
launch. Immerse yourself in a combination of these devices and
you will learn to see thermals coming.

I've heard of the manly rocketeers who took their shirts off
at launches so they could feel the wind at their back and find
thermals that way. Naturalists will trust hawks and turkey
vultures who love to circle in thermals, feeling them out by
the lift and warmth under their wings.

As for me, I use a strategy that is cheap, simple, and
modest. I fly with my brother, Bob, in a team called the
Bumbling Brothers Flying Circus. While Bob prepares his model
and sets it on the pad, I pay attention to wind and heat. It's
hard to ignore the wind, but I also notice the little warmings
and coolings. Once the model is on the pad, I ask myself 'is
it windier than average at this moment?' If it is, we wait. I
ask myself 'Is it warmer than average right now?' If it isn't
we wait. I rarely wait for more than five minutes. It's a
tough call, and I don't always get it right, but eventually,
the wind lets up, and I feel warmth.

I have a pet theory that the real lift hits at the end of the
calm interval. My theory is that the wind that comes blowing
in has to go somewhere, and the only place to go is up. At
least one experienced flyer has laughed at that one, so I
might be full of nonsense. But at more organized contests
where everyone flies from a rack controlled by an RSO, that's
a moot point, because the process of getting the model off the
ground can take so much time that you are almost always at the
end of the calm interval. I'm ready, you call to the RSO

Modeler's ready on pad five.

Timers ready?

Yeah we're ready.

OK, pad is ready with a an A8 is that an A8-3?

Yes it's an A8-3. I'm ready! you answer as you nervously feel
a little gust coming.

OK, ready on pad five. Oops do we have continuity oh the
switch was on pad four. We have continuity now. Timers ready?
Yes we're ready

Modeler ready?

Yes! Yes! Fly it now! The breeze is starting to pick up is it
starting to cool down? Oh Kay. A eight three on pad
five. Ready?

You can feel the thermal rolling past.

Fiiiivveee. Fffoooouurrrr. Threeeeeeeeee. Twoooooooooo

How can anyone count backwards so slowly! I'm losing my air

Ooooonnneeeee. Zeeeeeerrrrrooooo. Llllllaaaaaauuuuucccchhhhhh!

Whoosh. Pop. Unfurl Unfurl Unfurl Blossom.

It is satisfying to see a big silver parachute pulsate like a
jellyfish as it opens in a thermal--It's going up!

I love to watch a model rise on a parachute. It's almost like
watching a hot air balloon in flight. Sometimes the model goes
forever, or at least it rises until it is out of sight. I have
seen parachute models disappear into the bases of clouds. I
have seen them float for half an hour. I will never forget Bob
Kaplow's NARAM egglofter that hung for half an hour over the
Pennsylvania mountains (coincidentally formed by the ancient
collision of crustal plates propelled by convection currents
in the Earth's mantle) before flying away beyond the
horizon. Boost-gliders also can stay up for many minutes under
a thermal. Even a streamer model will catch a little lift off
a thermal and stay up longer than you might expect.

Thermals are a wonderful wild card in contest rocketry, though
you can't thermal away a model that sinks too fast'you need a
light rocket with a lot of wing or parachute area to catch an
express elevator in the sky. Also, you have to recover a model
in each duration event if you want those miraculous
anti-gravity seconds to count.

Every part of model rocketry can be an opening on a new
world. Scale modeling has introduced me to the history of
spaceflight and a deeper understanding of design. Predicting
rocket altitudes as a teenager prepared me for calculus. I
have come to see searching for thermals as a way of
understanding the motions of invisible air, like those
wonderful cool breezes. Recently, while visiting a local
nature preserve, I saw a bald eagle flap into the air, and
then circle effortlessly upward out of sight. Thanks to my
experience finding thermals at rocket duration contests, I
could appreciate exactly what he was doing. When you catch
your first thermal, you might just feel like that eagle, a
master of the air."

 

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previous page: 8.2.16 My glider never came down and flew away. What is wrong?
  
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