Tuesday, February 21, 2012

Spring time review. Thermal soaring 101, a return to basics.

Below is information that you may find useful. First and most important, thermals are thermals where ever their lives start at or ends. Meaning, they are no different in South Africa, the USA, or South America, i/e anywhere. What maybe the difference in what some claim to see is their change in climb rate as they get closer to the top of the thermal as the thermal starts to mushroom out. Meaning sloppy airman ship at lower altitudes with a shallow bank, not being near the core of the plume and then getting closer to cloud base where the plume starts to mushroom out and the shallow bank now gives the appearance of a stronger thermal. This has been put together for a clearer understanding which will pays dividends. Over the years I have spoken of "plumes" and some wonder what is I am saying. Well, I am saying "what is, IS."
Thermaling bank angles along with airspeed are very important as the change from 40 degrees to 45 degrees and 5 mph brings a quick exchange to a higher sink rate and a large circling radius.
The Boundary Layer
The short answer to the question is that thermals
are columns of rising air. A longer answer requires what may seem like a
digression into boundary layer physics. The boundary layer is the layer of air
near the earth's surface that is affected by the surface on scales of an hour or
so.
The sort of boundary layers we're interested in are
convective boundary layers, which
occur in the daytime over land in weak to moderate wind conditions. There are
other sorts, but they don't produce thermals as such. I'll also assume
relatively flat and uniform terrain, and at most fair-weather cumulus clouds.
Boundary layer physics is a sub field of atmospheric physics or
meteorology, but the scales (and therefore the forces) of interest are
different. It is easy to become confused if one tries to apply basic large-scale
or storm-scale meteorological concepts to the boundary layer.

A convective boundary layer is a few hundred meters to 3 km thick, depending on
the amount of incoming solar energy, the amount of moisture in the ground, the
larger-scale weather (high or low pressure), the wind speed, and other factors.
Call the boundary layer height ti. The bottom of the boundary layer is a
*surface layer* about 0.1*ti thick, say 100-200 m. The surface layer is heated
by contact with the surface. The top of the boundary layer is a temperature
inversion (hence ti, inversion height). So to first order, thermals are
columns of warm and therefore buoyant air that rise from the surface layer to
the inversion. The spacing between thermals is about 1.5*ti, say 1-2 km. The
thermals themselves are somewhat less than half that, say 200-1000 m in
diameter. Most thermals span the boundary layer vertically. There is, of course,
a distribution of sizes. Between thermals are broad areas of sink. The sink is
weaker than the lift because it covers a larger area. The opposite is true at
the top of the boundary layer.
There are,
as always, complications. Rising air in the surface layer (the
lowest 100-200 m) is in the form of small plumes, themselves a few tens of
meters in diameter. These plumes converge near the top of the surface layer to
form thermals. The surface layer to boundary layer transition is not sharp, so
we often find ourselves flying in either well-organized thermals or disorganized
plumes, or some of both.
Dynamic thermals evolve over time, are influenced by terrain, and
are shaped by and move with the wind. Boundary layer thermals form and dissipate
with time scales of 10-30 minutes, surface layer plumes faster. This can lead to
the apparent phenomenon of "bubbles" or detached thermals or plumes. Plumes and
thermals respond to irregularities in the surface (different amounts of
vegetation, houses, and so on) by forming more often in some places than others.
Dark ground (if it's not wet!) and sheet-metal roofs are well- known thermal
concentrators. If the wind is light, thermals may stay attached to the hot spot.
If not, thermals may form repeatedly over the hot spot and drift downwind.
Thermals drift with the average wind over their height, so they may travel at a
higher speed and in a somewhat different direction than the surface wind.
Thermals also tilt if the wind is stronger at higher altitude, the usual case.
Thermals are not uniform, nor do they have sharp edges. The edges
interact with the surrounding air, so thermals have a warm, usually fairly
smooth core surrounded by turbulent edges. The air around the edges may be in
the form of blobs and may be either rising or sinking. This leads to the common
idea that thermals are toroidal (donut-shaped). It's probably more accurate to
think of thermals as vertical cylinder, that is, that
thermals look like vertical sausages. Air detrained from the thermal edges is
cooled, and cannot be recirculated into the thermal except at the ground. Vortex
rings of the size of thermals are not observed. Real
thermals are not perfect columns of rising air, but twist and meander
horizontally and bifurcate and merge as they rise. The strength of
thermals is controlled by the amount of sunlight and the surface conditions. If
the surface is wet or moisture is being emitted by healthy plants, a larger
fraction of the incoming heat from the sun will be used to evaporate water than
to heat the air. Water vapor does contribute to buoyancy, but less than heat
does. These factors probably account for most of the difference between soaring
conditions in the western and eastern U.S.
Variations on the theme
So far we have described the situation in the
middle of a day with light wind and high pressure. I wish all contest days were
like that! If the wind is stronger, turbulence driven by wind shear (the
difference between the winds at one height and another) may interfere with the
formation of thermals and the lift will be light and spotty. If the barometric
pressure is low, there will likely not be an inversion to define the boundary
layer top. This will tend to produce larger thermals that are farther apart, at
least until the rain starts!
Do thermals rotate? They do, but not predictably!!!!!!!! Even dust devils
don't have a preferred direction of rotation. Thermals are
too small and too short-lived to be affected by the earth's rotation (Coriolis
force) or by the equator/pole thermal gradient. Their rotation is determined by
local terrain. Rotational velocity in the core of a typical thermal is small
compared to the vertical velocity.
The best climbs in plumes is staying as close as you can to the core of the plume with the lowest airspeed and shallowest bank. Remember, 45 kts. IAS, a 40 degree bank with keep you inside a 700 ft. wide thermal. Now try to understand what is 700 ft. using your vehicle. A picture is worth a thousand words, but this picture is even better. Imagine traveling 100 mph in your car, after you just pass entry into the thermal circle, wait 2 seconds for vario lag, then start to slow, see where your at. Also, remember, the horizontal component of lift is the best. so if you do these Joy of Soaring entry's, you be losing the horizontal component, going to the vertical component, which may result in not the best transfer of energy.
Maybe think as you see where you might think you'll be turning at, then start to slow down as you go thru the sink. You might see your closer to centering as you enter the thermal and have a better transfer of energy. Reaction time is critical as you can now see.
One more speed tip. When on the ridge and ridge running, if the lead glider pushes when thermals are encountered and the trailing gilder pulls to gain altitude, this dolphin maneuver he just did will never allow him to catch his friend who is now farther ahead. Every time you pull, you add mileage to the daily course, every circle is mileage added vs just using this energy in a new efficient way. When you finally get a thermal which is better than your current MC cruise, then and only if you need the altitude, stop and take the climb. If its a real big climb, remember this, your next climb is what is MC is used to be set by. You estimate what what maybe!!!!!! Stick thermals are not overall climb rates. I stress overall climb rate from bottom to top is what MC uses. Resetting the mc cruise now is something to think about.
Practice your bank angles, 22 seconds and 45 kts will get you in the desired area. Vary your bank angle giving the best overall climb rate BEFORE start, so now you have a better idea of what to use going out on course. Some luck, some thoughts and at the end of the day, another air adventure in which we played.
JUST MY THOUGHTS>>>OK.....OK. Next will be an article which is very important to many. It will appear shortly.
The source for the bases of this ariticle will be coming shortly. Some of the above post was from :
Copyright (c) 1995 Wayne M. Angevine. May be freely redistributed on Internet as long as this message is included.