On Sep 1, 10:38 pm, Allen Smith wrote:
Hello,
I just watched a movie online (Youtube) about gliders and saw one
thermaling and gaining altitude. How is this is done? I am a presolo
power pilot (Just finished with my 12th hour) and have been taugt
that a power plane will climb when excess power is availible.
(Steepen the angle of attack, and the airplane will start climbing
steady when all 4 forces are equal) How does a glider do that, since
there is no engine? Does the thermal change the relative wind?
(Which can\'t because the airplane creates the relative wind as it
flys through the air, right?)

Really interesting stuff.

Glad you find it interesting.

Generally, sailplanes have minimum sink rates from approximately 90fpm
to about 175fpm. Due to atmospheric instability and solar heating,
buoyant plumes of air will rise in columns, often combining into
larger plumes. Depending on the surface, type of vegetation or
coloring, slope, winds, and moisture content, the vertical velocity
and diameter and height of these plumes will vary widely. This is
what we call lift. In some areas, it may become usable for soaring
flight as early as 9 or 9:30am locally. Late morning or early
afternoon is a more common time for reliable lift to form. Sometimes,
convergence zones and other factors may delay local lift until mid to
late afternoon. The tops of the plumes, commonly call thermals, are
often marked by cumulus clouds. However, thermals may also be present
on 'blue' days. Like soaring birds, gliders need to find and circle
within the plume to gain altitude. Reaching the top of a climb, the
savvy pilot will have already considered the next two or three
developing thermals near the goal course, if cross-country is the goal
of the flight, and cruise in that direction. Lift generally builds
during the day, so starting out with 2-3knots of climb is okay as long
as the forecast indicates stronger lift on course. 1knot is 100fpm,
so weak lift may only sustain flight. However, in the US west,
thermal lift above 20knots has been experienced. 6-12knots is not
uncommon, thus allowing pilots to achieve cross-country average speeds
approaching 100mph at times.

L/D is what allows gliders to connect thermals together. L/D of about
23:1 is about the minimum for completing closed course thermal flights
that have an into wind leg, like a triangle. 30:1 L/D makes cross
country flying more reasonable. 40:1 L/D makes many long flights
possible as it keeps more landing options within reach. 50:1 L/D is
becoming more common in modern gliders with the super ships reaching
60:1 and 70:1. That's where the imagination runs wild.

By comparison, an SEL training plane like a Tomahawk or C150/152 has
about 3-3.5 minutes from 2000agl to arrival in an engine out
situation, so you pick the first reasonable spot and fly to it. A
40:1 glider at 2000agl has nearly 200 square miles of options for a
landing spot and is 15 minutes or more from arrival should the pilot
not find any more lift.

Visit http://www.onlinecontest.org/olc-2.0...ringId=1&today
to view flights submitted to the Online Contest. You may have to pick
a date. You can then pick a continent, country, or region to find a
list of flight. Then click the Info button on the right of a
particular flight. This will show you a map with the flight route
shown and an altitude/terrain chart. The files may be downloaded and
the flight replayed through a viewer. You can download a trial
version of SeeYou here http://naviter.si/index.php?option=c...9&Itemid =213

Others may tell you it's just f**king magic, which it is, of course.

Soaring, try it, you'll like it,

Frank Whiteley