View Full Version : Thermal Detection?
01-15-2007, 03:43 PM
Regardless of what type of Sailplane or Powered Glider you fly, there's one subject we all can use more help with; Finding and flying in, THERMALS!!
They're invisible, elusive, occur in various locations, shapes, sizes, strengths, and altitudes. Sometimes they're "Boomers" or "Brick-Lifters", other times it's a wisp of warm air that's barely noticable.
"Lift", "Updrafts", "Risers" "Bubbles" Where ARE they?
Technically, thermals are formed by Convection, explained here;
Which means, simply put, the warmer air in a given area will rise, hopefully taking our glider with it.
The best single article I've found for explaining the "Hows" and "Whys" of thermal formation is from Apogee Rockets (Yes, they use lift, also!);
And a Free Flight article, here;
http://www.f4bscale.worldonline.co.uk/Thermals.htm With some handy tips for using mylar streamers to "see" the thermal.
Other help can be gained from soaring birds, other gliders, mechanical and electronic devices (variometers, thermisters, bubble machines, cattails) but the best single factor is...experience.
Fly your glider as much as you can, in every weather condition possible. Fly at different sites, try different gliders.
A HLG or DLG is great for testing your thermal "sense' since you can launch right into the air you've picked. Problem is, they don't get too high and the thermal may be above your launch height. With a powered glider, you can try detecting lift at various altitudes, but they're heavier. A TD glider off a winch or Hi-Start isn't as quick in the response but yields better altitude, and they have a fairly low wing loading.
So they all have advantages and disadvantages. But you can learn from each one.
Here's a couple more links, the first part is about thermals, the second part describes search patterns and 10 tips for thermal flying;
And last, some soaring FAQs;
Hope this helps!
01-16-2007, 02:00 AM
Thermal hunting is a form of art all it's own. That's part of the love I have for this type of flight.
01-16-2007, 04:50 AM
Some years ago, back when I was teaching folks to fly model gliders, I did my best to encourage them all to buy a copy of Dennis Pagen's book "Flying Conditions - Micrometeorology for Pilots". It was written for the benefit of hang/para glider, sailplane, ultralight and balloon pilots, but there's a heck of a lot of stuff in this book that applies to us as well. In fact, any modeller ('cept for the indoor enthusiasts) would gain at least some benefit by reading this book !
As I often used to fly alongside (ie, at the same soaring sites as) hang glider pilots, I became very good mates with many of them, and we traded a wealth of information between us regarding the art of soaring. Naturally it was thru those friends that I was introduced to this book.
It covers all 4 types of soaring (ridge, thermal, wave and dynamic.. real dynamic soaring too.. the very rare occurrence that it is, not this pathetic and dangerous passtime U see many "slope nutz" indulging in today), with in-depth discussions on thermals, wind patterns and turbulence.
The amount of content that does not necessarily concern modellers is by far outweighed by the content that does, so it's still very much worth the few dollars (I bought a dozen or so for $AU12.95 each from a hang gliding supplier in the early 90's).
I did a search on Amazon.com and found the title, tho this book has a slightly different cover (see attached) to the ones I know. I'm sure it's the same book, however. It seems it may be out of print now, I'm not sure, but there are obviously still second-hand copies available.
I also found a more recent book by Pagen called "Understanding the Sky", but it's more expensive and, I'm guessing from the Table Of Contents and review, that it contains much more in-depth information... but I have not seen it so can't really comment on it.
Beg, borrow or steal "Flying Conditions" if U can (OK, OK... don't steal it!)... whatever trouble it is to find a copy, I guarantee U, it's well worth it.
Happy and successful soaring !
01-16-2007, 11:18 PM
TassieDevil (http://www.wattflyer.com/forums/member.php?u=10907) vbmenu_register("postmenu_138661", true);
Thakns for the tip on the book. I think I will have to look for that.
Here is my contribution to the thread:
by Ed Anderson
aeajr on the forums
A thermal is a column of warm rising air that occurs when one section of
the ground warms faster then other sections. As the air raises it draws in
more air. Think of a very slow moving tornado. Not exactly correct but
close enough for first approximation.
TIPS ON FINDING THERMALS
The best conditions are calm air, hot sun and low humidity. Some big dark
areas surrounded by lighter areas will help to create thermals, so look to
see if there is anything like that on or around your field. A freshly
plowed field is good. A parking lot works great! A large building
with a black roof is awesome.
However I have caught thermals at 35 degrees F in 15 mph winds. They can be
weak and they move fast, but the are there!
What do thermals look like?
Here are some thoughts on the hunt!
LISTEN WITH YOUR EYES
Get your plane up high, the higher the better. A long hi-start can get you
400-600 feet up which will give you several minutes of glide if you don't
find any lift.
If you have a motor, cut the motor and trim the plane for nice level flight.
Now, focus on watching the plane and keeping it on a nice steady glide.
Steady as she goes. Try to keep your hands off the sticks as much as
Let the plane ride with and across the river of air, giving it only
occasional input to keep it going in the general direction you want to go,
but don't be a stickler about it. Let it drift like a fly on the surface of
the river, waiting for a trout.
If you listen with your eyes, it will speak to you, but you have to listen
Glide across the wind, not directly into it and not with it as it can be
hard to see what the plane is doing when it is going directly away from
you or coming directly at you. Sort of a 45 -60 degree angle left for a
while then a 45 to 60 degrees to the right. Nice and slow and easy.
You want to cover the sky and search the moving river of air, like a
bird looking for food.
As you are flying, watch the wing tips the nose and the tail. If a wing
seems to bump up, or if the plane seems to become buoyant, floating up for a
moment, it could be a gust, or you might have just brushed a thermal. Go
gently into a slow turn in the direction of the wing that rose. Thermals
will try to push you away. If you see the nose come up for a little while,
then drop a few seconds later, you may have gone right through the middle
of the thermal. Turn gently to circle back into it. It will be moving with
the wind, so
don't try to keep the plane in one place in relation to the ground. As you
thermals it is natural for your plane to travel down wind as you circle.
Try to make a circle, but not too tight or you will lose too much altitude.
Try for about a 100 foot diameter at first. Complete a couple of turns
and see if the plane seems to be rising at some point in the circle. If it
is, just stay with the turn and try to find that area where it rose, the
focus the plane into that area. If you find the rising air you are trying
keep the plane in that air column. If it is rising you can apply a little up
elevator in your turns, but not much. You don't want to stall and you don't
want to scare the thermal, you want to bond with it.
Try to observe if the plane is rising steadily, or if it seems to rise and
fall. That could mean you are not centered in the thermal, so work your way
more toward the side of the circle where the plane rises.
Remember that thermals move with the wind, so you are not trying to stay in
one place in relation to the ground. The air is like a river and you are
trying to stay in a little whirlpool that is moving with the river.
If you go into the turn and make a couple of turns with no success, then
just resume the search pattern I mentioned. Angles across the wind. Not
into it and not with it.
A sailplane in lift - This is an AVA 3 M sailplane
notice he is hunting, then he hits!
As you watch the video, notice the light poles and how the plane tends to
drift from right to left. He is circling more than usual as he feels he has
lift in the area and is trying to find the center. That is the direction of
the wind. Using the
tops of the trees as reference you can see that he is rising.
If you are getting out too far, work your way back the same way, angles to
the wind. Remember it will take longer to go up wind than down wind and
you will be losing altitude all the time, if you don't find lift.
Finding elusive thermals
Unless you hit a boomer, you are not going to immediately know you are in
lift, so you have to watch the plane. Sometimes it becomes apparent because
you realize that your not sinking but appear to be holding altitude. The
only way to do that is to be in lift.
Remember also that thermal can vary in size and intensity. Some are fairly
narrow and some are so large that it seems a whole region of the sky is
in lift. I rode one area recently for 58 minutes where it seemed about 1/4
of the field was in lift. In this case, I didn't really have to circle. I
just flew back and forth and the plane rose beautifully. Those are really
nice, when you find them.
It is a hunter's game, if you are up for it.
Good luck pilot! May your hunt go well!
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