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Old Mar 03, 2009, 09:51 PM
Physics hate's my ideas
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Catching a thermal, whats it like, how do you know? what should you do?

Ok so i have a few questions mainly what i have above.
So with say under 40degrees is it possible to thermal? on my Spartan i feel like i have caught a couple mini ones but havent been able to stay inside them. Is it possible to catch one off of a hand launch? discus launched wing as in only 80ft or so. That sounds very possible. How do i know when i have 'hit' one? and then what should i do? put myself in a tight turn inside it? And then What type of plane thermals best? aside from wingloading but as in aspect ratio? are low aspect ratios ok? or should you have a higher aspect ratio?
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Old Mar 03, 2009, 09:54 PM
Physics hate's my ideas
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Also with a light enough wingloading can a KF airfoil thermal? i imagine so since im thinking that thermaling is just all about surface area
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Old Mar 03, 2009, 10:32 PM
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have a read through here, it's probably been answered + more.

http://www.rcgroups.com/forums/showthread.php?t=208889
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Old Mar 03, 2009, 10:53 PM
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Green, There's a lot to catching and holding onto a thermal. It's kind of luck of the draw at times. You can launch one time and not find any lift and launch again and have a hard time getting out of the lift to land. All in the same day!

The most important thing you need is to have your plane trimmed out very well. When it's dialed in it will tell you when you fly through a thermal. You'll either get a tip "Bump" or the entire plane will "Bump" When it's a tip you need to turn towards the tip that raised and start flying in a circle as you move down wind with the thermal. You have to remember that thermals don't stay in one place. They move with the wind.

Thermals also get larger in diameter as you get higher up in them. You have to increase the size of the circles you're flying the higher up you get. Think of them as an invisible gentle tornado. I'm sure that you've seen footage of tornados on TV. They twist and dance around quite a bit. Well a thermal does the same thing. It all depends on how the wind is blowing. You may have a 5-10 mph wind from the west on the ground but when you get up 500 feet it may be blowing at10-15 mph from the north. A lot of how the wind acts at different levels is going to be effected by the shape of the land around your flying area. Hills, trees buildings etc are going to effect the wind at ground level. Obstructions will make it swirl and split up and move any way but straight.

If you really ant to learn as much as you can about thermal, how they form and how they act, you need to get yourself a copy of "Secrets of Thermal Flying" from Radio Carbon Arts. It's a great DVD with TONS of good information on it. I've been flying since the early 70s and I was still able to learn more things from it.

Just about any plane can thermal if the conditions are right. Some easier than others. A larger wing area helps a lot as well as low weight. You can thermal over snow if the conditions are right. So 40 degrees can also be done. If it was a very cold night and the air is warming then you're almost sure to have some kind of thermal activity as the ground warms. DLgs can and often do thermal quite well. It's a matter of timing your launch and knowing how to ride the thermal at low levels is all. You don't want to turn too tightly. Turn but try to keep your wings as flat as possible. If you get too much bank into the wing it won't work as it is supposed to and you'll loose lift that is there to be used. Standing it on the tip is not the correct way to climb in a thermal unless you're in a real monster!

Hope this helps. There's tons more information but time is limited here tonight. Get the DVD and it will explain it a lot better than I can.

Jeff
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Old Mar 04, 2009, 10:10 AM
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Good advice above, especially about reading the longer discussions and checking out the RCA DVDs. Short answers (which hopefully inspire you to follow previous suggestions) follow:

Quote:
Originally Posted by GreenAce92
So with say under 40degrees is it possible to thermal?
yes.

Quote:
Is it possible to catch one off of a hand launch?
yes, though the lower you are the more difficult it is becasue they are smaller and weaker down low.

Quote:
How do i know when i have 'hit' one? and then what should i do?
You don't really hit a thermal- when you find one IT hits YOU. Any sudden uncommanded movement by the airplane indicated air currents near it. Most of these movements will try to turn the airplane *AWAY* from the thermal.

Quote:
put myself in a tight turn inside it?
Basicly, yes, but when you're turning you tend to lose altitude more altitude than in straight flight, so there's a balance to be struck.

Quote:
And then What type of plane thermals best?
Those with light wing loading and low drag.

Quote:
aside from wingloading but as in aspect ratio? are low aspect ratios ok? or should you have a higher aspect ratio?
That's a larger question- higher aspect rations tend towards lower drag, which is good, but they also tend towards narrower chords and lower Re numbers, which is not good.
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Old Mar 04, 2009, 10:16 AM
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Quote:
Originally Posted by GreenAce92
Also with a light enough wingloading can a KF airfoil thermal? i imagine so since im thinking that thermaling is just all about surface area
Even with a heavier wing loading a Kf (Klein-fogleman, right?) wing can thermal- but the Kf airfoils have much more induced drag than more conventional airfoils, and that can be a large penalty to overcome.

The worse your drag, the more altitude you must surrender to stay above stall speed. The more altitude you give up in flight, the faster your thermal currents must rise for you to break even.

The same principle holds in parallel for heavier wing loadings, but for a slightly different reason.

Also, the peak lift attitude is more difficult to detect on a Kf wing. That adds an additional challenge.
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Old Mar 04, 2009, 02:47 PM
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Ahhh Cool! thanks guys! hopefully i can catch one!
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Old Mar 04, 2009, 03:40 PM
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Quote:
Originally Posted by GreenAce92
Ahhh Cool! thanks guys! hopefully i can catch one!
GreenAce92,

It's not if, it's when you will catch your first killer thermal. When I started out learning sailplanes, I had read just about every post on this forum and really had no idea what it was really like. After some practice, you'll catch some small to medium stuff and think to yourself, wow, 8 minutes with no motor! Then one day your plane is going to get sucked up to 2000' and with your focal point adjusted to that altitude, you'll see God's creations up there with you playing in the lift. When this happens it's very humbling, and you my friend, will be hooked forever and ever and ever just like the rest of us.

Best of luck to you,

Don
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Old Mar 04, 2009, 03:58 PM
The ground's the limit.
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No fair man! How you gettin' god to build your gliders!!!

Shawn
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Old Mar 04, 2009, 07:06 PM
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Ace:

I fly gliders for two reasons.

First: there is a huge dividend to putting together the right shape and materials into an airplane the right way, much more so that in any power plane. Power plants of any kind are to the point that any mistake in design seems easily made up by adding more power - not so in a glider.

Second: my glider is a sensor, not an airplane at all. It shows me things most people can't ever see or imagine, including air that goes up! It was true when I flew full-scale gliders, and its true flying rc gliders. It's a magic world, and the glider is a way to sense the full magic.

Any airplane can soar: all it has to do is find air going up, faster than the airplane is going down. Sink rate is better when weight is less, and drag is less, too.

As the airplane goes along, and as you gain experience with a particular airplane, the signs will be there: nose up, for example, or the wing in rising air will go up (an airplane left to itself will usually fly carefully AROUND all the thermals). That's one of several reasons for flying very smoooooth: so you can see the airplane's reaction to the air, rather than to you.

Sunny days generate thermals, especially after a cold front passes. Cloudy days can have thermals, too. My first 30 minute flight was under a 100% overcast with the wind blowing about 15mph. But man-oh-man, that tennis court next to the flying field had a steady, non-stop thermal.

If you see the airplane go up while maintaining the cruise attitude, that's a thermal. Turn around and find it again, and start circling. I use a 45 degree bank as a starting bank angle in a thermal. Some days, ther thermals are larger, and I can fly a larger circle at a lower bank angle, with better efficiency. Some days the thermals are narrow little things, and even 45 degrees won't see a full circle in lift all the way around.

Look for Dave Thornburg's book: Old Buzzard's Guide to Soaring.

Yours, Greg
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Old Mar 04, 2009, 07:20 PM
Physics hate's my ideas
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wow thanks for the info! i'll trim out my bird and see what i can do, definitely need to get an airfoiled wing. Thanks for the info!

also what do you think about grapevines? like the area? i have lots of grapes and it seems that i can 'sense' that there are thermals there, my Spartan was getting the dips here and there and some updrafts that i would catch, but they were very very weak so i need a much lighter plane
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Old Mar 04, 2009, 10:18 PM
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You may wish to consider flying something other than a wing while learning to thermal. I'm not saying that it can't be done. In fact I love the challenge of getting my little Alula to seperate it'self from the local air, but I think it'd be a real discouragement for a newbee. Yes they go up fine.. it's just hard to fly smoothy enough to fully take advantage of lighter lift. So choice of where and when to launch becomes much more important to gain success.. You just need success period for now. I'd get a 2M or larger poly plane and a highstart if I were you. Then prehaps a Gamber or something similar. Once you can get it done with these.. I'd bet you can toss a tiny little wing out, get a bite, and set the hook so to speak.
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Old Mar 04, 2009, 11:47 PM
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Quote:
Originally Posted by GreenAce92
Ok so i have a few questions mainly what i have above.
So with say under 40degrees is it possible to thermal? on my Spartan i feel like i have caught a couple mini ones but havent been able to stay inside them. Is it possible to catch one off of a hand launch? discus launched wing as in only 80ft or so. That sounds very possible. How do i know when i have 'hit' one? and then what should i do? put myself in a tight turn inside it? And then What type of plane thermals best? aside from wingloading but as in aspect ratio? are low aspect ratios ok? or should you have a higher aspect ratio?
Also with a light enough wingloading can a KF airfoil thermal? i imagine so since im thinking that thermaling is just all about surface area
I have caught thermals in winter weather, and even when it was a bit windy, after sunset and under 40 degrees Farenheit. July tends to be better, though.

The lowest I have ever caught a thermal was about 8 feet, with a 42 oz. (maybe even 43 oz.) Sagitta 600 (that's a 2 meter) from a hand toss. But that was luck. 80 feet is plenty, although the higher you get, the wider and stronger thermals tend to be. You want to circle as wide as you can and still stay in the good lift, but that's hard to know. Obviously, the lighter the glider, the better it will thermal, but then the lighter it is the more trouble it will have flying through the sink before it hits the ground. There's more to it than that, of course. Manned gliders have wing loadings FAR higher than our models, but they don't sink much faster because they have long wings, they're very clean, and the Reynold's numbers are much better. (i.e. if something is big and fast, then air viscosity doesn't matter as much)

Big gliders with large wing spans, good construction techniques with lots of carbon, and precisely shaped airfoils should have larger aspect ratios. Small gliders with lousy structure and crappy airfoils should have smaller aspect ratios. Unswept flying wings should have small aspect ratios.

I have seen a paper grocery bag do a minute or so after it was sucked up off the ground, so I suppose a Kline Fogelman airfoil could also thermal. But the Kline Fogelman airfoil is voodoo aerodynamics. Look it up on the ntrs.gov (I think) report server if you don't believe me, where they recently posted an experiment. Besides, you're behind the fashion. Get with it and start extolling whale tubercules! (I'm not saying the latter can't work, but I think it's probably one of those things where you have to do everything just right or it makes things worse.)

I don't know the Spartan, but if you want to learn to thermal, get a big, slow floater. An Olympic 2 is excellent. I've heard Paragons are too, but I only have a couple of minutes of air time, as opposed to tens of hours with the Olympic. Plus I've had a number of students do better with it than anything else. Unfortunately, you'll have to either find a used example or build. But 100 inches is a LOT better for catching thermals than 2 meter or dlg, as far as floating goes. The best glider for catching thermals that I have ever flown very long is probably the Ava. It's not quite as easy to fly as the Olympic, and in fact to me it felt weird at first, but it really stays up well. But it's awfully pretty, costs a bit, and is probably somewhat harder to fly than an Olympic for a beginner.
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Old Mar 04, 2009, 11:50 PM
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P.S. Learning to catch thermals consistently can take a long time. Don't give up easily. I went to contests for two or three years before I ever got out of the bottom half, and most of that was not catching thermals reliably. On the other hand, I lost my first glider for a week or so in a big thermal only a month after I started to fly. If you can find it, read the Old Buzzards Soaring Book. Very good on finding thermals.
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Old Mar 05, 2009, 04:10 PM
Physics hate's my ideas
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Ahhh ok thanks alot guys! today is really really windy i mean its not bad but for my 5oz bird it is really bad, im making it forward CG heavy so it'll fight the wind...

As for thermalling i watched Geodes' video of his Birdwing Flying Wing DLG he threw it around 80ft or so and thermalled for 4+mins! it was amazing! i was like wow! so thats what im aiming for, but first i need the wind to stop, an airfoil, lighter plane and correct perfect flying platform
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