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Old Aug 20, 2012, 07:13 PM
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United States, OR, Corvallis
Joined Jan 2010
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Question
Does Disaster-Turbulance lurk around at "speck" altitude???

Hey, Yesterday I simultaneously had my best and worst thermal flight in the same flight. This was done with my newly completed completely modified 2M Spirit. Several times I took it to "speck", then did acrobatics, then floated around way, way high, did more acrobatics, etc. The location was an agricultural grassfield that had been cut and dried out and turned yellow/brown with lots of thermal activity during a 3 day freeflight contest at the same spot.

Side note: I've noticed before that when I do acrobatics pretty high to mess around when I have altitude to burn, that the plane seems to be moving REALLY fast and manuevers really fast with subtle movements of the control stick, or I'm moving the control stick more than I realize and manuevering more tightly than I should be. ?????

The Thermals were incredibly strong, there were constant thick, dense dust devils in a plowed dirt field about 1/3rd mile cross-wind from me. Boomers and hat-suckers, first time I've encounted thermals this strong and numerous(and so easy to find, it was almost a game to stay out of them, really).

"Best"- Longest time and highest altitude gain.: I didn't even think to start the timer on my watch until I was approaching "speck" altitude so it had been up there awhile already, then I ended up with 29 minutes after stopping the stop-watch after the crash.

"Worst"- At the highest point and the speckiest speck, it just disappeared while I was watching it(no I didn't take my eyes off of it), -possible retinal fatique but I doubt it because I'm very aware of that phenemonon and avoid it but still possible. Soon after I heard for the first time that gut-wrenching/blood-draining buzzzzz of a high speed wing flutter from way high up, I knew then it was in a dive, I tried to gently pull up but then right then things went silent, soon I saw the wing halves floating down then caught the sight of my fusalage way ahead speeding it's way down. In hindsight, I should have tried at least to slow its fall by putting the rudder hard left or right and the elevator full up or down to get it to spin and slow it down, but hindsight as always is 20/20 after the fact.

Amazingly, the winghalves were fine other than the totally replaceble center connector spar(which was alreading showing signs of damage, I know now, don't use a compromised center spar connector). The rudder and elevator are fine other than a wood control horn needing to be replaced on the elevator.

The nose of the fusalage went 5" into dry hardpacked dirt of a cut-and-dried grass field and is surprisingly repairable. I really appreciate the advice given to me long ago when I built my 1st plane: Fiberglass that nose while you're building it BEFORE you need to later. Words to build by.

****The Main Questions****
1) Is there sometimes severe turbulance to watch out for in 'boomer' thermals that would toss my plane in a multitude of bad postions such as dive a some point? Or.....

2) Did I just simply and most likely put the plane up too high to see what was happening and things went bad too fast? Or....

3) Both of the above?

4) Has anyone else had similar wing flutter or other disasters in strong powerful thermals up at "speck" altitude?

Thanks - Paul
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Last edited by Pauliwog; Aug 20, 2012 at 07:33 PM. Reason: Clarification, grammatically corrections, etc.
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Old Aug 20, 2012, 07:30 PM
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United States, MA, Waltham
Joined Dec 2001
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1. Not likely to be worse high up than low down, just stronger lift. Generally thermals are wider higher up, and the air is usually smoother. If you're in a full scale plane, I think the answer might be a little different.

2. I think this is the answer, most likely, especially if you moved the stick without being able to see the glider.

3. see above

4. Yes. Especially when trying to escape thermals by flying fast, although perhaps only halfway to speck height. Long ago and I don't remember a lot about it.

It's also conceivable that you had a little bit of a range issue, though unless your vision is exceptional, a good radio and installation probably wouldn' t have this problem.

I don't know what updates have been implemented, but I remember hearing things about weak center joiners in the Spirit kit before. Might be a good place to laminate many layers of straight grained, very strong wood or perhaps even carbon fiber in the next to outer layers. Sugar maple is not too hard to find and is very strong if the grain is straight. Just wet it to raise the grain when you can't sand it any more. Lots of info on other woods here:
http://www.westcoastpiet.com/construction.htm
Certain types of ash, locust and others are very strong.

Slop and flexibility in structures or control linkages can promote flutter.
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Old Aug 20, 2012, 07:48 PM
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United States, OR, Corvallis
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Good info...

Lincoln, As always you have some good input. What you said confirms some of my suspicions. I'm leaning towards my last input maybe being not quiet level, and it was specked out pretty close to straight overhead. I also fly with the CG back a little on the touchy side since I like the way that feels and flies.

As far as reciever/transmitter issues, the plane has a Berg 7 reciever, both the plane and transmitter where fully charged. I did 2 running hand launches and caught some thermal activity and kept the plane in the air far longer than any handlaunch before but still it should not have burned up much charge, the 3rd launch was with the hi-start and within 15 seconds off the hook I had a thermal, servo response post-crash was fine so I really don't think it was a radio issue.

I didn't think about any looseness/play in the joiner, I did have it shimmed with tape since it got looser since building the wing over the humid winter. Could very well be a contributer. I agree on making my next joiners out of stronger materials. I'm guessing the 2 -1/8" 3-ply plywood layers laminated with a 1/16" walnut veneer into a joiner may have been birch and would fail sooner than maple or even basswood if I made my own plywood with the grain running lengthwise (for the most part- I would bias them slightly to have some grain crossing).

Any other input is welcome too

Thanks - Paul
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Old Aug 20, 2012, 09:02 PM
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Grand Rapids, MI
Joined Apr 2004
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I agree that #2 is the most probable cause, especially if you have a rearward CG.

I have learned to figure out for each plane, when it is well in view, an appropriate "escape setting". That way if I lose sight of it, I can move the sticks to that setting and be confident that the plane is coming down without over-speeding. On most woodies this means putting the stick near one corner, with spoilers on.

Tim
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Old Aug 20, 2012, 10:52 PM
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Joensuu, Finland
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Quote:
Originally Posted by 2motheus View Post
I have learned to figure out for each plane, when it is well in view, an appropriate "escape setting". That way if I lose sight of it, I can move the sticks to that setting and be confident that the plane is coming down without over-speeding. On most woodies this means putting the stick near one corner, with spoilers on.
I fully agree with this. That same escape setting should also be your fail-safe setting, that way the decent will be "controlled" even in the unlikely event that you do fly out of radio range or have other radio problems. Spin (with spoilers) is a good strategy for emergency decent - it is stable, speed will remain reasonable and a rotating plane is easier to see than one which is flying straight towards/away from you.

As for turbulence at altitude, it can happen if there is a sharp inversion layer (warm air on top of cooler air). When a thermal hits inversion, it will penetrate some distance into the inversion under it's own momentum, then realize that it is no longer warmer than surrounding air and begin to sink again. Bouncing around the inversion boundary, the thermal can shred itself into pieces and create significant turbulence. I'm not sure if this is what caused Pauliwog's incident, but it could happen.
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Old Aug 21, 2012, 02:02 AM
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I guess I should have been more specific about the laminations. If you use a thin enough veneer, the layers can be horizontal, bend in the middle, and be continuous across the joint. This is likely somewhat stronger than with vertical laminations. Especially if you make a layer near the top and a layer near the bottom with carbon fiber. Don't use basswood for this. It's nice wood but not especially strong. Birch can be pretty strong if you're using the right type.

The laminations I'm talking about might look like this:
http://www.rickcorless.com/boat112.jpg
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Old Aug 21, 2012, 07:58 AM
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Charleston, SC
Joined Nov 2007
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A model looks like it's going way faster at low altitudes than up high. 70 mph in the landing pattern is screaming, but at 2000 feet it looks pretty leasurely. I once started to loose orientation well up in lift, slammed on the spoilers and gently pulled into a loop to buy some time to get reoriented. The ship did a nice round loop with full spoilers! Those spoilers were very effective. Later tests showed that the plane had to be going at an insane speed to do that loop. Luckily, that ship didn't have any flutter problems.
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Old Aug 21, 2012, 05:22 PM
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Wind gradients are a force to deal with . What you experience at 1500-2000 agl is worlds apart from what's going on at ground level.
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Old Aug 21, 2012, 05:26 PM
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Of course, but the further you get from the ground, the less likely the gradient will be steep. Doesn't mean the wind won't be MUCH stronger up there, just that it's not likely to be as rough as lower. I've been hanging around with some FPV guys a little bit, and helping some UAV guys, and we've measured a whole lot more wind at 1,000 feet or so than at, say, 50 feet. Can't remember the numbers, though.
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Old Aug 21, 2012, 05:31 PM
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This is a good reason to have flaps or spoilers, to make a slow controlled dissent. If you had spoilers you could have deployed them when you lost site of the model and put it into a spin.
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Old Aug 22, 2012, 10:46 PM
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United States, OR, Corvallis
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Good Info!!!

Thanks for all the info contributions you guys all put down here!

Spoilers, yes, they're built and ready to go but I built them a string pull system and ran out of room to put a servo in the forward compartment. So, whilst fixing the fuselage, I may just open up the wings and put a mini-servo in each wing and set up the wiring. Might be a good investment. Plus, I have a Berg 7 reciever that I think I can program in a "failsafe" configuration setting even with my nonprogrammable basic 4ch Futaba transmitter in case of transmission loss.

All other info is still welcome!

Adios - Paul
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Old Aug 27, 2012, 11:14 PM
R2R
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St Johns, FL, USA
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Found some related info in this thread ...
http://www.rcgroups.com/forums/showthread.php?t=1471514
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Old Aug 28, 2012, 01:37 AM
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Tennessee
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Forth years ago, I almost lost a sailplane in a monster thermal. I did not fly it again unil I installed spoilers. Since then, I NEVER build a sailplane without either spoilers or crow to get it out of killer thermals.
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Old Aug 29, 2012, 02:34 AM
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I think most sailplane pilots overestimate the altitude they're flying. I put a recording altimeter in a 42% Spacewalker (135" span and 24" chord), and I could just barely see it well enough to fly it at 3800 feet. A 2M sailplane would probably disappear at half that.

My escape plan is to deploy the flaps/spoilers and pull the stick to the lower corner. A stalled spinning plane doesn't fly very fast, but it comes down fast enough to escape all but the strongest thermals. Plus, it's a LOT easier to catch/keep sight of.
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Old Aug 29, 2012, 09:58 PM
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Depends on your eyes. We had a club member see a 2 meter glider of his go down. GPS said it was over a mile away when recovered.

An approximate judgement using the button on the end of a 72mHz tx compared to the wingspan, says that at over 2,000 feet away I get uncomfortable when flying a 3 meter glider.
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