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Sep 22, 2005, 02:59 PM
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Question

Thermal Turn Trim Settings


I have a 3.2 meter molded, 4 servor wing glider. Airfoil HN-354sm. For straight and level flight I need to carry positive left and negative right aileron trim; about 1.5 mm.

My question is with this trim setup will my glider naturally thermal better in one direction than the other? And if so, which way? At my skill level (not) I can't tell from one day to the next. Some days I'm thinking better left, other days better right. Also, should I set my flap trims to neutral? or halfway in between aileron trims to smooth out the junction.

By the way, the glider is laterally balanced. I thought initially that was the reason for having to carry extra trim, but not so.

Because this is a fairly fast glider, do you think I am carrying a large drag penalty with that much trim off set?

David M
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Sep 22, 2005, 03:45 PM
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slopemeno's Avatar
Wow- cant you move the incidence pins to correct that? At anything over a crawl that planes going to have a mind of its own.
Sep 22, 2005, 07:55 PM
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Slopemeno, "At anything over a crawl..." Not really. With the trim settings as is, it will go from a slow thermal flight to ripping across the sky without any further changes. Very solid. This is a 3 piece wing so the incident pins are on the outer panels only. Will I really gain any noticeable performance benefit by moving the pins compared to just keeping the ailerons trimmed as they are?

David M
Sep 22, 2005, 08:49 PM
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For greatest tip stall resistance, you want to turn in the same direction as the glider wants to go. For least induced drag in the turn, you want to turn against it.

But it's best to find the problem and fix it.
If it's not the incidence pins, then you have a significant spanwise warp twist.
This can almost certainly be removed by twisting the wing a bit the other way, and heating above the epoxy's glass transition temperature. For wet layup epoxies this will be anywhere from 140F to 180F. Hot water poured from a teakettle is a fast and safe way to apply the heat. A heat gun is slower, and more dangerous because of its relatively poor temperature control.
Sep 23, 2005, 09:30 AM
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Wow, what a concise, helpful and terrifying answer!

Let me state that whereas I am not new to modeling, I am new to molded technology. So in re the heat method, I had no idea you could do this. My mind flashes on all kinds of ways I could really mess this plane up. The water method does sound the safest but it seems in order to achieve a small degree of twist that I need to keep the glider hooked up and apply heat and twist to the surface simultaneously.

If I pour water over the wing does it become maleable that quickly? or does it need to soak. How concerned should I be of delamination? If you have or know of any passed threads on this process, please point me to them.

Much thanks!

David M
Sep 23, 2005, 12:38 PM
B for Bruce
BMatthews's Avatar
I had not realized that the temperature was that low.

If it was me I would want to first find which panel is the culprit. I would then make up a big jig out of 2x4's so that I could mount the offending panel into it with the leading edge upwards. Blue or pink styrofoam at the ends would be carved to fit decently around the root and tips and these would be clamped down using shims to bend the panel straight or perhaps a little bit past what is needed. A tea towels or other clinging and absorbent cloth would be draped over the wing and the hot water poured over. The towels would hold the water in place longer to allow the heat to soak in. The water is going to cool very quickly in such a case so mutiple kettles pouring at the same time to ensure the water stays hot for long enough to raise the epoxy past the critical flow temp would be needed.

Or.... what about frame and clamp panel as above. Build a blue styrofoam "oven" aroudn the panel with 3 or 4 100 watt lightbulbs inside and a meat thermomenter sticking through it and monitor the temp inside? I don't THINK the foam would soften at that temp. Once the temp is up to about 180 just cycle the lights to hold it there for about 1/2 an hour to make sure the panel is fully heat soaked and then turn them off and let cool.

But I have to agree David. Doing this to a model that is close but not "just" right may be more trouble than it's worth.

The other thing that comes to mind is if the rudder and ailerons are cross trimmed. Have you tried neutralizing the ailerons and use the rudder only to compensate for the trim? This largely depends on how much dihedral the model has. If there isn none then it won't work.
Sep 23, 2005, 02:45 PM
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Quote:
Originally Posted by BMatthews
The other thing that comes to mind is if the rudder and ailerons are cross trimmed. Have you tried neutralizing the ailerons and use the rudder only to compensate for the trim? This largely depends on how much dihedral the model has. If there isn none then it won't work.
Within my crude world of simplicity and experience, less than 1/16 inch deployment is hardly worth worrying about! If dihedral exists, compensating rudder trim for a compromise may set your mind at ease.
However, I am sure your flying skill would not be infringed by this miniscule error. Perfection is only an idea, whereas compromise is success!
Sep 23, 2005, 08:41 PM
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Quote:
Originally Posted by David McNeill
The water method does sound the safest but it seems in order to achieve a small degree of twist that I need to keep the glider hooked up and apply heat and twist to the surface simultaneously.
With three hands you really don't need a jig, so find a helper. Taping balsa sticks chordwise on the bottom surface at the ends of the panel lets you judge the panel twist while your helper pours the water. Hold the wing leading edge up over the sink (balsa sticks vertical), and pour the water slowly along the leading edge so it runs down both sides of the wing. Hold the wing with oven mitts so you don't get scalded. Tape over the servo or whatever you don't want to get wet.

Quote:
If I pour water over the wing does it become maleable that quickly? or does it need to soak. How concerned should I be of delamination? If you have or know of any passed threads on this process, please point me to them.
The water will heat up the surface composite skin very quickly, since the underlying foam is a good insulator with little thermal mass. It's virtually impossible to damage the wing this way, even with boiling water (212F). At the higher temps the composite fabric weave may become slightly visible in tthe smooth finish, but this is not aerodynamically significant. Or you may see no effect. If you're really nervous, I suggest doing a test in an unconspicuous location, such as on the bottom surface at the tip.

BTW, using boiling water is a standard method for removing dents in bagged wings. I've used it to add or remove twists in entire wing panels.
Sep 23, 2005, 08:46 PM
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Quote:
Originally Posted by BMatthews
I had not realized that the temperature was that low.
It depends on the epoxy. West epoxy has a very low glass temperature -- something like 130F -- which is a big problem in a hot car, BTW. MGS is reportedly much better, about 165F I think. I don't know about the others. Prepreg epoxy's glass temp is at least as high as its cure temperature, either 250F or 350F depending on type of prepreg.
Sep 24, 2005, 03:28 AM
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ooops


My Ellipse with a turbulated RG-15 probably had the same situation (possibly worse). In a traveling 5 year tour of duty, it was placing in TD events in Liechtenstein, Brasilia, Spain, and Russia.
My partner, and main pilot for the sailplane, refused to change a thing on it - and kept me away from doing anything - because it was successful!! As I look back, I believe he was smarter than myself.
Sep 26, 2005, 11:27 PM
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Thanks to all for your information and supportive comments. After examining my center section, I found an almost 1 degree twist and used Mark Drela's hot water method. I taped 24 in carbon rods on each end and held the wing over the bath tub while the wife poured 180 degree water over the leading edge. Took more than one try, but I am quite pleased with the results. Flew this afternoon and readjusted all trims to pretty darned close to their neutrals. I think it flies better; therefore, it does.

Good trick to have in my "tool box".

David M
Sep 27, 2005, 08:34 AM
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Quote:
Originally Posted by David McNeill
Thanks to all for your information and supportive comments. After examining my center section, I found an almost 1 degree twist and used Mark Drela's hot water method. I taped 24 in carbon rods on each end and held the wing over the bath tub while the wife poured 180 degree water over the leading edge. Took more than one try, but I am quite pleased with the results. Flew this afternoon and readjusted all trims to pretty darned close to their neutrals. I think it flies better; therefore, it does.

Good trick to have in my "tool box".

David M
1 degree?? what happened to 1.5 mm (0.059 in) deflection? Hmm... Both sides would equal 0.118 whereas 1 degree at 8 inches (or so) would equal about 0.140! Oops, thats close enough, you were right all along! Sorry. [I personally am not that fussy about 1 degree, and enjoy being flexable]
Last edited by histarter; Sep 27, 2005 at 08:45 AM.
Sep 27, 2005, 05:44 PM
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Thread OP
Histarter, Let me be frank (because David doesn't understand math at all!) you totally lost me!!! I think you're busting my chops, but I'm not sure... What 8 inches???

All I know is that my 127 in wing required APPROXIMATELY 1.5 mm of corrective aileron (each side) which was bugging the out of me! And neutralizing the twist in the center panel made a significant difference as trims are within a shadow of being neutral. I can only assume that the center panel twist, even though small, made a significant difference because it moved the entire wing, not just the control surface.

At any rate, I'm happy.

Thanks again. (And yes, my wife has been known to mutter terms such as OCD and Perfectionistic ....)

David M
Sep 27, 2005, 06:50 PM
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Quote:
Originally Posted by David McNeill
I found an almost 1 degree twist and used Mark Drela's hot water method.
See? That didn't hurt one bit

You might want to check the twist again some time later to see if the warp creeps back. Soaking in the sun or a hot car (next summer) will be the most likely cause. In that case, I'd repeat the exercise with 190F or 200F water. The 10-20 degree increase should make a big difference in how permanent the set it.
Sep 28, 2005, 08:42 AM
Permanently Banned

Simple is as simple does!


Quote:
Originally Posted by David McNeill
Histarter, Let me be frank (because David doesn't understand math at all!) you totally lost me!!! I think you're busting my chops, but I'm not sure... What 8 inches???

Thanks again. (And yes, my wife has been known to mutter terms such as OCD and Perfectionistic ....)

David M
8 inch is a value assigned as a nominal value for wing chord. Sin 1.0 degree X 8 inch = 0.140, a value close to your 1.5 mm corrective (right and left) error. First pass I didn't integrate both sides, and cut source problem in half. For me I am not that critical to a 1 degree error, and chuckle when folks wash out wing tips - programing in about 2 to 3 degrees, on purpose.
[I tend to use turbulation (or with my own designs - low aspect ratios) to eliminate that need. As a profesional engineer, I prefer simplicity!]


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