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Old May 11, 2015, 08:59 AM
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Trim for flying at slope and flat-field sites

As some of you might know, I have written a flight simulator that has a slight(!) bias towards sailplanes. One of my ongoing "problems" is how to trim the planes in the simulator so that they fly well both on the slope in plenty of lift, and on the flat when hunting thermals. The problem is that the gliders I never fly my slope gliders on the flat, and I never fly my thermal planes on the slope, so I'm interested to hear whether it is possible to set planes up in real life so that they'll fly well in both situations. If you do make adjustments, do you do it with elevator trim alone, or by adding nose weight (ballast is a separate matter)?

Thanks for any info - Danny
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Old May 11, 2015, 09:19 AM
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It sure takes me a lot of time to figure out cg/trim changes in real life going between the two...but havent done it enough to get any sort of system in place.
If simulator is reflecting real life, that's a good thing, no?
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Old May 11, 2015, 01:57 PM
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Why should they change?

I am no authority on this but I do fly my thermal gliders on the slope. Frankly it never occurred to me to change the CG from one to the other.
On the slope I fly them more thermal style than aerobatic style. My aerobatic talents are pretty limited. So I don't move the CG at all. But perhaps someone who is far more aerobatic a pilot might move his CG for the slope. I would not know.

Where a simulator would be of value would be to see how any given aircraft might change behavior as you move the CG. Also how they might change behavior as you change dihedral.

For example, my Supra has 3 sets of tip joiners: 5 degree, 2.5 degree and flat which leaves only about 2.5 degree dihedral in the center panel. How would my Supra fly on the slope or in thermals with a change in dihedral? Click for 5 degrees, 2.5 degrees and no dihedral in the tip joiners, while in flight. Cool!

How would it change if I shifted the CG? It would be wonderful if there were a dial or a slider that had CG 90 mm to 120 mm back from the leading edge. Let me tweak it while in flight in a thermal or on the slope.

Then you would have something that would help you decide how to tune your glider for thermal soaring and for slope soaring.
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Old May 11, 2015, 03:29 PM
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Quote:
Originally Posted by Mr Rowl View Post
As some of you might know, I have written a flight simulator that has a slight(!) bias towards sailplanes. One of my ongoing "problems" is how to trim the planes in the simulator so that they fly well both on the slope in plenty of lift, and on the flat when hunting thermals. The problem is that the gliders I never fly my slope gliders on the flat, and I never fly my thermal planes on the slope, so I'm interested to hear whether it is possible to set planes up in real life so that they'll fly well in both situations. If you do make adjustments, do you do it with elevator trim alone, or by adding nose weight (ballast is a separate matter)?

Thanks for any info - Danny
same CofG - (rearwards !) - different elevator trim for thermalling - cruising close to minimum sink - for slope racing its fast glide trim, faster, faster still...

Phil.
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Old May 11, 2015, 05:57 PM
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CG doesn't change in my experience.
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Old May 11, 2015, 06:11 PM
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well...I'm a CG tweaker!!!
I like a really aft CG. GREAT for the slopes, but makes it harder to fly "lazy-like thermal mode".

What I mean is, "instability" is great on the slopes, but when the neck is craned upwards looking up at the underside of the plane ONLY, it's too hard to tell if it's climbing or diving. With a more nose-heavy plane, it'll "auto-correct" better. I may not realize it's diving a bit until after it's picked up considerable speed...therefore, a single-speed-oriented, somewhat nose heavy/up-trimmed plane will be more pleasant in this situation.
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Old May 11, 2015, 07:05 PM
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Quick reply to say thanks for the replies (any more welcome!).

I guessed that tweaking the CG is a bit less usual, if only because it's more trouble!

With my planes, slope and DLGs, I tend to have the CG pretty far back so they're neutral in a dive test. Probably the DLGs should be further forward for the reason cityevader says - and also because it makes the stall softer, which helps if you're in a thermal and getting bumped around a bit.

What Phil wrote is how the gliders are in the simulator - though my feeling (which prompted the post) was that for a few the amount of trim required was excessive. However, working through things I've managed to pull the CG back on those planes, which then requires less trim difference, which feels much better to me.
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Old May 11, 2015, 07:25 PM
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altitude adjustment
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Once I dial in the CG on any plane, it stays there no matter where or how I fly. It seems some folks like to shift CG around. I've talked to people who shift it forward for slope which makes no sense to me. And others shift it forward for thermal flying as Cityevader described earlier. I guess if you're super far away, having the CG forward could help "self correct" if you're into that sort of thing.

Mark
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Old May 11, 2015, 09:34 PM
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Neutral to slow pull out is how my TD glides are set-up. Perhaps that is why I would feel no need to change them for the slope.

You want that light nose to make the glider responsive to lift indicators.

When you suggested that the CG might be changed I was thinking you would tell me that you would move the CG forward for the slope.
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Old May 11, 2015, 09:57 PM
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Hehehe...but on the slopes, I want to fly FASTER!!!
A slightly nose heavy/up-trimmed "stable" glider will only porpoise like mad if trying to fly fast.

I still prefer aft CG for thermalling...i just keep the altitude low (aka smaller DLG) to prevent undesirable neck response to high flying with a larger plane...the ONLY time i like a forward CG.
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Old May 12, 2015, 08:30 AM
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If you want to fly faster just trim in a little down elevator. No CG change required.
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Old May 12, 2015, 09:28 AM
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Aw, c'mon...at least i FINALLY gave a concession to the forward cg group...don't i get partial credit for that?

hehe...just erased a few sentences, as a CG argument wasn't intended.
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Old May 12, 2015, 09:41 AM
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altitude adjustment
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Ha. Yeah. Awhile ago in the UM Radian thread, there was a guy saying he packs a bunch of lead in the nose for flying off a slope. When I suggested he place the weight right at the CG instead of the nose, all hell broke loose. I guess some are pretty sensitive when it comes to talking about CG.

Mark
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Old May 21, 2015, 09:39 PM
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Check it and chuck it
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Correctamundo

Quote:
Originally Posted by cityevader View Post
well...I'm a CG tweaker!!!
I like a really aft CG. GREAT for the slopes, but makes it harder to fly "lazy-like thermal mode".

What I mean is, "instability" is great on the slopes, but when the neck is craned upwards looking up at the underside of the plane ONLY, it's too hard to tell if it's climbing or diving. With a more nose-heavy plane, it'll "auto-correct" better. I may not realize it's diving a bit until after it's picked up considerable speed...therefore, a single-speed-oriented, somewhat nose heavy/up-trimmed plane will be more pleasant in this situation.
In light or marginal lift conditions you want the plane slightly nose heavy (not so sensitive, less corrections=more efficient ), as lift conditions improve, (in thermals, or on the slope) move the CG aft.
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Old May 21, 2015, 09:47 PM
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On the contrary, the most complete, utmost contrary possible...the lighter the lift, the MORE important an aft CG is...that is all.

My comment regarding forward cg was for when thermalling high overhead, when it's difficult to determine the exact attitude of the fuse and you want to be "lazy" and let the plane "fly itself". That is all.
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