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Old Dec 15, 2012, 05:01 PM
Detail Freak
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Harbor City, CA
Joined Oct 2003
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The free flight plane will have it's CG likely pretty far forward, with a good amount of decalage built right into it (I suspect).

A simple light hand toss will see it take a nice glide slope. I fast toss, or throw into a stiff breeze will then likely see it nose up, or even loop, of thrown fast enough (rubber band catapult launch, anyone?)

If you get the AMA magazine, you might remember an article a while back about a free-flight gadget that was for a tip launched FF glider. If I recall correctly, the plane had a mechanism that allowed it to launch at high speed after a tip launch, with less decalage (straight up), then a weight would swing and allow the fuse stick to pivot a bit (also changing the CG a bit! Sound familiar??), and the plane would go into its "glide phase". Then, later, a timer would allow the fuse stick to pivot WAY MORE, to add a crazy amount of decalage, which made the plane super-draggy, so that it wouldn't thermal out of sight....

Its basically the same principles at work, to a more extreme degree, and NO R/C! Those crazy FF guys!

R,
Target
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Old Dec 16, 2012, 12:57 AM
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USA, KY, Louisville
Joined Oct 2002
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Flying wings?

Hey Target,

Seems like you have a lot of personal experience with them, so lets see some photos of your thermal wings?

Here's a few I have and flew in the last couple of months. You can also see them on my YouTube channel "GordySoar".

So what do you think? Maybe I know how to balance them?

I "do" - then write

If you need more examples, I guess I could pull out...a few dozen more :-)
Gordy (yep I sign my name :-)

PS, I spoke to DP yesterday...
You? :-)
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Old Dec 16, 2012, 11:16 AM
Daryl Perkins's Avatar
United States, VA, Falls Church
Joined Mar 2007
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Hey guys,

I haven't been ignoring you. Super busy at work, and haven't even booted up my laptop in 3 days. Now, it's a choice to respond to this thread, or clean my house.... Well, here I am...

There are some tidbits of information in this thread that are quite relevant. Some... well not so.

Quick question for you guys: When Joe and I used to fly virtually identical models, we would fly with identical CG's. During a "dive test," mine would tuck. Joe's wouldn't. How do you explain that?

There's no right or wrong trim. It's personal preference and can vary even with like CG's.

There's no right or wrong CG. It's personal preference. (Disclaimer - behind the neutral point is bad - well un-flyable) (Disclaimer 2 - more forward CG's cost performance) The one thing I do differently than most is define the most aft flyable CG position by going beyond it. If a model gets "twitchy," it doesn't necessarily mean you're behind the neutral point. It means you need to adjust your throws to get the feel you're looking for. And yes, you must also retrim for every new CG to your desired feel.

I don't use the dive test. It's mostly a trim test and aero-elasticity check. It only works on "like" airframes. Like somebody else mentioned, once the TVC changes, your dive test will behave differently.

With regards to full flying vs. articulated stabs. Pretty much a non-issue. No, you don't have to fly "in trim" on an articulated horizontal stab. As a matter of fact, sometimes flying out of trim slightly can eliminate the deadband or hysteresis around center. Selig even developed a cambered stab section designed to do just that.

Did I miss anything?
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Old Dec 16, 2012, 11:41 AM
Red Merle ALES
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United States, Mt, Helena
Joined Apr 2002
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+1 Daryl and Target!

Since this thread is about CG, Decalage and Trim and the first and last was answered by Daryl I have a question about Decalage.

I made up a nice drawing here: http://www.rcgroups.com/forums/showp...&postcount=108

So are you in agreement that if I DO NOT change the balance location, nor is it necessary too, that if I do change the angle the main wing is attached to the fuselage, that the horizontal tail whether articulated or not will make an accordingly similar angle change that the only difference that has been made to the model is the angle the fuselage is flying to the relative wind.

Thanks so much!

Curtis
Montana
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Old Dec 16, 2012, 12:00 PM
Daryl Perkins's Avatar
United States, VA, Falls Church
Joined Mar 2007
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Curtis,

For this specific question. Yes, the fuselage attitude is irrelevant. (As long as you're not going to extremes).

Although it can obviously get somewhat draggy if done incorrectly.
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Last edited by Daryl Perkins; Dec 16, 2012 at 12:10 PM. Reason: Standard disclaimer again
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Old Dec 16, 2012, 12:26 PM
Red Merle ALES
Curtis Suter's Avatar
United States, Mt, Helena
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Quote:
Originally Posted by Daryl Perkins View Post

Quick question for you guys: When Joe and I used to fly virtually identical models, we would fly with identical CG's. During a "dive test," mine would tuck. Joe's wouldn't. How do you explain that?
Was the horizontal tail trim verified prior to launch and exactly the same?
If only the balance was checked on the ground I wonder if prior to the "dive test" that one model was longitudinally trimmed slightly different.

Prior to the dive test we trim to what we believe is the best L/D speed. Since ground speed is our only reference perhaps the trimmed speed between the two models was a tad bit different.
So there's the big rub on the dive test, how do you really tell what the best L/D speed prior to performing the dive test? That's a rhetorical question. Daryl answered it in his response.

If not it, must be the "Virtually identical" models.

Curtis
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Old Dec 16, 2012, 12:37 PM
Red Merle ALES
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United States, Mt, Helena
Joined Apr 2002
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Thanks for everyone's time.
This thread has been more helpful to me than what may be realized.
I've flown gliders for over 20 years and models over 34 years. I have made spreadsheets that make complicated calculations simple, even Gordy's Flying Rainbow, the flying wing in his first two photos, their is a spreadsheet for calculating twist and balance location has been implemented using the late Dr. Panknin's formulas.

More to come in the following months from Tailwind Gliders on this subject. Stay tuned.

Happy Holiday's!
Curtis
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Old Dec 16, 2012, 01:34 PM
Phil.T-tailer
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Devon, UK
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Quote:
Originally Posted by Daryl Perkins View Post
Quick question for you guys: When Joe and I used to fly virtually identical models, we would fly with identical CG's. During a "dive test," mine would tuck. Joe's wouldn't. How do you explain that?
one word...
trim

Phil.
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Old Dec 16, 2012, 02:46 PM
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IHAVAWDY's Avatar
United States, MT
Joined Mar 2008
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Quote:
Originally Posted by Daryl Perkins View Post
When Joe and I used to fly virtually identical models, we would fly with identical CG's. During a "dive test," mine would tuck. Joe's wouldn't. How do you explain that?
Um Joe, while seemingly a honest competitor... lied about his CG.

Woody, *who also knows, when it comes to beating world class competition, ya gotta lie
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Old Dec 16, 2012, 03:37 PM
mostly gliders
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Skellefteċ, Sweden
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Quote:
Originally Posted by Daryl Perkins View Post
Hey guys,
...
Quick question for you guys: When Joe and I used to fly virtually identical models, we would fly with identical CG's. During a "dive test," mine would tuck. Joe's wouldn't. How do you explain that?...
Hi Daryl

I think you had different flap settings, or as Phil says, different elevatortrim .

/Ville
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Old Dec 16, 2012, 06:30 PM
Detail Freak
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Harbor City, CA
Joined Oct 2003
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Quote:
Originally Posted by Daryl Perkins View Post
Quick question for you guys: When Joe and I used to fly virtually identical models, we would fly with identical CG's. During a "dive test," mine would tuck. Joe's wouldn't. How do you explain that?
Your trim was set at a lesser angle than Joe's was, even if the CG was the same....
I was told the other day my plane (that I lent to a buddy for a TD comp) setup was "Good, but it needed a couple clicks of up in the Thermal mode".

I like my planes trimmed a bit more down when I feel the air is bumpy.....Its safer for me to keep the plane moving a bit faster, and might even be more efficient for the plane (who knows? Not me!).

In my previous references to flying wings, I specify PLANK wings, not swept wings.

R,
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Old Dec 16, 2012, 09:47 PM
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San Diego
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Quote:
Originally Posted by Daryl Perkins View Post

Quick question for you guys: When Joe and I used to fly virtually identical models, we would fly with identical CG's. During a "dive test," mine would tuck. Joe's wouldn't. How do you explain that?
This is a tough one to answer because I'm sure the models in question are no longer around. But if I had to guess, I would say the CG's were both very close to neutral but not identical. Yours was probably just a little closer to neutral than Joe's requiring a different trim. I don't know about everyone else, but I know that I can't claim the cg on my identical models have the exact same cg as some seem to imply. My guess is mine are within 1 mm of each other. I have a decent CG measuring system with ball bearings etc, but even so, there is still some friction and the models probably sit at slightly different angles when balanced. So, I would estimate most CG numbers given have an accuracy at best of +- 1mm. Slight changes in CG when getting close to neutral can mean the difference between a model tucking, going straight, or pulling up slightly when doing the dive test with a properly trimmed model for level flight. My guess is that your model probably was a little more work to fly than Joe's but also yielded a bit more performance. I've flown with you enough now to have noticed that your models always seem to perform just a little better than the others - even when just flying straight. I'm guessing your setup has something to do with that. . .

Tom

PS: Some of my models pull up slightly in the dive test, some go straight and some tuck - the ones that tuck have a lot of performance but are not a lot of fun to fly at distance. . . FWIW
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Old Dec 17, 2012, 01:51 AM
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United States, VA, Falls Church
Joined Mar 2007
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Thanks for playing guys,

Phil understood the point of my question. What I consider to be correctly trimmed, someone else may find too floaty... or vice versa. Joe and I used to drive each other nuts when sharing models. I used to hate to fly Joe's models. They had too much up trim in them for my taste. But back then, I was flying a lot of slope and F3B. So I trimmed all my models for the same feel - F3B or slope cruise. I didn't used to like pushing the plane around the sky. Soooo... Even though our models were CG'd and set up very close to identically, they were just trimmed differently. And those 1 or 2 clicks of trim were enough to alter the results of the dive test.

I've since gone the other way now that I'm mostly a TD guy... I fly with Wurts trim now...
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Old Dec 17, 2012, 03:14 AM
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San Diego
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Step 1 of the dive test - set the model for a slow trimmed glide (not a fast cruise or medium cruise).

If you don't do this, you are not doing the dive test.

From Mark Drela
http://tailwindgliders.com/files/Tips/CG%20Position.pdf

Slow glide is certainly open to some interpretation, but I think most are capable of getting close enough for the dive test to give them an idea of how close to neutral they are. By the way, you should have a pretty good idea if your model is close to neutral long before aero elasticity becomes an issue. If your model starts going fast enough during the dive test with a properly trimmed model that aero elasticity becomes a concern, then it is probably pretty close to neutrally stable - certainly close enough to not be at the dreadful nose heavy condition Gordy is crusading against.

my 2 cents
Tom
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Old Dec 17, 2012, 03:56 AM
Phil.T-tailer
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Devon, UK
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Quote:
Originally Posted by Kiesling View Post
Step 1 of the dive test - set the model for a slow trimmed glide (not a fast cruise or medium cruise).

If you don't do this, you are not doing the dive test.
Tom
Sorry, but you can do the dive test from any state of trim you like - so long as you return to that state of trim after changing the CofG. Re-read Daryls posts & think about it. What you are actually testing for, is dynamic stability/neutrality - at that particular state of trim.

If you only fly thermal, then start from slow-trim - thats fine. But if you were to fly slope F3F (and I do these days) - then you're going to start from a pretty fast state of trim - and your dive test has real-world significance - you start every F3F run with a high-speed dive-in (on a good day!) - and then you want to fly the speed course without the plane ballooning up. F3B multi-task - probably similar, but I dont fly F3B.

Phil.
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