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Old Feb 05, 2013, 02:04 PM
in awe of glide...
Kirkerik's Avatar
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Originally Posted by Kirkerik View Post
Hey Seba

How about a pic of that horizontal FF stab on that gold wing ship!
Are those EDGE tails?

Kirk
Hey, i found these great videos! Thanks Seba. They are on this thread: http://www.rcgroups.com/forums/showt...ght=edg&page=3

why is the V-mount so close to the leading edge? Would it not be best just before the aerodynamic balance point? I don't know the technical term for it but maybe you know what i mean?

Kirk
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Old Feb 05, 2013, 04:06 PM
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It's one of this when life experience goes against theory.
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Old Feb 05, 2013, 06:33 PM
in awe of glide...
Kirkerik's Avatar
United States, NY, DeRuyter
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Originally Posted by pyrsly View Post
It's one of this when life experience goes against theory.
So you have tried this? From this thread:http://www.rcgroups.com/forums/showthread.php?t=1072386

"Whatever stab design you use, the pivot pin location MUST BE slightly behind the 25% MAC (Mean Aerodynamic Chord). This is counter-intuitive. But if this is not done then the stabilator might flutter at high speeds and this would be BAD!"
GT

In your experience was 25% too far from lead edge?

Kirk

PS: Sorry if this already covered.
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Old Feb 05, 2013, 10:52 PM
G_T
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Being overly close to the leading edge will mostly just increase the load on the servo. If the servo geartrain develops slop or if there is otherwise slop in the linkages (bad plan for an elevator setup as we need real precision here) then flutter might be possible. Running the pivot point a little the other way essentially prevents flutter, but can allow aeroelastic divergence if one goes too far that way or doesn't use a strong enough servo for the job. It would also be a very bad plan if one tried a pull-spring setup with full flying. I think that is why some in Europe are scared of going in that direction.

BTW, since at least some full flying stabs have some camber to them, the assumption of 25%MAC being the neutral point of the stab becomes incorrect. It moves a bit off that location (rearwards, actually), and the position is a function of airspeed and lift coefficient (and Reynolds number of course, as for pretty much everything in this size and speed range).

Gerald

PS - Going a little rearward of 25% puts the pivot point a bit closer to the center of mass of the horizontal stabilizer, reducing inertial->pitch coupling. But this is probably an ignorable issue for our DLGs.
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Old Feb 06, 2013, 12:49 AM
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Guys from Momentum had that well tested and practice shows that forward of MAC is stable and at MAC or behind generates flutter. This is where I lisen carfully to Gerald and disagree with theory
Seba
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Old Feb 06, 2013, 06:53 AM
Father of Fr3aK, DLG Pilot
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Don't overlook the forces on the servo. Back in 2009 when we were playing with these, I had a ship with the pivot point around 15% by mistake. It flew great until it "locked up" and went in from altitude. The elevator servo was unresponsive and hot to the touch after the crash... so hot I could feel it through the fuselage when I picked up the wreckage. My assumption was that the pivot point was too far forward and the servo was working far harder than it should have been to maintain center. It should be noted that I was using a cambered horizontal.
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Old Feb 06, 2013, 09:27 AM
G_T
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One should pay attention to the difference between flutter and aeroelastic divergence. Flutter is oscillation induced generally by coupling of aero loads to momentum, with a surface that is flexible, yielding oscillation. To get this oscillation, there has to be stability in the system. In other words, the surface has to try to return to the "neutral" position on its own. The problem in flutter is somewhat a damping problem. There isn't enough damping so when it returns to neutral it overshoots to the other side. That is pretty normal anyway. Where it gets abmormal is that it overshoots to farther than it was off to start with. So the oscillations grow in amplitude - we call this flutter. Having the pivot point forward of the 25%MAC for a symmetric airfoil full flying horizontal tail establishes the stability portion of the requirements for flutter. That doesn't mean it will flutter; just that one of the preconditions is met.

The other direction, where for instance the pivot point is behind the aero center, the surface never tries to recover. It tries to pull farther from alignment. If the support system (say, servo, etc) isn't strong enough, the angle increases, the force increases, and it pulls apart. This is aeroelastic divergence.

These two are different problems and have different solutions.

Frankly, I use a stiff enough strong enough servo and a rock solid linkage system that is also very light. I don't worry about a small divergent force. I bias that way to avoid the momentum/flex coupling that can lead to flutter. Now if someone doesn't make such a system solidly enough, then of course it will fail. That is just poor building or engineering choices though. Anyway I've pushed such systems way beyond what most would attempt. No issues at all.

I prefer around 30-35%MAC as a good safe range. This is closer to the center of mass and likely closer to the max thickness - and both these reasons are good reasons to go there.

I mentioned, though it seems to have been missed, that with a cambered surface, the aero center is not at the 25%MAC and can be well behind in some situations.

But if it works ok however you have it, then no issues.

Gerald
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Old Feb 06, 2013, 11:34 AM
in awe of glide...
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Tom,

Certainly the servo could have been overloaded by the flying stab, overheated and failed. But, Is it possible that there was an issue with just the servo, and the location of the pivot point on the FFS had nothing to do with it?

Im not saying you're wrong... just asking for clarification.
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Old Feb 06, 2013, 12:23 PM
Father of Fr3aK, DLG Pilot
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It is possible. It's also possible that aliens decided that the color of my aircraft was offensive and shot it down with an electron beam from Jupiter. Like most crash scenarios, there were more variables than just the pivot point, but I have always felt strongly that the servo load was the issue.
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Old Feb 06, 2013, 12:27 PM
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That's crazy Tom Everybody knows Aliens prefer Uranis.
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Old Feb 06, 2013, 12:30 PM
Now fortified with carbon
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That's crazy Tom Everybody knows Aliens prefer Uranis.
LMAO good one Killer
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Old Feb 06, 2013, 12:51 PM
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Seba, for your Edge horizontals, are you using the SG planform (Root chord = 3.25",Tip chord = 1.625", sweep = 0.625", half span = 6.875") or are you using another?

Can you post the planform measurements (Root and tip length, sweep and span)?

It looks to be a linear taper so could we use the online simple MAC calculators (like this one: http://www.geistware.com/rcmodeling/cg_calc.htm) to get the distance from the leading edge of the horizontal for mounting the pivot for the v mount or is it too inaccurate a method?

Using the Super Gee planform, I got the following from the listed calculator:
Calculated Results
Mean Aerodynamic Chord (MAC) = 2.53" length tip to trailing edge

MAC distance from root chord (d) = 3.06"

25% MAC as measured from the root chord = 0.91"
30% MAC as measured from the root chord = 1.04"
35% MAC as measured from the root chord = 1.16"

Not sure if this applies to Seba's horizontal or not though
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Old Feb 06, 2013, 01:16 PM
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Stab is more swept back than SGII. I'll take pic of template with ruler next to it and post here. I have one somewhere with MAC marked on it.
But there simpler way to do it. Spar lays more or less in the middle of MAC. Ii usually cut half way true spar from LE when mounting anchors. It's also thickest part of stab.
During My trials I mounted stabs from 1/4" for LE to 3/4", measuring at hard point. All worked with no problems. Even at very forward placement I didn't observe any trouble. I go with thickest place for thickest hard points, they are the key to this setup.
Seba
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Old Feb 07, 2013, 07:20 PM
in awe of glide...
Kirkerik's Avatar
United States, NY, DeRuyter
Joined Dec 2012
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Originally Posted by tom43004 View Post
It is possible. It's also possible that aliens decided that the color of my aircraft was offensive and shot it down with an electron beam from Jupiter. Like most crash scenarios, there were more variables than just the pivot point, but I have always felt strongly that the servo load was the issue.
Please do not take my enthusiasm as sarcasm. (just in case you were) I do not question your judgement. Those aliens can be buggars! I am enjoying learning about this very much so. This is a great forums. I trust that your intuition on that servo load is correct. Was your flying stab mounted above or below the boom? I have a feeling that if the H stab. was mounted on the bottom of the boom, the more turbulent stream of air from the wing may have caused more stress on that stab and therefore the servo than if it were mounted above the boom. I see that Seba's FFS have been on the top of the boom out of the slipstream of the wing. I know not where Geralds stab was mounted. Could this have been part of the problem or solution?
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Originally Posted by G_T View Post
...But if it works ok however you have it, then no issues.
Gerald
Undoubtedly your technical theory is correct. Yet there is so much undiscovered or unknown. I'm glad you leave a clause for the mystery of flight...
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Originally Posted by pyrsly View Post
During My trials I mounted stabs from 1/4" for LE to 3/4", measuring at hard point. All worked with no problems. Even at very forward placement I didn't observe any trouble. I go with thickest place for thickest hard points, they are the key to this setup.
Seba
Wow! 1/4" from LE was working without any trouble. Glad to hear it. Just so we're clear, i do not doubt Seba's design. It appears he is having great success!
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Old Feb 07, 2013, 08:28 PM
Father of Fr3aK, DLG Pilot
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Not taken that way at all. I should use smileys more often.

My stab was top mounted on the deceased airframe... pretty normal.

I did play with bottom mounted stabs as well but didn't like them for other reasons.

Seba's stuff seems to work very well as it's designed.
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