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Old Jan 12, 2016, 03:06 PM
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Help!
why certain plank ocillate/flutter at speed while DSing?

Only certain designs do this.. lumberjack/ drongo (older Bluto clone) I think the Skua's do as well.

I am not talking about control surface flutter.
I am referring to the whole plank fluttering at high loads/speed.
Leading edge to trailing edge oscillation.

Anybody know why? thanks
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Old Jan 12, 2016, 05:20 PM
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I think you'd call it freedom body flutter and its just due to a lack of sufficient bending and torsional stiffness.
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Old Jan 12, 2016, 07:18 PM
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What Spencer said. It's what happens when the structure isn't strong enough through the Centre
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Old Jan 13, 2016, 08:36 AM
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There used to be a cool slo-mo video of the infamous Zagi flap.
Edit: found it. NOT one of mine
ZagiFlutter (1 min 50 sec)
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Last edited by 1000MPH; Jan 13, 2016 at 08:46 AM.
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Old Jan 13, 2016, 08:46 AM
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Swept platforms exacerbate the issue. A "plank" with a tapered wing and straight trailing edge is technically swept, so look at the planforms of the planes in question and reinforce as required. To heck with the foamy cop, if she doesn't like my carbon reinforcements she can discuss it face to face
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Old Jan 13, 2016, 12:04 PM
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Quote:
Originally Posted by 1000MPH View Post
There used to be a cool slo-mo video of the infamous Zagi flap.
Edit: found it. NOT one of minehttps://m.youtube.com/watch?v=Rbj90zHECz8
When looking at videos of flutter the actual speed of the flutter is often masked by the strobe effect caused by the frame rate of the camera. The excursion looks enormous so I wonder just how fast it actually was. I guess live audio might offer a clue.

Pete
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Old Jan 13, 2016, 12:20 PM
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The two times my Lumberjack folded, it survived exactly one oscillation before snapping the spars. Unfortunately the first video started rolling a nanosecond after the fold.

Implosion (0 min 9 sec)


Lumberjack death (0 min 40 sec)
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Old Jan 13, 2016, 12:30 PM
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I've had a few flappers.

Most notably Half-Pipe 60 & M60, both at about 154 mph, turning into sinusoidal jelly.
Though amazingly neither showed any visible sign of their high amplitude strains following landing & each continued to give service, albeit without ever again being allowed to reach those speeds.

The M60 was sufficiently built for DS but not with fast DS in contemplation.

The HP60 had been built for 60" racing & didn't even have a reinforced tape covering, just iron-on over foam. Tracked beautifully though on so many occasions before being pushed one day to its limit.
Reckon one built specifically for fast DS would get very close to 200.
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Old Jan 13, 2016, 02:57 PM
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With swept wings more often than not you need to add a spar as far back as possible, fairly beefy. Usually you can only get in about a 1/3 to half span before you reach the main spar. The first time I saw it was on an own design, in a dive, and not sufficiently strengthened over the middle near the TE. I probably still have a pic somewhere, it was before digital photography days. I taped a stick I picked up on the hill, to the wing over that area to continue flying for the day
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Old Jan 13, 2016, 04:02 PM
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Quote:
Originally Posted by dsman View Post
I've had a few flappers.

Most notably Half-Pipe 60 & M60, both at about 154 mph, turning into sinusoidal jelly.
Though amazingly neither showed any visible sign of their high amplitude strains following landing & each continued to give service, albeit without ever again being allowed to reach those speeds.

The M60 was sufficiently built for DS but not with fast DS in contemplation.

The HP60 had been built for 60" racing & didn't even have a reinforced tape covering, just iron-on over foam. Tracked beautifully though on so many occasions before being pushed one day to its limit.
Reckon one built specifically for fast DS would get very close to 200.
Ditto on my M60(Raven) which has extra carbon tow added to beef up the spar, heavy tape job and glassed elevons. Flies like a dream but right about 160+ it turns into a violently flapping noodle.
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Old Jan 13, 2016, 11:27 PM
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Models I have seen and flown that were most prone to it are the M60 and Reaper 60. Both seemed to be around the 160 mark.

Speaking of which, just noticed my old M60 speed from about 2006 isn't anywhere to be found, but damn if I remember what it was. Early 150 I think
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Old Jan 14, 2016, 05:32 AM
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Dear all

nice video!!

flutter of a swept wing and a plank are differet.
There is a flutter study of the SB13 sailplane (because the model of the eat sailplane fluttered badly already at slow speeds).

When the wing is swept, then there is a coupling between when the wing is bending and the twist in the wing. That is not the case for a plank! If you look at the video, you can actually see, that the pilot can start the fluttermotion by pulling elevator (bending the wing!)...and this induces twist into the wing. Twist on a swept wing is always the same as pulling or pushing elevator...and if this to Things act constructivly together :-) they destroy the plane.
In the study the main results are:
This coupling is worst for wingsweep around 15 degrees (SB13 and our own experiences with F3B flying wings ), so fx wing sweep of 30 degrees is in terms of flutter much better.
The main countermeasure is to make the wing as stiff as possible - no bending.
Secondly place the main wing spar at the root further back than at the tip (chordwise).....but if you go too much back you will loose at lot of stiffness as the thickness of the airfoil is less at the trailing edge.
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Old Jan 14, 2016, 07:32 AM
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The Sonic spar is exactly as you describe Peter. Pat Bowman had incredible insight with his foam designs.
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Old Jan 14, 2016, 09:05 AM
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yes!
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Old Jan 14, 2016, 10:52 AM
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[QUOTE=Peter Wick;33719061
..............
Secondly place the main wing spar at the root further back than at the tip (chordwise).....but if you go too much back you will loose at lot of stiffness as the thickness of the airfoil is less at the trailing edge.[/QUOTE]

Useful tip, thanks Peter.

No doubt particularly so if applied to an HP60, given it does rather well even with a solitary spar installed in contrary fashion.
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