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Old Sep 26, 2006, 01:19 PM
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THUREN's Avatar
United States, OR, Bend
Joined Jun 2005
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Quote:
Originally Posted by Daemon
I'm not surprised it went 159 at all. Having DS'ed it in the mid 120's
in light air, I knew it would go faster. As for the ailerons. I wonder
if the following factors keep them from fluttering.
1. Goop hinges offer some damping
2. The wings bend fairly uniformly which may actually be stiffening up
the ailerons
3. The bend may also be locking out the tips a little bit.

ian

Great pionts ian, didn't think about the wing bending helping hold the ailerons..

Oh and yeah, I wasn't surprized the "plane" did 159 but that it took that long to induce some flutter. It wasn't bad flutter either, very very mild considering it was seeing an approximate 200mph airspeed...

I think I may have noticed some tail flutter in there too, which scares me a little as I don't want to loose control. At this point I'm probably going to call it good with DSing this thing, as to go faster is going to require building a stronger tail, cuting/redoing ailerons, etc and I like it as my frontside mauler anyway...

Also, when it did the 159mph pass it was later in the day when the wind/thermal shift was making the backside pretty rough. At this same time IIRC the Extreeme's were in the 225's and the Destiny's were hitting 175ish...Bill L's carbon Mach Dart did 170 right before me after working hard... So not too bad in comparison...
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Old Oct 02, 2006, 12:57 PM
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AndreasMergner's Avatar
USA, NY, Albany
Joined Dec 2003
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Very cool report, Don. Sorry about joining in late as I've just gotten back from globetrotting. I've sent out a bunch of 4 servo wings that would raise the speed of aileron flutter onset significantly. The surfaces are held much better than with a two servo setup. I'd be surprised that the tail is fluttering at the same speed of the ailerons. The ailerons should be much more prone considering they can be tweaked quite easily at the tips while the elevator is much stiffer.

Was it difficult to get to 159, or were you limited because of flutter?

Andreas
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Old Oct 02, 2006, 01:10 PM
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THUREN's Avatar
United States, OR, Bend
Joined Jun 2005
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Quote:
Originally Posted by InsaneEPP
Very cool report, Don. Sorry about joining in late as I've just gotten back from globetrotting. I've sent out a bunch of 4 servo wings that would raise the speed of aileron flutter onset significantly. The surfaces are held much better than with a two servo setup. I'd be surprised that the tail is fluttering at the same speed of the ailerons. The ailerons should be much more prone considering they can be tweaked quite easily at the tips while the elevator is much stiffer.

Was it difficult to get to 159, or were you limited because of flutter?

Andreas

Naaa it was and easy 159 Andreas...

I knew you were out n' about...


I really do think the tail is a limiting factor though. Not the control surface but the entire vertical/horiz assembly... It's just a little too "wiggly"...

The ailerons would be an easy fix though yeah...

Here's the main issue....

I think there was a consensus that day that wing loading has it's place on the fontside, but on the backside it can hurt the wind-up with the wind not strong enough... For example, Joe Manors 60" dynamic was barely breaking 220mph when JOe's 41oz Carbon destiny just did 250mph... Not sure what Joe's Dynamic weighs but it's probably right at 50-60oz. I think the Thorn will really come alive in 40-50mph constant wind, but I wouldn't feel safe going over 160 without beefing the tail surfaces a bit... In anything under 30mph of wind it's just too heavy to really get going.. Earlier in the day my 31oz Devastator EPP wing did 154mph FWIW...

The Thorn was SOLID though when it was turbulent having the weight...

-Don
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Old Oct 02, 2006, 01:55 PM
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AndreasMergner's Avatar
USA, NY, Albany
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Thanks Don. This is the kind of stuff that helps me make changes to the design.

Is the tail boom twisting, or are the H and V surfaces in need of more thickness (or glassing)? The vertical (and horizontal) are probably bigger than what is absolutely necessary, so decreasing their size would be advantageous for DS.

Interesting about the wing loading and DS max speed... I know that the wing loading theoretically drops out of the equation for top speed for DS. I had also heard that lighter planes accelerated quicker than heavier planes in DS, but were not necessarily faster. A light weight would help a lot on initial laps, however. Also if you got one or two perfect turns on a light plane, you could have a higher max speed as opposed to having to string more of them together on a heavy plane to accelerate it up to the same speed.

Do you think the weight was the only difference between the two planes? With a 54" span, the C-Destiny probably has a higher loading that you'd think.

Seems like you could lock out the tips of the ailerons with some carbon sheet (top and bottom) and some tape if you wanted to avoid aileron flutter.
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Old Oct 02, 2006, 02:53 PM
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THUREN's Avatar
United States, OR, Bend
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Not the boom but the actual H and V.... But really, they are fine Andreas...

If they are taking 200mph turbulent airspeed(shear shock) there is nothing to worry about, on the frontside...

"IF" someone wanted to make a Thorn for PURE DS, you could allways shape the tail into more of an airfoil, and bag it, along with cutting and bagging the ailerons...

I'm good with it the way it is though and no need to touch it...



On the weight thing... This is just my "feeling" about it so take it FWIW... I think that at a cirtain(high) wingloading the bottom turn suffers so much trying to retain all the energy. I think of it like a lead sled trying to pop a hard F3F turn... Ain't happenin'.... With a thin airfoil and high weight it just likes long sweepers more in lighter air... In 50-60mph air it's going to need the weight to penetrate the shear and make energy where a lighter plane will slow down hitting it...

Hard to say whats truth though and all planes are different...

-Don
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Old Oct 02, 2006, 02:58 PM
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Oceanside
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Has anyone tried to replace the poplar ailerons with bagged balsa to lighten them up? Should help to reduce flutter a bit.
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Old Oct 02, 2006, 03:04 PM
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THUREN's Avatar
United States, OR, Bend
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Quote:
Originally Posted by sl33pdirt
Has anyone tried to replace the poplar ailerons with bagged balsa to lighten them up? Should help to reduce flutter a bit.

Yeah I dunno'... But honestly, I was thinking they would flutter at 130 as heavy and "technically" flexy they are... Getting to 160mph with uncovered wood full-span ailerons says something... Not sure if "lighter" is the answer anymore...

-Don
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Old Oct 02, 2006, 03:29 PM
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Considering how thin the ailerons are, I wouldn't use balsa. It isn't just how light or heavy they are, but also where the weight is. A skinned/composite aileron has more weight at the TE than a solid aileron. This is because most of the weight of the aileron comes from the glass/carbon and it is centered at 50% of the aileron chord. A solid aileron is thicker at the LE of the surface and so balances out at 35% or so. Given the same stiffness, the solid surface will do better due to the more forward CG of the surface.

Also, the solid wood is flexy, but strong (it will flex a LOT before breaking) so you don't get surface failure with flutter. With a carbon skinned surface, you'll more likely get a delam after flutter and thus lose all control.

Don, I think your explanation of heavy loading DS makes sense. I wasn't aware that you were not able to get as tight a turn on the bottom as you would have wished on the lower wind days. It sounds like a combo of the airfoil and loading for sure.

FYI - I'm working on a "light" Thorn that should come out around 30 ozs that should do better on the light days - both front and back side.
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Old Oct 04, 2006, 09:01 AM
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minifly's Avatar
Cape Town, South Africa
Joined Jan 2003
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Quote:
Don, I think your explanation of heavy loading DS makes sense. I wasn't aware that you were not able to get as tight a turn on the bottom as you would have wished on the lower wind days. It sounds like a combo of the airfoil and loading for sure.
I think he was able to 'get the turn', but a lot of energy was bled off when he did.
Correct me if I'm wrong.
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Old Oct 04, 2006, 10:22 AM
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THUREN's Avatar
United States, OR, Bend
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Quote:
Originally Posted by minifly
I think he was able to 'get the turn', but a lot of energy was bled off when he did.
Correct me if I'm wrong.

That's it exactly....
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