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Old Jul 09, 2012, 07:55 PM
Fremont, CA
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Bruce, would adding another 6mm foam surface on top of the front of the wing, making it a KFM2 foil, be a good start for stiffening?
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Old Jul 09, 2012, 09:00 PM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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Franky I would not bother. The whole KF airfoil thing is so wound up in folklore with little real understanding that you would be more likely to end up producing more drag than anything. And if the foam is quite flexible making it thicker won't do much at all but make for problems when you to add some skins to stiffen things up.

But one option would be to run a carbon tube of the same diameter as the foam is thick along the leading edge. Adding something that stiff along the leading edge will do a lot to prevent the foam twisting in flight. I would still plan on the upper and lower bond paper or newsprint skins to stiffen the foam though. For high speed you want a truly stiff wing to avoid flexing and flutter.
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Old Jul 09, 2012, 11:53 PM
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Garland, Tx.
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I should have thought of that one. Carbon tube LE is a great idea - once you get the yaw stability issue resolved. On second thought, do both, as the exrta stiffness and strength will be good once you start doing more than level flight.

J.P.
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Last edited by TheAeronut; Jul 10, 2012 at 01:47 AM. Reason: spelling and grammar
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Old Jul 10, 2012, 12:07 AM
Fremont, CA
United States, CA, Fremont
Joined Jan 2009
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One more carbon tube comin' up!
Thanks guys, much appreciated.
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Old Jul 10, 2012, 02:45 PM
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The 'Wack, BC, Canada
Joined Oct 2002
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I'm still not sure that there's a yaw stability issue. The model is fine until it "tripped up". If there was a yaw stability issue it should have shown up right away with the launch. But it grooves out for the initial climb and even into a turn or two fairly well. It was only when the speed came up that all hell broke loose.

Now the way it tumbled after it tripped without any apparent desire to straighten out suggests that there's something else going on. But both vids showed the model "tripped" at fairly low altitude. So perhaps there simply wasn't time for the gyrations to damp out and let the model get back to flying normally with a diving recovery and pullout.
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Old Jul 11, 2012, 07:54 AM
Needs brain lubrication
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Aachen, very western Germany
Joined Dec 2004
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From the videos I'd say, you have two problems combined, an imbalance in lateral area (accounting for the initial yawing) and a bad kind of flipping dihedral effect (accounting for the following sudden roll).

The big fuselage in front of the CG may overpower the fins at some yawing angle.
Their proximity to the big duct will not help either.
The fins may work just good enough, as long as a critical yawing angle is not exceeded, until they stall and the forward fuselage continues to be there, playing its destabilising part now without opposition.
From that point on i suppose, its all going out the window.

As long as the airplane is slow and the delta wing has to put out some lift it will have proper yaw-roll-coupling, just like any airplane with dihedral would be supposed to have.
That's because of the swept leading edge.
The annoying thing about the dihedral effect due to the sweep of a wing is that it is tied to the lift it generates.
At higher speeds the delta wing doesn't have to make a lot of lift (actually lift coefficient would be the correct term here) and thus the effective dihedral is small.
The fuselage, EDF, and fins sitting all on top of the wing may have their own negative influence on the effective dihedral and may overpower the low positive dihedral effect of the wing at higher speeds.

Some things you could try out, if what I suspect is correct:

You could try out the somewhat bent up wingtips like the original Jettiger obviously has them.
In addition to that, I'd move the base of the fins ouboard, right to the inner end of the elevons.
Make sure there's no toe in, nor toe out.
And maybe it is better to not cant the fins outboard.
Try to shorten and flatten the fuselage to minimise its lateral area, maybe try to round off the corners a bit more to even lessen the impact.
Maybe scale up the fins a bit, if the fuselage can't be downsized.

Generally, stiffening all surfaces to avoid warping under aero loads can't be that bad, either.

Good luck

biber
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Old Jul 11, 2012, 09:02 AM
Fremont, CA
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Thanks Biber, I'll make a note of those points.
Dan
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Old Jul 11, 2012, 11:07 AM
Herk
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Virginia USA
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Dan - if you get tired of endless suggestions, just do something and fly it
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Old Jul 11, 2012, 11:50 AM
Fremont, CA
United States, CA, Fremont
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HerkS, I already did some mods and waiting for some free time to try it again. Will report on the outcome.
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Old Jul 11, 2012, 02:05 PM
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Garland, Tx.
Joined Mar 2002
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Well, don't just keep us in the dark! Tell us what you changed so the wild speculations of what the effects will be can pour forth.

Actually, I am curious as to what you chose to try first and why.

J.P.
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Old Jul 11, 2012, 02:31 PM
Fremont, CA
United States, CA, Fremont
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Sorry, J.P., I'll do that as soon as possible.
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Old Jul 11, 2012, 07:09 PM
Fremont, CA
United States, CA, Fremont
Joined Jan 2009
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Here are a few changes that I made and waiting for a good time to fly...
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Old Jul 11, 2012, 07:25 PM
Herk
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Virginia USA
Joined Jun 2007
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Dan - watching that slow final tumble in both videos, I really think there is little risk in adding a bit of nose weight. Planes that go out of control like that are almost always tail heavy. Adding weight just means a bit more "up" elevon and a slightly higher landing speed.
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Old Jul 11, 2012, 08:53 PM
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Good observation Herk. I get that now that you brought it up. One clue is the elevon position at neutral flight trim. On most tailless designs the TE should be deflected upwards a bit, especially on an untwisted wing.

Djacob, I was under the impression that BMatthews intended to reinforce the LE of the wing rather than the vertical fins. Not that the fins will be hurt by the reinforcement, but I would think that they would be adequately stiff as they were. Similarly, my concern with the duct has more to do with changing the shape of the space between the duct and the fins than the various problems caused by the intake capacity grossly exceeding the flow capacity of the fan unit. Still, your mod will change the airflow over the airframe and may have an impact on stability.

J.P.
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Old Jul 11, 2012, 10:42 PM
Fremont, CA
United States, CA, Fremont
Joined Jan 2009
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HerkS, yes indeed, I adjusted the elevons to have a bit of reflex and placed the battery about an inch forward.
J.P., I'm trying to stick to the original design as much as possible and it also has a narrow gap between the duct and the fins. Also, I want to change as few things as possible at a time so that I can narrow down the reasons for improvement (or worsening).
I'll launch it tomorrow morning.
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