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Post by GotrekGurnisson on May 29, 2008 20:51:31 GMT 10
I should say that the breaking of the ply TE piece and the wing tear I experienced both occurred during crashes.
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Post by Pij on May 29, 2008 20:54:01 GMT 10
Good point. We need to be thinking, then, about the forces at that point during the most brutal moment.
In a nose-in crash, what happens with the momentum of the wings? Does their mass try to carry the wings forward after the centre section has stopped, even reversed (bounced)? If so, where will the sheering force be greatest? No, not sheering, sort of ripping apart, like an angular tensile force.
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Post by felix on May 29, 2008 20:59:47 GMT 10
pij my experience goes against the theory of 2different materials causing it.my first bee had a balsa tail and the latest is the original foam.....both tear.without the tail it doesn't tear at all. the shear is only caused by the upward twisting force of the trailing edge (the tail) against a downward force of up applied elevon hence the shear? as for the radius mod,if you were to look from above the tail would have a curve from where it meets the elevon hinge to the trailing edge.by doing that the upward force of the tail is distributed gradually away.could also curve the elevon which would do the same for the downward force. make sense??? sorry all for the boring rant......but i love this stuff
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Post by felix on May 29, 2008 21:06:50 GMT 10
very good point!!
but i really think the crash shows the final effect rather than causes the failure. reason i say this that even my wing without a crash showed the covering cracking in a manner that shows shear force and not tensile. i think that the foam gets badly weakened by the shear in flight over time.then when that bad "arrival" happens the nose hits the ground.then the center mass of foam that contacts is trying to go backwards whilst the outer is trying to go forward (hooray kept that bit simple lol).that force is applied at the weakest point which is the tear already formed causing it to pull apart?
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Post by GotrekGurnisson on May 29, 2008 21:16:13 GMT 10
Felix which direction do the the curves you are you referring to go?
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Post by Pij on May 29, 2008 21:29:19 GMT 10
I love this stuff too. Drives most people crazy when some of us want to "get inside" to see how it's really happening. Oh well, they can just skip it.
No, I din't mean that the difference in materials was causing the problem - just that having 2 different flex/rigid properties would prevent that particular cure from working. I think that if the ducktail was made in a single piece continuous with the body of the glider, of the same stuff, the problem would still occurr UNLESS the inside corner was radiused as you suggest. In other words, your number 2 solution would work for a uni-material situation, but may not for a di-material situation.
The more I think about it, the more I find it hard to imagine that the aero forces are sufficient to cause this damage. Though there will clearly be a difference in TE forces between a deflected surface (elevon) and non-deflected (ducktail), which will result in sheering forces expressed as tension in the specific zone, only the covering material lacks sufficient flexibility to withstand that.
My cover is showing the beginnings of the tearing after 1 day!
But I don't think the flexible materials should fail when subjected to only aero forces. Not at the small scale our models would experience. Let's face it, much as we get obsessed with them, they 1) are small and low mass 2) have small control surfaces 3) have low airspeed 4) are over-engineered for the things that happen to them in the air (except combat). They are engineered for ground impact! WAY too strong for the forces of air at 100km/h or so.
But constant stressing of the same part, the same way, day after day, week after week - that is another matter. Maybe, just maybe ...
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Post by felix on May 29, 2008 21:34:19 GMT 10
gotrek the top one is the way to go.....on a little more thought i think an angled straight line may be more effective than a radius? basically make the angle as shallow as practible (at least 45degrees though) with a slight curve where it meets the trailing edge.do the same treatment on the elevons but mirrored (would end up with a triangle cut out with the base away from the direction of flight along the trailing edge)and i think the forces would be distributed enough to solve the problem.
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Post by Pij on May 29, 2008 21:34:45 GMT 10
This is how I pictured the idea.
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Post by felix on May 29, 2008 21:41:53 GMT 10
hey pij i used to think the same of the aero forces in models but here's a trick i discovered by accident (honestly i wasn't making whooshin noises at the time ) that changed my perception......grab your wing at the leading point and the trailing edge and as fast as you can roll the wing.you wil notice the force required to do so is quite high.will see that the roll you just did static is similar to the roll rate in the air! also you may have noticed the covering stretch,wrinkle and do all sort of funny things.add g forces,high drag forces and slope induced buffeting and the forces these things cop is amazing! .....or you could pop your wing out of your car window at speed and move the controls a bit lol (that's a new idea i may just try...now who do i know who has a sunroof? mmm)
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Post by Pij on May 29, 2008 21:48:37 GMT 10
OK,Felix, another good point. The difference in air resistance between the wingtip and the root is high, proportionally. In fact, the root should have no extra force from a roll, and the tip should have force proportional to the rate of roll. (Or the square of the rate of roll? I dunno? No wait, proportional to the rate of roll and the square of the distance from the root?) Here is the problem as I saw it. No, that sunroof idea is scary! I used to do aero experiments as a kid with just my hand out the car window. My dad didn't like it, but he couldn't stop me. I think it was his suggestion in the first place, anyway.
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Post by Pij on May 29, 2008 21:53:12 GMT 10
So, what I mean is, the EPP WILL move up at the one point, relative to the bit held in place by the nearby ducktail. And in between will have to cope with the difference between the part that does move and the part that does not.
If the ducktail was of the same material, no glue joint, all as one, the force would be evenly spread. As it isn't, it all comes to a head at the junction.
I liked solution 4. sort of. Effective, but ugly.
Have you realized that if the inner end of the elevon is not a forward-backward facing edge, then it will have a new LEADING surface exposed to the airflow when deflected? Draggy.
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Post by Pij on May 29, 2008 22:00:53 GMT 10
I just had an idea how to flexibly reinforce the problem part without redesigning the Duck.
Cut a slot into the foam at 45 degrees across the problem part (perpendicular to the typical rip).
Shove some loose glass fibres or carbon tow in the slot.
Glue it in with flexible glue.
Or even better, lay the fibres on the TE surface, from the end of the ducktail edge, round the problem corner, and part way along the foam sub-trailing edge, then glue it on with not-completely-rigid glue.
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Post by felix on May 29, 2008 22:09:30 GMT 10
pij funny how the smallest point can hold so much fascination and conversation lol. what i was gettin at with my last comment is the overall forces on the model and how even a small part of wing can be under alot more stress than one would think and how much force the aileron must apply to roll the wing at the rates that we see in the air against many other forces. that picture is pretty much right (a straight line as stated previous would be better for force distribution but that picture will do as an example). here goes: on your pic if we were looking at the wing from above and in uninverted flight....the left most text change to upward force (caused by the wing twisting at positive angles of attack) the right most text change to downward force (caused by up aileron and resultant negative camber) that would change the center text to shear failure point. by having that curve/taper the force at any given point will be lower and lower towards where the tail and aileron meet which i now believe will reduce the forces enough to bring the shear forces to within the foam and coverings capability. i also would after more thought say that the aileron would be the far greater of the two forces so tapering the aileron towards the joint would be more needed than the tail.
one other point is that if a failure was caused by a crash the infinately more likely failure point would be where the two cores are bonded together at the trailing edge (never happens).
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Post by felix on May 29, 2008 22:19:17 GMT 10
your carbon tow/glass fibre idea is what i was gettin at with point 1.....i tend to think of what i have at hand though and is dirt cheap or free lol.another substitute for glass is cotton cloth and white pva to form the flexible strength.
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Post by Pij on May 29, 2008 22:21:51 GMT 10
Mmm. Must sleep on that. But I don't think I would want to be handling those forces with a tapering-towards-zero piece of something pretty rigid like balsa. If it was able to flex along with the EPP, I'd be all for it. Though it would be nice to think we were able to make the whole line rigid, knowing that hinges just don't work around corners So I can see the attraction in an indestructible drag spar, which even a week ago I didn't understand. Anyway, enough brain strain for me tonight. I can either dream of forces in flight, or just of the fun I had today. ;D PS I do have carbon tow, kevlar yarn and my favourite , Dyneema, all on hand, but no glass fibres. I'd probably make a chunky thread out of one or more fibre type and glue that in with flexible CA. Have to be chunky to get a good grip in a short section. For Dyneema, see www.smallflyingarts.com/cgi-bin/yabb2/YaBB.pl?num=1204284493
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