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May 22, 2019, 10:40 AM
Call me Frosty
TheRealFrosty's Avatar
I've seen what they call T-Post Sleeves at farm supply stores that are square. Here is an example: https://kencove.com/fence/T+Posts_detail_JR1755.php
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May 23, 2019, 12:07 PM
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Thread OP
That must be the most extensive crash write up i've ever seen...

Sorry to hear about all the damage wow.. my alu frame was built so that it bends instead of breaks (it used to break) and the esc is placed in such a way that the wing flies over it on a hard landing so it does not rip out the servo wire from the pcb (which used to happen before) rx is at the rear of the wing, It's now almost bullet proof.

But a different concept than the gutter you were flying so the 2 cannot really be compared.

Rubberbands tends to save the wing better as well i guess.

Thanks for showing the "posts" i will have a look if i can see anything similar in our builders warehouse store... So far Alu is my go to, but would like to at least try a few different ideas for a little variety.
Jun 02, 2019, 01:29 PM
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Thread OP
Quote:
Originally Posted by TheRealFrosty
This is a cut/paste/edit of one of my posts from spadworld --

The first flight with the new wing started out promising, but I was not pushing for speed at the beginning. I was just making easy flat speed passes. I decided to try to beat my previous max speed of the other flight which was 132MPH. I got up high and started a dive to gain speed before pulling up for a flat pass. Never made it to the flat part. The plane rolled left during the dive and full right would not correct it. Backing off the throttle did not help because I was diving at about 45 degree angle and the plane did not slow down much. I got disoriented on control corrections and the plane went in hard, almost straight in. My Hitec Aurora 9 shows the max speed returned from the onboard GPS telemetry station. It was 132MPH.

I was getting a measured (with watt meter) 1500watts out of the Scorpion 4020-12 motor running on 10S A123 cells. The prop was an APC 11x10 E-prop. Theoretical max speed of 137MPH according to the ScorpionCalc program. I went with the high voltage to keep the amps down because I already had, and was using, a Castle Creations 45HV ESC. The prop/voltage was not recommended in the prop charts Lucien at Innov8tive Designs made for Scorpion motors back then, but I asked him if it would work and his answer is here: https://www.rcgroups.com/forums/show...postcount=7199

1500 watts was at the max watts rating for that motor, but it was hanging out in the wind on the front of the Flipper and I figured cooling would be ok like that.
Ok so i decided today i am going to take out the big 2 meter carbon tube spar spad wing and put it on my normal fuselage that i fly my 1.2m span plane.

The wing has incredible lift i was very surprised it wants to really fly, not come down it just keeps on going resisting to be forced down !

Anyway i have not put in a carbon spar for nothing so i pushed it a little further, from up high i point the nose down and start to dive, it goes solid, pulling up not too harshly makes that wicked sound as the wind blasts over at speed on this big wing, very nice...

SO.... What do i do.... by now i'm getting real stoked and since it passed the dive test successfully it though to myself full power on the 15x13 prop straight and level should not be a problem, awesome...So

I fly in this big park and there is a dam nearby i decided to go full throttle over the dam and as i apply throttle eventually i got to full throttle. very nice.... Up to the point where all of a sudden i hear a very alerting,horrifying, unearthly ! sound, and until now i almost still thought my eyes were deceiving me !

The whole wing on straight and level flight, full speed, Twisted like a snake ! It vibrated so badly you could physically see the whole wing flexing like spaghetti ! ! With that super solid spar in there ! !

It looked like Jelly wobbling ! ! But in a twisted form (like twisting a rubber-band winding it up !

the motor made strange noises as by now the whole frame was vibrating and i knew right there and then, this plane is going down... Trying as hard as i can to keep calm, but freaking out like nothing... i kept a steady hand with no movement just keeping it level. To my pounding heart after 2 seconds the vibration stopped and the plane flew just fine ! !

Until Now i still can't believe that when the wing was twisting ever so horrifically it still flew and i did not lose control, it flew straight, however i do believe that if it carried on for only slightly longer than it did, i would have lost complete control ! !

I'm still shaking as i type, it was a close call that very rarely ends up positive.

NOW i guess i reached limits of correx, Is there anyway i could reinforce the wing so it does not happen again or would i have to rebuild from scratch with perhaps correx ribs ?

The wing is perfect, really don't want to waste it
Last edited by Axefly; Jun 02, 2019 at 01:43 PM.
Jun 02, 2019, 06:00 PM
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As soon as I don't shake after a near crash, I will have to find another hobby. Skydiving maybe? You could shorten the wing, that would kind of strengthen it, or at least there would be less force twisting it.
Jun 03, 2019, 01:20 AM
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Thread OP
lol Exaggerated on the shaking part, just wanted to emphasize that it was a really crazy thing to see happen knowing that impending doom is very likely a split second away, and then... it did not happen !

Yes i have a shorter wing that flies just fine, but i want to make this one work, anything that can be done without adding another spar towards the trailing edge ? i suspect the area from the rear of the spar towards the trailing edge to be the culprit... Spray in foam ? Something to stiffen up the skin.
Jun 04, 2019, 04:39 AM
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Extreme Sports's Avatar
Sounds like a resonance effect, which would most likely be flutter related. I doubt very much that this represents 'the limits of correx' but rather you were unlucky enough to hit the resonant frequency of your wing structure. Google 'flutter' and 'Tacoma Narrows bridge' and if that looks like what you saw, then look for solutions for changing the resonant frequency. They can sometimes be very simple. On real planes, even a coat of paint applied incorrectly on the ailerons can cause a significant change.

I've only once had flutter and that was on a 2mm correx tailplane of an F16. To solve it I simply gave the HS some anhedral (which moved it out of the dirty air off the wing) and added a stiffener (which makes it 'stiffer', but more importantly changes the resonant frequency).
Jun 04, 2019, 05:12 AM
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Thread OP
Tacoma Narrows bridge That is Exactly what it looked like ES,Exactly ! Just much faster, you could clearly see the whole wing visible twisting not just a little, it was a big movement.

Unbelievable that this stiff wing changes into Jelly just like that ! And almost more amazing is that the plane continued to fly straight while in this wobbly state !
One could imagine thinking your mind is playing tricks because its so strange, but it wasn't !

This now makes more sense because why did it not flutter on the high speed dive from up high pulling out, but it fluttered in straight and level flight, of course i was on full power with the 15x13 prop but that was on 4s and the speed less than what i get from a dive prop folded. So it must be Resonance.

Thanks for this info.. at least Now i know it might be curable, now only to find the right solution, ideas i am thinking about: Adding 2mm carbon sticks inside the correx wing skin from TE to LE. say every 20-30 flutes ?

Or Adding a 2 meter long, about double spar width (in my case say 2 or 3 carbon tube widths of correx about 4 cm aftwards from the spar on the inside of the top skin or bottom skin (correx strip flutes spanwise) (currenlty the 2mm flutes of the wing skin runs chordwise.)

Or Adding another spar, Not really want to add another expensive carbon tube in there and a 2m wooden spar will make the wing too heavy for my taste.

Would it help posting more detailed picture of the wing for you guys to try help find me a solution ?

Wow can't believe you summed up the problem ES, brilliant. One other thing i must add which might have been the cause for hitting that unstable resonance frequency, the wobble/flutter happened about exactly when the plane was at the very start of a waterpump building (it's a big building) right next to the dam, exactly when the plane was at the edge of the start of the roof.

I went insanely low over that roof, (unintended) (ok i'll admit, i misjudged the height i thought it was going to go over higher than it should've) so it was about a meter or so went at the exact start/edge of the roof (so it was in clear open air and all of a sudden it hit the air surrounding the roof, the roof is triangular it starts low and inclines)

So perhaps that had something to do with it ?
Last edited by Axefly; Jun 04, 2019 at 05:23 AM.
Jun 05, 2019, 05:41 AM
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Thread OP
Wing flutter especially on bigger wings seems to be a harder cure than simply control surface flutter:

I have also heard that the twisting effect that the prop has over the wings at high speed can induce flutter, and the solution they say is to bring the motor closer to the wing which i cannot do.


To better understand what flutter is, see the structure as a mass-spring system like a spring pendulum that has a damper attached to it. The aerodynamic forces both make the spring stiffer and dampen the movement. Crucially, they will act with a delay to the motion of the spring pendulum. Since we are dealing with oscillatory phenomena here, this delay causes a phase shift. The amount of damping increases with dynamic pressure and the phase shift angle decreases with flight speed.

Things start to get interesting when the phase shift angle approaches π. Now the damper will not work against the movement of the spring pendulum but will support it. Instead of reducing the amplitude of the movement, it will increase it - damping will become negative*. Flutter happens.

There are several ways to shift that point outside of the flight envelope:

Reduce the mass or inertia of the mass-spring system: Now the eigenfrequency of the system will be higher, so the phase shift will approach π at a higher speed. Moving the engines closer to the fuselage will increase the bending eigenfrequency of the wing, so any flutter that involves wing bending will be suppressed. A typical case would be the flutter resulting from the coupling of the fast-period mode with the wing bending mode in swept-back flying wings.

Make the spring stiffer: This again raises the eigenfrequency of the spring, so a stiffer structure will shift the flutter speed up. This is no panacea, however: When the Rockwell Commander 112 was developed, flight test revealed flutter of the horizontal tail. A professor from Kansas recommended stiffening of the tail spar and was so convinced that this would cure the flutter that he joined the next test flight, only to crash with the aircraft. As it turned out, the stiffening shifted the flutter speed up such that the forces involved broke the spar while flutter was non-destructive with the softer spar at a lower speed (like in this video).

Change the Eigenmode of the spring: If the motion of the bending wing will at the same time add a torsion, and that torsion will result in lower angle of attack and, consequently, lift on the up-moving part of the cycle and vice versa, the aerodynamic damping can be increased. By shifting the engines forward, the engine mass will introduce such a torsion moment into the wing. Of course, when placed further outboard, the engine mass will cause a larger torsion angle with the same torsion moment, so shifting the engines out towards the wingtips makes this more effective.

Shift the resonance speed below minimum flight speed: This might sound silly, but works sometimes: When flying fast enough, the phase shift can be small enough to turn aerodynamic damping back into the positive region, and the plane will not flutter with that particular flutter mode. But it might still flutter at a harmonic

The part as per the above whereas moving the motors to the wing tips can add to flutter is being strongly argued and very much in question, however i just have a single motor so that part is not relevant to my situation.

Now i was thinking adding extra servos for the ailerons, however now that i read that wing flutter differs from aileron flutter wondering if it might actually work ? Part of me wants to believe that the flutter actually starts from the ailerons then works it's way into the wing, however reading on the topic i'm not sure of that is the case ? So would adding extra servos actually be able to help cure the wing flutter. The wing pictured below.
Jun 05, 2019, 05:44 AM
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Thread OP
how about a carbon rod near to the trailing edge on the inside of the correx skin ?
(never mind the clear tube the rod will be taken out of it)
Jun 05, 2019, 08:36 AM
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Shouldn't hurt. Maybe shorten the servo horn a couple holes too. I would snip off most of the yellow tips. Pretty high aspect ratio for corro.
Jun 05, 2019, 01:02 PM
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Extreme Sports's Avatar
I'm no expert on flutter, so take what follows as well intended, but potentially BS advise:

What I would not do:
  • Add another spar: The CF spar is very stiff already, so I suspect that a torsional resonance is initiating the flutter. A second spar adds little torsional stiffness without other measures as well
  • Use 3mm correx - increases the weight, but won't change the stiffness much, so could even make the flutter happen at a lower speed
  • Add another servo. This could help, but see my caveat below
  • Do a power off dive to see if it was the prop that initiated the oscillations. Once the flutter starts you could lose control and in a dive speed and flutter will increase until something breaks or you hit something.

What I might try:
  • Cut a chunk off each wing tip. For the first test I'd just leave the spar sticking out - no need to cut up a CF spar until you are sure this works. This will reduce the bending moment on the wing root, which should change the resonance frequency
  • Increase the taper of the wing - same logic, but also changes the aerodynamics a bit
  • Glue thin CF strips diagonally across the wing - this will increase the torsional stiffness. Not sure if you can get a good enough bond, but even a single layer of CF tow might help. Needs to be a very inelastic material, so plastic, tape or GRP won't be enough.

Regarding the low pass initiating the flutter: Quite possible, but a red herring. If the wing is flutter prone at a certain speed, it is basically at an unstable edge. It will find any excuse to go over that edge. This time it was the roof, next time it was something else. It will happen again.

The caveat is that tinkering might just move the flutter speed slightly and you might not be as lucky next time and lose the plane (or worse hit someone very hard). So big changes like cutting big chunks off the wing should be less risky. That said, please do any further test flights somewhere where there is LOTS of space and NO people. You'd deservedly get taken to the cleaners on this if you hit someone.
Last edited by Extreme Sports; Jun 05, 2019 at 01:07 PM.
Jun 05, 2019, 01:43 PM
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Thread OP
Thank you both.

I'm confident but could it be 100% that the flutter happened due to the plane hitting the air 1m above the pump building, because that's when it started. Right when it hit the beginning of the roof area of the building.

i did do a dive before the full power level flight and pulled out, no flutter.

cutting the wing shorter does not sound like fun.. BUT the advice is good if all else fail i would probably have to try it.

I'm wondering if i should add the extra servo's, couldn't hurt i suppose if it all started with aileron flutter then it might cure the whole flutter ordeal.

i did not hear an aileron flutter first, but then again, these things happen really quick, the plane was around 120 meters away when it struck. (in the past i had aileron flutter on a shorter wing before looong ago so do know what that awful whirr sound sounds like)

BUT again the motor was on making huge noise with that prop 120m away, so who knows if ailerons started it...

Where i fly is quiet and safe when doing these experiments... so all good
don't have carbon tow, could glass fiber tape over the wing be used instead perhaps ?

I also read that helicopter blades contain lead in the leading edge to avoid flutter, thinking how about adding a little weight to the LE change the cg of the wing ?
Last edited by Axefly; Jun 05, 2019 at 02:00 PM.
Jun 06, 2019, 01:53 AM
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Extreme Sports's Avatar
Take a look at this old thread: https://www.rcgroups.com/forums/show...ional-rigidity

Glass fiber is not very stiff, but might be enough. I'd try some fiber tape on the top an bottom wing surface - probably insufficient but cheap and easy. The tape I have in mind is that clear form of duct tape that Builders Warehouse sells. Not sure exactly how to run the tape - search the composite forums - IIRC they often add CF tow at 45 degrees to the wing chord.

I suspect though that more taper and lower aspect ratio will be the most promising options.
Jun 06, 2019, 04:43 AM
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Thread OP
Thanks i will have a look, i do have duct tape, but is suppose you are reffering to other tape, i have seen the fiber tape its clear tape with glass fibre strips embedded in them, not sure if those are the ones you reffering to at builders, but like you say let me put a post up in the carbon/performance section here and see what they say thanks.

One guy in the construction section told me that adding winglets might cure the problem, so that could be the best bet at this stage,

Changing the aspect ration and taper, will basically be ruining the wing, and i am not at my workbench where i used to build the planes.
Jun 06, 2019, 11:02 AM
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Thread OP
Ok so the science guys tells me i need to add a wing extension, not a conventional winglet.
Something to stabilize the main wing stationary from the ailerons so it acts as a horizontal stab for the wing.

Does anyone have an idea how i could add say a 10cm extenstion to the tips without making it airfoiled, i would like to add about 10 cm at each tip of 3mm flat correx but how...?

here is his explanation with a link to the picture:

""Sorry it was not clearer! The tip should remain FLAT with the rest of the wing since its purpose here is not to deal with the vortex per se and open it to reduce induced drag .... but to act as a horizontal TAIL-stabilizer for the wing itself. Anyway all you need do is provisionally TAPE-on the HORIZONTAL flat-with-wing in front view wingtip extension shown as option A above and see if it ameliorates the flutter (make sure the extension is reasonably stiffly attached to the existing tip!).""

https://static.rcgroups.net/forums/a...r%20bye%21.JPG


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