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Was this overspeed or mech failure?

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Old 08-08-2010, 09:05 PM
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starfire73
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Default Was this overspeed or mech failure?

I recently crashed my second U-Cando .46 on it's third flight, and there is some debate between myself and another pilot at the field. I am trying to determine if the cause was CA hinge failure or pilot error.

CONDITIONS:
I had just pulled out of a low throttle dive. It was not a straight down nose dive, and I am not sure where the throttle was, but I know it wasn't full throttle. I had pulled out of the dive, leveled off for about 10 yards, and was intending to slow down in order to set up for a hover. At this point I heard a flutter sound and saw something come off of the aircraft. The aircraft banked right, and as I was unable to controll it, it descended about 30 feet and crashed into the ground. I saw in the wreckage that the left elevator (two piece elevator) had ripped off of the stab. There were three CA hinges at that joint, and they had no fiber on them, just soft, smooth, plastic.

A guy standing next to me said he "saw the wings flutter" prior to the malfunction, but I did not see that. I know a 3D plane isn't made for extreme nose diving, and I have flown dives at this moderate speed dozens and dozens of times on my first U-Cando. So I am not sure what the cause was... I am trying to nail that down. Maybe I was diving too fast? Or maybe CA hinges really suck sometimes?

HAS ANYONE HAD CA HINGES FAIL LIKE THIS WITHOUT BEING IN AN OVERSPEED CONDITION?

INFO:
U-Cando .46
Magum 70 4-stroke
14x4w APC prop
Stock pushrods, control horns, and other linkages
Stock CA hinges (with plenty of CA, and the center hing hole drilled as instructed)

Thanks!
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Old 08-08-2010, 10:02 PM
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ChuckW
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Default RE: Was this overspeed or mech failure?

Sorry about losing your airplane. It is a rotten feeling.

If the U-Can-Do has a downfall, it is flutter. Mine did it and I have seen a lot of others do it. You can literally see the wing vibrate sometimes.

It is a fun airplane but it needs you have to carefully manage speed.

Your elevator could have fluttered too. Were the broken hinges the cause or the effect of the accident... that can sometimes be tough do determine after a crash.

In my experience, I've had CA hinges break but never pull out unless I've put the plane into the ground.
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Old 08-09-2010, 04:33 AM
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Default RE: Was this overspeed or mech failure?

The first catostrophic airframe failure I ever saw was a U can Do 60.....heard a noise and the air was full of confetti......same same as you, some minor aerobatics, pulled out of a dive and failure......
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Old 08-09-2010, 07:02 AM
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JohnBuckner
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Default RE: Was this overspeed or mech failure?

Absolutely flutter and more common than one might think. What it is not is a simple hinge failure.

It is also more common with some designs as already noted. It can be eliminated simply with Mass balancing if one survives an episode. But most just will not do it even flying airplanes with a known history of flutter.

John
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Old 08-09-2010, 07:10 AM
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Default RE: Was this overspeed or mech failure?

Usually when CA hinges are stripped clean like that its because someone added extra glue after the initial dose had dried.
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Old 08-09-2010, 08:05 AM
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Default RE: Was this overspeed or mech failure?

Hi!
Don't think so! Have done so (added thin Ca glue) for 35 years on many models and no failure!
on the contrary! To little Ca glue! !!!
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Old 08-09-2010, 08:12 AM
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Default RE: Was this overspeed or mech failure?


ORIGINAL: starfire73

There were three CA hinges at that joint, and they had no fiber on them, just soft, smooth, plastic.
I'm certainly not an expert here, so maybe this should be considered a question from me more than an answer, but I've done a number of CA hinges over the past couple years and have not had a lick of problems. Then again, I've not seen ordinary CA hinges that were just soft smooth plastic that didn't get some kind of pin in them. The only CA hinges I've used have been a fiber type hinge. I assume that's for the CA to wick into the hinge. If the hinge was actually plastic and didn't have a couple pins, then maybe that's the reason the elevator made an untimely departure?
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Old 08-09-2010, 08:21 AM
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Default RE: Was this overspeed or mech failure?

To clarify the hinge questions:

- The hinges are the standard fiber mated to plastic that Great Planes includes in thier kits. I always thought they were fiber throughout, but I recently learned (the hard way) they are a fiber coated peice of plastic.

- Two hinges were still attached to the left elevator (one attached to the stab), but they were stripped clean of the fiber.

- I may be guilty of using too much CA, but definately not too little. I usually put on about 12 drops per side instead of the recommended 6.

- I DO NOT go back and add more CA to the hinge. It's all done at one time.

...By the way, what is Mass Balancing?
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Old 08-09-2010, 09:09 AM
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MinnFlyer
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Default RE: Was this overspeed or mech failure?

Too much can be as harmful as too little, 5-6 drops per side is plenty!

As for the fiber pulling off the plastic, the only times I have ever seen that happen is after the hinge has been subjected to a traumatic situation such as flutter or a crash
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Old 08-09-2010, 09:13 AM
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Default RE: Was this overspeed or mech failure?

You said 12 drops per side. Clarify that a little as it could be part of the problem in addition to flutter. Do you mean 12 drops on each the top and bottom, or 12 drops in the wing then install the control surface and an additional 12 drops. If you are doing the latter then you will have a failure because the hinge must be in both surfaces prior to adding the CA so you glue the entire assembly at one time.
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Old 08-09-2010, 12:45 PM
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Default RE: Was this overspeed or mech failure?

Mass balancing is the addition of weight forward of the hinge line, to offset part or all of the weight of the control surface. Most often used on ailerons, to a lesser degree elevator, and rarely on rudder, flutter can also be helped by having a good gap seal, and using more hinges than normal, also adding structural stiffness to the flight surfaces can help the situation. If you look at a three view or photo of a P51 mustang you will see mass Balances on the control surfaces (even the rudder). The balances on the ailerons are hidden inside the wing. This is why many WWII aircraft had fabric covered control surfaces (the lighter the control surface the less weight it takes to balance it).

Richard
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Old 08-09-2010, 02:15 PM
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JohnBuckner
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Default RE: Was this overspeed or mech failure?

ORIGINAL: starfir

...By the way, what is Mass Balancing?


This is a flutter induced failure, it is not a simple hinge failure. This would have happened even if all hinges were perfect. When an airplane demonstrates known severe tendencys toward flutter, mass balance is the only simple fix and it will work every time. No matter what.

As Tweety correctly noted mass balancing is simply is extending fixed weight forward of the hingeline to partially balance that surfaces weight behind the hingeline. It does not have to be anywhere near the weight behind and it does not have to be anywhere near 100% balanced.

As little a a half ounce of lead imbeded on those aerodynamic balances of those elevators for that airplane would have prevented this elevator flutter. There are other ways of doing simple mass balances on different kinds of surfaces too and I have posted pics many times but mostly ignored. It mostly ends up in hinge discussions that really are not the culprit.


John
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Old 08-09-2010, 02:56 PM
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jaka
 
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Default RE: Was this overspeed or mech failure?

Hi!
It sure is flutter!!
Flutter makes a wing explode! Seen it many times over the years!
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Old 08-09-2010, 07:04 PM
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starfire73
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Default RE: Was this overspeed or mech failure?

Ok... so why did the elevator flutter? Is it just a quirk of the design, or is it from overspeed. I was wondering also if maybe the servo went goofy on me as it tried to center. I have had some problems with my servos sitting still when at center and there is no input.

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Old 08-09-2010, 07:52 PM
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ChuckW
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Default RE: Was this overspeed or mech failure?

Those big surfaces on the U-Can-Do are just prone to flutter even at what you might think are moderate speeds. People try all sorts of heavy duty pushrods, overkill servos, sealing the gaps, etc. Most of that has little to do with the flutter and it is wasted effort. The counterbalancing that John Buckner mentioned looks like it might be a good solution. I have never tried it personally but I might if I ever own another airplane with flutter issues.
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Old 08-09-2010, 08:53 PM
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JohnBuckner
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Default RE: Was this overspeed or mech failure?

I am not any kind of engineer and have only learned from the school of hard knocks models and full scale. Flutter analysis is a full engineering discipline in itself so I am not about to try and explain it.

I do know though that with our model airplanes at the speeds we normally fly at (a rather narrow range of speed) If flutter rears its ugly head we can delay the onset of flutter to an airspeed that is higher that that to which our airplanes will be capable of reaching just by mass balancing.

It definately works. The trouble with flutter is other things are often blamed and we go off to extremes fixing things that truly will not help the problem. A classic example occured to one of our local gentlemen not to long ago. He had a simple hundred dollar 3D arf of some sort
and one day he landed and one of the aileron servos was striped and the servo screws were loose (Hmm major red flag). Someone told him to get a fancy servo so he did, first flight same result - striped/ loose. That is flutter (in this case was not audable in flight) so I suggested he stick in one of my little balances. Naw not about to, instead bought even more big bucks servo and super linkage. Result the servo fell clear out of the wing this time. This ended up with his total investment in fancy servos at around 300 bucks for just one servo in one wing with no end in sight.

Finally he relented and let me poke a hole in the ailerons and stick in one of my simple balances and guess what: never fluttered agine and the airplane still going strong with another owner and no attention paid to limiting the speed . The picture of the clear blue wing shows the balances on this airplane.

This is just one of many examples that all have been completely successful. I have been doing this since the days of our Fun fly aeroshaft airplanes with the ridiculous surfaces. combined with those wet noodle fuselage shafts you could never open the throttles. My more extreme (closer to 100% balance) balances on those ships once agine was like magic. I just have used them only on airplanes that have demonstrated this flutter tendency and on any of the three axis's that needed it ever since.

John
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Old 08-09-2010, 09:48 PM
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Default RE: Was this overspeed or mech failure?

Hey John - Thanks for the reply, and the pictures. I would not have been able to properly visualize what you were talking about without the pics. How do you make your balancers? And how do you determine how much they should weigh?

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Old 08-09-2010, 10:38 PM
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Default RE: Was this overspeed or mech failure?

There are numerous ways to do the job with many different types of control surfaces. For example on a U-cando with the aerodynamic balances at the tips of both the elevator and rudder that protrude forward of the hinge line its easy to just make a hole in that and imbed some lead and drip a little thin CA.

On that airplane with the flat aileron tips at the outboard ends you could have a wire extrending forward at the very end with a bit of fishing weight on the end. Another options is the simple L shaped weights I use anywhere and just have them extend and inch or inch and a half forward of the hinge line. They also can be used on rudder if required and do not half to be on both sides.

As far as how much weight and how much in front of the hingeline, All I can say is I don,t know. It does not seem to be critical, remember The weights do not even remotely need to fully balance the surface and we are only trying to give the onset just a little boost to a higher speed. Those L shaped wires which are 4/40 rods with the threads on the short leg for nuts, I will bend them so they can get a little thumb sized cone of fishing sinker about a inch and a half forward of the hingeline. that fishing sinker (the kind with a hole in its is just crimped onto the wire with a hammer and then a little thin CA is dripped in.

John
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Old 08-10-2010, 06:38 AM
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Default RE: Was this overspeed or mech failure?

If you really want to get rid of the problem then go with ball links, pro links, and aluminum servo arms to remove all the slop from the system, also run powerful servos, something like 8411s in a little airplane like this.

3D airplanes have huge surfaces and the best way to keep flutter mode from starting in the first place is to have the right equipment and proper setup. Using too much CA will cause cracking in the hinge. I have not used CA hinges in a very long time but when I did, I only used about 3 drops per side, you want the hinge secured in, but you also want the hing to flex freely and when the CA has a heavy buildup, it starts to crack from day one of flying, so don't get crazy with the glue.

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Old 08-14-2010, 11:23 AM
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Default RE: Was this overspeed or mech failure?

The U Can Do is known for flutter and other damages in flight. A guy at our club yanked up on his UCD 60 while coming in to land the whole rear section ripped off. The wreckage showed that hot glue was poorly used to build the model. The Funtana 40 had a bad reputation for wings ripping off when flown at moderately high speeds.

Flutter can be induced by uneven air coming around and over control surfaces. Sealing the gap goes a long way in flutter prevention. Also, making sure the surfaces are even along their span helps. A crookedly installed aileron is asking for problems. Then, make sure the linkages are tight and slop free.

I got a Fantasy 40 from a buddy and it fluttered twice. The buddy had lost his first one to elevator flutter and built the one I had. Later, on RCU, the owner of Skyshark wrote about buying the rights to the Fantasy, and sort of became a folk hero for the pictures of Angie holding up the model on his website. He also mentioned a redesign of the rear formers, etc to prevent chronic elevator flutter problems in the design.
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