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Dec 10, 2005, 02:13 PM
Team Cobra!!! Ssssssss!
lmopar69's Avatar

The 7' twin is dead.. flutter, flutter, flutter --- BANG! RIP


Well, maiden flight went great, until that fatal moment! Wish I woulda had video! Nice smooth takeoff, couple laps to get it trimmed perfect. Man it flew smooth with lots of control authority. Decent power with the S400 DD with 5x5 props. Top speed maybe 30ish, not really fast.

Then the sequence of events that followed, well, it looked cool at least... did an aileron roll, pretty quick roll too for nearly 7' wing span. Decided to try a roll followed by a loop... ugly, ugly, ugly. Did the roll and into a dive, pulled back and FLUTTER, FLUTTER, FLUTTER... BANG! Ailerons and rudder fluttering like mad all the way down from about 150'. Tried everything to save it in that 5 seconds too. Oh well, back to the drawing board.

Too bad, it was a neat plane. Wonder whats next.
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Dec 10, 2005, 02:41 PM
Wow, that sucks!

But, you can't stop there, you have to re-build it and get her flying right!
Dec 10, 2005, 03:13 PM
Balsa is for doll houses
Skrogg's Avatar
HAHAHHAHAHAHAHHAHA
wait, um sorry for the loss!
NO VIDEO! NO VIDEO?
Dec 10, 2005, 03:17 PM
An itch?. Scratch build.
eflightray's Avatar
Great concept, nice build, good looking model.

The flaw is one that many people fail on, myself included, not designing to withstand the intended flight envelope. I assume you intended to do aerobatics.

If we intend to do aerobatics, we need to consider the stresses involved not just pulling 'g', but speed as well. I have seen plenty of light weight models, gliders especially, go into a self destruct when diving. Flutter is a very difficult thing to design out for light weights.(edit - and big spans).

I know that if I try to bring my electric glider down too fast the wings will start to flutter, I am pushing it outside it's design envelope.
I now like to scratch build semi scale models, and they are build to withstand flying, but only gentle non-aerobatic flying.

Flutter is a nasty one that can take you by surprise, hope you can rebuild the model, it doesn't look too bad from the pictures. Just remember that next time it flys, no aerobatics.
Dec 10, 2005, 04:32 PM
Ascended Master
Sparky Paul's Avatar
6 reasons:
1) elevator chord WAY too much!
2) elevator pushrod insufficiently stiff
3) booms insufficiently stiff
4) Aileron chord WAY too much
5) Aileron pushrods not stiff enough
6) Wing way too flexible due to 4)
Dec 10, 2005, 05:39 PM
Team Cobra!!! Ssssssss!
lmopar69's Avatar
Quote:
Originally Posted by Skrogg
HAHAHHAHAHAHAHHAHA
wait, um sorry for the loss!
NO VIDEO! NO VIDEO?

Thanks for the words of encouragement there Skrogg! You shoulda seen it, it was cool! And this after having to wait 3 days to fly it because I had to order my props and adapters. The suspense KILLED ME!!

Yeah, I know the ailerons and elevator were too big... but they were SUPPOSED to be too big, the idea was supposed to be over kill on control authority. It worked, cuz man it was quick and sensitive, but they ailerons/elevator just werent stiff enough. I knew it would be a problem, and figured it would flutter, but didnt think it would be THAT bad! The intent was to adjust as necessary until I got it all to work right. Guess that wont be happening!

I would rebuild it, but, there just is too much damage to it to really worry about it. So, I will pull all the electronics, take all my carbon fiber out and build something else.

Any ideas on what I should do with either one or two DD S400s? Im thinkin fan fold pusher jet type thingy.
Dec 10, 2005, 06:26 PM
Registered User
Sorry to see that you had to find out the hard way but can't understand you saying that you knew you'd get flutter. Once it starts it does not normally go away as plenty of full size pilots have found out. If you are really lucky and can slow the aircraft the moment it starts - sometimes hard to recognise - you might get out of the speed range where it occurs but it's unlikely. Frankly if I thought flutter could be a problem on a specific model before take off the flight would not take place. The reason for that being that once flutter on any surface takes place but it seems to usually be ailerons on the aircraft I've seen affected you are going to lose control and hence have a potentially dangerous situation.
Dec 10, 2005, 06:29 PM
Team Cobra!!! Ssssssss!
lmopar69's Avatar
lol Yeah, common sense would say to fix it before its a problem huh. Never claimed to have much of that stuff!

I have had other planes start fluttering and was able to slow it down and recover, this thing, nope, it was gone. Besides, even after the first 3 min of flight, I knew I wouldnt be keeping it as it was long. Didnt do what I was hoping it would. So, a crash was just a nice way to put it out of its misery.
Dec 10, 2005, 10:50 PM
I broke my kerpellor.
Artemetra's Avatar
Cool, really neat one. What next? It looks way worth rebuilding and beefing it up! Maybe instead of carbon booms, some kind of ladder frame booms or something.

Really nice, thanks for the posts and stuff. I hope you do it some more. It's great to have a unique plane. No one else has one of those, you can be sure!
Dec 10, 2005, 10:59 PM
Resistered Loser
bicmedic's Avatar
Why give up?? That is a REALLY cool design, just needs reinforcement.
Dec 10, 2005, 11:00 PM
Team Cobra!!! Ssssssss!
lmopar69's Avatar
Yeah, for sure no one else has one like that! It wasnt really worth fixin. Too much damage. So, its been gutted now, and Im about halfway through my next "project". This one is a pusher jet lookin thing. About 32" span. Hope it flies better than that big twin did, well, it flew fine, just not quite "stiff" enough in the control surfaces.

Pics will follow in new thread sometime soon.

Laien
Dec 11, 2005, 08:12 AM
Baptised Heathen
batman's Avatar
I tgink Sparky Paul is right. Too much elevator and aiteron chord. Just reduce these in size and you should correct most of your problems.


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