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Old 01-02-2011, 11:41 PM   #1
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near fatal failure

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went out flying today and had a freaky moment with my 120 adrenaline.
wasn't sure i was getting enough cooling to the engine so i opened up the cowell a little bit just to make sure,(see pic 1)well i got her in the air ( second flight) and made a few rolls and such.
from the video you cans see the tail section blow apart in the air and almost riped striped one of the 2 elevator servos while doing the funky flapping dance in the air. right after you can see the canopy come off from the air pressure in the fuse.take in mind that the canopy is held on with 8 rare earth magnets,not a easy one to get off.because there is no fire wall in this air frame and that i opended up the cowell there was no release or the air flow going into the airplane and something had to give.i was lucky to get her down in piece.
you can see in the video the tail section fluttering bad.
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Last edited by PurgatorY; 01-02-2011 at 11:50 PM.
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Old 01-03-2011, 01:57 AM   #2
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Re: near fatal failure

Man, you did well to bring it in...nice going. Looks like she took quite a blast.
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Old 01-03-2011, 08:44 AM   #3
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Re: near fatal failure

Very interesting case here. Without seeing it in person I'm taking a guess, but I think your cowl work did not contribute to this failure. To help determine, it might be good if you could elaborate if this was the second flight of the day, or the aircrafts second flight total.

I'm asking to try and narrow down if this avenue may be the correct one, but here is what I think occurred:

Most likely is that the tail servo failed first, removing direct and tight control of the tail surface on ONE side only. The resultant extreme flutter caused the fuselage to torque beyond structural limits and partially come apart. This is what ejected the canopy.

The real difficulty is in determining if the tail surface was fluttering before the failure and, if so, why. I believe it was and this is what took out the servo geartrain.

Flutter is a notoriously damaging occurrence and frequently causes partial or total structural failure in real aircraft. Lots of different causes and the most common are loose control linkages or anything else that can allow the surface to move freely. You can also get into it if the control surface is improperly designed or balanced, (even painting can contribute).

In your case I suspect the control surface was free to move slightly and/or improperly designed or balanced, (either or both mass and aerodynamically). This is what starts the oscillation and a loose linkage system or too flexible tail or fuse structure allows it to move enough. Hit it with the right frequency and instead of dampening it out the oscillation very quickly builds to destruction. The fact it occurred on one side only may eliminate the likelyhood there may have been a design problem, but something was different there. It could have been the forementioned items and also prop wash and aircraft speed.

This is why without knowing the general condition of the aircraft, control system, flying conditions and aircraft speed I am taking a guess based on the results of flutter analysis on real aircraft.

In the spring of this year I had helped in the first run up of a friends Cap with an OS FS90. The laser cut ARF was perhaps rigid enough for normal flight loads and an electric power system, but I was frankly horrified by the amount of flex in the motor mount box and structure. 4-strokes generate quite pronounced power pulses and I believe unless this was reinforced at least back to the wing mount area, this was a recipe for disaster.

On a final note you mentioned this aircraft did not have a firewall. Unless your 4 stroke is one heck of a lot cleaner than any of mine, you may have gotten enough oil into the structure to weaken it. Another reason I ask the airframe history.
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Old 01-03-2011, 09:07 AM   #4
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Smile Re: near fatal failure

I think Cougar is right' from the vid. you can see the canopy was edjected after the flutter started. Like he mentioned flutter is very distructive & you are very lucky to get your plane back in basicly one piece. When reassembling make sure all linkages have absoutly no slop & your servos are up to the task.
Good luck on your rebuild, & upcoming flights.
Charles L
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Old 01-03-2011, 10:51 AM   #5
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Re: near fatal failure

the aircraft was new with 2 flights total on it. i flew it the day before with no issues.having a concern about engine cooling i opended up the cowell that evening.the video is the second flight in total for the airplane.
one of the reasons i suspect it was air pressure is that the canopy used 8 rare earth magnets with 4 have to pry the sides wide to remove it.its quite a effort.there may have been to failures happening here.
make note that the fuse is fiberglass.

the elevator servos are located in the elevator itself.its possible that one failed and the air pressure inside the fuse blew the canopy any case i will be watching for such issues in future just happy i got my saito 150 down in one piece.
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