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Author Topic: Something else to check on the T-Rex  (Read 1984 times)

Offline Steve Fitton

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Something else to check on the T-Rex
« on: March 24, 2010, 06:47:39 PM »
This past Sunday, two ARC T Rexs in our club suffered broken wings.  Neither crashed and both are being repaired at this time.  Both models used glass cloth on the fuselage and ultracote on the wings.
Jimmy Welch noticed it first on his PA powered T Rex the day before we went flying.  His outboard wing was cracked on the upper surface, about 2" out from the fuselage, from the training edge sheeting all the way forward through the top spar and the leading edge.  The cracked area was as straight as a ruler, and after detecting it Jimmy made a patch out of carbon veil.
After two flights post repair, Jimmy noticed that the wing had cracked again, about a half inch beyond the repaired area.  Around this time Willis noticed that his plane had a crack in the inboard wing, in this case right where the center section ended.  John Tate managed to get a great close up of the inside of Willis's wing showing the broken spar, which looks like some light a grain wood.  Both Willis and Welch's planes had about 15 flights each on them.
  Of course, Welch's orange ARF T Rex continues to be indestructable, after 91 flights it has no fuselage or wing issues, which hints that once again communist wood selection continues to sometimes undo even the most careful engineering HB~>
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Steve

Offline Wynn Robins

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Re: Something else to check on the T-Rex
« Reply #1 on: March 24, 2010, 07:57:36 PM »
so the magic T-rex is not so flash after all - I have not put my ARC together yet and looks like there are more issues to sort out before gluing it up....
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Online Brett Buck

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Re: Something else to check on the T-Rex
« Reply #2 on: March 24, 2010, 08:53:16 PM »
Quote from: Steve Fitton on March 24, 2010, 06:47:39 PM
After two flights post repair, Jimmy noticed that the wing had cracked again, about a half inch beyond the repaired area.  Around this time Willis noticed that his plane had a crack in the inboard wing, in this case right where the center section ended.  John Tate managed to get a great close up of the inside of Willis's wing showing the broken spar, which looks like some light a grain wood.  Both Willis and Welch's planes had about 15 flights each on them.

   That's a weird-looking break - maybe wind-checked?

      Brett
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Willis Swindell

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Re: Something else to check on the T-Rex
« Reply #3 on: March 24, 2010, 09:02:15 PM »
This is an easy repair a  cut down Popsicle stick on the side of the spar through the rib. doing  top and bottom spar. I am also doing the out board wing for safety
« Last Edit: March 24, 2010, 09:50:28 PM by Willis Swindell »
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Willis Swindell

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Re: Something else to check on the T-Rex
« Reply #4 on: March 24, 2010, 09:23:33 PM »
I was thinking about what Brett said. The line slider came loose I don’t know if I broke it adjusting it or it just popped loose. that is the probable cause for my break.
Willis
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Offline Steve Fitton

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Re: Something else to check on the T-Rex
« Reply #5 on: March 24, 2010, 09:52:28 PM »
This picture of Willis's plane in a maneuver shows the point of maximum stress on the T Rex wing.  While its not uncommon to see a film covering showing ripples during a maneuver, the point you see the big ripples corresponds to the point where the spars have been breaking.

Its also very possible that taking an ARC wing and covering it in silkspan may eliminate the problem completely.  I just don't think anybody has done that yet.
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Offline Wynn Robins

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Re: Something else to check on the T-Rex
« Reply #6 on: March 25, 2010, 01:06:42 PM »
Quote from: Steve Fitton on March 24, 2010, 09:52:28 PM


Its also very possible that taking an ARC wing and covering it in silkspan may eliminate the problem completely.  I just don't think anybody has done that yet.

I am about to do this........but I think I might strengthen the spar anyway - just to be sure
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Offline Bradley Walker

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Re: Something else to check on the T-Rex
« Reply #7 on: March 25, 2010, 01:14:43 PM »
Quote from: Wynn Robins on March 24, 2010, 07:57:36 PM
so the magic T-rex is not so flash after all

That is an odd comment...

What the manufacturer chooses to use as wood is one of the difficult issues facing any ARF producer.  Considering this is the first run, and the weights came out ridiculously light...  I am not sure I would bitch.  It is much easier to add than to take away.  The best myself and Brodak can due is make note of the improvements needed, inform customers, and improve the next run.  None of the issues I have seen are design related, they are all wood selection related.

Also, it is not known if there were not some other extenuating circumstances that caused the spar to break.  I wonder waht would have happened if everyone who bought every other ARF had posted every field failure on the Internet.  I ma not sure anyone would have bought an ARF.

If I wanted a T-Rex (or any other Asian made ARF) to be in the league of a full blown stunter, I would check everything I could.  Glue joints especially. That is why i made the design in a manner that forces the user to glue in their won bellcranks. I made a list of additions I would make to the model on the other thread, I think it is a pretty short list, most are just to make sure everything comes out nice and straight and will last a long time, not really for performance.  This spar issue would have been covered with the addition of shear webs that I mentioned.

I think the T-Rex is doing OK.  I think it will do BETTER than OK when some hopped up versions get in the hands of great pilots.
« Last Edit: March 25, 2010, 01:35:12 PM by Bradley Walker »
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Offline Wynn Robins

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Re: Something else to check on the T-Rex
« Reply #8 on: March 25, 2010, 01:55:58 PM »
Brad,

I am not hasseling the design at all - I bought one as I believe your designs show great promise - I guess buying the "first run" of any ARF will always show up problems in the manufacturing, which is evident in this case.....and those posted   

Balsa selection in the ARC I have is dodgey at best - this is not something that can be controlled by Brodak etc, given that they are not overseeing every ARF/ARC being cut and assembled - it is just a kick in pants when you spend $400 on one, expecting to be able to put it together and go fly.  ($400NZ dollars)

I have made the mods you suggested to my one, and will be assembling it in the next couple of weeks - covering is SLC and silkspan - like your original so I am sure it will be well capable.

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Offline Robert Organ

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Re: Something else to check on the T-Rex
« Reply #9 on: April 05, 2010, 09:38:54 AM »
Since I am just starting to get back into CL after many years in RC, I cant speak with a lot of exper. about CL ARFs, I do however have quite a bit of exper. with RC ARFs of all sizes and I have yet to see one that didnt need at least a few changes, mods, repairs, etc. done when assembling. I know we tried to keep CL airplanes as light as we could back in the Fifties and Sixties but with todays competition and designs, acft weight has really come to the forefront in importance, that being said, when you consider the airframe stress from CL(or RC) aerobatics,there has to be a balance between weight and strength so some beefing up of stress points, glue joints, loose covering etc. seems to be the norm rather than exception, if we dont understand the "whys" of this and complain too much, the ARFs wont sell and the manufact. wont make them and the only ARFs left will be trainer types. Just my thoughts on this. BOB
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Offline Bradley Walker

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Re: Something else to check on the T-Rex
« Reply #10 on: April 05, 2010, 09:45:55 AM »
Quote from: Robert Organ on April 05, 2010, 09:38:54 AM
RC ARFs of all sizes and I have yet to see one that didnt need at least a few changes, mods, repairs, etc. done when assembling.

That is what I heard from my RC friends also.  The better builders and fliers go through each model with a fine tooth comb just to be sure.  It still is a significant time saver over a kit build.

I think that there are some people that are going to have failures no matter what or how they build.  The guys that know what to look for will not.  There is a learning curve to this, same as building your own.
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"The reasonable man adapts himself to his environment. The unreasonable man adapts his environment to himself, therefore all progress is made by unreasonable men."
-George Bernard Shaw

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