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HelicopterGasser Model RC HelicoptersOther › Cracked Predator frame - 0 crashes
10-07-2005 07:02 AM  12 years agoPost 1
PullPitch

rrApprentice

Bend, OR

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Crap!

13 hrs on my new SE, no 3D, just AP. I've never even dinged a blade or been inverted. I was cleaning 'er up a bit a noticed a crack in the left lower side frame. Its a 3/4" vertical crack at the hole for the 4x10 socket screw that bolts through to the z231. This bolt often had an oil ring around it which always puzzeled me I was wiping it again when I noticed the crack.

If it weren't for the routine mystery oil appearing around the bolt/washer, I would just blame it on my usual overtighting

I have a spare but something doesn't seem right.......

It seems vibration free, has always performed flawlessly and takes great pictures!

???????????

PullPitch
www.skybercam.com

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10-07-2005 10:37 AM  12 years agoPost 2
Al Magaloff

rrMaster

12,199 Posts- Enough​Time Wasted. See Ya!

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No offense, but did you ever wonder why aluminum frames are not used on modern helicopter designs? Of course you'll get the cheerleaders who will tell you that they never had a problem, but some of us can trace the cracked aluminum frames , back to the 1980's. Especially in the motor mount area.

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10-07-2005 01:10 PM  12 years agoPost 3
SteveH

rrProfessor

Texas

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Of course you'll get the cheerleaders who will tell you that they never had a problem,
And....of course you will get the Century bashers also.

It seems to me the "mystery oil" was trying to tell you there was a problem. I suspect the bolt was not tight, certainly there was some movement there, and I'd also recheck the fan alignment, but that's just me.

The government cannot give you anything without first taking it from someone else.

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10-07-2005 01:15 PM  12 years agoPost 4
nicco

rrVeteran

Sweden

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What are your needles settings. Mu muffler come loos (due to vibrations) when whent to leen on the low needle. I open it up around 1/8 and now it's stays on.

Only reason for cracking is vibration.

/N

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10-07-2005 03:19 PM  12 years agoPost 5
rckrzy1

rrElite Veteran

Hurst Texas

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Only reason for cracking is vibration.
Not true.
I cracked the left and right and lower pieces on my pmax due to flexing and semi hard auto's.



Wildcat Fuels

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10-07-2005 03:23 PM  12 years agoPost 6
nicco

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Sweden

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I guess a hard auto can be catigorazed as one hard vibration :-)

/N

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10-07-2005 03:56 PM  12 years agoPost 7
steve9534

rrKey Veteran

yakima, wa.

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Aluminum frames

I'd agree with Al that the aluminum frames are almost gauranteed to eventually crack, given enough time and vibration. Some hard 3D and a few less than perfect landings can help accelerate the process. I'm putting up with it because I love the way the Pmax flies and the replacement parts aren't terribly expensive, but this is pretty much predictable given the materials used. steve.

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10-07-2005 04:19 PM  12 years agoPost 8
avator

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New Jersey

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IMHO

Even if you ship appears to be smooth as glass, there is still enough vibration and flexing going on to crack aluminum frames. It's just a matter of time.

Smoother = longer

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10-07-2005 05:05 PM  12 years agoPost 9
PullPitch

rrApprentice

Bend, OR

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Guess what I found...

You know the four little M3x6 button head screws that hold the clutch shoe to the fan? One of mine is missing its head! It failed and bailed. Its threaded shaft is still stuck in place. Could the weight of one button head cause these problems?

Now I can't get my clutch shoe off, which means I cant get my fan off

I tend to OVER TIGHTEN things. I'll bet the button head clutch screw was too tight, failed and then caused high frequency vibrations.

Also, the oily motor mount bolt hole actually penetrates into the engine cavity, I didn't realize that at first.

PullPitch
www.skybercam.com

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10-07-2005 05:22 PM  12 years agoPost 10
Al Magaloff

rrMaster

12,199 Posts- Enough​Time Wasted. See Ya!

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And....of course you will get the Century bashers also.
Only if you're overly sensitive Steve. But reality is reality.
Even if you ship appears to be smooth as glass
Very true. Hold that motor in your hands at nine grand, and tell me it's smooth. The Pre-Dater is a decent machine, with carbon frames. I would never put together an aluminum framed machine.

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10-07-2005 07:23 PM  12 years agoPost 11
steve9534

rrKey Veteran

yakima, wa.

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Al

Pre-Dater or no, I love the way it flies. It took me a while to get it sorted out, but I've had a couple of Freyas, an R90, and an Extreme, and i like the way the Predator flies the best. It would have been nice if they had seen fit to do something different with the frames. The advantage of the current iteration is that the aluminum acts as a heat sink to help cool the motor and the frames are very rigid. The down side is that the Al cracks, and in a crash, gets all bent up. I talked with a Century rep this weekend who said they had something different in the works. i have no knowledge of what it might look like. steve.

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10-07-2005 07:36 PM  12 years agoPost 12
Heli-Driver

rrElite Veteran

Arlington, TX

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I suppose if you don't build one right or have the motor tuned improperly the metal will eventually fatigue and crack but NORMALLY it's not a problem.

I've seen plenty of metal frame birds fly countless hours without problems, saying you'd NEVER build a ship with aluminum frames is verging on paranoia IMHO. Build it right and fly it like ya stole it!



Raymond

Predator Gasser SE/231 X 2
Century Helicopters

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10-07-2005 08:08 PM  12 years agoPost 13
steve9534

rrKey Veteran

yakima, wa.

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Raymond

Al. frames crack. 15,000 cycles per second vibration gaurantees it will crack. Rolled corners at the bottom where the struts mount ensure that it will happen even sooner. I've been building helis for 17 years and don't have a clue how to put them together. I've been tuning two stroke model engines for over 40 years. Al's probably even older and more clueless than I am . Believe what you want. Happy flying. steve.

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10-07-2005 08:30 PM  12 years agoPost 14
JKos

rrProfessor

Redondo Beach, CA

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> Also, the oily motor mount bolt hole actually penetrates into the
> engine cavity, I didn't realize that at first.

I don't think that is normal!!! Some mounting systems don't use those holes at all.

- John

RR rules!

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10-09-2005 05:06 PM  12 years agoPost 15
AirWolfRC

rrProfessor

42½ N, 83½ W

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A quick comment about aluminum and cracks.

Yes, I know about Century aluminum frames. Been there, done that.

Given enough repetative stress (like vibration), most anything will crack. Aluminum will crack easier than most materials.

Keep the stress below about 1/3 of the tensile strength of the part and it will last a long, long time. You can do that by reducing the stresses or making the part stronger.

Single cylinder two stroke motors will vibrate no matter what you do, period. Even if they are "perfectly" ballanced, that only means that the vibration has been minimized. It can never be eliminated.

Wolfgang

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10-10-2005 04:21 AM  12 years agoPost 16
Pre-Mix

rrApprentice

U.S.A.

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check the motor mount for parallelism

I just recently got through assembling my Predator and what I found is this:
when I was in the process of aligning the starter shaft/clutch I would tighten the M4 screw which attaches the frame to the motor. Once the M4 screw was tight the starter shaft would bind. What I found was the surfaces on the motor to which the frame attaches are not parallel, they are actually angled. So when you tighten the M4 screw it flexes the frame so bad that it causes binding at the clutch region.
Check to make sure there is no stress imparted on the frame section when tightening the M4 screw. I had to make some wedge shaped washers in order to tighten the frame section without flexiing it. This will definitely cause premature failure in the form of cracks.

Johan

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10-17-2005 07:46 AM  12 years agoPost 17
PullPitch

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Bend, OR

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Thanks Pre-Mix - I'll give that a close look.

The CF frame upgrade is sounding more tempting too. Odd though how aluminum is used through out other forms of aviation, but it gives us trouble.

I had a spare SE kit so I robbed the parts and I am back in action.

PullPitch
www.skybercam.com

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10-17-2005 08:18 AM  12 years agoPost 18
FCM

rrElite Veteran

Surrey, England

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Whilst I agree that aluminium alloy is prone to cracking and by the way, full size aviation has been dealing with this fact since it was first introduced, whether it will crack or not will largely be down to the alloy's make up i.e. is it aluminium zinc, aluminium copper, heat treated to T3, T6 hardness etc. Alu/copper has good fatigue properties and is used in areas of a plane where stress reversal is prevelent i.e. lower wing skins on an airliner. Alu/zinc has huge compressive strength that can match some steels of a given thickness but will crack during repeated stress reversals so this is normally used on the top surfaces of an airliner's wing as it will normally be in compression.

The Benzine uses an alloy engine mounting plate that has loads of holes cut in it, the engine is overhanging and can be easily flexed up and down and to me, looks like it should crack especially around the waisted areas such as the carbi cut-out, but it doesn't. This is because of the type of alloy that Vario have used, although not very strong/stiff, has good fatigue properties which will probably mean it is a copper based alloy but that's only a guess.

I have noticed a tendancy for helicopter manufacturers to use the hardest, strongest alloy available. This is good for strength/weight but will, in time, fatigue and crack. The biggest worry for me is what alloy is being used for metal blade grips. Use T6 for these and you are flying a time-bomb!

Paul.

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10-17-2005 10:42 AM  12 years agoPost 19
Al Magaloff

rrMaster

12,199 Posts- Enough​Time Wasted. See Ya!

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I don't think we've ever heard of a blade grip breaking, that was made by a quality company, such as QUK. Maybe some of the low end offerings from someone such as HHI, QWW, etc. The Bergens have had excellent service from G10 side frames.

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