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HelicopterGasser Model RC HelicoptersOther › Bergen Twin Industrial Tail Failure
08-06-2007 08:16 AM  10 years agoPost 1
ahh_me2

rrNovice

Edmonton, Alberta,​Canada

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I am a test pilot and heli mechanic for a university that owns a Bergen Twin Industrial.

This heli was brought to me for the purpose of setup and flight, and had been previously flown by a couple of our best pilots in our local club. This was a few years ago.

It has barely had any fuel run through it, and therefore the engine is not really broken in. Bergen states it takes approximately 8 gallons of fuel through the engine before break-in is complete. So I only wanted to do light circuits such as figure eights, remote circles some hovering and what not until we had the engine broken in.

So we went out this afternoon and ran close to a gallon of through her.
It was great! Compared to my blade CP and MX400 and My TRexe's it was quite a treat! LOL

Anyways the post flight inspection failed!

I could rewrite the report, but I figured I would just copy the report I wrote to the owners of the heli. So if it seems geared towards newbies, it's because it is! But it's also to make people aware that failures in the sky are often preventable!

So here's my tale of a failed tail!

Some content edited for names and phone numbers, irrelevant to the issue at hand.
************
Hi All!

Potential Disaster Averted!

The Bergen is grounded!

It cannot be flown until we get a few parts!

Post flight inspection tonight revealed a very strange binding with regards to the tail assembly.

Since there is a clutch assembly, we can manually rotate the main gear with the engine not running, this means it can rotate either way. One direction of rotation causes the main blades and tail rotor to turn, the other direction causes only the tail rotor to turn ( due to the one way bearing). Bearing this in mind(no pun intended! LOL), if we wish to examine the tail operation we simply rotate the main gear in the direction required (CCW) for only the tail to rotate.

So I'm standing there, looking over the heli for any signs of loosening of bolts or whatever that might look out of place, and I keep rotating the main gear, for something to do while I inspect the heli. And I keep getting this little binding action every so often, 4-5 times per full rotation of the main gear. This bothers me somewhat, because, I don't recall it being this way before. So I decide to loosen the tail boom, and maybe take it off for closer inspection.

Well, there are 3 plastic block clamps that sit in the frame and lock the boom in place. 2 of them look like they have melted plastic around the metal boom, but a boom isn't supposed to heat up!

By playing with the position of the tail boom I can hold it at a point where the main gear drives the tail with no binding or resistance at all. So I begin to wonder if I have an alignment issue on the tail, but the melted plastic is nagging me to look further. So I pull out the tail boom assembly and the first thing I see is a bent pin sitting halfway out of the plastic female U-Joint that is attached to a shaft that connects with the main gear though a bevel gear.

Not only is the pin bent, but it sits in a widened circle with evidence of heat distortion.
So I grabbed a pair of pliers, and pulled gently on the pin. To my amazement, it just lifted out with no effort and was bent approximately 30°.

Was it ever inserted correctly? I don't know for I didn't assemble the tail-boom.

Is it a design flaw? I don't know that either.

So while I'm pondering over all of this, I kind of start looking at the forward end of the boom, at the male side of the U-Joint/torque tube. It looks fine, but something doesn't look right.

So I look closer, and I can see that the tail boom tube wall has some serious signs of something gouging into it, almost a full circle worth.

It turns out that as the pin worked it's way out, it started gouging out a path/trough in the I.D. of the boom tube.

I figure no big deal, after all I have a crash kit here. So I grab a new boom, stick it on, and what! The male U-Joint is sticking out the front end of the boom! The other one didn't!

So I compare the 2 booms, this one is shorter! I'm screwed! LOL

I might have been able to cheat on the female U-Joint repair, even on the melted plastic blocks, but there is no way I can extend a metal tube.

So since we have to order a new boom, we might as well order the plastic blocks and a new female U-Joint.

Until then, We have no flight capabilities!

What would have happened if I had not done this post flight inspection?

At some point in the next 30 mins of flight of the Bergen, the tail would have failed!

What happens when a tail rotor system fails?

The helicopter goes into an uncontrollable pirouette, and a crash is virtually inevitable!

There is a way out out, and it is called throttle hold and autorotation!

I haven't fully set the throttle hold or tried the autorotation setup. I have just begun breaking in the engine.

We ran close to a gallon of fuel through the Bergen today over a 4 hr period at the field, and I must say it was flying great! The engine was starting to gain power, as we were closely monitoring headspeed, and it increased by 40-70 rpm over this period. This simply means more power was being developed by the engine as it gets broken in. Prior to this, it may have had a gallon at most through the engine.

It's a very nice flying heli, kind of like a Cadillac, a soft smooth ride, great for a Sunday afternoon! Too bad our field is so small! Due to it's large size, it tends to eat up distance in a hurry!

Things were going so well!

The best laid plans of mice and men! LOL

I'm just glad I found it before a crash occurred! While crashes are inevitable at times, some prevention is always possible, Dave aptly named it "Due Diligence!"

I weighed the Heli, it turns out it's heavier than I thought! I figured 12-15 lbs, but it comes in at 18.5 lbs, and that hurts when it hits the ground!

Sorry for all the detailed info, but at least you may all realize there is more to a heli than just flying it!

*********

The question now begs itself, was this a fluke? How do I prevent this in the future? Was the pin incorrectly fitted? Or did it just work it's way out? Is it a design flaw? If so what is the fix?
Any and all feedback would be appreciated!

Thanks for reading!

Andre

Blade Cp / Spektrum Rx - TRex #1 Blk CF 6100 rx- TRex #2 Slvr CF 6100 rx /GY401-9650

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08-06-2007 10:47 AM  10 years agoPost 2
Autoeject

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Ashtabula, OH, USA

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Not installed or loose setscrew in the end of the dogbone? My pin has never budged with much more fuel than that through it.

I'd check everything over more thoroughly before flying again. It doesn't take much to bring it down when improperly assembled.

I'd also get the throttle hold setting done. Especially during first flights you should be prepared for the worst

Mark Webber
wai-rc.com
Spartan RC Distributor
Outrage Helicopters

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08-06-2007 01:44 PM  10 years agoPost 3
FCM

rrElite Veteran

Surrey, England

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I haven't fully set the throttle hold or tried the autorotation setup. I have just begun breaking in the engine.
So, when do you intend to set the throttle hold? I would say you were very lucky to have got away with flying such a big powerful heli without it being properly set up and inspected first. Pre-flights are as important as post flight inpections.

Paul.

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08-06-2007 03:15 PM  10 years agoPost 4
Chris Bergen

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cassopolis, MI USA

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A very good detailed report, and I must say a very good catch!!

That cross pin is supposed to be secured by a setscrew that is installed in the end of the shaft, looking into the delrin coupler where the dogbone sits, you can see the end of the shaft.

Verify that the setscrew is there of course and if it is tight, loctited, etc.

Give us a call at 269-445-2060, we'll get you squared away on the parts you need immediately.

Chris D. Bergen

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08-06-2007 04:50 PM  10 years agoPost 5
ahh_me2

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Edmonton, Alberta,​Canada

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"I haven't fully set the throttle hold or tried the autorotation setup. I have just begun breaking in the engine."

Ok guys, I kind of glossed over this issue, but it's not quite what I meant.

I have a fully enabled throttle cut switch that I tested and it indeed kills the engine.

I have throttle hold function enabled but haven't determined the optimum throttle idle for "practising" auto's. It just goes to engine idle for now.

I also have a basic "Hold" curve in place, based on Normal flight mode curve for now.

The point is, I haven't optimised the setup to practise auto's but basic parameters were set for safety reasons so I could do basic hovering and low level forward flight with basic autorotation capability.

My next outing with the bird was going to entail trying some hovering autos to see the response and then verify that I can disengage throttle hold and verify engine spool up before taking her to altitude and doing some practice auto's.

Hopefully this clears up the misunderstanding!

Blade Cp / Spektrum Rx - TRex #1 Blk CF 6100 rx- TRex #2 Slvr CF 6100 rx /GY401-9650

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08-06-2007 04:50 PM  10 years agoPost 6
ahh_me2

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Edmonton, Alberta,​Canada

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Chris, I am not sure if I explained it properly, but I see no place for a setscrew to hold this pin in place.

I will be getting some pics today and I will post them here.

It should be much clearer then, as a pic is worth a thousand words!

Imagine a couple of pics! LOL

Thanks

Blade Cp / Spektrum Rx - TRex #1 Blk CF 6100 rx- TRex #2 Slvr CF 6100 rx /GY401-9650

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08-06-2007 05:08 PM  10 years agoPost 7
Autoeject

rrKey Veteran

Ashtabula, OH, USA

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The hole for the setscrew should be right in the end of the dogbone to retain the pin.

Mark Webber
wai-rc.com
Spartan RC Distributor
Outrage Helicopters

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08-06-2007 05:49 PM  10 years agoPost 8
Chris Bergen

rrElite Veteran

cassopolis, MI USA

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Delrin coupler Mark, not dogbone.

Andre,

Peer inside the delrin coupler, at the end of shaft. It is hard to see, so far down in there. Take your 1.5mm allen driver and insert it into the area of discussion. You will find a setscrew.

Chris D. Bergen

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08-06-2007 07:43 PM  10 years agoPost 9
ahh_me2

rrNovice

Edmonton, Alberta,​Canada

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Thanks Chris, I will take a closer look at the internals of the delrin coupler.

I am including a picture of the disassembled parts.

Chris, our crash kit has a tag that states Twin 130 crash kit, yet as you can see by the picture, the tail boom is 1-1/8" shorter.
Can you advise on this please?

Also we will be needing new plastic boom clamps as you can see they have severe heat distortion.

I will have the guys call to place an order today or tomorrow, it's actually a holiday for us today in Canada.

Thanks, looking forward to comments on the pic!

Blade Cp / Spektrum Rx - TRex #1 Blk CF 6100 rx- TRex #2 Slvr CF 6100 rx /GY401-9650

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08-06-2007 08:05 PM  10 years agoPost 10
ahh_me2

rrNovice

Edmonton, Alberta,​Canada

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Chris

Is this the setscrew location you are referring to?

Thanks

Blade Cp / Spektrum Rx - TRex #1 Blk CF 6100 rx- TRex #2 Slvr CF 6100 rx /GY401-9650

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08-07-2007 02:21 AM  10 years agoPost 11
Autoeject

rrKey Veteran

Ashtabula, OH, USA

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Doh Misunderstood the original issue...pardon.

Mark Webber
wai-rc.com
Spartan RC Distributor
Outrage Helicopters

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08-07-2007 04:59 AM  10 years agoPost 12
ahh_me2

rrNovice

Edmonton, Alberta,​Canada

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I located the setscrew on the end of the tail transmission shaft, but couldn't loosen it. So I removed the tail transmission cradle assembly.

The setscrew was flush to the end of the shaft, yet looking through the hole where the coupler pin goes through there was no sign of the setscrew. In other words, the setscrew didn't appear to have been threaded in far enough to lock the coupler pin.

I then tried again to remove the setscrew, had to apply a bit of heat first. This time I was able to remove the setscrew.

After cleaning up the threaded hole and the threads on the setscrew, I re-threaded the setscrew in, put the coupler pin into place, and tightened the setscrew until it just grabbed the coupler pin. I could now feel some resistance when trying to pull out the pin.
I looked at how deep the head of the setscrew was in relation to the end of the shaft, it was recessed by 1-2 mm.

This indicates to me that even though the setscrew was loctited in place, it was not threaded in far enough to actually lock the coupler pin.

And this is obviously how the pin worked it's way out and caused the rest of the problems as described earlier.

Blade Cp / Spektrum Rx - TRex #1 Blk CF 6100 rx- TRex #2 Slvr CF 6100 rx /GY401-9650

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08-07-2007 12:49 PM  10 years agoPost 13
Chris Bergen

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cassopolis, MI USA

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Exactly what I suspected.

We have changed the length of the tail booms to 37" as standard on the Industrial Twins.

Give me a call so I can talk you through what needs doing.

Chris D. Bergen

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