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