Back Side Idler Pully

This item to my knowlege was first created for the Scorpion's by someone in California
and later copyed by Eric Cymlbaliuk and then Al Banacheck
or perhaps it was the other way around.

 
This item, depending on who you talk to, is either a good thing or a bad thing.
The folks who sell them and their customers seem to like them.

On the other hand the majority of what I call Old Timers I have met in my travels say they wouldn't have one on their ship because they are known to have caused several cracked air frames over the years dating back to the days of the Scorpion.  From what I gather from Orv's post below it's a possibilty that all these cracked frames may be the result of improper installation but then again maybe they aren't.

Your the owner of your ship and it's up to you whether or not you decide to go with this mod just like every other mod you choose to add to your ship so be carefull and take your best shot.

From: Orv Neisingh 

8/12/2002 

  Warning to all RotorWay flying pilots

Recently I have flown several RotorWay helicopters with backside idlers. The idea is good in that they help reduce the vibration from main drive belt resonance during run-up and low or power off conditions. I have one on mine and it has worked great for several hundred hours.

The last three installations that I have seen have the back side idler actually intruding past the tangent line that runs from the outside circumference of the secondary to the outside circumference of the main engine belt drive pulley.

This means that with the belts under tension by the engagement of the clutch, the idler actually forces all four drive belts to bend inwards toward the center of the helicopter and then back out to travel around the engine drive pulley.  As the engine fires each cylinder there is a power stroke that pulls the main drive belts around their intended path.  At full power there are over 8,000 power strokes per minute transferred to the main drive belts and into the secondary drive.

The backside IDLER should be just that, an IDLER that sits back from the belts and only engages them when they begin to resonate or vibrate from their intended straight path while traveling from the secondary pulley to the engine drive pulley.

On the last 3 installations that I have viewed on my students helicopters the backside idler intruded past the tangent line by approximately 5/16" and thereby were in constant tension against the belts. Each power pulse of the engine would apply outward pressure against the idler forcing it toward the frame tube that the idler is attached to. The frame on your helicopter was never designed to carry that load!!!!

My concern was that the frame of the RotorWay was not engineered to withstand these lateral power pulses and was subject to metal fatigue. I just yesterday returned from a flight instruction trip where the helicopter had this same backside idler condition present.  I voiced my concerns to the owner/builder and said that we needed to closely monitor the installation. This helicopter had around 75 hours of flight time on it with no problems when I arrived.  After 6 hours of flight instruction and during our post-flight inspection we noticed that the clutch pivot arm bracket support had broken. I told my student that I suspected the back side idler as the cause but he felt that it may have been due to trailering the helicopter.

We spent the entire next day tearing the helicopter down to the point that we could get to and safely repair the cracked bracket, this cost us a full day of planned flight instruction that was wasted as well as the lost time he took from his work. Once repaired and the rest of the frame closely inspected we planned to fly the next day. After only 2 hours of flight instruction the following day we discovered that the 1" frame tube itself had cracked at the point where it intersects the square drive tube that the lower secondary bearing attaches to.  This crack had traveled about 350 degrees around the circumference of the frame tube.  It did not show up initially most likely due to the tendency of the powder coat finish on the frame not cracking while the metal fractured beneath it.

This again required that we tear the helicopter down to get access to the frame for welding and cost us another full day of flight training while we worked on and repaired the helicopter. This was a very expensive idler in that it cost him two full days off from his work, my time, and frustration that his helicopter was falling apart on him.

Here is a photo of the idler with the frame tube crack to give a perspective on the location.

The backside idler was removed and we flew another 20 hours in the next 4 days with no further problems at all with the helicopter.  If you have a backside idler, take notice of this and take appropriate action before this happens to you while in flight. The RotorWay as designed is a very reliable helicopter. Nearly all of the failures that I have experienced I'm my students helicopters have been from building errors, or associated with after-market items. There are a number of proven after-market products available. Should you decide to install one of them, just be sure that you are aware of their safety history and that they are installed correctly.

Be careful
Orv Neisingh
R/H CFI

End