About a week following
this break, I recieved a phone call from Bruce in Main. He informed
me that he had also experienced a similar cog belt break on his
Kevlar belt running on the same AP cog belt system. We now know
of two such failures with this drive system. There has been an active
discussion on ROG about the reason that the belt may have broken.
Surprisingly, after my
initial call to the factory to report this failure, they have not
attempted to contact me for any follow up or findings, suggestions,
or assistence. Since I have received no feed-back from the factory,
I turned to Andrew Burr of Vertical Performance helicopter products
to help with a solution--I just did not feel comfortable flying
this system again in this configuration following this break without
knowing what had actually caused the cog belt to break.
I tend to lean toward the
fact that the cog belt is not brought up to proper tension until
the engine is actually running and oil pressure is acheived to activate
the tension mechanism. VPHelo, LLc is testing a much stronger belt
and a totally different elastomeric tension system that does not
depend on the engine oil pressure to keep the belt tensioned at
all times.
The VPS,
LLc cog belt tensioner and stronger belt are shown in the
below
photo.
As one of only two pilots
that has experienced an in-flight cog belt failure in flight, I
have put a lot of thought during sleepless nights trying to figure
this one out. Besides doing some damage to Larry's skids, tail rotor,
and denting of the tail boom when the helicopter landed backwards,
the outcome could have been a double fatality had it happened a
few seconds later while higher, faster, and near the lake.
I am not sure that the
check valve in the oil lines that some have suggested would be the
answer only because I usually see some oil seepage around the plunger.
Since the oil volume inside the plunger is very small, it would
only take a very slight seepage to release the tension held on the
belt even if the check valve held firm.
On the first AP system
that I installed in PA quite a few years back the plunger did not
extend at all. We called Al and he agreed to send a new tensioner
ASAP but due to customs issues enroute from Canada, it was still
going to take several days. Since I was there to conduct flight
training, I suggested to my student that we modify the tensioner
to work under spring tension. We went to the local Home Depot and
purchased a stiff spring that was designed as a screen door closer.
We cut the spring to length,
installed it over the plunger so that it provided a constant 10
pounds pressure on the tension unit, and began hover testing. We
closely monitored the tensioner at start-up, idle, and at full rpm.
It worked well with the exception that the piston pulsed like the
engine oil tensioner due to the lack of dampening that the elastomeric
tensioner exibits. The spring did give the added advantage of providing
the proper tension to the cog belt during engine start pulses. It
did have more movement during operation than the VP elastomeric
tensioner.
At that time I did not
know how important it was to keep the belt properly tensioned during
the start sequence because I had no Idea that these belts could
break due to hypertensioning (high tension transfered to individual
cog belt segments due to a momentarlily loose belt riding up on
the drive cog sprocket). I am now convinced that the quick fix that
we did in order to get the ship flying was most likely a vast improvement
over the original design because even when the engine was not running
the spring held the cog belt under a tension load. I am sure that
AP had no idea that these belts would break or that the hydraulic
tensioner may actually be the cause due to no tension during the
start sequence.
We now know of two cog
belts that broke. One, a Kevlar belt from AP, broke a week before
the carbon fiber belt broke with Larry and me. The factory was aware
of the first break but did not know the reason. If we had known
that the cog belts could break it may have caused this discussion
earlier and stimulated the same thought processes. Since two belts
have broken now, each of a different material, there is a common
thread. The one that broke on Larry's ship was installed by Mark
P. and it was installed perfectly and was at the proper tensions
that are called for on the RW instructions.
Since we have 4 Rotorway
helicopters at our Sho-Me Helicopters, LLc traning center, Larry
and I decided to run a couple of tests. We pulled two Rotorway 162F
ships out of the hangar at the same time. Both cog drives were at
the same temperature in the same location. Both cog belt systems
were installed correctly and the cog belts were set to the exact
same tension as directed by the factory. Both drive cog wheels were
set with the required .1 degree aft and left tilt.
The ship that the VP tensioner
is running in is Larry Stuckman's who had the cog belt break in
flight here at our training facility. Larry installed new landing
gear and skids, and new tail rotor belts, TR shaft and bearings,
and new TR blades. He installed the high-load cog belt that VP is
now supplying along with the tensioner system that Andrew built
for Larry to test.
Below Larry
stands next to his ship during his phase II flight training at our
training center. We also were flight testing his new VP cog belt
and tensioner following the repairs to his ship after the cog belt
break.
We shot a short video of
each cog belt tensioning system during the start sequence. On the
VP system the start pulses are absorbed by the elastomeric tensioner
as it is designed to do. Note on the AP/Factory engine oil tensioner
the tensioner does not even apply any tension to the flopping cog
belt until the engine is actually running and builds oil pressure.
Also note that the oil
pressure tensioner never seems to stabilize during operation. The
"axel" bolt on the cog wheel also continues to spin in
it's aluminum bracket, that will cause wear and slop in time.
Check out the video links
below to see each system in operation:
To view the AP engine oil
cog tensioner in action:
CLICK
HERE
To view the VP Elastomeric
Cog tensioner in action: CLICK
HERE
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