free wheel units

[540DEGREE TorqueTurn]

KH4/3b1 recently had a hard landing at a station in the N.T,the pilot said that he was in powered flight at approx 80 feet when erpm screamed out the top and seemed to start to settle . The exprienced pilot upon realizing the rrpm needle was decaying , he was settling /overpitching even with full erpm entered auto contacting the ground with a force that collapsed both skids ,luckily not hurt at all .
The engineers where quoted to say ,sounds like the free wheeling unit has let go, and used to happen all the time at vrd .
Can anyone confirm that this has happened to them in the past .

[Nigel Osborn]

Yes, I had a freewheel let go in PNG, fortunately within auto range of a strip. Had quite a few slip on start up. Usually we just stopped, restarted & all was well. With luck you would find an engineer who would deglaze the shoes, normally caused by poor starting technique.:rolleyes:

[sprocket]

With luck you would find an engineer who would deglaze the shoes

Nigel, you might be thinking of the centrifugal clutch.

[212man]

http://www.aaib.dft.gov.uk/cms_resources/AS355F1,%20G-XCEL%2007-06.pdf

Very sad and sombering look at free wheel slippage:(

[FlightOops]

For what its worth...

The cause of G-XCEL was an incorrect coating on the ramp and roller bearings on the combining gearbox.

Still debated whether the "correct" coating was used according to spec or whether the spec was incorrect - I think it will be a long wait for that one.

FO

[Brian Abraham]

Once doing the pre shut down cool down in a 205 and noticed the rotor at very low rpm - like count the blades going by - and engine way above normal idle rpm. Maintenance gave it the thumbs up and organisation kept flying it with no further problems.

[noooby]

Nigel, you might be thinking of the centrifugal clutch.
Sprocket, that is because Bell 47's have a centrifugal clutch. Well, the G3B1's and 3B2's did anyway. If the clutch shoes are glazed, ERPM goes up while RRPM starts to decay.

[540DEGREE TorqueTurn]

ok, i understand this upon starting ,where the clutch is glazed erpm is higher ,and rrpm is slowed by glazing of the cluch,to fix hold blades by hand upon starting and hang on for a few seconds and hopefully this will burn of glaze .............. but this helicopter was and had been flying for some time .... i was with the understanding that once the clutch was engaged it could not disengaged due to centrifugal force,,,now what about the rollers, plate, cushion, springs in the inner race assy in the free wheel unit? how could this let go
if this theory is wrong then ,unfortunately my faith and all my time approx 1500hrs in B47/KH4/3B1,has me a little worried ,

[brett s]

If the clutch is just plain worn out it can also slip, so glazing isn't the only way it'll occur.

[Shawn Coyle]

Free wheel units have failed in the past, and then worked absolutely fine for ages afterwards.
Bell UH-1N in Canadian Forces had a problem with the freewheel between the combining gearbox and the transmission failing at least twice and it was eventually removed from the fleet.
It was the 'this can't be happening' failure - power turbine speed above rotor RPM...

[topendtorque]

Joining the freewheel club is like joining the caterpiller club, frightens the living @@@@ out of you and you're lucky to live thru it.

Basically the donk goes to valve bounce speed instantly, which is deafening and it feels like you have stepped into a lift that is not there.

In my instance (a bell 47 3B1) I tried to re-engage twice from eighty feet, just after translating in a ten knot downwind at the time, just bent the skids and plucked the rear short shaft out due to deliberate bottoming of collective to avoid going over a cliff in front of me, that is when the freewheel re-engaed as if nothing was wrong. We put a short shaft into it and flew it home, the engine became a tin mine in seventy hours.

snap re-engagements we believed, a) will do lots of damage and can b) break the inner race.

Rule of thumb - always check free wheels GENTLY, so as they can demonstrate that they will work without duress and NEVER snap them back on. like anything mechanical.

I do believe that many earlier freewheel slips (Nigel ?)may have been put down to clutch slips until someone started pulling them apart with a micrometer. Although the heavy weight F/W was a relatively recent addition.

We found that a freewheel that did not engage on start up was a warning sign. I have yet to see evidence of heat on a clutch drum that would indicate clutch slip.

There are three types of '47 freewheels, lightweight, which is all that's called up for the KH4, and of course the G2 range.

Medium weight which is a wider and a bit heavier unit and the heavy weight, which is distinguished by the extra ring visible on the exterior on xmon which has holes all the way round on outside. Heavy model has extra planetaries as well.

use heavyweight only, read the worksheet from a trusted engineer that miked the rollers and inner and outer race within limits, fly within the hour limits and do a proper 600 hrly inspection and they will never fail.

Put a leightweight on a KH4, which is turbo charged and will pull much more power than a leighweight can handle and you will find the limit when you least expect it.

[tbc]

I had two freewheel units fail or start to fail during my time as a chicken leg high speed pursuit pilot!!

First one just went bang and caused me to enter auto from about 700 feet agl at about 100 kts whilst in a gaggle of 4 ships in transit. I prepared for an EOL and 'advised' the formation that my mount was feeling somewhat "tired and emotional". I checked the instruments and didn't see anything untowards so tried a little bit of collective which worked so carried out a conventional albeit rather lumpy and bumpy arrival in a field.

The decking area was littered with some of the internal ramp and roller bearings from the unit. Never did find out why it failed.

The second was again straight and level and was simply a short but sharp pronounced 'loud' buzz with some yawing. Happened a couple of times in short succession so did the decent thing and dumped it on the ground. No sign of debris this time but another freewheel bit the dust.