Schweizer 333...Again

[CJ Eliassen]

Stuck valves have been attributed to improper cool down which is also pilot error.  Leaning a hot engine to detonation will cause an engine failure, but that is also pilot error.  When looking at catestrophic engine failures, turbine helicopters have a higher incident of failures than the Robinsons.  Turbines were never designed to fly at low altitude.  They have no filtration system and suck in all the contaminants in the air causing excessive component wear.  

 

Its interesting that the NFS is looking at the ADAMS-500 for fire observation and patrol because it is piston and can fly through the  smoke and ash with far less problems then any turbine.

[flingwing206]

Turbines have no filtration system and suck in all the contaminants in the air causing excessive component wear.  

 

Its interesting that the NFS is looking at the ADAMS-500 for fire observation and patrol because it is piston and can fly through the  smoke and ash with far less problems then any turbine.

You're dead right, CJ - not to mention that a piston will run for longer even if it is having to directly breathe dirty air (maybe not a whole LOT longer, but long enough...).

[longhaul]

What's a new triple three cost all decked out?  Anybody got any operating cost estimates?

 

Thanx

[flingwing206]

Schweizer 333 info (click here)

Note this is SACUSA's website, sure to be a little optimistic but a good place to start.

 

Also note that the 333 is getting an upgrade to its power ratings which should result in a substantial performance boost:

333 upgrade (click here)

 

Hope this helps!

[Ga. Chopper]

Stuck valves do occur on the de-rated Lycomings used in Robinsons and not just because of improper cool down or leaning. These engines operate at a full rich mixture which causes them to load up with carbon in the cylinders heads and causes the valves to stick "open". Not only can this bend the pushrod/pushrod tube, which can also cause an oil leak and loss of power, but it can also wear the associated cam lobe which gets real expensive; i.e. complete engine teardown with a cam replacement.

 

Most piston powered aircraft engines were designed to operate at a 75% power setting and the fixed magneto timing is based on that power setting.  De-rating the engine is easier on the engine, but it also affects the cylinder's operating efficency and can cause excessive carbon build-up.

 

We've seen this on a few R-22's and R-44's at my friend's Robinson service center. The most recent one occured last month on a R-44 Raven with about 300 hours since new. Most of these stuck valve incidents are not documented or reported unless it is still under warranty, so I wonder how many of these occur fleet wide?

[CJ Eliassen]

hese engines operate at a full rich mixture which causes them to load up with carbon in the cylinders heads and causes the valves to stick "open". Not only can this bend the pushrod/pushrod tube, which can also cause an oil leak and loss of power, but it can also wear the associated cam lobe which gets real expensive

 

An stuck open valve will not cause the push rod to bend.  A stuck closed valve will.  A stuck open valve may be struck and damaged by the piston.  

 

Robinsons do not operate at "full rich" for a couple reasons.  The first is the venturi and carb installed is designed to operate more efficienctly at the lower power settings.  Also, you don't get full rich until the econimzer valve closes.  Any Robinson I have ever worked on or flown that has been properly rigged had a nice clean exhaust.  No carbon build up.  Even at 6,000 feet we don't get excessive carbon from exposure.

 

The carbon that builds upon the exhaust valves is from coked oil, not combustion.  If combustion caused the valves to stick, then you would see the continentals sticking as much as the lycomings.  But due to the sodium filled stems and higher operating temps, the lycomings are more susceptible.  

 

Sure there are some stuck valves encountered when the engine is cooled properly, however, the majority of stuck valves are from an inadequate cool down and early shut down of the engine.

[Ga. Chopper]

An stuck open valve will not cause the push rod to bend.  A stuck closed valve will.  A stuck open valve may be struck and damaged by the piston.  

 

 

CJ Eliassen,

 

Thanks for the correction on the stuck open valve, I had meant to say that push rod damge occurs with the valve stuck closed.

 

Maybe you can help me with something else you said. I don't understand how a normally aspirated carbuerated aircraft engine can not run on the rich side, on atleast 2 of the 4 cyinders due to the induction system and unequal fuel atomization. A fuel injected engine has the same problem even though fuel is injected directly in each cylinder.

This can be seen on a Graphic Engine Analyzer with unequal EGT's on all of the cylinders at any given power setting. Having an engine shop flow out all the cylinders does not eliminate the difference in EGT's either.

 

I would appreciate your explaination on how this is different on the Robinson.

 

Thank you.

[CJ Eliassen]

I never said they don't operate rich of peak.  I said they don't operate at full rich.  Also, its not a rich mixture that causes carbon build-up, its the low temps associated with a rich mixture.  If you have an over heating engine and you are operated at full rich, you are not going to get carbon build up.  If you have an engine running cool witha slightly rich mixture, you will.  An airplane taking off is at full power and a very rich mixture, but does not experience a great deal of carbon build up because the combustion temperatures are higher and that extra fuel is needed for cooling.  An airplane in a low powered descent is at a leaner mixture than takeoff, but because the engine is cooler, the carbon builds up.  Lead fouling builds the same way.

 

The R-22 doesn't experience extra cooling in the descent like an airplane.  It has a relatively constant flow of air accross the cylinders.  So you rarely see carbon or lead build up in the R-22 unless they are performing a great deal of practice autorotations.

 

GAMI has precise flow fuel injection nozzles to maintain a more equal fuel/air mixture across the cylinders and it works quite well.  However, EGT's can also be different among cylinders from compression differences.  Never have I done a compression check where every cylinder had the same compression.