[NSRCA-discussion] 120 AC issues UPDATE

Dave Harmon k6xyz at sbcglobal.net
Mon Jul 14 19:17:12 AKDT 2008


Stuart...here is an old email from Mike McCormick that is interesting.

 

>>>>Because the YS piston compresses in both the up and down stroke, the
ring is 

serving double duty.  It must not only seal the mixture being compressed,
but 

it must also separate the two compressed mixtures.  Once the piston begins
to 

rock and the ring loses its seal, then surge begins.  If you think about it,


the YS engines with the highest compression are the most problematic (SC and


the L; the L is really just an SC version of the FZ).  Why do the pistons 

rock?  No side wall.  Plus (as Dick stated) the pistons are made of soft
2017 

aluminum and will wear quickly (especially if the engine is run lean or fuel


with low oil content is used).  This piston design has been in use since the


AC and always been a pain.  But in the 1.2's we could alway install an FS 

Piston and cure the ill's.  FYI, I once did a test on an AC just to see how 

much more boost the AC piston created vs the FS piston; it was less than 1/4


psi!  I also noticed that any engine in which I installed a FS piston ran 

better than it did with the compound type piston.  I figured this occured 

because the FS piston ran "truer" in the bore; it created less drag because 

it was less prone to rock or cock in the bore (more side wall).  But along 

comes the 1.4 and we no longer have the FS piston to fall back on,  Plus the


1.4 piston was shorter and bigger in diameter,i.e, more rocking force 

(around) the wrist pin is created and there is less length to stabilize the 

force.  Things didn't get too bad until YS increased the compression of the 

engine (L version) and then the piston really started to get beat up.  

Because of the higher compression, the ring seal is even more important and 

the piston/ring fit has to be right for the engine to perform optimally.
And 

the optimal fit is within a narrow range.  I set the piston/sleeve clearance


at .003 to .0035".  Max clearance is .005"   .0035" is tight, and engines I 

set up must be run rich for a long time to properly break in.  I also set
the 

ring end gap as tight as I can get it, usually at .0055", though I like it 

best if I can get it at .003".  But when set up at these clearances and run 

rich, they will last 500 plus flights.

So what is boils down to is the old piston is soft and has very little side 

wall. The forces on the piston are high and attack it from both ends.  This 

creates high side forces on the piston.  The minimum side wall works hard to


keep the piston straight, but even in the best of conditions (running the 

engine rich and using lots of oil) it is a losing proposition for the
piston. 

 The new piston has 4 times as much side wall area under the ring groove 

(where the old style piston experienced the greatest forces and therefore
the 

most wear).  It runs very smoothly and I believe this is because it too
tends 

to run very true in the bore.  I am sure it will last longer because of the 

increased side wall area.  

Sorry for the length of this reply, and I hope it answers your question.
But 

heck while I am at it I might quickly address one other issue I see a lot of


problems with.  Many flyers tend to set the mixtures too lean on 1.4's.  

Because of the different plumbing in these engines, the regulators do not 

respond as quickly as did those on 1.2's.  To set these, you must do it 

slowly and wait a few seconds for the regulator to equalize with the change.


If you move the needle quickly, and not wait it is very possible to get a 

false reading on the mixture and usually it ends up too lean.  Kills the 

engine quickly.  

 

Mike McCormick<<<<<

 

 

Regards

Dave Harmon
NSRCA 586
K6XYZ[at]sbcglobal[dot]net
Sperry, Ok. 

-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Stuart Chale
Sent: Monday, July 14, 2008 10:07 PM
To: adriancwong at earthlink.net; General pattern discussion
Subject: Re: [NSRCA-discussion] 120 AC issues UPDATE

 

I have been mostly using Cool Power 15%, but tried 30% heli today with the
same results.  No special YS blends available locally.
This plane is for my son to fly.  Sure makes my electric Beryll seem real
easy :)
Stuart

 

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