[NSRCA-discussion] 120 AC issues UPDATE

[Michael Cohen]

Sounds like what I was attempting to describe, but Mike McCormick's write up is better......
 
Mike Cohen



From: k6xyz at sbcglobal.netTo: nsrca-discussion at lists.nsrca.orgDate: Mon, 14 Jul 2008 22:20:42 -0500Subject: Re: [NSRCA-discussion] 120 AC issues UPDATE




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<<<<<
 
 

RegardsDave HarmonNSRCA 586K6XYZ[at]sbcglobal[dot]netSperry, Ok. 
-----Original Message-----From: nsrca-discussion-bounces at lists.nsrca.org [mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Stuart ChaleSent: Monday, July 14, 2008 10:07 PMTo: adriancwong at earthlink.net; General pattern discussionSubject: 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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