Doug Ireland Posted November 2, 2009 Author Share Posted November 2, 2009 Thanks for the PM and attachment Steve, much appreciated and will be acting on your advice! Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted November 2, 2009 Share Posted November 2, 2009 Posted by Bruce Richards - Moderator on 02/11/2009 17:34:27:The Saito 82 in my Osmose (intalled by the previous owner) has a piece of pipe from the breather nipple right through the full length of the fuselage (about a meter). The previous owner said he had done it to help keep the model clean. It does not appear to cause any problem and all the oil does end up over the tail wheel. But is the lubrication system working properly? It was while researching the reason for an air pump fitted to the crankcase of a Saito twin (180 degree crank - no change in crankcase volume as the pistons oppose each other - therefore no airflow drawing oil mist over the camshaft) that I discovered (from Saito literature) why there was a maximum length specified which is that which will allow proper airflow. Quote Link to comment Share on other sites More sharing options...
Doug Ireland Posted November 2, 2009 Author Share Posted November 2, 2009 I can't see why there is a "critical" length for a piece of open pipe. The crankcase is continually sucking and blowing (in a two-stroke anyway) so gravity alone will carry the waste oil away; eventually! Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted November 2, 2009 Share Posted November 2, 2009 I imagine that the air stagnates/oscillates in a longer piece of piping meaning that there isn't a continuous flow of oil mist past the cam and followers, which are extremely prone to wear if starved of copious lubrication. The differential between positive pressure from the pumping action of the piston and backflow into the crankcase as the piston rises against any piston blow-by would result in a small net flow outwards per stroke. Remember that it is NOT waste oil - the oil mist that migrates along this path is lubricating the entire bottom end!Edited By Martin Harris on 02/11/2009 21:23:48 Quote Link to comment Share on other sites More sharing options...
Bruce Richards Posted November 2, 2009 Share Posted November 2, 2009 Ill keep you posted on how the Saito survives long term Quote Link to comment Share on other sites More sharing options...
Shaun K Posted November 3, 2009 Share Posted November 3, 2009 We've kind of crept off the original topic here a little, but a theory on the breather tube length could be that it's similar to the issue of having a your skin diving snorkel too long. The problem with having a snorkel too long is that the volume of exhaled air retained inside the snorkel takes up most of the volume of your lung capacity - when you breathe in again you're sucking back the last exhaled breath getting no circulation of fresh air. Moving across to an engine breather tube could be a similar principle - if the tube holds a volume of 5 mls of air and the crankcase pressure only pushes out 4mls per cycle, you're not actually depressurising the crankcase - whatever gets pushed out gets sucked back in again therefore not allowing negative pressure in the crankcase to draw oil through the oil galleries. Does that make sense? Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted November 3, 2009 Share Posted November 3, 2009 It does to me! Quote Link to comment Share on other sites More sharing options...
Doug Ireland Posted November 3, 2009 Author Share Posted November 3, 2009 Me too, I'll admit it never really crossed my mind. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted November 3, 2009 Share Posted November 3, 2009 Its an interesting thought Shaun & I guess we'd have to ask a guy who actually designs these engines to fully understand the reasons. It's interesting that most new engines route the breather via the carb manifold or for the new OS Alpha series with internal galleries & things. From practical experience I know that anything in the breather tube (I mentioned I put after run oil into the engine via my breather) either gets sucked into the engine or blown out (dependiong which way the the piston happens to be travelling!!!) at a great rate of knots as you turn the engine over. Also on my installations I cut the breather tube at an angle such that the angle of the tube is with the slipstream/airflow over the model (make sense??). The amateur aerodynamicist in me is then confident that this airflow will cause a low pressure area at the end of the breather tube thus keeping it clear!!! Certainly there is always a trace of oil running from this pipe after a session.... Quote Link to comment Share on other sites More sharing options...
ken anderson. Posted November 3, 2009 Share Posted November 3, 2009 hello all - here is my 5 pennyworth-----i have been using 4st motors-OS and Saito....since 1990...allthough i dont study the instruction's on a daily/weekly or even monthly/yearly....basis-i understand that the 4st depends on it's bottom end lubrication after the ignition combustion...and as the residue oil has to go down the cylinder wall-do its bit then away out the of the nipple on the base of the crankcase----i was allways told that if the tube was too long the engine wouldn't be able to get rid of the excess by product oil and would suck it back into the crankcase-and so keep the tube you add to it-to a certain length......(2st get's it's oil as the fuel passes through the lower crankcase on it's way to the top/combustion chamber-4st has to wait till it's been upstairs and burnt-before it get's any.....................my view... oily hand anderson the 1st....... Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted November 3, 2009 Share Posted November 3, 2009 The rebreather systems will work to pull the mist through more efficiently. Whether they are an attempt to improve oiling of the bottom end or a token gesture to the green lobby is something we would need to ask the designers! Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted November 3, 2009 Share Posted November 3, 2009 Ken, I think that as the oil built up in a long tube it would expell through hydraulic pumping (it won't compress unlike the air) a little at a time. What would stop is a free flow of oily air over the moving parts. As well as making things "slippery", an important function of an oil system is to carry heat away from parts. I haven't managed to track down the documentation explaining the system that I found a few years ago but I'm 99.9% certain it was from a Saito publication.Edited By Martin Harris on 03/11/2009 14:54:49 Quote Link to comment Share on other sites More sharing options...
ken anderson. Posted November 3, 2009 Share Posted November 3, 2009 bearing in mind that i have to buy a paper to find out what day it is-...i might try and find some of my old instruction's for 4st's...i think they were on parchnent....but i recall that they the4st engines-early on were knocking out bearings as though they were growing on tree's--even more so if you used nitro-because one of the residue's after combustion was Acid...that used to Kapow them quicker...and the early bearings were open cages...now they are sealed for life affairs..... oily anderson the elder.......'ss Quote Link to comment Share on other sites More sharing options...
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