OS FP .20 ABN

fredvon4 wrote:Thanks George

This project of the two SkyRay35s are because I found the Bret Buck notes for this Plane and The OS FP 20 BBTU formula

Brett Buck wrote:
I know I am beating a dead horse here, but when I or someone else says *use stock parts*, it means to use the stock parts that came with the engine!  AGAIN, this weekend, a very knowledgable modeler came to me and said that they couldn't replicate my results with the 25LA (or in fact, the 20FP, either) and that neither had the sort of power I was claiming. So I take a look at the LA, and what do I see but yet another damn ST clone needle/spraybar instead of the superior and much smaller *stock* OS spraybar. 

   The stock ST spraybar is .157" in diameter. The *stock* spraybar is 3.5mm/.137". With the stock .257 venturi, you lose about 25-30% of your power, and THAT is why it won't go over 11.000 RPM on the ground and why your engine is wimpy!   Of course, there is no way to put a stock part back in the engine because the case and venturi had to be drilled out to get the larger spraybar to fit.

   So, OK, what is wrong with the 20FP?  At first blush, looks OK, clearly an OS needle/spraybar. Except on close examination, it's the old OS35S spraybar that is 0.155" in diameter, causing EXACTLY THE SAME ISSUE. With the SAME ISSUE THAT CANNOT BE FIXED AT THE FIELD BECAUSE THE CASE HAD ALSO BEEN DAMAGED BY DRILLING IT OUT.

   The solution in this case is to take a brand new 25LA out of a shoebox in the back of my van, modified it in *no way whatsoever*, bolt on an APC 9-4, fuel, attach battery, and flip prop. One run on the the ground. Instant nearly perfect runs, excellent power, very good competitive flights that later come in second in the contest over a NATs-level competitor.

   Later in the same contest - another very experienced modeler, another 25LA. This one has a chip muffler, and a 10-5 and would barely run in the air, eventually quitting inverted. I helpfully go get the airplane after the crash, flip it over, and guess what - yet ANOTHER ST spraybar. Guess we know why it needs a 10-5, and who knows how much more power was lost using a chip muffler. In this case, again, no way to put it back to normal even though I have stock venturis and stock spraybars right out of the package.

    The most practical fix to this issue - ASIDE FROM LEAVING THE DAMN THING ALONE SINCE IT WAS FAR BETTER BEFORE IT WAS "IMPROVED" - is to drill out the venturi to recover the lost choke area. To match it up, you need a .275 venturi bore with an ST or the old OS35 spraybar. Maybe I should start bringing a Letter I-sized hand reamer to contests. A different solution is to sleeve a *stock* OS 20-40FP spraybar to fit the oversize hole that was erroneously drilled in the case. This is pretty tricky because you have to make the sleeves long enough to engage in the case and the venturi, but not so long that the intrude on the bore of the venturi.

  BTW, as a general rule, the OS spraybar is *superior* to the ST or ST-clone types. The needle in the OS spraybar is very nicely supported on the inner bore of the spraybar, and the needle itself is pretty stiff. The ST and ST-Clones support the needle only at the pinch end of the collet and it otherwise hangs from the threads like a tuning fork. This was notorious i the good old days for problems, and you want to run the collet as tight as it can be and still be able to turn it. The needle itself is also thin and very pointy, meaning that if the point is slightly different, bent ,etc. (and no two were ever really alike) you have no idea what the effect might be. The PA and RO-Jett needles are much superior to the ST, if nothing else, because they have teflon/nylon compression fittings for the seal, which also damps vibrations nicely.

   The purported advantage of the ST and similar needles is that they are infinitely adjustable. That is at best a double-edged sword, for stunt at least, because while you can get any setting, you can also easily make a change in the needle too small to be of any use. A click-type needle forces you to make a distinct change, and can be repeated. If there was some way to put an OS needle in my RO-Jett, I probably would.

  Bottom line - just run the damn engine (any of them) as they come, and only modify them if you have identified a genuine problem that requires a solution!

    Brett