Originally Posted by
sigrun
Actually, by O.S. convention hailing way back to the 1950s, the "S" suffix indicated "stunt" (C/L) being an understood. By the time of the appearance of the FP Schnuerled series of engines superseding the MAX series, C/L was arguably even past its Indian summer. O.S. continued to use the -S suffix, but by then to indicate the motor a C/L venturi and spraybar fitted circle turner. FP sans suffix fitted with an R/C carb was the "standard" FP. By then, the capacity range of C/L motors was already becoming restricted although .10 and .15 FP-S were still made.
As I recall they sleeved the.40FP down to .35 for the C/L version FP-S to fit with C/L capacity and class expectation, but the Schnuerled FP-S was never popular as a stunt motor with stunt fliers. Around this time I recall the .20 and .25 were consolidated and one or the other eliminated from the lineup. I can't recall which now. Eventually they were all superseded by the LA-S series for C/L and the line gradually rationalised. None of this was helped by R/C twin ballrace engines becoming so inexpensive relative to what they had been, that the only buyers for a PB .40 in R/C were beginners for their trainers. And of course, it wasn't long before even the .40in³ which had been the standard entry level for an aeon was superseded by the perception of a need for the .46 LA because of the success and reputation of O.S. .46 class in the TBR SF and subsequent FX.
Aftermarket proprietary venturis for conversion to C/L can be had, at a price. For a beginner not understanding the importance fo adequate suction through manoeuvre and its relevance to inner diameter, unless from a source like Just Engines, it's a minefield. Yes, a wired open R/C carb can be used, with the caveats already mentioned. More weight and usually, less effective venturi area which can result in lower peak power. With contemporary Schnuered engines, the latter usually isn't an issue. NVAs are usually an easy fix. When I could source them, I used to use standard .10FP-S N/V on my .15FP-S' and even Fox 35 Stunt classic. Cheaper, lower profile more easily protected in a 'mishap' by a N/V protector, and finer tuning.
"Peacemaker" was a redesign of Flite Streak by George Aldrich for APS to accomodate British 2.5cc diesels of the day. It's still a fabulous design off the plan and well suited to any contemporary PB glow motor for sport aerobatics. "Smousen" (Modelhob) was designed later, and is better. They are .15 sport-pattern profile models which are capable of performing the pattern if built right and engined right in adequatelly skilled hands. "Akromaster" is similar, but for motors of lesser HP output. As already iterated sufficiently, it is MTOW weight critical which is its Achille's Heel as the builder has to save weight in either the build or motor choice. Because its airfoil is thin relative to the other two, other aerodynamic characteristics not withstanding, it is also faster if overpowered which doesn't compensate for a high wing loading once you start turning.
"Flite Streak" was a great design in its day, and like "Peacemaker" still is. Sets the standard in class by which all others are judged. Flite Streak Jnr suffered by intention from the same thing which plagued Mike Gretz no doubt when he designed Akromaster. They both wanted a model which kids could afford and had an engine for which looked "just like a real Flite Streak". In the dominant US target market, that meant targeting the kid with a ubiquitous PB Fox 15, not exactly a powerhouse. Remember, back then motors weren't a virtual dime each as they are today post the Chinese market entry.
The above is a straw argument nonsense for the obvious sake of either intended ridicule or provocation which backfires for anyone with ½ a clue thinking about it. No-one said thinner sections can't fly well. As already iterated, the Akro's does. But having a thinner section gives a model certain characteristics encumbering it with restrictions or offering it advantages e.g. Goodyear, depending upon purpose. Like mathematics or physics, aerodynamics don't lie. They just are what they are. Either one understands it or one doesn't. That said, I'm sure the 'contest winners' did and built light and engined right to accomodate that section, just as I have been advocating the importance of with Akromaster. If the end user understands and implements that, s/he'll have a first rate flier on his hands. If he doesn't, he won't. I can't for the life of me understand exactly what it is that's difficult to comprehend about that when spelled out so clearly!???
Good info there. I would like to add that one of the primary issues with these OS offerings is they were R/C engines with the carb removed and a venturi added. The important matter here is that they were time for all out RPM's. Where the stunt guys wanted that 4-2 break. Not possible stock from the FP line because of the port timing. I have never been much of a stunt flier. Combat, and pylon racing mainly. Now in my advanced age I fly mostly RC scale. But I am flying CL combat again of late. SO I always wanted the full bore tuning you get with a bladder. In fact, I am not sure I have an engine that will run a 4-2 break. I have an old Sig Magnum, with an OS SF .46 installed. But again, I could never get it to hold the 4-2 the whole flight. But it is box stock.
Not knowing the experience of the OP, I am going to guess if he builds his Akromaster box stock, and just throws a .15 on it, he will be fine. His biggest concern will be completing all of those laps without hitting the ground. Turning radius concerns are gonna come a little later.