With FP40s and Tower 40s I have kept to the magic 10.5x4.5 apc. No doubt other props can be tuned in, but these work and are available. I know the FP40/Tower40 frustrations first hand, ran the gamut, experienced run aways, inconsistent runs, good day, bad day, plane/engine combinations. The mysterious changeability of a domesticated FP40s. Very serviceable strong runner morphing into a nasty unusable beast. Engines that run great in Philly, running away at Muncie or Carmichaels. I also had a few modded, ports cut, this didn't work. The result anemic and erratic. In other words I went through the learning process. Why did I keep at it. Things curved upward, getting better gradually. Besides that my good flying buddy Jack Weston had the touch, his FPs working well almost all the time. Competing effectively against the high priced dedicated stunt motors. (Which I like, by the way, just haven't felt the need to go there, yet. Soon. Perhaps.) Too bad Jack was attacked by the electric virus infection. Caught it from our in house Typhoid Mikey. Jack has the ears... for setting a needle. A waste. A waste. Of talent. Now he flicks a switch. Dan Banjok's unremitting determination to make a specific engine/plane combo work, always a process model in my face.
I have found most of the issues with the engines to be traceable to the accessories, the systems that keep these engines lit and running well. Bad fuel tanks that leak or make crud or are plumbed wrong, poor fuel choice (we use Powermaster 22% (50/50) 5% or 10% in almost everything, including Foxes and 4 strokes), clogged filters, dirt in the needle, weak vibrating noses (profiles in particular, but full fues planes can also have this problem), pin holes in tubing, inappropriate, low quality, or worn out plugs (we're into Ernya#3s and Thurderbolt RC longs, at the moment), loose engine bolts or engines mounted up against a vibration enhancer, definitely bad news, and... and... make up the list. Get these right. Build the profile nose stiff. I have been using 1/8" five ply doublers, a tripler closing up the inside side of the engine cut out, good epoxy, checking the doublers and fues, making sure all the parts fit without gaps, weighting the glued elements over night, and so forth. Good wood work adds to longevity, tamps down the vibes. Like I said. Add to this list. Make sure everything outside of the engine itself is right. Amazing how many poor engine runs originate from issues outside of the engine. Often on these forums advisers jump to engine mods without cautioning to check all the other contributors to poor engine runs, factors outside of the engine itself.
I regulate the break with head shims. Usually I start out with 30thousands of shims. Important to measure the thickness of the shims. OS shims typically measure 15 thousands of an inch, Tower 40 shims, (and Leonard Neuman shims -- a great deal), and other after market stampings, typically measure ten thousands of an inch, so, two OS or three Tower. Prop, fuel and plugs specified above. Over the last few years my newest insight into these simple appearing machines, which function complexly, is the regulation of power via venturi. One day the obvious hit me. Venturis are throttles, fixed throttles. If the engine is putting out too much power for a given plane, throttle it back, put in a smaller venturi. Nothing like the obvious to trip up the deep thinkers of stunt. Lately I've grown fond of .272 venturis, midway between the big FP40 and the small FP40. I like Jim Lee's nylon versions, since they are somewhat longer than stock. I think it smooths out the mixture flow. Whether it does so or not. Dunno. If necessary go down to .265. If you use a Supertigre type needle valve, include that in your calculations. The fatter needle further closes down the throttle. If your engine is not overheating and your plane is flying too fast. Throttle back. Smaller opening for air/fuel mix. Set the needle on the ground, at first, after the engine has come up to operating temperature, fifteen seconds at least, set it a bit rich. Observe the plane as it flies. If it leans out too much. Back the needle out a few click. If it's too rich, click it in one or two. Make these corrections increment by increment. Don't be ham handed. Sometimes a click or two in either directions will find the sweet spot. It is the performance on the plane that counts. This may or may not match up with how the engine runs on the ground. Often it does not. Listen to the engine as it flies.
Note: if you ever luck up on an FP-S40 NIB you might find extra head shims and a number of venturis. OS knew what they were doing, including these tuning elements.
Best of luck. No doubt I left stuff out.
Topic: Prop recommendation (Read 4028 times)