As to the 40FPs. No, I will not be drilling the cases just to get a boutique NVA in them. The OS parts are fine, I'd like some venturis as I have none. Maybe in a couple of different diameters. Come to think of it I wonder if I could print some up? A buddy and I build race carbs and we've been printing booster venturis and other parts in nylon. Maybe I can do that.
Tim, I was used to running 10-5W or 6W Rev-up props on a Fox .35. Are the OS and current engines that much more powerful that they can use props in 11 or 12" diameter?
A 40FP will probably spin a 12-4 fast enough to fly the airplane, but that's not what you want to do. In the days of Fox 35s ad ST46s, the only way to get more power was to add diameter. You don't do that any more. You reduce the pitch.
Modern engines are much more powerful, but at higher RPM, and they want 4" of pitch. Almost all of the large contest winners since about 1990 have used props in the range of 3.25-4.5".
Something like a stock 40FP will end up being FAR too fast with a 6" pitch prop, and people have had nothing but frustration when we tried to run them like they were Fox 35s with better fit/finish. They are notorious for "runaway", where no matter how you set the needle on the ground, at some point in the flight, they take off into a 2-stroke and never come back. This is seen as a defect, however, *this is precisely how they were intended to run*, that's how they are designed, and you have to make drastic modifications to change it.
If the engine makes the airplane too fast, the logical solution is to reduce the pitch. This will permit the engine to rev up more like it wants, you run it in a 2-stroke all the time like it wants, and everything is a lot more reasonable. However, you will quickly find that a 40FP is *way* too much power to deal with and it's still too fast even with a 4" pitch prop. You can get fiberglass props that you can adjust to have less than 4" (try 3.25" to start with) but with any common prop, you still are too fast/have too much power. In fact, if you set the engine at the desired/intended operational setting (peak out lean, then richen up so it peaks out at some point in the maneuvers), it's close to *twice* the power you need.
When this was discovered, about a million (it seems...) guys came out of the woodwork with dremel tools and drills, trying to "fix" it. It wasn't broken, but that never stopped anyone, and eventually people came up with a bunch of modifications, some of which actually caused it to work OK with props like a 10-6EW most of the time. The problem is, then the performance is no better than a Fox 35, either.
You can deal with the "runaway" problem a better way by reducing the venturi size until the power is reduced enough to fly with the props you want. An alternative, if you have twice as much power as you need, is to use one about the same but half the size. Like a *20FP*. I did that equation sitting out in the 100+ degree Gilroy, CA sun one afternoon, and sure enough a bone-stock 20FP with an APC 9-4 is just about exactly what you want. It doesn't have the vast overkill power of a 40FP, that is hard to deal with lacking a tuned pipe, and it performs much better than a Fox 35. I did nearly back-to-back flights on a Skyray35 with a very good Fox 35 and the 20FP, and flying it with a 20FP was like gravity had been turned off, with vastly better vertical performance and dead-steady pace in the maneuvers.
This is exactly how top competition systems work, except you are doing it with a muffler instead of a tuned exhaust. That means you don't have nearly the sort of pinpoint control you have with a piped system so you have to be a little more picky about getting the right-sized engine.
The led to very extensive experiments with similar small RC engines, with the rules of using commonly-available 4" pitch props and *minimal* or preferably no engine modifications. This turned our very well, and I found that almost any of the smaller RC Schenurle ABC engines at the time worked fine if you ran them how they wanted to run. The 20FP (available for $49 at the time) and the 9-4 APC prop ($1.94 at the time) stood out, but they all worked with slight adjustments and NO GRINDING OR DRILLING whatsoever, and far better performance than any engine from the good old days and a 10-6EW.
Brett
Topic: Fancherized Twisters (Read 3033 times)
