Does that kind of answer my question (reply #29) also? The Fox 35 is a "6"-pitch prop engine" and an APC 10 x 5 would be running at too low of an RPM to "do the job"? In short, folks are still running 10 x 6s on Fox 35s because they have to? (doesn't sound right, but you get it)
Yes, I get it, and to a certain extent, you are right. A fox with 5" of "pitch" is on the edge. It will spin it fast enough to fly the airplane, but it doesn't have a lot of breathing room left. 6" of pitch puts it in the "zone" where it can pretty well loaf along in level flight and have something left for maneuvers. 4", forget it, won't come close. With a 25FP, you can spin a 9-4 or 10-4 fast enough to fly the airplane (maybe 12000 in the air) , and it's still got plenty of room to rev up more. And I have tried this experiment more than a few times - same prop (Rev-Up 10-4), same airplane (Skyray 35) and switching engines between the 20FP and the Fox. Match the level flight speeds. To comparison. Both cases were vastly better than the Fox with a 10-6, but the 20 was much better than either, not even close. Not to mention that the Fox self-destructed and damaged the nose of the airplane from vibration, in just a few flights.
That's also why something like an APC works so much better than, say, a Rev-Up of the same dimensions on the 20/25FP. The APC takes very little to spin it at 12000 RPM, so the engine is not laboring so much. With a Rev Up, it will do it, but you have to run it much harder.
It's not particularly unique to the Fox, either. A typical prop for an ST46 was a 11.5-6 or 12-6 rev-up. It had no problem getting a 11.5-4 up to launch revs, and would match speeds with the 11.5-6 in level flight. But it was totally gutless in the maneuvers. It didn't even work with an ST60, same problem, even though you had 1/3 more displacement still. Take the same prop, put it on a 40VF in essentially the same airplane, and you get 6 National Championships and the current "Gold Standard" of stunt power. The only real degree of freedom you had in improving performance was adding diameter, which is what everybody did. It wasn't like people in 1970 didn't know what they were doing or couldn't figure out what was going on - Wild Bill was talking about pitch in the 50's. But the engines weren't there to make it work, the way to improve the run was to run more diameter, you needed all the power you could get (since it was never enough), and so you used your 6" of pitch, and run the biggest prop you could manage.
Some still haven't grasped that things are different - but I note that David went from a 51 to a 61 to a 65 and now a 75 and is using more-or-less the
same prop.
The real breakthrough with current propulsion is
low pitch. "Schneurle Wars" engines from the late 70's/Early 80's had enough poop to make it work, for those who clued in, but tended to be difficult to control. Adding the pipe just made them controllable.
Brett
Topic: Fox .35 vs. OS .25LA (Read 23804 times)