When you hang a big motor like an ST46 on a small plane like the original Tutor, do you think you are in essence "fooling" the wing into thinking its bigger, like blown flaps on some full size jets? I had a Tutor with a Fox when I was starting out, and often heard people say that the Tutor was the perfect example of a good design with too small a wing. Arguably, Fitzgerald's Worlds plane would fall under the small wing/big engine family, even if the 75 is considerably detuned. Plus there was that guy in Oz who hung a Stalker(?) 60 in a Nobler. There is plenty of anecdotal evidence of this method working, but I'm not sure where the physics or aerodynamics come in. How does one define "too small" (a wing) when people keep making them fly better with progressively bigger engines? And then how could one go backwards and redefine the Tutor for an FP-20 vs a ST-46?
I don't think it has much to do with the wing size or making it seem bigger. The ST was an experiment, in one way, and the most practical solution to my other problem - that is, I knew an ST46 would keep running through outside corners! That's a lot more than I can say about the Fox, in those pre-bypass-stuffer days.
The experiment part was to see if what was, at the time, a grotesque increase in "power" would make it work better in the wind. The answer is - sort of. To get the full effect of the ST, you needed a pretty larger-diameter prop. That killed the cornering, even compared to the Fox, and the resulting break caused the wild power burst problem. It certainly worked better than it did with a Fox, but it was not significantly better than the larger airplanes with half the power/weight ratio. At the time (and this was a long time ago, 1982) I had little idea how to go about fixing these problems. Running less diameter helped both issues, but reduced some of the beneficial effects. It certainly ran a lot more consistently that way. But in any case, as long as it was running 6" of pitch it was still going to whip up, and it was no better in that regard than airplanes twice the weight with the exact same engine.
Of course, what I thought was a "grotesque" increase in the power really wasn't. An ST is certainly stronger than a Fox, but both are feeble and ineffective compared to even a Magnum 36. And although I hadn't quite worked through the math at the time, there's no way to make use of the superior capability of the ST46 on that airplane. Certainly the level flight power of the same airplane, flying slightly slower, was less than with the Fox, add in 2" more diameter, and it's down on shaft HP, too, compared to the Fox. Lower speed = less hp applied to the airplane = less thrust, more diameter = more efficient = less shaft HP required to maintain any particular speed. The static thrust was drastically higher, the acceleration out of the corners was somewhat improved, but that, too was a bit of a double-edged sword, since if you tried to get much power out of it, it tended to leap out of the corners, and the breaks were too strong. The only thing it did much better than with a Fox (aside from *continuing to run*, which is always a plus) was penetrate into the wind in the vertical 8, hourglass, and overhead 8. Although I couldn't put my finger on it at the time, the cornering was actually worse because I was swinging a lot of prop on a tiny airplane, and particularly with relatively tiny tail. And there was a bunch of extra weight in the nose, and a bunch of solder wrapped around the tail wheel strut to balance it, greatly increasing the pitch MOI.
The bottom line, and this was well-understood at the time, airplanes running 4-2 break motors needed to be in a very small range of sizes to optimal performance. Too big, too much power variation, too little, poor handling characteristics.
The 20FP/9-4 would solve a lot of those problems, mostly, by being able to run a smaller prop *with less pitch*. The combination of the low pitch and gentle response in a rich 2-stroke completely solves the "power burst" problem, and the smaller diameter at high revs permits the use of more shaft HP in level flight. The control response with the small prop would certainly be superior and much more consistent, and not cause a lot of trim problems. It would fly faster, but the smooth engine response would make that a very minor issue. So I expect that it would fly MUCH better with a 20FP than it did with an ST46.
High rev/low pitch motors, even more so those with tuned pipes, don't seem to care very much what they are attached to. That's why you can put a 40, or a 91, in an Impact or similar, and get relatively small change in performance. You have to drastically detune them anyway, so when you use different motors, all you are doing is selecting for the run characteristics, not how much "power" they have. I don't really concern myself about being able to get enough power any more, it's a moot issue. But *how* it runs is critical.
I don't think a lot of people have fully clued in to this situation, but I think my analysis is correct.
Brett
Topic: Top Flite Tutor, the old original 35 kit, not the later ARF; how good ? (Read 25557 times)
