Hi Brother John,
Generally I agree with your thoughts on these matters, but with the trend toward bigger engines in all top level planes (it seems), using a pair of .15-.25 size engines on something like the F5F-1 goes against the trend of the day. Trimming is what takes away the enormous pull as you well know. If it were not so, David could never handle his little 630 sq.in. model with the PA .75 in the nose, or people flying a Shark .35 with a ST G.51.......
I would not feel comfortable with a pair of .25s in a 700 plus sq.in. model the size of the F5F-1
BIG Bear
RNMM/AMM
Hey Brother Bill.
Being a manly man myself, I used to think the same thing. After all, PJ uses a .61 in a Nobler , and it works pretty good I've been told, but we sorta enter another world when it comes to multi engined stunters.
I believe that what is working so well with larger engines in modern stunters is the effects we get from the increased prop disk area. There are several examples we can look at to help make the case.
We can figure the prop disk area for a normal prop as used on the average .40 engine, say an 11 inch prop. It figures out to slightly under 95 sq inches. It'll haul a decent 600 sq inch stunter without breathing too hard. For the average .60 size engine, we will figure a 13 inch prop, and wind up with just shy of 137 sq. inches of prop disk area.
If you think of the disk area as drag racing tires, the big, (wider) tires get better traction, or power to the track. Similarly, bigger disk area gives more drive to the plane.
Now, let's consider a pair of .15s, running 9 inch props. We've got about 63.6 sq. inches of disk area for each prop, a pair of them has slightly more than 127 sq. inches of disk area. This is very close to the .60, running a 13 inch prop, listed above. A pair of .25s running 10 inch props, is even more impressive, with the pair winding up with 157 sq. inches of disk area. A single prop would have to be longer than 14 inches to equal that.
Now, that pair of .40's winds up with a combined disk area of 190 sq. inches. so the question begs to be answered. Why isn't that even better? Well, it is, until the wind comes up. That's the root of the problem of mucho power on a stunter, but its even more noticeable with a multi engined stunter.
You can trim out the excess line tension, until the wind comes up because, wind up occurs, and you are hanging on for dear life. Gone is your ability to use the positive things about multi engined designs. You have great difficulty finessing the plane through the maneuvers in the wind. All thoughts are towards surviving the flight, not doing the maneuvers correctly.
So I hope you consider the idea that there is a real relationship to disk area driving our planes. It's more than running larger engines, rather larger props they can turn. Multi engine designs inherently get a great ratio just because they are swinging more props and therefore have more disk area than a single engined design.
I'll always remember Paul's B-17 at over 100 OZs performing on 4 OS .15fp's It never lacked for power, as far as I could see.