Thanks guys for your thoughts and once again years of experience. I just can't seem to get my head around the difference between 10x4 wide paddle pop blade and a skinny 10x4. Surely the wide blade shifts more air and has better bite? But does that also mean it revs slower because of the greater load. What are the actual physics involved? I suppose at the end of the day we say "Who cares? Whatever works best ."
The physics are largely the same as any other airfoil, but that's not very helpful. What is missing, particularly from hand or CAE programs, is that the feedback the engine sees during maneuvers will not be addressed. And that is one of the most important factors for regular engines.
I also caution you about getting misled by ground RPM measurements - what it measures on the ground, i.e. "revs slower because of the greater load", can sometimes be *very misleading*. For instance a 10-6 will "spin slower" on the ground if you put in place of an otherwise identical 10-5 and leave everything else the same - but it takes much less shaft power in-flight at the same lap time - so it is much less load and less demanding of horsepower than a 10-5.
That brings up the next wild misapprehension. The 10-6 requires less of the engine to go the same in-flight speed because it is better at transferring power from the engine to the airplane - which is the definition of "more efficient". Unfortunately a lot of people have been trained or led to believe that "more efficient" is the equivalent of "better" - which is possibly true if you have limited power, or if you goal is to go as fast as possible for a given airplane and maximum power.
Back in the good old days, and for some of the "retro" engines and approaches today, you need to run as efficient a propellor as possible to make use of the maximum power. That's the problem, in many cases, for full-scale airplanes. You only have so much power, the engine (geared or otherwise) can only spin so fast at a the maximum power, so you choose a prop that gives you the best efficiency at the maximum shaft HP, so you go as fast as possible.
With feeble engines like the ST46, Fox 35, etc, getting enough power was the primary issue, so highly efficient props were the norm, and you dealt with the effects of it.
Since the advent of schneurle engines (late 70's) and the knowledge of how best to use them (mid-late 80s), getting enough power to fly a stunt plane hasn't been an issue - you have far more power than you could use, the problem is using it up or suppressing it somehow, while taking the maximum advantage of it. So you typically use *extremely inefficient* props, completely different from before, and count on the efficiency changing with speed in a favorable way to maintain the maneuvering speed.
This has completely transformed how stunt works, and how much performance you have.
To answer your question directly, on 4-2 break engines (ST46, etc) wide-blade props increase the efficiency and the feedback of load into the engine. They usually work well in hot and calm conditions because you can use all the break you can get. In windy conditions, wide-blade props are notorious for encouraging "whip-up" for the same reason, your load feedback tends to be "out of phase" with the drag and dynamic wind effects, causing wild acceleration in some maneuvers (particularly the round loops).
With narrow-blade props of nominally the same "load" and level-flight speed , you have the opposite effect, it gets wimpy in the hot air and is generally better in terms of whip-up.
If you want to know what you need or might want to try in a specific condition, then we need more details. The question cannot be answered in a completely general case, because prop selection is mostly about how the engine responds, much more than how the prop works itself.
Brett
Topic: Toothpicks or fence palings (Read 1110 times)