ORIGINAL: D Bronk
ANYBody have any test records ,as to a prop size/rpm etc.I`m especially intrested in theBIG MIG 0.074" at the moment but the 0.049" + 0.061" would be nice to have in here too
Depends on the prop brand. An APC 7 X 3 and a Grish Tornado will not turn the same rpms and performance will not be the same. For sport applications, the best prop that I've found for the Norvel .074 is the Cox 7 X 3 1/2. Like the 6 X 3 on the .049/061, the Cox prop will turn slower but the THRUST will be there,,,, for sport applications.
This does not apply to a "need for speed" although, in a sport plane, the Cox will sometimes out fly some other brands spinning much faster.
Pictured is a collection of 8 X 4 props tested on a
dieselized Norvel .074 on the same afternoon on the same plane on the same fuel. The Cox actually turned faster than the APC and also out pulled it, in this case.
The issue, when dieselizing an engine and going one prop size up, is that the up sized prop is built for the rpms (faster) of the next sized up glow engine. As those rpms are much higher, compromises have to be made sacrificing efficiency for just keeping the blades attached to the hub. Cox props take the Clark Y airfoil all the way to the hub. The APC goes VERY symmetrical for nearly a third of the blade. Grossly inefficient,,, but they'll never break. Somehow though, Cox props, despite the "thin" blades near the hub, stay together just fine. I suspect that most props are built with a 3 or 4 to one safety factor, just to keep the lawyers away.
I've taken APC props and carved the blades flat all the way to the hub. The result was a free 500 rpm along with extra thrust and efficiency. I've done the same to Grish Tornados and the result was the same. Recent experiments with the newer Master Airscrew props have given similar results. I disliked the old MA props but the newer ones are much better, particularly the wide blade series.
I DO NOT recommend modifying props in this manner unless you're prepared to take responsibility for doing so. In other words, IF you do it, be careful, as always, and in any case.
Also pictured is a collection of 6 X 3 props. You can see a variance of prop shapes and prop blade area. All of these factors along with the plane you're flying determine the best prop to use. Props are cheap, best to get a pile of them and experiment.