Wrong Propeller--How Important?
 

I have a question concerning the prop on my old, old Carl Goldberg Piper Cub. First let me say that this is a total junk plane in every way. I don't care about its well-being beyond being basically flyable. I bought it well used with it's shortened wings and a KB .61 and no cowl. It was always the dickens to fly. Just too sensitive and powerful. The .61 finally gave up the ghost so I bought a used $30 ASP .46, just to tone it down some. I just want to fly more leisurely than it would let me.<br />While mounting the .46 I and dismounting the prop off the .61 to put on this one, I noticed the prop is an 11x7 Master Airscrew nylon job. It appears that either engine called for 10x6 or 11x6. But I'm just going to put this 11x7 on it because I don't want to spend another dime on this piece of junk. Is there any reason I can't get by with it?

Very important to have the right size prop. A prop to large will load your engine to much. Cause it to wear faster. Make it hard to tune. Cause it to over heat. Burn out glow plugs faster. Give you dead stick landings. You will never get your engine up to its maximum HP with to large of prop. For no more than a prop that size would cost it would be well worth getting the proper size.

Just cut 1/2" off each tip and re-balance it.

My son used a 11x7 prop on his .46 for years with no ill affects. just don,t run it lean, the larger prop will pull better

Sounds like it should be retired. When the .61 gave out, it would have looked cool hanging from the ceiling at, say a Cracker Barrel or someplace like that.

Sure, you can get away with an 11x7. It's pretty close but you wont get the best performance. The thing is, props are relatively cheap. To a degree you can consider them a consumable item. Even if your LHS is not conveniently close you can mail order a couple of props and have them in a few days.

Two cycle engines are designed to run at a high RPM. If they are over prop'ed they will seem to run good and you'll fly for years that way. If you tune your prop for RPM's you'll be amazed how much more performance you'll get. I tune my engines for a 12,000 to 14,000 RPM's. Props are cheep. Buy several and test them. Large diameter / low pitch = more pulling power with lower speed (like first gear in a car). Small diameter / high pitch = less pulling power with higher speeds (like fifth gear). Depends on your plane and flying style.

Skyhawk

NEVER trim a prop diameter!!!! Not a good thing to do!

Never say never. Modelers have traditionally modified props, especially competitors who may need a little more thrust than a stock prop provides.
There are some additional considerations, especially balance. But I wouldn't hesitate to modify a wooden prop. A Master Airscrew with relatively straight edges can be easily and safely trimmed down equally on each end a small amount and rebalanced, even under field conditions.
As usual, common sense is required and YMMV.

I trim props all the time . Never had a problem . Sometimes because I don't have the size I want . Other times to get the rpm I want.