How do I tell if I'm overpropping my engine?

This may come off as naive to some, but how do you tell if you are carrying too much diameter on your prop? I'm not talking about gross overpropping like putting a 13x6 on your Fox 35, but need to know the more subtle signs that might tell you to cut down a prop of find one a bit smaller. Obviously brute force trial and error of taking props and successively cutting them down would eventually yield useful data on which diameter is ideal for a given motor, but I'm really just trying to learn whats too much, and how to detect that.

LAST EDITED ON May-12-04 AT 12:39 PM (CDT)
 
Steve,
I'm not an expert, but the way I can tell is if the motor starts to sag at the top of the maneuvers. This is assuming the engine isn't to lean from the start. Good example is the 25LA with a 9x6 prop vs the same engine with a 9x4. The 9x6 will sag and the 9x4 will run perfect all day in all weather and is very easy to adjust the needle. The other thing about the 25 LA is that with a 9x4 prop I can get almost a 4-2-4 engine run, the 9x6 wants to screem all of the time...Brett have you observed this with your 20 to 25 LA/ FP

The more pitch you add the harder the engine will have to work and the more heat that is generated causing the engine to sag. With too much pitch it is impossible to get a good needle setting.

Again, not an expert but I have been paying attention to the experts!

When you release an airplane, the engine may well speed up as the prop starts flying at a lower angle of incidence. If there is a lot of speed up, I suspect that the engine is overpropped. Not sure what I mean by "a lot". All I know to do is try different props around what you think will work. I've been surprised by some of the results I have seen. Grunt upstairs is indeed one of the things you look for.

I've been running a TF 11-5 wide blade on a modified Fox 36. It can't handle it. Even in a 4-2-4 run the rpm sags up top. Not badly but detectable. I'll try a 10.5-5 next.
I detect the overprop condition by the calibrated ear technique. If the rpm drops in a climb, you are over propping. Another indication, overheating and leaning out after mid tank. Extra castor can help. But extra castor and smaller prop can "cure" the problem. Every engine has its limits. My Fox 36X runs a nice 4-2-4 @ 10,800 on a 10-4, but can't take a big prop because of the combat timing. On the other hand, an ST 46 can turn a 12-6 if need be but is happier on a 11.5-6 or 11-6. Just listen in the wingover.
Happy Landings

Don't forget airplane weight. I have two planes with LA 25s. The Skyray is heavy and will not move with the 9x4. I now run a 9x5 and like the performance much better. The Magician is a feather and really performs with the 9x4. I have other planes with the same engine that require different props.

Even if the engine doesn't overheat or go lean, you can sometimes get a sense of the problem right after the flight. Ask the person helping you if the engine seems too hot. If the spinner is hot (metal), you are probably overloading the engine.

Of course, all this is predicated on decent cooling through the cowl and a good needle setting.

My two cents toward higher education

Ron King