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Old Oct 12, 2004, 08:05 PM
Firebreather
Mendnwngs's Avatar
Napa, California, United States
Joined Jul 2004
330 Posts
What a difference an inch makes.. (.46 BB)

Ive been flying the Tower hobby .46BB on my Avistar for a couple months now, and altho I havnt been unhappy with the engine, Ive been less than fully confident in its reliability. Ever since the initial 3 tanks I ran through it at the break-in bench, Ive been flying a 10X6 prop.

Since I was Just learning to fly, I didnt bother with even thinking about experimenting with different prop combos, as just flying straight and level posed enough of a challenge

So the reason I wasnt too confident in the engine: Id did "funny" things. At WOT, it would go from Top RPM lean, to "Four stoking" (missing) with almost no adjustment of the Top-end needle. It never transitioned without hesitating no matter how many fliers looked at it, and tried to tune it. There was a hard line at abou 30% throttle where one click less on the TX and the RPMs would drop to almost half, and it would start Four stroking again.

So, I took off the 10X6, and mounted an 11X6 on the engine, and Wow, what a difference. The top RPMS have been cut by a bit, I can hear that, but Transitions instantly became snappier, the engine throttles back smoothly from WOT to idle, and the engine lost its "Pingy" sound. Plus, it seems like the plane got a good amount more power. Landing approaches are only a couple clicks above idle, and take-off @ 50% throttle only needs half the runway.

So, from the standpoint of a newbie, I ws actually quite amazed at how big an influence the prop has on the running charecteristics of the engine.

So, how would say an 11X7, or 11X8 change the flying of the plane?

-jason
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Old Oct 12, 2004, 10:42 PM
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NW Ohio
Joined May 2004
23 Posts
I've had similar strange problems. I'm now on my third airplane which is my first with a four stroke engine. Until this plane, I never really experimented with props that much. Generally a smaller prop with more pitch, like a 9x8 will give you a higher speed but lower thrust. On the other end a larger prop with low pitch, say a 12x4 will give you more thrust with less speed. It is much like using a higher or lower gear in a car. 11x7 and 11x8 may be a little too big for a two-stroke .46, but the best thing to do is to just try it. Not only should you experiment with different sizes, try props made of different materials. With my .40 four stroke, I started out using a 12x4 wood prop which wouldn't idle well and the rpm seemed to wander without changing mixture or throttle position!! Sometimes it would just quit when I went from full throttle to idle. I thought maybe it was something I was doing wrong due to my inexperience with four stroke engines. It had me very confused until I tried 11x5 and 11x6 nylon props. They are heavier and the weight acts as a flywheel which improves idle. Sorry, that's probably more than you want to know!! Just experiment... it's fun!!
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Old Oct 13, 2004, 12:00 PM
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mikester's Avatar
Los Angeles Intl, California, United States
Joined Oct 2004
368 Posts
I think the 11x6 prop would likely be your best prop for the trainer. An 11x7 might be worth a try but I think an 11x8 would load it up too much. You could of course try a 10x8 or 10x9 as well.
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