TCrafty

Just wondering...

I've got a Kadet Senior that has a .91 four-stroke on it with a 15X8 MAS prop. I love the combination and can keep altitude with about 3 clicks of throttle. It'll do just about anything I ask of it and I love the sound of a 4-timer just puttering along. Now, my question is this, IF you have a plane that has the clearance for a larger prop and rarely fly at 3/4 throttle or higher, could you conceiveably over-prop the engine to keep the rpm down? I'm sure this would be best on a 4-stroke and their better torque but, I'm thinking of getting something like a Stearman with the radial engine and I think the sound is much more scale-like with it like that. I've been playing around with some planes on my RealFlight sim but that's not real world. I know that the plane will fly more scale like due to the lack of full rpm but whereas I read a LOT of guy's posts who are infatuated with high-rpm, I don't use it much. Only when I'm huckin' around my Senior or one of my other planes. I just won't plan on going vertical at WOT. Anyone had any experience with doing this?

jetmech05

If you love the combination, and the plane flies great. Does what you ask.
then I have a question for you.
If it aint broke why you tryin to fix it?

scratchonly

To me that is a 120 4S prop, but if it runs fine go ahead. I like my 4S to turn about 9000 backed off slightly.

TCrafty

I wasn't necessarily meaning to do it on my Senior, I was just using that as a reference. No offense to scratchonly, but, when you prop an engine to run a specific RPM at WOT, what bearing does that have on how the plane will fly at lower RPM. Especially if you never wind out the engine to WOT. I was just out flying my Senior tonight and except for those times when I was hovering or going vertical, it was pretty much 1/3 throttle and lower. I asked this same question at the field tonight and they added that I might see some overheating of the engine if the prop goes too much larger than suggested. I was just curious if anyone had done this and what the differences were. Thanks for the replies Jetmech and scratchonly.

Sport_Pilot

If you don't want to go faster than 3/4 throttle and you want to turn lower RPMs. I would suggest useing a smaller engine on the new plane say a .70 four stroke and turn the same or nearly the same prop with it.

broke_n_bummin

Too big of a prop isn't going to look very scale either. That 15X8 is about the biggest I'd run on that .91. Actually, it might be a little big for it. The longer the prop is, the heavier it is and the harder the engine has to work to turn it. If you think it needs a longer prop, you'd be better off with a larger engine ... but then it probably wouldn't sit still on the runway.

Sport_Pilot

I thought the issue was that he wanted the engine to simply fly slow and yet have enough power. Might be better to simply throttle down an oversized engine rather than to use an oversized prop.

carrellh

A lot of .91 four strokes tend to peak around 10k with a 14x6 prop, and are typically richened a bit from there. A lot of people are looking for maximum thrust, or maximum speed, from any combination. So, they play with diameter and pitch to try to keep the engine in that range at full throttle.

If the same engine peaks at 8k (I made up the 8k number. The true number is irrelevant) with the 15x8 prop, and you richen it a bit, I do not see any reason why you'd have overheating issues.

If any setup will pull any plane around the way YOU like, then it's a fine setup (for you). I may or may not like that combination, BUT, it's not mine so it doesn't matter if I like it or not.

2slow2matter

My Feedback: (1)

If you run that prop Wide open for any length of time at all, then the motor will get excessively hot.
But that's not what the OP was insenuating. As long as you aren't pushing the engine to any limits with an oversized prop, then you should, in theory, be OK. but I would check the head temp after a typical flight and make sure it isn't over-heating. Typically, you can touch the head of a four stroke after it's been ran for a while and not burn your hand. Not saying it won't be hot, but it shoulnd't be scalding hot. If it is, then you need to re-prop it. Over-heating is the only issue I can see other than prop clearance on the model. If it isn't over-heating, then you should be fine.

TCrafty

Thanks for all who responded. I like the response of my 4-strokes, especially when it only takes a few clicks to make a big difference. I know that some guys are looking for the maximum RPM from their engines but I like just puttering around (especially with my Senior) since I think it's much more scale-like in sound. I'm currently working on a Goldberg clipped-wing Cub in L4 trim and I am going to put a Saito 72 in it. Again, puttering but with plenty of prop. Thanks again to all.