Swinging a large prop

nitroairplane wrote:Well you want the smallest air intake you can get.
Highest cooling head to allow to run as lean as you need.
A throttle ring is good.
You need diesel lol.
Also no spi as it feeds of itself and needs higher rpm to benefit.
I swung an APC 7x5 on a pee wee and got nearly 10 min on a tank.

Yeah yeah..... I need a diesel

apart from that I see where you are going with the SPI but, the small air intake would be for? I am not worried about duration as I plan on using an external tank. Unless I am missing something.

Was that pee wee diesel?

The pee wee i sent you?
Yes it was. The pee wee that turned a huge prop was glow.
Small diam intake for more steady run and reliable needling at Lower rpm.
Or you could convert it to co2.

I see,

Yeah I knew that pee wee was a diesel, took me forever to clean off all the goo!

A small diameter venturi hole may be an issue. I may have to fill a horseshoe one in and drill a smaller hole. You wouldn't happen to know what size the hole is would you?

you can always make something like a texaco engine, which is designed to swing bigger props.
It has pretty much every thing nitro suggested the engine should have:
a VERY small venturi intake and a high 5-fin glow head.

cribbs74 wrote:Ok guys,

I want to build an engine that will swing an 8x4 prop. I am not looking for high rpms but, enough to be able to pull about 15oz around the sky. I know the Texaco is an option but, I want to build one using spare product engines. My thoughts are an SPI cylinder from Bernie or Matt, as well as a Texaco head...

SPI will do nothing for an engine at less than high RPM. You'll probably develop better crankcase pressure with a non-spi set. Low crankcase pressure doesn't mix the fuel and air as well. At high RPM SPI increases pressure, but at low RPM it'll only vent it.
I might defer to any experts that disagree, but I think I've got that right.

Hi Cribbs,

One thing that has not been mentioned and I think it’s a must, is lower the compression. Compression acts much like ignition timing, by lowering compression the combustion will be delayed somewhat and there will be more fuel in the combustion to push that big prop., think tractor engine vs. Indy engine. I'd run a standard head with four gaskets, you don't really need a Texaco head as your not going to be running that lean. Bernie's carb seems to be working pretty good from what i hear, but I'm got to wonder about throttle response on that big (load) of prop. Definitely try to find a wood prop, maybe a 8x3 instead of a 8x4? Don't worry about reducing the intake, leave as is.

A cylinder that would be great if you could find one, would be from the old Space Bug Jr. The SBJ cylinder ports were cut low and narrow, to make it easy for a kid to start, the side benefit was it could swing a large prop. I remember reading in a magazine test review they ran 7", 8" even a 10"x6" prop! The 10"x6" didn't turn enough RPM to actually use, but it did run!

Happy New Year!

Mark

http://hobby1.com/store/product/109209/Slow-Flyer-Propeller%2C8-x-3.8%2C/ I know it sez, for electric only..but who reads that stuff!

I think that's so someone would not bolt that to a .25, but for a .049, which would probably turn less rpm then a electric, I'd think it would be fine.

Indunnonit is a slowly prop and APC slowflys area bit thin.

Thanks Mark,

Loads of good info, and I appreciate the link, that prop would look good on the front of my solution.

Thanks to you too Indra! Your comments are always appreciated.

nitroairplane wrote:Indunnonit is a slowly prop and APC slowflys area bit thin.

What?????

Almost forgot Beez and Rusty! Thanks. Rep points all around!

nitroairplane wrote:Indunnonit is a slowly prop and APC slowflys area bit thin.

Exactly, that’s why it might work. Do you really think a .049 will throw a blade?