INFO NEEDED: Franklin engines and Aeromatic props

Let me test your knowledge of older engines. I have a Franklin O-425 (I believe it is a -9 model) on my Starduster Too. It has a 90" Aeromatic prop (220 hub). Due to a careless hangar mate (not sure which one) at it's previous storage location, one of the blades was damaged and I need to replace it. The Aeromatic shop has 88" blades and 93" blades, but no 90" blades. He can make some new 90" blades, but that is significantly more expensive.

I find myself wondering if should I go with 88", 93" or fork out the extra mula for new 90" blades. In researching my engine, it says that peak HP occurs at 3300 RPM. At this RPM, the prop tips would be well past the sonic threshold (on all three blade options). Further research shows that many (all???) Franklin O-425 engines were built with an internal reduction of .623 to 1. This would put the tips at only .75 Mach.

For anyone unfamiliar with the Aeromatics, they are a self adjusting prop that uses a system of flyweights, centrugal force, aerodynamic force, and an internal viscous dampener to control the pitch of the blades without engine oil pressure or pilot input. The blades are wood core with platic or fiberglass skins, and are thicker in cross-section than a metal blade. They are an old, certified design that worked well.

QUESTION #1: Does anyone know if all of the O-425 engines has the built in reduction? The engine is a horizontally opposed 6-cyl rated between 245 and 260 HP (depending on which reference you use) and, though heavy, is said to be nearly bulletproof and very reliable. If there is a built in reduction, it must be compact, as the crankcase does not look overly large or extended.

QUESTION #2: Should I go with the 93" blades (assuming I have the ground clearance for longer blades)? I don't care as much about top speed (the flying wires get pretty loud at 140 mph), but I do want the climb. However, I don't want to strain the engine by lugging it down. Will this engine handle the longer prop?

Sorry, I know these are some pretty specific questions, but if I don't ask, I won't know if anyone has the answers.

Thanks,
Bruce

From what I can find digging through some books all the 0-425s were geared at 0.623. All were rated at 3300 rpm as you state. Without that gearing they would never have been able to turn a 'normal' sized prop, never mind those huge Aeromatics you have. I am curious - your tach presumably reads RPM directly off the crank? I am certain your engine is geared through planetary gears. You can always confirm the gearing by taking out a spark plug and putting a stick down there and seeing how often it goes up and down per prop revolution.

Originally Posted by PTAirco

You can always confirm the gearing by taking out a spark plug and putting a stick down there and seeing how often it goes up and down per prop revolution.

Great idea PT. Thanks.

Bruce what ever you do make sure that prop is in top shape. I had one on my Cruiseair.
A friend in MO had one on his Monocoupe with a Warner, he used to say it would be O'l blacky that would get him if anything would. It has not been proven but they say it did. Keep an eye on it.

Originally Posted by vortilon

Bruce what ever you do make sure that prop is in top shape. I had one on my Cruiseair.
A friend in MO had one on his Monocoupe with a Warner, he used to say it would be O'l blacky that would get him if anything would. It has not been proven but they say it did. Keep an eye on it.

Not to worry, these are all new blades we're talking about, and the hub is like new. The only difference between the options is that he has 88" and 93" in stock, but the 90" would have to be carved and glassed.

I know that there can be some problems with poorly maintained Aeromatics and with some manufacturers (they have been manufactured by several companies). According to my research so far, I believe that I have a sound setup and have gone to the source to make sure.

Still, I would be interested in hearing about what happened with your friend's prop (here or by private message). You can never have too much knowledge.

Bruce

Bruce,

Please give me the following data and I give you the diameter.
Max SL (or low level speed)?
Max RPM on your setup?
Max thickness and blade width at 75% of prop radii?
Max blade width of blade (around 40-50% of radii)?
Your field altitude?

Jan
www.JCPropellerdesign.com

Originally Posted by Jan Carlsson

Bruce,

Please give me the following data and I give you the diameter.
Max SL (or low level speed)?
Max RPM on your setup?
Max thickness and blade width at 75% of prop radii?
Max blade width of blade (around 40-50% of radii)?
Your field altitude?

Jan
www.JCPropellerdesign.com

Unfortunately, I don't have all of this information available, especially since the prop is at the Aeromatic shop.

If 140 MPH is max - I would go to 93" diam

Jan

I'd go big too, Bruce.

For instance, the 80' Hartzell CS will out pull just about anything on a o-360, but crank harmonics are an issue on most engines. That said, I doubt the Aeromatic with it's well dampened wood blades has such issues, so longer is almost certainly better.

When I get around to doing my Too, it will have a Lyc. 435 with 320 cylinders (480), and a real long wood prop.

I was leaning in that direction too, so I finally bit the bullet and told him to build it with the 93" blades. Thanks all!