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
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
