Propeller Design

[Michael Hope]

You are EXACTLY right! The propeller is only at its maximum efficiency at one one speed for a given pitch. There are three modes for which a change-speed gear-box is desired but the one case that generated the most intensive development activity concerned the USAAF's determination at the close of the 1930's that a bespoke, ultra-high output (5,000-8,000Hp) 'bomber engine' was required for super-heavy, long (very long!) range bombers and transports. These engines would typically have low specific outputs due to their huge displacements so they required maximum thrust for take-off and climb-out but once settled in at cruise altitude the power gear-ratio was simply too high to maintain propeller rpm's once the engine was throttled back to lowest sfc point on the rpm range so a second optimal gear ratio was necessary (torque to spare!). Blade pitch change was still available on both ratios. gear-box fabrication was undertaken by Merz Engineering, Ohio. For further details, consult GOOGLE patents, Wright's designs at least, are there.

So if I read this correct, the propellers are variable pitch, it is the engine that has a gear box so the propeller will operate in its most efficient RPM. So it is the engine has a TWO SPEED propeller output shaft, so the propeller itself is not TWO SPEED, it is just the engine/powerplant, is this correct. If so your posting about engine design in a propeller design post. It is an interesting subject though. The largest diameter propellers that I have worked on are only 17 feet in diameter from a L-1649.

 

[special ed]

I really appreciate the knowledge and responses in this thread. As an "apprentice assistant propeller repairman" for three years (!963-65) in the USAF reserve, I was amazed at the condition of the balls in the first teardown I witnessed of a prop from our C-119. The many small flats on the balls left it not a true sphere. Prior to the USAF, I only had some knowledge of fixed pitch two blade metal or wood props on light planes. Again thanks for everyone's explanations.