OT: Prop question for B-25, B-26

for a layman, what is the reasoning behind the fact that the B-25 had a 3 blade prop and the B-26 had a 4 blade? Same basic size, probably different engines, was
just flipping thru a catalog and it caught my eye....
what would happen if the B-25 used a 4 blade prop?
Craig
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Off the top of my head I'm thinking the Marauder used an R-2600. It had the power to make use of a four bladed screw.
WmB
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snipped-for-privacy@earthlink.net wrote:

Craig
It has to do with the amount of horsepower that the prop has to absorb. The B-25 had R2600 engines with about 1700 HP, while the B-26 had R2800's with about 2000 HP. You have a choice of longer blades or more blades. If ground clearance is a porblem, then more blades is the answer.
Mike
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There was a good article a few years back in one of the magazines where somebody wrote about an attempt by NAA to do just that, so they did put a couple of 2200 HP radials with four-bladed props on a B-25 to see what would happen. It was a rocket ship, but from what the article said while it was making a low altitude pass at around 390 mph the airframe disintegrated and the crew was killed.
Good reason why they didn't go any farther with that idea...
Cookie Sewell
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snipped-for-privacy@earthlink.net wrote:

    Besides the other observations stated, I'm pretty sure the design of the aircraft had a ton to do with it; by this I mean the B-26 was much 'cleaner' than the B-25 with less drag and was designed for different requirements. Also, if a prop with too much pitch or added blades were used on a given engine, overheating and excessive wear could occur. There are definite reasons for using any propeller.     Not unlike photography, aircraft design has many variables to control for acceptable results. The narrower the meaning of 'acceptable', the tighter the tolerances but greater the rewards...
Frank Kranick
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snipped-for-privacy@earthlink.net wrote:

The more blades you have, the more you suffer a slight loss in efficiency. However, you can only make the blades a certain diameter before, at operating rpm, the tips get to a critical mach number and you lose efficiency there. The ratio of blade width to diameter also affects efficiency. So, as you increase hp, there is a point reached where you need to go to another blade. Some piston engined planes after WW2 had five or six blades. The efficiency loss in adding a blade is often less than that suffered by increasing width or length of blade.
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