The email-bag brings questions from two readers about propeller blades. John E. Peters of The Villages, Florida asks: “Why do some aircraft, particularly of World War II vintage, have a three-bladed propeller while others have four? Is there an advantage to having one over the other?”
And Jeff Rankin-Lowe of London, Ontario, Canada asks about the blades on two more modern aircraft: “The P-3 Orion and C-130 Hercules (pre-J model) have essentially the same engine, so why do Hercs have square-tipped props and Orions have rounded tips on their propellers?”
For the answers, we turned to Jeremy Kinney, a curator at the National Air and Space Museum, who explains that the use of four blades on World War II aircraft (such as the North American P-51 Mustang) increased the blade area, which produced more thrust, without increasing the overall diameter of the propeller.
Kinney says the reason why in the late 1930s the designers of the Vought F4U Corsair used the inverted gull wing was to accommodate the big 13-foot, three-blade propeller—the largest used on a fighter up to that time. That propeller was right for the largest fighter engine at the time: the 1,800-horsepower Pratt & Whitney R-2800. When four-blade propellers came along early in World War II, they alleviated the problem of getting enough ground clearance for the tips. (Shorter, four-blade props replaced the Corsair’s three-blade props on the less curved-wing F4U-4 and –5 models in the Korean War.)
As for the Orion/Hercules propellers, “this is a tricky question and there are a lot of ‘opinions’ out there” Kinney writes in an email. “Many old-timers would say that square props are best for low speeds and takeoff performance, while round props are best for efficient cruise, which reflect the actual uses of these airframes.”
The longer answer, he says, is that rectangular, wide blades emerged as engine power increased in the late 1940s because they absorbed energy more efficiently than the traditional narrow-tapered, round-tip blades, increasing the airplane’s overall thrust-to-weight ratio without increasing its propeller diameter. Both the P-3 and C-130, says Kinney, have rectangular blades; the difference is in the tips.
“It is all about application,” he explains. “The C-130 needs high thrust at takeoff” and a square tip provides that along the entire length of the blade. The C-130’s bulbous shape shape and lower cruise speed (around 335 mph) “deters the tips from suffering from compressibility burble -- the setting off of small sonic booms as the propeller rotates at high speeds [since the tips move faster than the roots at the hub]. The P-3 is a long-distance airplane with a cruise speed that's 45 mph higher, and those rounded tips help offset compressibility burble.”
The new C-130J, Kinney points out, has “super-pointy tips” on the six blades of its two Dowty R391 scimitar propellers.