Are more propeller blades better?
- By Paul Hoversten
- AirSpaceMag.com, February 27, 2012
A sailor hoses down the propeller on a P-3 Orion during an aircraft wash on the flight line at Naval Air Station Whidbey Island, Washington.
U.S. Navy
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.
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Comments (8)
Straight-wing Corsair? I went looking for pictures of that bird and I found a forum on Finescale Modeer Magazine's web site in which one commenter mentioned a friend of his dad's who talked about a straight-wing Corsair being based on the Pacific island on which he served, but the responders were all but unanimous the friend had seen a Hellcat. Closest thing I found to a picture was on aeroknow.com, belonging to a guy who mashes parts together from different model plane kits to make up funny planes. (The jet-powered Spruce Goose is a hoot.)
Posted by Don Lee on March 8,2012 | 01:13 AM
Straight wing Corsair? No such thing.
I also think you will find that the C-130J has FOUR Dowty props, not two!
This article misses some key points. Firstly, a larger diameter propeller is more efficient than a smaller diameter one, but is limited generally by the speed of the propeller tip. If a combination of the propeller rpm and the airspeed of the aircraft causes the air at the prop tip to go supersonic, it breaks away from the tip and causes a significant loss in efficiency and thus forward speed.
A solution for a given engine power/rpm is to reduce propeller diameter (lowering the tip speed for the same max rpm) and to help absorb the power, add another blade to the prop disc. But if the blades are too close together, the airflow over preceding blade will interfere with the next blade and reduce efficiency, particularly at lower airspeeds.
6 blades seems to be a reasonable maximum number, but ongoing propeller blade shape may mean more blades may soon be practical.
An 'H' model C-130 Hercules and the P3C orion may have essentially the same engine but the applications are different. The Orion is designed to spend most of its time on patrol at low altitudes where the air is dense, and the propellers are optimized for this. The Herc does low-level stuff too, but in general spends most of its hours transiting at relatively high altitudes to and from bases, and needs a prop blade optimized for this. And guess what, the Lockheed Electra (the airliner base for the P3C Orion) has square tipped props, just like the Herc. Why? Efficiency at high-level cruise, just like the Herc.
Posted by Noel Puzey on March 11,2012 | 10:18 AM
Also apparently there were two brands of props used - the hollow ones are square ended because that is the best way to seal off the hollow inside. They are lighter but less efficient than the rounded end props. Rounded tips help to reduce tip vortices and thus increase the efficiency of the prop.
Posted by Noel Puzey on March 17,2012 | 04:06 PM
This article and comments about propeller design is very interesting. I have a question. How does propeller design affect the speed of an aircraft? Specifically, I've noticed that in air racing, some of the aircraft will use different propellers in the quest for speed and the checkered flag. I've seen the unlimited air racer Rare Bear, a modified Grumman Bearcat, use both 3 bladed and 4 bladed props. Which is better for speed? Another interesting design is the Hawker Sea Fury which uses a 5 bladed prop with its Bristol engine. Interestingly, most Sea Furys that race use 4 bladed props mounted to a Pratt & Whitney engine. If anyone has information on these designs I would be interested to hear it.
Posted by Rich Stalter on March 22,2012 | 11:07 AM
Longer blades are more efficient up until the tips start developing shock waves. Once the shocks form, overcoming the drag on the blades begins to absorb a large portion of the engine's power and it becomes advantageous to reduce the diameter and increase the number of blades. There are ways of reducing the effect of shocks on the blades - see the scimitar blades proposed for some of the propfans of the 1980's and un-ducted or open fans being proposed today.
Posted by Mike Osborne on March 25,2012 | 07:18 PM
Oops! Of course the C-130 has four propellers! Good catch, Noel!
Posted by Jeremy on April 20,2012 | 11:15 AM
Rich,
The Unlimited racing community has a lot of opinions on propellers. The spirit of adaptation that drives the teams to experiment with propellers from different airplanes is rooted in that quest for more speed. The more efficient the propeller, the more energy from the engine is being turned into thrust.
You can find an interesting comment from Rare Bear's creator, the late Lyle Shelton, about propellers here:
http://www.rarebear.com/bearfacts/april-may_LS.htm
Overall, deciding which propeller is better is pretty much determined through trial-and-error, a sophisticated knowledge of the technology, and what suits the needs of a particular airframe/engine combination. Hope that helps.
Posted by Jeremy on April 20,2012 | 11:28 AM
I would add, in the case of the modified prop the Rare Bear used to run... it was more than simply going from 4 blades to three. The P-3 Orion blades are of MUCH wider chord than the 4 blade Skyraider aeroproducts prop. As such, they are much heavier so it required moving to the C-130A hub with only three blades. Even with just three, there is MORE blade area than with 4 of the Aeroproducts blades. They have since moved back to the Aeroproducts 4 blade (which has less rotating mass) as the course has shortened over the years and the 3 blade became too much of a handful.
Posted by Rich Henry on July 26,2012 | 01:34 PM