The tip going supersonic depends on the radius and rotation speed.
What determined the number of prop blades?
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The tip going supersonic depends on the radius and rotation speed.
GregP
Major
Here's a quote from an article in the Smithsonian Air Space magazine."
"The Bearcat was painstakingly reconstructed with a donated R-3350 engine of unknown provenance shoehorned into the cowling. Nine months after he had trucked the wreck to Compton, Shelton flew the airplane to Reno for the 1969 races. Hickle remembers people looking at the big engine, shaking their heads, and muttering, "You stupid suckers." Nobody in air racing had tried such a radical conversion before, and the Wright radial was still dogged by a reputation for setting B-29s on fire during World War II. Sure enough, the first engine blew up. In 1970, another 3350 of uncertain vintage also went kerplooey.
For the 1971 race, Shelton got an engine customized by Mel Gregoire, who had been servicing Wright radials since 1950. Gregoire worked for Aircraft Cylinder Turbine in Sun Valley, California, and company owner George Byard donated the engine to Shelton. "I don't know if there's anybody alive who's worked on those engines longer than I have," says Gregoire, who is, at 91, still Cornell's guy for engine advice. Gregoire knew that during the 1950s and '60s, ultra-rugged versions of the 3350 had been developed for airline and military use. The engines were too big and heavy for air racing, but their pieces were stout enough to withstand extreme stress, so Gregoire mixed and matched components to create a one-of-a-kind monster. From a Lockheed L-1649 Starliner, he took a nose case designed for a slow-turning prop and mated it to the so-called power section—crankcase, crank, pistons, and cylinders—lifted from a Douglas DC-7 (which also provided the Bear's engine cowling).
Shelton and the hot-rodded Bear won their first race at Cape May in 1971, then, beginning in 1973, finished first three times running at Reno (though he was disqualified in 1974 for not pulling up during a caution). But boom was followed by bust. After a blown oil line, then a gear-up landing at Mojave in 1976, there was not enough money to repair the airplane, so the Bear sat forlornly at Van Nuys Airport, without an engine or obvious prospects.
One of the witnesses of that spectacular gear-up landing was Dave Cornell, attending his first air race. A self-taught engineer who created special effects for the movie industry, Cornell saw the airplane again a few years later while he was taxiing at Van Nuys Airport during a flying lesson. He volunteered to help get the Bear back in the air, and he apprenticed with several of the aging wizards of air racing. But even pumped with plenty of nitrous oxide and a witch's brew of nitromethane, Rare Bear couldn't keep up with newer, more sophisticated warbirds. "It dawned on me that if we were going to get in the hunt, we needed a much more powerful supercharger," Cornell says.
He snagged a blower from a Lockheed EC-121. The supercharger had been designed for direct-head fuel injection, a technology that wouldn't fit inside the Bear, so Cornell re-engineered the supercharger to work with the existing pressure carburetion system. Normal rated power of a stock 3350 was 2,800 horsepower at 2,600 rpm and 45 inches of manifold pressure. With Cornell's mods, the engine made 4,000 ponies at 3,200 rpm and 80 inches of manifold pressure—4,500 horsepower with a shot of nitrous. With that engine and a slicked-up airframe, Shelton kicked holy butt, demolishing the three-kilometer speed record and dominating at Reno from 1988 through 1991.
In fact, it was the 1991 Gold race that best showcased the formidable partnership of man and machine. As soon as he heard the traditional call—"Gentlemen, you have a race!"—Shelton hammered down the chute and led the field around the first pylon. Riding his tail were Bill "Tiger" Destefani in the P-51 Strega and Skip Holm in Tsunami, the great coulda-shoulda-woulda scratch-built racer that never caught a break. Destefani and Holm dogged Shelton for 73 miles, but the Bear ran like a scalded 'cat, maintaining a winning gap the entire race. "That was the best race I've ever seen," says Pete Law, a longtime Lockheed Skunk Works thermodynamicist who has provided engineering support for virtually every Unlimited winner at Reno since 1966. "It was the race of all races." Lyle Shelton never won another Gold.
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I do not know the actual diameter of the P-3 prop, but I bet Dave Cornell does.
- Greg
"The Bearcat was painstakingly reconstructed with a donated R-3350 engine of unknown provenance shoehorned into the cowling. Nine months after he had trucked the wreck to Compton, Shelton flew the airplane to Reno for the 1969 races. Hickle remembers people looking at the big engine, shaking their heads, and muttering, "You stupid suckers." Nobody in air racing had tried such a radical conversion before, and the Wright radial was still dogged by a reputation for setting B-29s on fire during World War II. Sure enough, the first engine blew up. In 1970, another 3350 of uncertain vintage also went kerplooey.
For the 1971 race, Shelton got an engine customized by Mel Gregoire, who had been servicing Wright radials since 1950. Gregoire worked for Aircraft Cylinder Turbine in Sun Valley, California, and company owner George Byard donated the engine to Shelton. "I don't know if there's anybody alive who's worked on those engines longer than I have," says Gregoire, who is, at 91, still Cornell's guy for engine advice. Gregoire knew that during the 1950s and '60s, ultra-rugged versions of the 3350 had been developed for airline and military use. The engines were too big and heavy for air racing, but their pieces were stout enough to withstand extreme stress, so Gregoire mixed and matched components to create a one-of-a-kind monster. From a Lockheed L-1649 Starliner, he took a nose case designed for a slow-turning prop and mated it to the so-called power section—crankcase, crank, pistons, and cylinders—lifted from a Douglas DC-7 (which also provided the Bear's engine cowling).
Shelton and the hot-rodded Bear won their first race at Cape May in 1971, then, beginning in 1973, finished first three times running at Reno (though he was disqualified in 1974 for not pulling up during a caution). But boom was followed by bust. After a blown oil line, then a gear-up landing at Mojave in 1976, there was not enough money to repair the airplane, so the Bear sat forlornly at Van Nuys Airport, without an engine or obvious prospects.
One of the witnesses of that spectacular gear-up landing was Dave Cornell, attending his first air race. A self-taught engineer who created special effects for the movie industry, Cornell saw the airplane again a few years later while he was taxiing at Van Nuys Airport during a flying lesson. He volunteered to help get the Bear back in the air, and he apprenticed with several of the aging wizards of air racing. But even pumped with plenty of nitrous oxide and a witch's brew of nitromethane, Rare Bear couldn't keep up with newer, more sophisticated warbirds. "It dawned on me that if we were going to get in the hunt, we needed a much more powerful supercharger," Cornell says.
He snagged a blower from a Lockheed EC-121. The supercharger had been designed for direct-head fuel injection, a technology that wouldn't fit inside the Bear, so Cornell re-engineered the supercharger to work with the existing pressure carburetion system. Normal rated power of a stock 3350 was 2,800 horsepower at 2,600 rpm and 45 inches of manifold pressure. With Cornell's mods, the engine made 4,000 ponies at 3,200 rpm and 80 inches of manifold pressure—4,500 horsepower with a shot of nitrous. With that engine and a slicked-up airframe, Shelton kicked holy butt, demolishing the three-kilometer speed record and dominating at Reno from 1988 through 1991.
In fact, it was the 1991 Gold race that best showcased the formidable partnership of man and machine. As soon as he heard the traditional call—"Gentlemen, you have a race!"—Shelton hammered down the chute and led the field around the first pylon. Riding his tail were Bill "Tiger" Destefani in the P-51 Strega and Skip Holm in Tsunami, the great coulda-shoulda-woulda scratch-built racer that never caught a break. Destefani and Holm dogged Shelton for 73 miles, but the Bear ran like a scalded 'cat, maintaining a winning gap the entire race. "That was the best race I've ever seen," says Pete Law, a longtime Lockheed Skunk Works thermodynamicist who has provided engineering support for virtually every Unlimited winner at Reno since 1966. "It was the race of all races." Lyle Shelton never won another Gold.
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I do not know the actual diameter of the P-3 prop, but I bet Dave Cornell does.
- Greg
swampyankee
Chief Master Sergeant
- 4,022
- Jun 25, 2013
Sorry for the necroposting, but this myth -- that fewer blades is more efficient -- needs to die. I did prop aero at HSD, and asked my manager and the other propeller aerodynamicists about this very issue: no, fewer blades are not more efficient, as the induced losses decrease with the number of blades for a given diameter: generally, more blades is better if diameter is fixed (more diameter is always better if aircraft geometry permits). Since induced drag is the source of (roughly) half the drag on a propeller, reducing induced losses will increase efficiency; that is, if you need a total blade area of X, you are better off spreading that X among more blades, however the minimum area per blade was usually dictated by structural -- not aerodynamic -- requirements at takeoff. When I was doing sizing studies (I was a propeller aerodynamicist; I'm in recovery
), one of the variables we looked at was the number of blades, and we never ended up with two or three on any modern aircraft. Other issues with increased number of blades include increases in complexity -- each blade requires a pitch-change bearing -- decreased resistance to FOD, and increased cost, as blade cost is less than directly proportional to blade area, so each blade on an 8-bladed prop will cost well more than half the price of each blade on an four-bladed prop. The pitch change mechanism, by the way, won't be much more complex: there is one hydraulic piston controlling all the blades. One of the other things going on is that propeller blades usually pivot about 50% chord; increasing chord increases the loading on the pitch change mechanism.
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The argument about fewer blades just ignores the physics of an aeroplane in the real world. Model race planes may use a single blade with a counter balance but they are transmitting a fraction of one horse power for which a plastic prop is good enough and tips going supersonic is not an issue. If you scale up that single blade plane it would need bearings and mountings from a battleship, the weight may be counterbalanced but the forces aren't.
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swampyankee
Chief Master Sergeant
- 4,022
- Jun 25, 2013
Corrected.
Thankyou.
GregP
Major
Hi Swampyankee,
Not to start anything, but if more blades is better, why is it that almost ALL control line speed records are held by 1-blade efforts?
The 1-blade guys RULE in control line racing and always have. The issue is not in doubt, at least there. Maybe there is a diameter where "more is better," but is isn't a small diameter!
Also, the 2-blade version of most Van's RV are a just a but faster than the 3-blade versions. That sort makes it interesting. The 3-blade guys quote 2 or 3 blades, without ever talking about equivalent blade area. If they have equivalent blade area, you contention doesn't hold up. Perhaps if the 3-blade HAS more area.
Not to start anything, but if more blades is better, why is it that almost ALL control line speed records are held by 1-blade efforts?
The 1-blade guys RULE in control line racing and always have. The issue is not in doubt, at least there. Maybe there is a diameter where "more is better," but is isn't a small diameter!
Also, the 2-blade version of most Van's RV are a just a but faster than the 3-blade versions. That sort makes it interesting. The 3-blade guys quote 2 or 3 blades, without ever talking about equivalent blade area. If they have equivalent blade area, you contention doesn't hold up. Perhaps if the 3-blade HAS more area.
That is what I read in swampyankees post. A bigger diameter is always better until it becomes impractical. RC aircraft are very small and things dont scale up in a linear fashion. A scale model aircraft can take off vertically and land almost vertically without damaging the air frame.
An early spitfire with a two blade prop has 500 hp acting on each blade to push it forward, if it were a single blade you have all the force acting on one side of the prop shaft. Although the blade weight may be counter balanced the force isnt. on models the forces and turning moments are small.
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GregP
Major
Yes. but do the propellers have EQUAL balde area? If they DO, then compare them. If they don't, the 1-blade is at a disadvantage naturally.
A single balde with equal area is FAST, produces great thrust, and has less totla tip turbulence, but not many general aviation manufacturers make a 2-blade prop with a very wide blade chord to equal the blade area of their equivalent 3-blade unit.
If they did, we'd SEE, post haste! Since GA aircraft don't have to shoot bullets through the prop, it wouldn't interfere at all. They could ALL do it. Then question is whether or not they will, and what are the ramifications of same?
Same blade area for 2 and 3-blade units ... and then performance of both.
Tough to find!
A single balde with equal area is FAST, produces great thrust, and has less totla tip turbulence, but not many general aviation manufacturers make a 2-blade prop with a very wide blade chord to equal the blade area of their equivalent 3-blade unit.
If they did, we'd SEE, post haste! Since GA aircraft don't have to shoot bullets through the prop, it wouldn't interfere at all. They could ALL do it. Then question is whether or not they will, and what are the ramifications of same?
Same blade area for 2 and 3-blade units ... and then performance of both.
Tough to find!
I was simply making the point that if you put (for example) a single blade from a C 130 on a 2000BHP engine the force has no counter balance, instead of pushing the plane forward it would act as a lever on the prop shaft with the moment of force going around with the propeller blade. Two three four and multi blade props are balanced in this respect and the force exerted by each blade goes down as the number of blades increases. There maybe other factors too, are multi blade props quieter for example, they seem to be on helicopters.
What about german late war propeller design? There blades were getting progressively wider and the chord had its maximum width in the middle of the blade being wider than those of allied propellers. The tips were rounded compared to the square ones of the allied props.
Once read that the german solution gave advantages in climb rate while the allied design was better for speed.
After the war the german prop design all but vanished as far as I can tell.
Postwar there were no opponents for piston engined fighters to get an energy height avantage over that would justify such a prop design.
Speed was more important.
I guess the german approach might have been better at altitude also.
The late/post war soviet planes kind of still had older blade design (almost tooth stick) that might be more advantageous at low altitudes for these planes were traditionally built to perform best down low.
Once read that the german solution gave advantages in climb rate while the allied design was better for speed.
After the war the german prop design all but vanished as far as I can tell.
Postwar there were no opponents for piston engined fighters to get an energy height avantage over that would justify such a prop design.
Speed was more important.
I guess the german approach might have been better at altitude also.
The late/post war soviet planes kind of still had older blade design (almost tooth stick) that might be more advantageous at low altitudes for these planes were traditionally built to perform best down low.
swampyankee
Chief Master Sergeant
- 4,022
- Jun 25, 2013
For r/c models, I would suggest two things: Reynolds' number and structural requirements permit a one-bladed prop to be sized without far too much blade area, reducing skin friction drag.
If the two- and. three-blade props for the RV are the same diameter and properly twisted, the three-bladed prop probably has too much blade area. Increasing the number of blades only reduces induced drag.
If the two- and. three-blade props for the RV are the same diameter and properly twisted, the three-bladed prop probably has too much blade area. Increasing the number of blades only reduces induced drag.
Shortround6
Major General
For every propeller there is a condition at which it is most efficient (most thrust for power input) but this condition varies not only with the speed of the aircraft but the air density. The Prop is trying to convert power from a spinning shaft into thrust by means of accelerating a quantity (weight/mass) of air to a speed proportional to the desired aircraft speed. There is no one propeller (or even blade design) that is ideal under all conditions.
On a model airplane you have very high rpm engines (in general), with the control line speed engines perhaps going over 34,000rpm?
even a small model propeller is going to have a a lot of drag from an an extra blade and obviously the amount of blades needed to "grab" the air is rather small if the prop is making around 2 revolutions for every foot or less the plane moves. A 400mph fighter with a prop speed of 1500rpm on the other hand is moving 23.4 ft for every revolution of the propeller. Now divide the 23.4 ft by the number of blades. Now throw in the fact that the air is about 1/2 the density at 20,000ft than it is at sea level and the prop has to move twice the volume of air at the same speed to get the same thrust and we start to see the problems with propeller design. A bomber or transport often used a larger diameter prop with a different reduction on the same engine as a fighter in order to suit the thrust to the speed of the airplane.
On a model airplane you have very high rpm engines (in general), with the control line speed engines perhaps going over 34,000rpm?
even a small model propeller is going to have a a lot of drag from an an extra blade and obviously the amount of blades needed to "grab" the air is rather small if the prop is making around 2 revolutions for every foot or less the plane moves. A 400mph fighter with a prop speed of 1500rpm on the other hand is moving 23.4 ft for every revolution of the propeller. Now divide the 23.4 ft by the number of blades. Now throw in the fact that the air is about 1/2 the density at 20,000ft than it is at sea level and the prop has to move twice the volume of air at the same speed to get the same thrust and we start to see the problems with propeller design. A bomber or transport often used a larger diameter prop with a different reduction on the same engine as a fighter in order to suit the thrust to the speed of the airplane.
Robert Porter
Senior Master Sergeant
I recall there was a great deal of current research being done on ducted fans, which I guess would qualify as propellor driven? Seems to me the current thinking is lots of blades with adjustable angles that work in concert. Takes a humongous amount of real time processing power but the result is an extremely stable vtol capable platform.
Shortround6
Major General
There was quite a bit of development going on in the late 40s and 50s between the Piston airliners and turbo props.
And development has continued at a low level for even relatively small planes.
DC-7 props
C-130 prop
C-130s have been going to 8 bladed props.
General aviation has "Q-Tip" props
No, not a result of a prop strike. Slightly better performance and quieter than normal props.
Scimitar shaped blades are also being marketed for light aircraft.
And development has continued at a low level for even relatively small planes.
DC-7 props
C-130 prop
C-130s have been going to 8 bladed props.
General aviation has "Q-Tip" props
No, not a result of a prop strike. Slightly better performance and quieter than normal props.
Scimitar shaped blades are also being marketed for light aircraft.
GregP
Major
We could take a few pointers from marine engineers. Ships spin a propeller ina much thicker medium, but it has similar erquations with vastly different densities. Ducted fans were mentioned above, and we should do some experimenting.
In the marine world, ducted propellers are many times called Kort Nozzles and the "fast version" is called a Rice Speed Nozzle. Kort Nozzles give some of best bollard pull (static thrust) with the Rice Speed Nozzle being just a hair better, but the rice speed nozzle is the fastest if forward motion (dynamic thrust), by long shot.
Here is one comparison. There are others:
A look at the ducts will reveal:
On the surface, it looks like the Kort Nozzle is sort of a Clark Y, flat-bottom airfoil and the Rice Speed Nozzle is more of a semi-symmetric airfoil, with some under camber in it. The underwater ducted fans all lift toward the inside of the duct. Maybe someone whould try a laminar duct airfoil.
Another really innovative solution is the Voith Water Tractor. But it may not be possible to make a aircraft propulsion system with this technology. Here is a small ship with two Voith units:
And hare is a Voith without the protective shround:
Basically, as the Voith prop spins, you control it like a helicopter blade, making it produce lift in the direction desired on both sides of the circle, and the lift can be directed in a 360° arc, in any direction that is 90° to the prop blade span.
I'd love to see some experimentation with the same airframe tested with various propeller configurations, and would speficy all props have the same total blade area at first, then experiment to find the bext combinatrion. Seems like something the old NACA would do in aheartbeat, but NASA only spends on esoteric crap of limited immediate use. SOMEBODY should find out which prop configuration results in the best static and dynamic thrust. Propellers are sure to be around for a long time to come. Since that's true, why not use the best prop combination for a specific task?
If you recall, the Custer channel wing had a LOT of advantages. Ease of manufacturing wasn't one of them. But we now have materials that can be molded and are both strong and durable. Might be worth another investigation by someone. Question is, who would pay for it?
In the marine world, ducted propellers are many times called Kort Nozzles and the "fast version" is called a Rice Speed Nozzle. Kort Nozzles give some of best bollard pull (static thrust) with the Rice Speed Nozzle being just a hair better, but the rice speed nozzle is the fastest if forward motion (dynamic thrust), by long shot.
Here is one comparison. There are others:
A look at the ducts will reveal:
On the surface, it looks like the Kort Nozzle is sort of a Clark Y, flat-bottom airfoil and the Rice Speed Nozzle is more of a semi-symmetric airfoil, with some under camber in it. The underwater ducted fans all lift toward the inside of the duct. Maybe someone whould try a laminar duct airfoil.
Another really innovative solution is the Voith Water Tractor. But it may not be possible to make a aircraft propulsion system with this technology. Here is a small ship with two Voith units:
And hare is a Voith without the protective shround:
Basically, as the Voith prop spins, you control it like a helicopter blade, making it produce lift in the direction desired on both sides of the circle, and the lift can be directed in a 360° arc, in any direction that is 90° to the prop blade span.
I'd love to see some experimentation with the same airframe tested with various propeller configurations, and would speficy all props have the same total blade area at first, then experiment to find the bext combinatrion. Seems like something the old NACA would do in aheartbeat, but NASA only spends on esoteric crap of limited immediate use. SOMEBODY should find out which prop configuration results in the best static and dynamic thrust. Propellers are sure to be around for a long time to come. Since that's true, why not use the best prop combination for a specific task?
If you recall, the Custer channel wing had a LOT of advantages. Ease of manufacturing wasn't one of them. But we now have materials that can be molded and are both strong and durable. Might be worth another investigation by someone. Question is, who would pay for it?
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Robert Porter
Senior Master Sergeant
Start a rumor that the Russians and Chinese are working on propellor technology for their Mars missions and NASA will be all over it!
swampyankee
Chief Master Sergeant
- 4,022
- Jun 25, 2013
Hey! Careful, there!
NASA has been working on long-distance rovers for operations on Mars; some of these have been propeller-driven. Some other schemes included a helicopter(!!!), balloons, winged rockets, and straight old rockets.
GregP
Major
Here is a tugboat with a Voith prop maneuvering. It weights many tens of tons.
Imagine if we could mount a Voith-type prop perpendicular to the airflow on an airplane!
Maybe it could hover! ... and VIFF ... ?
View: https://youtu.be/w6lg9GM5Ogs
Imagine if we could mount a Voith-type prop perpendicular to the airflow on an airplane!
Maybe it could hover! ... and VIFF ... ?
View: https://youtu.be/w6lg9GM5Ogs
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