3 blades or 4?

[Deleted]

I’ve often wondered why the Lockheed engineers stayed with 3 bladed propellors even when they put bigger and bigger engines into the Constellation series of aircraft when they could’ve fitted 4 blade props, with their lower tip speeds and the opportunity to introduce shorter and lighter undercarriage legs. Those guys weren’t dumb, they must’ve had their reasons. I wonder what they were? Douglas went for 4.

[Tom/CalClassic]

Hi,

The two negatives I've heard about 4 bladed props is that they are not as efficient as 3 bladed props (something about the wake of one blade getting in the way of the next?), and they cut the amount of air into the air intakes (that's why the turbo Connie had 3 bladed props).

Douglas had to go to 4 bladed props due to the tight space between the prop and the fuselage, as well as between the prop and the ground. The Connie had a greater distance to both of those, and could keep using the (supposedly better) 3 bladed props.

Hope this helps,

[railrunner130]

I think generally the difference (correct me if I'm wrong) is that a 3-blade tends to be more efficient in cruise, whereas the 4-blade is better for climb.

I believe this is what happened with the C-130. The old hats at work claim that the 3-blade A-models were faster than those that had been converted to 4. The six blades on the J-model are more efficient for high-altitude cruise, but I don't know how they do on climb performance. There's an 8-bladed prop in the works that promises to increase take-off performance as well.
www.snowaviation.com/current.htm

[ozbeowulf]

Regarding the number of prop blades, my understanding for a long time has been that the fewer blades a propeller has, the more efficient it is. That theory is based on exactly what Tom mentioned; that the prop blades should remain in smooth air, not in the turbulence from the preceding blade. Obviously, rpm and aircraft speed (i.e. the rate of blade advancement) are contributing factors.

I recall reading a piece on those WWI replicas build by TallMantz Aviation for the spate of WWI flying movies around 1970. It outlined the problem they had when TallMantz tried, at first, to use 85hp Continentals in place of the original 80hp Rhones. Waaaay underpowered, basically because they lacked the torque of a low-revving prop with lots of blade area. After all, the Daimler-Benz in the Fokker D-VII maxed out at 1100 rpm and cruised at 900, IIRC.

AFAIK, multi-bladed propellers were/are used only when the propeller diameter is limited by ground or fuselage clearance and tip speed. When the diameter is too big, especially with modern high-revving GA engines, the propeller tips go supersonic and become useless. That's why Cessna 206s, 210s, etc, howl so much on takeoff. So, if diameter is limited, you can only get the total blade area needed by having more blades. It is possible to get some additional area from fatter prop blades (C-130, Convair 580, Electra) but often not enough to avoid increasing blade numbers.

There are all sorts of new innovations these days; scimitar blades, ever-increasing blade numbers, etc. I don't know, but I'm guessing they get away with it because of the also-increasing rate of blade advance that comes with higher aircraft speeds.

Cheers,

Glenn

[capflyer]

One thing with the props too is that size is determined not by amount of air needing to be moved, but rather the amount of torque needed to be absorbed by the prop to properly "load" the engine and keep it within its peak operating range. Blade count then comes in as the designers reach the maximum size to maintain proper clearances and tip speeds, they add blades to keep the size down as the torque increases.

One thing to note on the Constellations is that the power increase did not equate to a torque increase in the R3350s used. This is why they remained with a 3-blade prop. They were able to deliver the additional power without additional torque, so no increase in blade diameter was required. All aircraft have props of 15' 1" in diameter, the difference was in the width and design of the blade. The later aircraft had a wider chord blade with slingers, cuffs, and reversing props. This was the only real change. Had there been an appreciable increase in torque being delivered by the engine, they would have needed to increase blade diameter and then that would have resulted in the addition of a 4th blade. It's also good to point out here that the R3350 in the B-29 was the same as the early model Connies and the R3350 in the Skyraider and C-119 were the same as late model Connie (sans the Turbines) and they used 4-blade props for clearance reasons.

[volkerboehme]

Hi,

here's a discussion of this topic on Aerospaceweb.org:

www.aerospaceweb.org/question/propulsion/q0039.shtml

Best regards,
Volker

[Deleted]

Nov 28, 2008 10:53:21 GMT -5 Tom/CalClassic said:

Hi,

The two negatives I've heard about 4 bladed props is that they are not as efficient as 3 bladed props (something about the wake of one blade getting in the way of the next?), and they cut the amount of air into the air intakes (that's why the turbo Connie had 3 bladed props).

Douglas had to go to 4 bladed props due to the tight space between the prop and the fuselage, as well as between the prop and the ground. The Connie had a greater distance to both of those, and could keep using the (supposedly better) 3 bladed props.

Hope this helps,

Tom is right when we were looking for maximum speed out of rare Bear back in the ninties we had a Constellation Hub built up with modified P3 Round tipped blades with a wide chord. Carl Friend engineered the propeller and spinner for us for maximum straightline performance. Climb performance and acceleration did not suffer. Ultimately the Bearcats fuselage length and a hard knocking effect in the rudder pedals caused it to be removed. But in essence what Tom describes is what the engineers said were the strong points.

[Deleted]

Nov 29, 2008 0:08:34 GMT -5 capflyer said:

One thing with the props too is that size is determined not by amount of air needing to be moved, but rather the amount of torque needed to be absorbed by the prop to properly "load" the engine and keep it within its peak operating range. Blade count then comes in as the designers reach the maximum size to maintain proper clearances and tip speeds, they add blades to keep the size down as the torque increases.

One thing to note on the Constellations is that the power increase did not equate to a torque increase in the R3350s used. This is why they remained with a 3-blade prop. They were able to deliver the additional power without additional torque, so no increase in blade diameter was required. All aircraft have props of 15' 1" in diameter, the difference was in the width and design of the blade. The later aircraft had a wider chord blade with slingers, cuffs, and reversing props. This was the only real change. Had there been an appreciable increase in torque being delivered by the engine, they would have needed to increase blade diameter and then that would have resulted in the addition of a 4th blade. It's also good to point out here that the R3350 in the B-29 was the same as the early model Connies and the R3350 in the Skyraider and C-119 were the same as late model Connie (sans the Turbines) and they used 4-blade props for clearance reasons.

Not to be argumentitive for a brand new poster but the R-3350-26WA, WB, and WC engines used in the Skyraider had lighter weight crankshaft assemblies and had much lighter weight cylinders and cylinder hold down bolts. Some of the WD engines enjoyed heavier cases and crankshaft assemblies at the depot level overhauls as parts started being exhausted. but none of them got the heavier duty blower drives, and cylinder heads. The later model R-3350-42 Navy engines recieved the three planetary blower drive, Air force transport engines that had the R-3350-93, had the three planetary drive.

The L1649 with the 988TC18EA-2 actually had more torque available due to the reduced RPM of the slower nose case in comparison with the other 988TC18EA series engines.

[capflyer]

I don't think that you are being argumentative. I was being general in the statement as I don't think there's a huge need to go deep into the specifics on the differences as there were of course differences (as they always are between military and civilian versions), but their performance and specs were comparable and many parts interchangeable. I've come to know this because I've been learning about the R3350 in conjunction with the CAF's program to re-engine "FiFi".

"FiFi's" engines will be using the Case from the Skyraider, Rotating Assemblies from the C-119, and exhaust from the Connie. This will provide us with R3350s that are capable of over 3000 HP per engine, will have 2400 HP per engine available, and won't be flown regularly above 2200 HP per engine. This on an airframe that originally had only 2000 HP per engine. The result - an engine that should last us well over a thousand hours (and this on an airplane that usually gets less than 100 hours a year).

[Deleted]

Dec 12, 2008 7:41:25 GMT -5 capflyer said:

I don't think that you are being argumentative. I was being general in the statement as I don't think there's a huge need to go deep into the specifics on the differences as there were of course differences (as they always are between military and civilian versions), but their performance and specs were comparable and many parts interchangeable. I've come to know this because I've been learning about the R3350 in conjunction with the CAF's program to re-engine "FiFi".

"FiFi's" engines will be using the Case from the Skyraider, Rotating Assemblies from the C-119, and exhaust from the Connie. This will provide us with R3350s that are capable of over 3000 HP per engine, will have 2400 HP per engine available, and won't be flown regularly above 2200 HP per engine. This on an airframe that originally had only 2000 HP per engine. The result - an engine that should last us well over a thousand hours (and this on an airplane that usually gets less than 100 hours a year).

I think it is genius to get rid of the older Wrights with all their issues, Having been involved with many Hybrid R-3350 engines over the years it sounds like you guys are following the right track. I'd still go for the heavy cases and cranks though. Good luck with the project.

[capflyer]

It may be a heavy case and crank used, but the primary consideration is that the new engines have to mount on the existing B-29 mounts and fit within the stock cowlings. It's really cool though for me as an enthusiast as we have Nelson Ezell doing the engineering and modification of the cowlings to fit the new engine and Kermit lent him one of his cowlings for the pattern work. The idea here is that when (if) Kermit ever decides to fly his B-29 again, he will have the patterns already in hand to modify his cowlings to fit the new engines which he will use. We offered to let the "Doc" guys in on it, but they have chosen to stay with the stock engines as they want as close to a 100% original airplane as possible (good for them, I'd rather have engines that won't ever overheat, but that's just me).

BTW, Gary calls the engine a "Rare Bear Light".

[riogrande]

So let's see....less blades - more efficient - how 'bout a single bladed prop?

[Tom/CalClassic]

That would be interesting...

[Deleted]

I thought about that....and i think while the efficiency would be near 100% you would lose the maximum amount of thrust producible due to the added counter weight for balance which a 2 bladed prop does quite well.

[Deleted]

"So let's see....less blades - more efficient - how 'bout a single bladed prop?"

That's an extremely broad statement , but single bladed folding props were fairly common on rubber powered models when I was a lad. But tip speed was low, they were hand-launched and sometimes they even flew long enough for the prop to fold before landing. ;D