4 bladed Hellcat props

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Blade solidity has never been shown to have a great effect on efficiency. Several studies could not come to a conclusion as to whether it was better to have a propeller with many small blades or one with fewer wide blades, both of the same area. There are several NACA Reports on the subject, and blade solidity (swept area divided by total blade area) was never shown to be a significant factor in any of the reports I have seen to date.

Much more important were the selection of airfoil sections and a few other factors. I see more reference to blade solidity in helicopter analyses than in aircraft analyses, and very little with respect to WWII, high-horsepower aircraft.

I'm SURE there is a point where more blades is better, but exactly where that point is may be a very good question. also, blade solidity should be a factor, but high-area props have never caught on. Much more frequently, we see solidity ratios of around .04 - .09, with helicopters getting up into .11 - .16 or so.

I have never studied boats propellers, but they are MUCH more solid than aircraft propellers .... go look at one.

The German were obsessed with having armament in the fuselage, and stayed with 3 blades longer than anyone else. Their WIDE blade props on some Fw 190s have certainly caught everyone's attention in here. But the performance they achieved was never significantly better than, or sometimes as good as, the Allied 4+ multi-blade types. Again, solidity is a confusing variable just by looking at pictures of the various propellers used to achieve performance.

Look at the blade area here:

As opposed to here:


The Spitfire has more blades, but they are each more narrow than the Fw 190 D-9 blades. It would be interesting to know the solidities of both, but I can't find any reference to it in typical wartime reports.
 
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GregP said:
The Spitfire has more blades, but they are each more narrow than the Fw 190 D-9 blades. It would be interesting to know the solidities of both, but I can't find any reference to it in typical wartime reports.
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Great post GregP. With propeller rigidity, as a layman I have seen a helicopter on the ground with engines not running and the blades waving gently in the wind. However on take off they are a perfect rigid circle. Rotating at high speed obviously makes the blades come under a tensile load making it rigid in one plane at least. Is there any name for this effect, any way of calculating it and could they/did they know this in the WW2 period. (general question not just to Greg)
 
I have no idea if blade material or manufacturing has anything to with it. Americans used hollow steel and solid and hollow aluminium.
British used metal blades and composite wood ones. I believe the Germans used both also.

Another question (which just came to me looking the 190D picture posted by Greg) is that we talk a lot about the airfoils used in the wings and yet propellers are airfoils. That prop on the 190 may look thick but given it's broad cord it might have thickness ratio close to a thinner and skinnier propeller?

and put it together with pbehns question, did the materials some propellers were made of, and their tensile strength (or other properties) have any influence or limits as to size (diameter) or shape of the blades?

Or any limits on manufacturing? As in if you wanted to put 4 bladed props on the B-17 you need 16 blades instead of 12 and then you multiply that by thousands of bombers, 4000 more blades per 1000 aircraft.
a 4 blade hub is going to require more machining than a 3 blade hub.

Propellers were not cheap or easy to make and more than one nation had propeller shortages at one time or another.
Sometimes solved by using a different manufacturers propeller, like the P-51K using Aeroproduct propellers instead of Hamilton Standard props.
 
The engineers who design the props calculate the tensile stresses and make sure the blades will stay attached. I don't think it was a major issue in WWII or any other time, really, unless you had battle damage and lost a blade.

Prop solidity is the ratio of the swept area to the prop blade area. Rigidity is something else. Most aircraft props are quite rigid, but there is blade movement when the prop pitch is cycled at high power on the ground. 2-blade props especially are susceptible to it.

Rather famously, the P&W R-1690 Hornet had the issue with a particular aircraft. Howard Hughes' R-1 racer had prop flexing issues that could be seen on the ground, and he lost a blade in flight. Later, as we all know, Jim Wright built another one and used a P&W Hornet and the stock 2-blade that Howard used. He also lost a blade in flight, unfortunately fatally. You could see the prop flex as he exercised the pitch mechanism on the ground, and several people told him it was an issue that should be addressed ... right then. It wasn't that the prop was flexing as much as the extent to which it was flexing.

ANyway, the fighters of WWII used 3, 4, and 5 bladed props and didn't have the issue very often. The last 2-blades were on the very early 1939 and earlier planes. The Bf 109 and Spitfire prototype come to mind, along with some PZLs and the early Ki-43. By the time 1940 rolled around, everyone with 850 HP or more was using 3-blade props and the issue was gone for the most part.
 
Another variable not mentioned here is RPM. The B-36 had much larger diameter, slower turning three blade props than the four blade props on the B-50 even though engines were similar. Slower props are more efficient than faster ones so you like to go as big as possible while maintaining a specific tip speed. Props also have effective side are as a function of their blade area so mounted in front they are destabilizing. The Mustang was designed for three blade props and when they went to the four blade some directional stability was lost but the cure was put on the back burner (Hucknall actually tried several different fin styles on their Merlin prototype before NA). The turbulence behind the D's canopy worsened the problem which spurred the Army and NA along a bit quicker.

An interesting side by side comparison of three and four blade props are the late P-39Qs. With the four blade it was found that neither speed nor climb were noticeably improved but longitudinal stability was slightly improved (CG moved forward a pinch) and directional was definitely worsened so the recommendation was to go back to the three blade on the assembly line and in the field as props were available. The P-63's larger fin is the result of both the four blade prop and the prop being somewhat further forward of the aerodynamic center relative to the P-39.
 
We talk about HP, but let us not forget about torque. Just found my book by G. Jenkins on the B-36 and the R-4360's were putting out over 7,000 lbs/ft of torque at 3,000 RPM at the crankshaft. But, after the gearbox, rated torque went up to over 20,000 lbs/ft at a propeller speed of just over 1,100 rpm! Hauling that much metal around, those 19 ft. props had to be strong. Reminds me of my last Pontiac GTO, a 1972 455 HO that was only rated at 310 HP, but 510 lbs/ft of torque. On your average city road, that is what makes acceleration happen. Something most Harley Davidson riders don't understand why they like their bikes, but the Ducati riders do. Just my thoughts, I like them all.
 
YF12A said:
On your average city road, that is what makes acceleration happen. Something most Harley Davidson riders don't understand why they like their bikes, but the Ducati riders do. Just my thoughts, I like them all.
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They are slightly different things. When people talk about a "torquey" motorcycle it means that it produces a good torque output throughout its rev range so it has pretty much the same urge at 2000RPM as it does at 7000RPM. I had Triumph Bonneviles and a Norton that were like that. They are most satisfying to ride in standard tune. When you tune them they get more and more "cammy" The torque output is low at low revs but increases sharply when it comes "on cam" that is when the port timing gas flow and ignition timing are all working at optimum. They are great to ride on a race track but cough wheeze bang and splutter in normal use and give the clutch a really hard time., every road test on the old Ducati 900SS said it only wanted to go fast.


I raced 250cc and 350cc two strokes. At 2000RPM the they hardly had enough power to accelerate but when they hit the power band torque and RPM were both increasing at the same time and they lifted the front wheel without using the clutch just with the throttle fully open. It was great to race but a danger on public roads.

For prop driven aircraft constant speed/variable pitch props meant that the engine could run at its optimum for power and the propeller would adjust for it.
 
Hey sorry to revive an old thread, but I'm being asked about this pic that has been floating around the interwebs for years. Aircraft on the Top Left looked a bit like a Hellcat to me, but I was thrown by the four blade prop. I thought maybe a Bearcat, but it looks too big compared to the B-25. I gather two F6Fs were made with four blade props, but the odds of this being one of them seem remote. I figured y'all would know already and have seen this pic before.

 
What is that fuselage below the A-1D, another A-1D? A P-47?

It's such a weirdly compelling image I can't stop looking at it.

For a while when I had to commute I used to stop at this one small rural airfield where they had some old Lockheed PV1 (or maybe PV2) Venturas, still with their engines, guns and everything, partly broken up just sitting out in the weeds and the weather. I told a guy on a forum who was obsessed with Venturas, and he told me they were probably 'just' Hudsons, as if that wasn't cool. He said he knew where all the Venturas were in the world. His whole website devoted to them.

Next time I made that commute, I went out there again, walked a half mile through the snake infested weeds in the heat, and took pictures. They were Venturas, it said so on the fuselage. I sent the pics to him and he acknowledged it, but I never heard back and I stopped having to make that commute shortly afterward. I always wondered if he put a buyer in touch with the airport and came and collected them. I have mixed feelings on that, since it was kind of cool to see them out there (you could spot them from the highway, just barely). But it would of course be better if they ended up restored etc..
 
The airport off US20 going into Greybull Wyoming had an a/c graveyard. There was some twin engine/twin tail a/c.

Google map shows a couple of PB4Ys.
 
Guns? Really? Can you post the pics please?
 
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