All About Propellers

sequoiaranger · March 12, 2009, 05:24:31 PM

I guess I just have an abiding curiosity about the use of aircraft propellers and types. I have a few specific questions, then a general one on the subject:

Why didn't we see five-bladed propellers on OTHER than British aircraft? What prompted the British to use them on several different aircraft (obviously NOT an experiment, but "regulation")?

The Hawker Hurricane started out with the Merlin and a two-bladed propeller and went to three blades, but not four. The Spitfire started with a Merlin engine, too, and went from two to four, then FIVE with the Griffon engine. Why?

Six-bladed propellers were used by the Japanese on quite a few late-war projects. Why only them?

American aircraft never used more than four blades. Why? And the Hellcat retained the three-bladed propeller even though it had the same engine as the four-bladed Bearcat and the Corsair (went from three to four).

The Germans never used four-bladed propellers on their single-engined fighter aircraft, but had four-bladed propellers on some of their bombers. Even the powerful late-war Ta-152 had a three-blade propeller. Some EXPERIMENTAL fighter types had four-bladed props, but not field models. Why?

So what determines the number of blades on a propeller for a certain engine or airframe? Merely power? Engine RPM's?

I just hope to stimulate discussion and learning about this common mode of aircraft propulsion.

jcf · March 12, 2009, 06:06:45 PM

From what I've read it is primarily a matter of absorbing the available power without having to increase the diameter
beyond an operationally feasible point. Widening the blade is another tack, and the one preferred by the Germans.

While the F4U, F6F and F8F all used an R-2800, they were not the 'same engine', there being differences between the dash
numbers. The XF6F-2 & XF6F-6 both used four-bladed propellers with their, respectively, XR-2800-16 and R-2800-18W engines.
R-2800 Double Wasp specs:
http://www.enginehistory.org/P&W/R-2800/DoubleWaspIndex.pdf

Jon

PR19_Kit · March 12, 2009, 07:14:54 PM

In the case of the Hurricane and Spitfire, it's all to do with the engine power and the length of the landing gear. It's darned expensive in time and money, and could even be impossible, to change the landing gear length and position after you've already started production on a basic design, even more so in war time.

Both of them used a Merlin of only just over 1000 hp in their early Marks, thus the 2-bladed props on them. Both of them moved to 3-bladers to gain from the variable pitch systems soon afterward, but the maximum engine power that the Hurricane used was the 1620 hp Merlin 24 in the Mk IV. The Hurricane had a much taller undercarriage than the Spitfire and could get away with a 3-bladed prop to absorb that power.

As time went on the Spitfire had more and more powerful Merlins installed and eventually switched to the Griffon with the Mk XII. The undercarriage still remained almost the same length though and they had to use more blades to absorb that power, over twice as much as the original Merlin in the Mk XIV and XIX.

On the other hand it could be just because we Brits like being different.........

kitnut617 · March 13, 2009, 07:11:33 AM

I'd like to add to Jon' comments, it wasn't just the increase in power that dictated prop diameters, by the end of the war prop powered fighters had increased their forward speed from around 350 mph at the beginning to nearly 500 mph at the end, all this in around six years of development. At these speeds you're getting into serious problems with blade tip speed with these going supersonic for maybe the last six inches of the blade. This degrades the performance of the blade along with serious vibrations setting in.

I've noticed that the optimum diameter was around 12' to 13' in diameter on many later prop powered aircraft especially in fighters, were as in bombers it was more around 16' .

Hobbes · March 13, 2009, 09:39:48 AM

I suspect the prop type used also depends on the altitude the plane is optimised for. At low altitudes, you can get away with less prop area than at high altitudes.

sequoiaranger · March 13, 2009, 09:41:27 AM

>I've noticed that the optimum diameter was around 12' to 13' in diameter on many later prop powered aircraft especially in fighters, were as in bombers it was more around 16' .<

Yes, I too have noticed that "bombers" often have larger diameter propellers. Is this because most "bomber" engines peak their power at lower RPM's, thus the tips are slower relative to fighter, or higher-RPM engines? Or some other reason?

>From what I've read it is primarily a matter of absorbing the available power without having to increase the diameter
beyond an operationally feasible point. Widening the blade is another tack, and the one preferred by the Germans.<

Yes, we all notice the "fat" blades of the "Dora" Fw-190, etc. For the American P-47, the propellers eventually had "cuffs" on the usually "skinny" part of the blades nearest the hub.

Hobbes · March 13, 2009, 09:53:58 AM

Bombers use larger propellers because
- props are matched to the airplane's cruising/top speed. A bomber is slower, so you can use the available power to move more air at a lower speed.
- prop diameter is less of a problem on a large bomber than on a small fighter (ground clearance, maybe gyroscopic effects)

sequoiaranger · March 13, 2009, 04:11:39 PM

>>I've noticed that the optimum diameter was around 12' to 13' in diameter on many later prop powered aircraft especially in fighters, were as in bombers it was more around 16' .<<

In measuring my models with a 1/72 scale ruler, the "rule" is generally 9'-12' for fighters, and 13'-14' for bombers. The Supermarine S6B racer/fighter whif I did had only a 8 1/2' diameter prop! I don't have a B-29 or He-177 that I know both had large-diameter props--maybe they would get up to 16 feet.

kitnut617 · March 13, 2009, 05:04:10 PM

Quote from: sequoiaranger on March 13, 2009, 04:11:39 PM
In measuring my models with a 1/72 scale ruler, the "rule" is generally 9'-12' for fighters, and 13'-14' for bombers. The Supermarine S6B racer/fighter whif I did had only a 8 1/2' diameter prop! I don't have a B-29 or He-177 that I know both had large-diameter props--maybe they would get up to 16 feet.

I did say 'later' prop powered sequoiaranger, Spitfires, Hurricanes, Bf.109's etc. were as you say between 9 & 12 feet. Typhoons, Tempests, Sea Furys, MB5, Thunderbolts, Corsairs, Hellcats were all around 12-13 feet

sagallacci · March 15, 2009, 07:26:56 PM

You've got a whole shopping list of dynamic factors in a prop, rpm versus pitch, which crosses over to issues of tip speed and diameter, which also plays on expected altitude, etc..
As said, if a limiting factor, like diameter limits come in, then you have to do a work around, like more short blades. And if even more power is applied, contraprops to counter the torque issues as well.

As for national preferences, I wonder if there isn't some data somewhere on how each arrangement did in practice as far as efficency . And if there was some aspect of resource preferences or limits that also played a part. For example, most Jumo engines got some form of fat wooden paddle bladed prop, to make the most of their torque-y but somewhat slower output rpm(?) and not using scarce light alloy.

Weaver · March 16, 2009, 05:51:44 AM

Quote from: Hobbes on March 13, 2009, 09:53:58 AM
Bombers use larger propellers because
- props are matched to the airplane's cruising/top speed. A bomber is slower, so you can use the available power to move more air at a lower speed.
- prop diameter is less of a problem on a large bomber than on a small fighter (ground clearance, maybe gyroscopic effects)

Plus bombers don't generally get involved in high-speed dives which are standard practice for fighters. By the end of the war, most fighters could get into compressibility problems in a dive, and if the wing's getting into trouble then the prop's already there..............

dy031101 · September 09, 2011, 10:25:03 PM

In ALVAMA's excellent Steampunk Warships I've noticed a propeller design I remember seeing only in the League of Extraordinary Gentlemen movie (used by Nautilus, IIRC)......

Was the design really in actual use by any historical vessel? Did it serve as a predecessor of or a parallel with the propeller that we commonly know?

pyro-manic · September 10, 2011, 10:23:35 AM

A propeller is a screw (as in Archimedes' Screw). I seem to recall that the most efficient screw is one that has only one turn, which is what a propeller is. I think early designs used screws with multiple turns before it was realised that single turn ones were better.

PR19_Kit · September 10, 2011, 03:56:32 PM

The 'Turbinia', the world's first turbine powered ship, originally had multiple turn screws arranged in triplicate along it's three shafts!

Even when developed they still kept the three props in line ahead on each of the three shafts. They certainly wanted to absorb the power available.

Pablo1965 · September 10, 2011, 05:15:15 PM

Quote from: sequoiaranger on March 12, 2009, 05:24:31 PM

American aircraft never used more than four blades. Why? And the Hellcat retained the three-bladed propeller even though it had the same engine as the four-bladed Bearcat and the Corsair (went from three to four).

I just hope to stimulate discussion and learning about this common mode of aircraft propulsion.

Well, I remember a swedish mustang with RR Napier Griffon engine and contrarotative props.