Propellers on Aircraft

[wbraswell]

I've been wondering about the different propellers on aircraft, mainly WWII warbirds. How were decisions made about which type were used. I know the early P-51s, with the Allison engine had 3 blade props, while the later Merlin types sported 4 blades. The later P-38s with Merlins, still had 3 blades. I know the 3 blade wasn't quite as bulky for carrier use. I've seen F4U Corsairs with both 3 and 4 blade, and wonder what the performance differences were. B-29s had 4 blade props, but I can't recall ever seeing them on B-17s or B-24s. Martin B-26s had 4, and Mitchell B-25s had 3. Early Spitfires had 3, and later ones had 4. I know some the characteristics of outboard motor propellers, but not airplanes. What do you think?

 

[Dennis The B]

Although I don't play an aeronautical engineer on TV, nor did I stay at a Holiday Inn Express last night, I'd hazard a guess that the more blades, the more efficient the propeller, up to a point. As the efficiency increases, the amount of thrust increases, increasing aircraft speed. But, none of this happens in a vacuum. The efficiency of the wing, ailerons, fuselage, etc., all come together as a unit. So, increasing the efficiency of the propeller may not provide all the benefits of its design, unless it fits within the design.

However, to gain efficiency, the engine must generate more horsepower to turn the heavier prop. Adding bigger, more powerful engines also adds its own problems in terms of weight, aircraft handling, etc.

 

[coltle6920]

Just a stupid thought here but before weaponry was mounted in the wings wouldn't more blades be detrimental when firing a machine gun that was mounted in the fuselage?

I probably just watch too much Wiley E.Coyote and Three Stooges.

 

[carpriver]

look at the WW i aircraft they only a two blade prop. early machine gun had to be times to fire through the moving prop.

There is also the pitch of the prop that had to speed and lift. But I don't know anything other then the props go around and the aircraft goes up

 

[chief38]

While I know little about aeronautics, I would think that there are many factors that determine what kind of prop they use on a plane.
* weight
* length of blades
* material they are made from
* shape and pitch
* weight, size and shape of the aircraft they are attached to
* horsepower and torque of the motors
* how many engines are on the plane
* what the plane is designed to do

During WWll there were different engines used on the same planes. For instance the P51 Mustang originally had Allison engines and then they switched over to the English Merlin engines giving the Mustang even more speed and climb. Different engines can and do determine what kind of prop to use just like boats.

 

[venomballistics]

it works according to power, RPM, and ground clearance.
The optimum propeller would be a single blade to create a maximum active thrust area.
This theoretical ideal prop would, of course, make for one rough ride if it stayed together, or rather, a very rough ride till it came apart.

two blade is the real world ideal propeller, as it can be balanced and provide the largest thrust area.
The glitch is that as engine power gets up there, that acting disk grows as well, until it extends into the tarmac in ground operations.
To put the power to use, we add more blades to offer a suitable load for the engine, while keeping the prop out of the ground.

Add RPM into the mix and theres more fun to be had.
When the RPM is high enough, they can, and do, drive the prop blades into supersonic speeds.
The result of this is a dramatic loss of efficiency.
Thus for a higher RPM, the prop size must be reduced in order to keep the tips out of the sound barrier. Again, looking back at engine load requirements that must be met, the only option is to add blades.

this, omits the attribute of prop pitch .. which is yet a whole other bag of rats adding to the prop selection equation, which was purposely omitted as the above is sufficient to gain a fair grasp of the general concept

 

[DR505]

The later P-38s with Merlins, still had 3 blades.

Point of reference...P-38's NEVER had Merlin engines...from start to finish they had Allison engines. The CEO of Allison sat on the War Materials Board and vetoed the use of Merlins in the P-38.

Venom has it right.

The Hamilton Standard Propeller was a subsidiary of the same corporation that owned Pratt and Whitney (engines) so you see most P&W engines sporting HS props.

 

[steveno]

the only thing I really know about props is the C-130A had three bladed props that were quite long and you could really tell the difference from a distance as they had a higher pitch sound to them compared to C-130B and later which had 4 bladed props. the change was made to get tips from going supersonic.

 

[therevjay]

"I don't know anything other then the props go around and the aircraft goes up"....Carpriver


That covers the basics very well. I like it, simple is good.

 

[WuzzFuzz]

it works according to power, RPM, and ground clearance.
The optimum propeller would be a single blade to create a maximum active thrust area.
This theoretical ideal prop would, of course, make for one rough ride if it stayed together, or rather, a very rough ride till it came apart.

There was in fact a single blade prop made...It was tried out on a Cub...The one side had the full length of a prop. The other side was just a weighted stub, to counter balance...Not too good of a idea, but it did work.



WuzzFuzz
two blade is the real world ideal propeller, as it can be balanced and provide the largest thrust area.
The glitch is that as engine power gets up there, that acting disk grows as well, until it extends into the tarmac in ground operations.
To put the power to use, we add more blades to offer a suitable load for the engine, while keeping the prop out of the ground.

Add RPM into the mix and theres more fun to be had.
When the RPM is high enough, they can, and do, drive the prop blades into supersonic speeds.
The result of this is a dramatic loss of efficiency.
Thus for a higher RPM, the prop size must be reduced in order to keep the tips out of the sound barrier. Again, looking back at engine load requirements that must be met, the only option is to add blades.

this, omits the attribute of prop pitch .. which is yet a whole other bag of rats adding to the prop selection equation, which was purposely omitted as the above is sufficient to gain a fair grasp of the general concept

Venom has it correct....Originally the large 2 bladed prop was for under powered planes...6 foot long propellers....Those that had less than 150 hp, and didn't turn more than 1800 rpm. HP went up, props got smaller as the rpm's turned more in the 23 to 2500 rpm range...Sort of like driving a original Volkswagen with their only 36 hp...They would really have to grunt to go up hill....Not toooooo bad on level ground.


WuzzFuzz

 

[Big Cholla]

The number of blades required for a particular airframe and engine combo is determined by a lot of engineering problems. The two foremost are horsepower available to spin the prop system and the apparent airspeed of the tip of the propeller blades. At the speed of sound a stall starts at the tip of the blade and starts working back down the prop. So, it behooves efficiency to make sure the tips don't exceed the speed of sound. Shortening the radius of the props is one way to do that. Shorten the blade and one must make up for the loss of that blade area someplace. Usually it is to add blades to the prop system. There are other reasons to add blades; i.e. on a sea plane to minimize the water spray damage to the props, on a carrier based plane to increase clearance between the deck and the prop tips, smooth out prop wash over the wing.

There was an actual airplane that was flown with ONE prop. The other side just had a weight to balance the assembly. I don't recall why the engineers tried that.

The latest in prop design calls for small vortex tips on the end of the prop that increase the efficiency of the prop blades. ............. Big Cholla

 

[steveno]

I guess the other that should be known about props is when you are on the flightline you don't walk into them whether they are turning or not

 

[JonF]

WuzzFuzz is right about the venerable VW. It was my first car. A 1956 bought in early 1957. 36 roaring horsepower, 4 on the floor and 37 miles per gallon. On anything more than a slight incline, you were maxed out while in 3rd gear at a bit less than 45mph.
I drove from Detroit to Long Beach Calif. for $25.10 in gas, and no oil needed.

Didn't mean to HiJack......

 

[Texas Star]

The P-47 Thunderbolt gained a lot in performance when they added fatter/wider propeller blades. In particular, it could climb much better, previously a major shortcoming of the design, although its powerful Pratt & Whitney 2800 radial engine and its weight made it a great diver.

Robert Johnson, a leading US ace, tried the new prop out by inviting a friend in the RAF to pit his deadly Spitfire MK IX in mock battle against Johnson's re-propped P-47. In a hard climb, the Spit naturally pulled away as expected. But once the P-47 really got going, it outclimbed that Spitfire!

This made the P-47D an even more deadly instrument. I think the wider prop gave more lift.

 

[venomballistics]

The P-47 Thunderbolt gained a lot in performance when they added fatter/wider propeller blades. In particular, it could climb much better, previously a major shortcoming of the design, although its powerful Pratt & Whitney 2800 radial engine and its weight made it a great diver.

Robert Johnson, a leading US ace, tried the new prop out by inviting a friend in the RAF to pit his deadly Spitfire MK IX in mock battle against Johnson's re-propped P-47. In a hard climb, the Spit naturally pulled away as expected. But once the P-47 really got going, it outclimbed that Spitfire!

This made the P-47D an even more deadly instrument. I think the wider prop gave more lift.

I noodled with a similar concept when I couldn't find any off the shelf props for a model airplane running an OS 120 SP
It's an oddball that forced me to cut down 18 inch props till the local shop finally started to stock a few I could use.
I never noticed any real difference. Scale was still kinda small to show any resolution.

 

[LVSteve]

The big "paddle blade" props come into their own at altitude. Even the Thunderbolt motor could not wind it hard enough at low altitude to match a Spitfire MkIX for initial climb. The B-29 had similar props IIRC.

When the RAF initially got the Griffon engined Spitfires, there were all manner of problems. First, the tail fin and rudder were too small to have proper control authority at low speed to counter the massive torque of the new engine and to make matters worse, the torque was the other way compared to a Merlin engined Spit. The five-bladed prop needed a much larger fin and rudder to keep it straight due the torque and massive slipstream.

The most extreme props I have seen are the six-bladed sickle shaped devices on the C-130J. They are allegedly 15-20% more efficient than the last series of four blade screws.

The P-47 Thunderbolt gained a lot in performance when they added fatter/wider propeller blades. In particular, it could climb much better, previously a major shortcoming of the design, although its powerful Pratt & Whitney 2800 radial engine and its weight made it a great diver.

Robert Johnson, a leading US ace, tried the new prop out by inviting a friend in the RAF to pit his deadly Spitfire MK IX in mock battle against Johnson's re-propped P-47. In a hard climb, the Spit naturally pulled away as expected. But once the P-47 really got going, it outclimbed that Spitfire!

This made the P-47D an even more deadly instrument. I think the wider prop gave more lift.

 

[Skeetr57]

When I was in VN, 1969-70, my unit was located just down the road from the revetments of the 1/20 Aerial Rocket Artillery unit. One day when passing by, I saw a bunch of civilians working on their AH1G Cobras. I asked about what they were doing and was told that they were moving the tail rotors from one side of the tail boom to the other, because the tail rotor was more efficient on the other side. On one side it was operating in tractor mode and pulling and on the other side it was in pusher mode. I don't recall which was more efficient,, but before the change, the tail rotor would run out of power to counteract the rotational force of the main rotor in a max climb, so the helio would begin to rotate in spite of the tail rotor.

 

[Mcwsky09]

Don't forget to consider whether you can feather the props or not which impacts how much area you need to fit in a the parts that accomplish feathering and that some older smaller aircraft have engines with only one power setting meaning to slow down you actuall but power to the engine rather than adjusting a throttle.

 

[Frank46]

Not to hijack this thread but have always wondered why in the WWII movies you see guys turning over the propellers by hand prior to engines being started. Was there a specific reason for this?. Frank

 

[DR505]

Not to hijack this thread but have always wondered why in the WWII movies you see guys turning over the propellers by hand prior to engines being started. Was there a specific reason for this?. Frank

I thought it was to get oil moving about in the engine before start-up.