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Hello Aero-experts, 

 

I am an historian by trade, but I am not an expert (by any stretch of the imagination) an airplane historian, nor do I understand or know all of the technical aspects of airplanes. However, i am curious by one thing. I noticed watching an air show on youtube with my 2 year that some planes have 5, 4 and 3 blades. Can someone explain the advantages and disadvantages/ or the rationale behind different number of blades of an aircraft? 

 

Thanks in advance

 

 

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As you may have guessed: the number of blades increased over time. It's due to the increase of engine power over time. For efficiency, less blades is preferable as they influence each other less. Problem is, as I understand it, the engine power will become too much for each blade. So the load from the engine has to be distributed across more blades to avoid having to increase the structural strength of the blades (stronger heavier/more material being needed).

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different design philosophies leading to similar outcomes. to extract more power more efficiently from their engines throughout the war (broadly speaking), USA designs used longer blades, German designs used thicker blades, and British designs used more blades.

 

All three approaches have advantages and disadvantages. You pays your money and takes your choice.

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I would guess so - given that USA & UK moved away from wooden blades throughout the war & it seems only Germans persisted with them. Same with wooden tail fins & rudders on later 109s?

 

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Blades interfere with each other, so a smaller number of blades is better. Two is the minimum.

 

However, you need to make the best use of the engine power. There are a few ways to do this.

1) Make the blades longer

2) Spin the blades faster

3) Make the blades wider

4) Have more blades

 

Making blades longer results in higher speeds at the blades tip, so both option 1 and 2 run into problems as the blade tips start to approach the speed of sound. At such speeds, their efficiency drops dramatically.

Option 3 also has drag penalties, so it is sometimes better to add an extra blade instead.

 

 

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38 minutes ago, I./JG52_Woutwocampe said:

Why wooden blades instead of metal blades though? Cheaper and saving strategic materials?

 

Wood is a pretty decent natural composite material. Well suited to the stresses on a propeller. The biggest drawback is probably the reduced durability when compared to metal ones, which wasn't an issue with late war German aircraft. It's also, possibly, not as easily produced using industrial production techniques. Also not much of an issue with Germany, as their aircraft industry was to a large extent based on manufactures, as opposed to assembly line production of the Allies. For Germany the pros of saving aluminium and making use of additional skilled labour (wood workers like carpenters) outweighed the cons, for the Allies it didn't.

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2 hours ago, JtD said:

 

Wood is a pretty decent natural composite material. Well suited to the stresses on a propeller. The biggest drawback is probably the reduced durability when compared to metal ones, which wasn't an issue with late war German aircraft. It's also, possibly, not as easily produced using industrial production techniques. Also not much of an issue with Germany, as their aircraft industry was to a large extent based on manufactures, as opposed to assembly line production of the Allies. For Germany the pros of saving aluminium and making use of additional skilled labour (wood workers like carpenters) outweighed the cons, for the Allies it didn't.

 

Still, many spitfires used wooden blades like the mark V and IX. I think many ( all? ) english merlin powered aircrafts used wood for propellers.

Edited by I./JG52_Woutwocampe
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14 hours ago, [DBS]Browning said:

Blades interfere with each other, so a smaller number of blades is better. Two is the minimum.

 

Not necessarily so:

https://www.aopa.org/news-and-media/all-news/2018/november/pilot/singular-sensation

https://en.wikipedia.org/wiki/Single-blade_propeller

As I recall from my flying model airplane days control line speed planes often used single blade props.

 

One of the reasons for multi blades (5 or more) is to make use of the increased engine power without having the prop tips go supersonic which reduces their efficiency as well as noise (that's what makes that loud "Brapp" on the T6 for example). Also, more blades means possibly better balance as well as more "beats" per revolution hitting the windshield which serves to reduce cabin noise.

Another is for ground more ground clearance.

 

Here is a CL speed plane with one blade ( and one wing to boot).

 

Edited by JG1_Vonrd
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13 hours ago, I./JG52_Woutwocampe said:

Still, many spitfires used wooden blades like the mark V and IX. I think many ( all? ) english merlin powered aircrafts used wood for propellers.

 

But not exclusively, I can't name a type from the top oy me head that didn't use / wasn't cleared for wooden and metal airscrews at some point.

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Wooden propellers by default were used initially in the fixed pitch Mk.I. Problem is when you have a million oafs using them, they tend to be a bit fargile. Metal propellers are more durable if it comes to abuse, hence DeHavilland supplied duraluminium ones for the two pitch and constant speed ones. Thus, most of the planes would be issued with metal three and later four blade propellers.

 

The problem was that good quality duraluminium was scarce. Hence, they also resorted to making wooden composite propellers. They are different from the laminated wooden propellers as seen on the old biplane kites, as their laminated (usually pine I guess) wood core was treated by the Schwarz process that presses a hard metal mesh and cellulose layer on top of the wooden core by using special machinery. This way, the later "wooden propellers" are not just wooden, they are a composite of wood, metal and cellulose. Post war, there was enough metal and the wooden composite propellers were discontinued.

 

The propellers used now are usually composite and most high performing propellers these days are (modernized) composite ones. You can see here, this is the aircraft that I flew in in 2019, picture taken 2015:

 

151_Kent2.png

 

(Warbirds crash all the effin times.) Hartzell makes composite propellers for the Spitfires besides being one of the main advocates of composite propellers. Wooden (composite) blades also break in this same manner. You can see here:

 

05478.jpg

 

This is a laminated wooden four blade prop on Polish RAF pilot Kazimierz Chomacki's (him sitting on the wing) Spitfire Mk.XVI. But earlier three blade propellers were aleady made of metal:

 

7jdIx0O7U84dz4DdRjBQIUFXgI2EQzyEufJm9ZFZOLM8d1aLxtfK90ZkhJI1Hx3CfBS82lABaC-Sq6gldy43x1E7kSgYG8EwiGTE9yZUoSBJszWvx6Ncfzh4lF1HEkczML6B

 

Yet some three blade propellers were also made of wooden composite:

 

Spitfire_Mk_V_GI-H.jpg

 

All broken clean off when coming down with the engine running. The famous Spitfire 944 has also a wooden composite propeller:

 

Spitfire_944_fest_poster_a.jpg

 

Wooden propellers are actually a good choice for high performing aircraft as they are lighter and as some say give better performance, they supposedly allow the aircraft to accelerate faster. I think this was a rationale to put on wooden composite propellers on the PR Spitfires. Those PR variants were a very small number, hence I figure they would have any choice they wanted, it wouldn't matter in terms of supply. And they went for the wooden ones as they needed the last bit of performance from the aircraft. And today, composite is of corse a bit more refined than what they could do in 1940.

 

Metal propellers are heavier and are more prone to vibration. But they can take a lot of damage which is probably the killer agument for must Spitfire use back then.

 

s-l640.jpg

 

He wouldn't have come home if he had a wooden composite propeller.

 

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