Why Didn't Late Generation LW Fighters Use Four Blade Props?

[Boomerang]

I hope those of you with good aeronautical engineering knowledge (unlike me) can clarify why late generation Luftwaffe fighters,such as the Fw 190D and the Ta 152, didn't use the four blade propellors used by their Anglo-American contemporaries, such as the P 51D, P 47D/M/N, Tempest and Spitfire XIV (which had a five blade propellor).

The He 177 had a four blade propellor and I've seen a four bladed Do 217 (and even the middle engine of a Bv 138 flying boat so equipped), so I assume there was no technical limitation barring the use of four bladed props by Luftwaffe aircraft.

Also, as the published data don't seem to indicate that eg the Ta 152 lagged in performance compared with Allied equivalents, was there an inherent advantage in four, compared with three bladed props?

Regards,

Boomerang

[pstrany]

I seem to recall this topic came up before, some time ago. The upshot was it is blade width versus number of blades. The Germans opted to make the blades of their props wider, while the Allies (mostly) chose to increase the number of blades.
Thats a radical oversimplification I'm sure, but that is the essense of it. I'm sure others will flesh that out a bit......

Paul

[Tony Kambic]

I have been doing research on the FW-190 prop and what I found is exactly what pstrany stated: that VDM engineers focused on blade design versus adding another blade. Results showed three blades could do as much as four. In my readings I did not find any evidence of keeping it at three to maintain weapons firing through the prop cycle. I also learned that the cuffs on the American AeroProducts props on P-47s were to induce air flow, similar to the fan on the BWM801, and results were positive. They used the same props on P-51s but no improvements were noticed (duh).

[Tony Williams]

Originally Posted by TonyK

In my readings I did not find any evidence of keeping it at three to maintain weapons firing through the prop cycle.

Nonetheless, the bigger spaces between the prop blades would have made gun synchronisation simpler.

This would have become a more significant issue as gun power increased, as the larger cartridges had slightly less predictable burning times. As it was, the Germans had difficulties in synchronising the big 30mm MK 103, although such an installation was proposed for some Ta 152 variants.

Tony Williams: Military gun and ammunition website and discussion forum

[bluebiggsey]

I forgot about the the Focke Wulf ta153. The proposed prop for this development used 4 blades.

Just on side note. Perhaps a few reasons for the MK IX Spit had for a 4 blade propeller was to counter the level speed of the Focke Wulf 190A, plus the change from the defensive to a more offensive role?

[Graham Boak]

To my understanding, the sole reason for moving to a 4-blade propellor for the Merlin 60-series Spitfires was to retain ground clearance. had it been to gain more speed I think it would have been mentioned somewhere. The preference would have been for a larger diameter propellor, but this was not possible on the Spitfire. The same can be seen in the progress to a 5-blade propellor on the 2-stage Griffon Spitfires, and the later Sea Fury.

One advantage of a multi-blade propellor is that it reduces vibration, as on the Typhoon.

I have not seen any suggestion that the British companies even considered wider blades rather than multiple ones, but my year's intake was the one that missed out propellor theory.

[Six Nifty .50s]

Originally Posted by George Hopp

The wider -- wooden -- blades gave a better climb, and the narrower -- metal -- blades gave a better level top speed.

A propeller also must be durable enough for a combat aircraft. Some of the pure racing blade designs were not practical for military use. The same principles applied to motor boat propellers for the coastal forces, always resulting in a design compromise between maximum speed and maximum acceleration.

[Dénes Bernád]

Originally Posted by pstrany

The Germans opted to make the blades of their props wider, while the Allies (mostly) chose to increase the number of blades.

Does it mean that the sum of the propellers' surfaces is roughly the same?

[Dick Powers]

One primary design parameter of propellers is “Activity Factor”. The larger the Activity Factor, the more power a propeller can absorb, and turn into thrust. A wide blade has a higher activity factor than a narrow blade. Hence, three wide blades may be able to absorb as much power as four narrow blades if the activity factors are similar. The wider blade, however, may be heavier requiring a beefier hub but fewer blades reduce the number of mechanical parts required – movable pitch mechanisms, etc. So prop design, like aircraft design is balancing the compromises.

There are many other parameters that effect the total propeller efficiency, such as blade airfoil thickness, and twist along the blade length.

[Kurfürst]

I think it was just a matter of different choice on design as other put it. Certainly the Allied and German prop effiency curves and trial results with more blades (Allied) or redesigned blade structure (German) does not show any mentionable difference in gains with either approach.