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

[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.

[Christer Engdahl]

It's difficult to compare two propellers since so many parameters are different. The basic aerodynamics of a propeller is not much different from the aerodynamics of a wing or rather a number of ½-wings. There are a few more components such as swirl (outwards acceleration of the air flow) but the predominant factor for aerodynamic efficiency is the aspect ratio (for a propeller = diameter² / blade area). If a propeller has all parameters equal, except the number of blades and the chord of the blades (to maintain equal blade area), the higher the number of blades - the higher the aspect ratio - the higher the efficiency.

The germans must have known about this and other considerations must have dictated their choice of less efficient propellers and my bet is the gun synchronization issue.

There are many examples of landing gear design to accomodate the longest possible leg in the wing. The shock absorbers of twenty-series Spitfires were compressed when the gear was retracted. The F8F Bearcat had "articulated" legs which were "folded" into the wings. The shape of the F4U Corsair wing was to acchieve higher ground clearence with shorter landing gear legs. This indicates that the diameter of the propeller is vital and the higher the diameter - the higher the aspect ratio at any given number of blades.

Christer

Edited: Propeller tests to determine the effect of number of blades at two typical solidities

[Tony Williams]

Originally Posted by Christer Engdahl

The germans must have known about this and other considerations must have dictated their choice of less efficient propellers and my bet is the gun synchronization issue.

I agree, particularly since some multi-engined German planes and projects did have four-bladed props.

Tony Williams: Military gun and ammunition website and discussion forum

[Harold Lake]

IIRC the Germans almost always selected 4 blades if the fighter had a turbo, e.g., Fw 190 V18/U1 and Bv 155. I seem to recall that some larger types such as the Ju 288 and He 177 also had 4 bladed props for their coupled engines. Then too, the Ju 388 had them, plus a turbo, for their BMW 801Js. Can anyone draw any conclusions from these examples?

Hal

[Christer Engdahl]

Hal,
I think that turbo chargers and four bladed propellers both were introduced to increase high altitude performance. The discussion is on propellers and the aspect ratio becomes even more important with altitude. Compare the high aspect ratio wing of the TA-152H, designed for high altitudes.

Christer

[Boomerang]

If I have understood a number of the contributions, there seems to have been an element of design compromise between the wider 3 bladed wooden props used on eg the Fw 190D/Ta 152 and the four bladed props on their Anglo-American competitors.

Could the realities of the late war economy also have played a role in not moving from the 3 bladed props, which by then would have been well entrenched in the German aircraft industry, whereas the introduction of 4 bladed props would have called on scarce design/tooling/ production resources? In other words, if you are producing a 3 bladed propellor which essentially does a good job, it wouldn't have been worth the disruption and extra resources needed to introduce a 4 bladed prop which might some, but not critical, advantages.

Boomerang

[Graham Boak]

I don't think so. Generally, each design needs a dedicated prop. There'd be no difference between making a blade for a 4-blade hub than for a 3-blade hub design. The hub design for a four-blader is perhaps a little tighter but doesn't require more exotic technology, and had already been done for aircraft such as the Do 217M, He 177 or later Ar.240s.

The suggestion that 3-blade props were retained for the benefit of fuselage-mounted armament makes sense to me. Which would perhaps explain why the twin-engine designs went to 4 blades first. The Russians retained 3-blade props too, presumably for the same reason. Also, they were mainly interested in low-level performance. For their more powerful fighters the Japanese went to 4-blade props and wing-mounted armament.

[Crumpp]

The germans must have known about this and other considerations must have dictated their choice of less efficient propellers and my bet is the gun synchronization issue.

There is nothing to choose from in terms of efficiency between the Allied or German propeller designs.



All the best,

Crumpp

[kennethklee]

Not all Japanese fighters had 4-blade props without fuselage-mounted weapons. The late-war (1944-1945) Nakajima Ki. 84 Hayate ("Frank") had a 4-blade propellor and fuselage-mounted 12.7mm machine guns.