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

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

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

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

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

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).

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

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

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

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.

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

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.

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

I know next to nothing about aerodynamics. But from what I can gather fewer blades with the same surface area give a better climb rate but more blades increase level fight speed. It would explain the 3 on a 152(interceptor)and the 4 on the p-51 (escort).Also the most efficent prop is with 1 blade but you try balancing it!!!!!!!!

Can an expert confirm this?

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

Nay, from what I remember it really does not matter. At last Spitfire IX, interceptor, had 4 balded prop. It is true that one bladed prop is the best thing, but only in theory. Real life shows several problems like interference of blades, complications of mechanism, behaviour of blades' profile at transsonic speeds and many, many more.
I believe increase of number of blades was a better sollution anyway, as it was followed by both British and American, despite of access to German experiences.

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

The wider -- wooden -- blades gave a better climb, and the narrower -- metal -- blades gave a better level top speed. RR and Dehavilland had a discussion about this for the Mosquito. And, in the report on Fw 190 V17 -- the prototype for the 190 D-9 -- FW noted that amongst the several losses in top speed would be -6 km/h for the wide, wooden-bladed props.

Sorry, that doesn't answer your basic question about the use of 4-bladed, metal props. For what it's worth, they were testing them on the 109 K-4.

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

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

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

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?

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

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.

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

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

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

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

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

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

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

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

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

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

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

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.

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

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



All the best,

Crumpp

Japanese fighter with 4-blade propellor and prop-synchronized weapons
 

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.

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

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.

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

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

Neither have I, but I have seen letters exchanged, as I mentioned above, on the matter of narrow blades vs. wide blades. RR was leaning towards the wide wooden ones because they increased climb, reduced level speed only marginally and on the Merlin 61 Spitfire, because of their lighter weight combined with less tail ballast, reduced a/c weight by ca. 150 lbs.

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

As I went through the back corners of our flood-ravaged basement, I came across a water logged report on "Propellers for the Me 109G with the DB 628 engine," dated 21.9.42, and produced by VDM. In assessing 5 types of propellers, of which the 5th was a 4-bladed unit, it noted that 3-bladed props could not equal the performance of 4-bladed props. But, the 3-bladed props could get to within 1.5 to 2 percent, and the the 3-bladed prop units weighed 50kg less. The difference in weight appears to be a prime reason for sticking with the 3-bladed propeller at that time. Now, that might have been a reason specific to the installation of the DB 628 in the 109, but it still gives one reason for the continued German use of 3-bladed props in its fighter aircraft.

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

An interesting comment, George, but it leads straight to the query of what percentage of the weight of the fighter is 50kg? How much percentage of thrust is worth how much percentage of weight? This points again to the description of 4 blades providing more speed (where weight is not significant) but 3 blades being possibly better in the climb (where weight is highly significant).

The moment of the weight is significant, being as far in front of the cg as it is possible to get. This would affect the balance and hence stability of the aircraft. 50 kg on the nose may mean 30-40 kg at the tail, or double the apparent penalty.

One other point I don't think I've seen raised is that a 4-hub will be more complicated, and hence more expensive to produce, perhaps also to maintain.

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

Would not 4 blades vs. 3 also increase the overall drag of the aircraft (given my basic understanding of aerodynamics, props work like wings, so increasing their number increasing the drag - or is this already factored into effiency?)

Considering the extra weight vs. extra thrust effiency of the props, it looks rather similiar. 50 kg would be around 1.5-2% weight of a roughly 3-ton aircraft, which is exactly the same as the 1.5-2% propeller effiency gain from the bigger prop - it seems to balance it out.

An important issue could be the development principles and custums; German designers, from what I've seen, seem to prefer effiency, effiency and for the third time, effiency of the design over brute for approach. IOW, keep it simple, and factor the extra performance vs. vs. weight, drag increase. Recalling Mankau's book, this was the exact reason behind the rejection of the (waaaaay more powerful) DB 628 vs. the raher simple DB 605AS solution.

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

I noted in some 1936 footage of the Dornier Do 23 twin engine bomber (high wing type with fixed undercarriage and the engines mounted in nacelles under the wings) that it was fitted with 4 bladed props.

Presumably this indicates that German designers were well versed with 4 bladed props, so the design choice of using 3 bladed props for late war fighter aircraft was based on design philosophy or pragmatic factors, rather than lack of familiarity with 4 bladed designs.

At least my sad lack of aeronautical knowledge is receiving a (very belated) boost.

Cheers

Boomerang

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

Basically, a propeller works in the same way as a wing, creating lift by displacing air downwards or backwards (in the case of a propeller). If we consider two wings with the same area, the one with the higher aspect ratio (longer span, narrower chord) has the lowest drag. The same is true for a propeller. If you consider two propellers, one three-blader and one four-blader, of the same total blade area and diameter, the one with four blades has the higher aspect ratio and the lowest drag.

Christer

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

A propeller only has drag when it is stationary - it produces thrust.

What a 4-blade prop does (normally) have is a greater blade solidity, and hence will be destabilising. Beyond that my education will not take me.

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

Drag is by two components, zero lift (depending on the area in contact with the air) and lift induced (depending on thrust when a propeller is considered). A stationary propeller does not fly, it is pushed by the aircraft totally out of the designed range of angle of attack and that is the reason for the extreme drag of a stationary propeller. (If the propeller is not under power but wind milling the drag is even higher because the energy to turn the engine is extracted from the airspeed.)

If the propeller is under power and producing thrust (within designed range of angle of attack), it still has the zero lift component and also the lift induced component. That is what keeps the engine from over revving.

We have to compare propellers of the same solidity (total blade area), if we don't we are not comparing apples with oranges but apples with melons.

When the Spitfire went from the Merlin and a four-blader to the Griffon and a five-blader, engine power and propeller solidity were increased and in this case you are right about the destabilising effect of the propeller. At the same time, low-back fuselages became more common which added to the problem by reducing the area behind CoG. The fin and rudder was increased in size but it was not solved until the Spiteful-type fin and rudder was introduced with the Mk.22.

Christer

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

If efficiency, efficiency and efficiency was so dear to German über-designers, two questions:
-why did they keep the utterly inefficient Maybach petrol engines for tanks?
-why was German large surface warship steam machinery noted for its poor efficiency???

Short answer: K´s theory is fatally flawed.

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

Which questions, if true, show only that even über-designers can have "off" days.

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

Certainly. Blade area is the key in this case.

I just recently had to purchase a new propeller for my aircraft. In speaking with numerous prop shop's some interesting insight to the German Engineer's decision to stick with 3 blades instead of 4 came to light.

I wanted to put a 3-bladed STC'd prop on my aircraft. Surprisingly not one prop shop recommended it nor did the company engineer's. The first question asked was, "How do you fly the plane?” As I do not fly above 18,000 ft ASL, the answer was to stick to the 2 bladed props because they offered the best performance in the envelope I used the aircraft.

As we all know props are extremely complicated pieces of engineering. Generally speaking the performance differences on the same airplane can be summed up as follows:

Adding more blades will increase high altitude cruise speeds, Vy, higher top end acceleration, and high altitude level speed on the aircraft.

However at lower speeds the more blades you add, the more blade interference decreases the propeller efficiency. So for lower stall speeds, better Vx, faster low altitude speeds, and better low end acceleration, you will find fewer blades is generally the better choice.

To clarify:

Vy = Best climb rate or the greatest gain in altitude over a given time.

Vx = Best climb angle or the greatest gain in altitude over a given distance.

All the best,

Crumpp