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

[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.

[Crumpp]

[George Hopp]

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.

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.

[George Hopp]

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.

[Graham Boak]

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.

[Kurfürst]

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.

[Boomerang]

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

[Christer Engdahl]

Originally Posted by Kurfürst

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?)

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

[Graham Boak]

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.

[Christer Engdahl]

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

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.

What a 4-blade prop does (normally) have is a greater blade solidity, and hence will be destabilising.

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