Luftwaffe and Allied Air Forces Discussion Forum - Why Didn't Late Generation LW Fighters Use Four Blade Props?

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

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Originally Posted by Kurfürst (Post 32305)

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.

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?

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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?

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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.

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