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In order to absorb more power for a given prop diameter, we can design the propeller with a higher activity factor-- basically what amounts to wider blades-- or we can also design it with more blades. Both increase the solidity, and thus the ability to produce more thrust, but what drives optimal design? Longer chords mean more drag on an individual blade, but more blades mean more disruption of the airstream. Why and when choose one over the other?

Low blade count, low activity factor:

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Low blade count, high activity factor:

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High blade count, low activity factor:

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High blade count, high activity factor:

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One way look at it is your engine is power and prop is load. The amount of thrust you get will depend on the efficiency of the propeller x engine power output.

So, if one is going for the best fuel economy, the two blade is most efficient, but if you want the more thrust from a larger engine, you either increase the number of blades or increase the chord of the blades.

Both approaches reduce efficiency but increase thrust output, resulting in increased performance.

More high aspect blades, as in a helicopter, will be more efficient at lower airspeeds, but will suffer structural issues (bending) as forward airspeed increases.

Fewer low aspect blades offer a much higher forward speed range but will create a lot more blade interference turbulence at lower airspeeds. These are commonly seen on turboprop freighters such as the C-130.

It is extremely important to evaluate the design speed envelope of the aircraft when choosing, as forward airspeed reduces interference turbulence.

I took the liberty of adding the A6 Intruder jet aircraft to your list, and found the expected improvement in higher airspeed performance, but a take off run almost 5 times that of the A2D-1 Skyshark.

Enough said there. Contra rotating coaxial props, and keeping prop speeds below supersonic, also help improve efficiency.

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  • $\begingroup$ Any chance you could update this with some more detailed examples of optimization? I don't quite follow why high activity factors will improved forward speed range. P.S. I don't understand the reference to the A6. $\endgroup$
    – Kenn Sebesta
    Commented Feb 1, 2021 at 5:29
  • $\begingroup$ @Kenn Sebesta the jet engine, with its rows of blades, is an extrapolation of high blades, high activity. It is better to have more, higher AR blades, but lower AR are stronger. Forward speed reduces blade/blade interference. $\endgroup$
    – Robert DiGiovanni
    Commented Feb 1, 2021 at 5:45
  • $\begingroup$ High activity, high blade count enables more horsepower (more fuel/unit time) to make more thrust, increasing forward speed. Comparing prop to jet was irresistible. Just swinging the 2 blade faster and faster is constrained by the sound barrier. $\endgroup$
    – Robert DiGiovanni
    Commented Feb 1, 2021 at 5:51
  • $\begingroup$ so, you have a choice: more thin blades or increasing chord. More thin blades are better, but not nearly as strong. $\endgroup$
    – Robert DiGiovanni
    Commented Feb 1, 2021 at 5:57
  • $\begingroup$ (Notice the biplane also has 2 wings. It is much stronger per unit wing area, able to handle higher +/- G loads). $\endgroup$
    – Robert DiGiovanni
    Commented Feb 1, 2021 at 6:02

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