Influence of propeller blade shape and tip shape on its behaviour?

Question

Newbie here, still learning how to fly and having to buy new propellers. Of the various properties to be chosen among, there are two that have me a bit baffled:

• symmetrical vs. undercambered

and

• swept-tipped vs. straight-tipped

Looking at pictures of historical aeroplane propellers (my model is a Piper Cub, the original of which was first flown in 1938), there only seems to be the symmetrical+straight variant:

So when did undercambered, swept-tipped propellers come up, what were designers trying to achieve with them, and what are their advantages and disadvantages?

Here's an example of a prop sold by a popular Hongkong-based brand:

As another example, the model I'm learning with came with an APC 7x5E prop, which has these two properties even more pronounced:

_{(source: netdna-ssl.com)}

(Full disclosure: this question evolved from that one over on Aviation.SE where I was told I'm off-topic and lacking focus and to go here instead. Please let me know if either is still a problem!)

Answer 1

The main thing the designers are trying to achieve is reducing tip losses by making the chord of the tip of the prop small relative to the rest of the blade. Tip losses are due to air moving around the tip from the high pressure area behind the blade to the low pressure area on the front.

Swept tips theoretically delay transonic effects as the tip approaches the speed of sound. Air speeds up as it flows around the curves of an aerofoil, and so shockwaves start to form before the whole blade reaches Mach 1. This is about 30,000rpm for a 7" prop, so well about the RPM limits for glass-reinforced nylon prop. However carbon 'race' props can spin that fast. I guess that shaped developed for high-performance props have just been copied in cheaper props for marketing reasons.

The bulge (wide chord at about half span) in the last prop is because it's designed for an electric motor and doesn't have to deal with the forces that occur when an IC engine fires. The thick part of the blade is extra area away from the tip (reducing losses because more of the prop is now 'not the tip'). A prop for an IC engine would need a massive thick root to support the extra weight, but electric props don't need to be as strong, and saving weight is important.

Finally, the last prop would be a difficult shape to make from wood, where you want the grain of the wood to align with the tension.

A note about under-camber. This doesn't really do anything directly, but it's a result of picking a thin aerofoil with lots of camber. If you play with an aerofoil plotter you can see that at 2% camber and 12% thickness, the bottom is approximately flat. If you increase the camber or decrease the thickness, an under camber appears. Here again the difference is mostly because electric props don't need to be as strong as ICE props. The camber is there because the prop is optimised to work in one direction (as planes don't fly backwards) and thinner props are lighter and have less drag.