5
$\begingroup$

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:

Historical Piper Cub propeller

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:

HobbyKing Master Airscrew S-2

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

APC 7x5e
(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!)

$\endgroup$
3
  • $\begingroup$ Welcome ot the site! As far as being on-topic, I suspect it is on-topic here, though I'll let the community weigh in! However, I would personally remove the "bonus question," as it really has little to do with your original question, and consequently makes it more difficult to post a clear succinct answer. Also its direct link with drones rather than search engine optimization and/or linguistics is a bit dubious. Thanks! $\endgroup$
    – anonymous2
    Sep 23, 2021 at 11:33
  • $\begingroup$ Hi, and thank you! Removed the "bonus question" part but kept the third image in as another example, as I like the progression from first over second to third picture. I did not understand your reference to linguistics. As for SEO, do you suspect me of working for either of the manufacturers of the props in pics 2 and 3? If so, I'll gladly find different pictures...? As for drones, I didn't understand that either ... I'm not interested in drones and all three props shown are for use on traditional fixed-wing aircraft. $\endgroup$
    – Sixtyfive
    Sep 23, 2021 at 12:00
  • 1
    $\begingroup$ Oh, appologies for being unclear: I was just giving reasons why it might be best to remove the bonus question. It looks great to me how you've editted $\endgroup$
    – anonymous2
    Sep 23, 2021 at 12:22

1 Answer 1

5
$\begingroup$

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.

$\endgroup$
5
  • $\begingroup$ Wow, thank you for addressing all the points! One question though: what you call "bulge" is the top-part of what the manufacturers call "undercambered", i.e. both refer to the same airfoil shape as it would appear in a cross-section? Or are "bulge" on the top and "undercamberedness" at the bottom features that need to be talked about separately? $\endgroup$
    – Sixtyfive
    Sep 24, 2021 at 10:16
  • $\begingroup$ I was talking about the wide chord at about half span. It's not related to the aerofoil. I think part of the odd shape is that this extra width is mostly achieved by extending the trailing edge (compare with an APC 'slow fly' prop) but I think that is to keep the thickest part fairly straight. $\endgroup$ Sep 24, 2021 at 10:23
  • $\begingroup$ Okay, so it's only about what you said with regard to "making more of the prop not its tip". Then perhaps the purpose of the undercamberedness (as I understand it, the "negative bulge" on the back side of the prop, but not a native speaker of English!) is to have increase the lift of the prop's airfoil, so to speak ... more air moved for each rotation? $\endgroup$
    – Sixtyfive
    Sep 24, 2021 at 10:31
  • 1
    $\begingroup$ 'undercamber' refers to the cross section, specifically on the side of the prop that faces backwards (or the bottom, when the prop is sitting on a table, as in the photos). If the prop was a wing, it would be the lower surface. 'Undercambered' means the same as 'concave', but specifically for the lower surface or an aerofoil. It requires slightly more advanced manufacturing than a flat bottomed aerofoil, if you're carving it from wood. Undercamber doesn't really do anything specific, it's just something that happens when an aerofoil has a lot of camber and is also thin. $\endgroup$ Sep 24, 2021 at 12:19
  • $\begingroup$ Awesome, thank you Robin, that was the last part of it that made me scratch my head. Hoping your answer will get some more upvotes! $\endgroup$
    – Sixtyfive
    Sep 24, 2021 at 13:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.