The Schempp-Hirth Duo Discus T powered glider has a five-bladed propeller, with blades of different lengths.

What is the purpose of such an arrangement?

  • $\begingroup$ They look to me of the same length (google.com/…) $\endgroup$ Jun 30 '19 at 23:54
  • $\begingroup$ I tried to find a conclusive source for the differing length, and funnily enough, only ran across it in this accident report: assets.publishing.service.gov.uk/media/5714f22aed915d1601000000/… It seems the propeller hub had some fatigue issues which were not detectable with the suggested inspection procedures. The manufacturer does not state anything about different lengths on the open portion of their website. $\endgroup$ Jul 1 '19 at 9:27
  • $\begingroup$ Asymmetrical fan blades are nothing new. Automakers have used them for decades to reduce noise from radiator fans. They, however, are the same diameter because they are ducted. $\endgroup$ Jul 2 '19 at 19:36

You are right. They are unequal length blades. Schempp refers to the propeller as being "noise optimized" and you can see that the blades are free to flap (forward) like helicopter blades (which flap up) so that the asymmetric thrust axis from having the propeller disc offset from the rotational axis doesn't place bending loads on the hub. And obviously, the blades will be mass balanced to place the center of gravity at the hub axis.

I'll go out on a limb here and say that some genius figured out a way to reduce the horrific noise level caused by such a small propeller by having the blade tips each run in a slightly different path of rotation. Brilliant.

enter image description here

And here it is in action where the eccentric blade disc can be clearly seen.

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    $\begingroup$ They look like they are also unevenly spaced around the hub, which is a known technique to reduce noise. escholarship.org/content/qt9q75v9t9/qt9q75v9t9.pdf $\endgroup$
    – MikeY
    Jul 1 '19 at 1:13
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    $\begingroup$ Excerpt from the conclusions of that paper: "The results showed significant increases in total acoustic intensity for propellers with uneven blade spacing, with very high concentrations of energy in the 0.5BPF tone. [...] This is beneficial for noise metrics such as flyover noise, as noise at low polar angles are “weighted” less than noise near the plane of the propeller." So it is more noisy, but in a lower frequency which is less annoying to humans. $\endgroup$ Jul 1 '19 at 7:41
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    $\begingroup$ Noise is wasted energy, so my guess is yes, it is less efficient. Regarding safety, only as much as other unusual designs, by merit of being unusual. The early model prop hubs developed fatigue cracks that went undetected, but this seems to have been fixed. $\endgroup$ Jul 1 '19 at 9:31
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    $\begingroup$ @AEhere: It's not that the noise is at lower frequencies, it's that the noise is "thrown" forward (along the axis) more than it would be with a symmetric design, and correspondingly "thrown" less in directions perpendicular to the propellor axis, including down towards the ground. $\endgroup$ Jul 1 '19 at 11:56
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    $\begingroup$ @MichaelSeifert that too. I realize that the part I quoted is not the best as it touches on both topics, so reference page 39 of the report: "The shift of acoustic intensity into the low frequency 0.5BPF tone is beneficial because the human ear is much less sensitive to low frequency sounds." and the accompanying graphs; you can clearly see additional peaks in the 1-5 kHz range. $\endgroup$ Jul 1 '19 at 12:18

Just a guess, but it might be an attempt to reduce the chance of resonance at the blade frequency.

The tail will receive pressure pulses from each blade, but they won't be evenly spaced.

A motor like this is designed to be started and stopped during flight, and will this accelerate and decelerate through a range of speeds. Somewhere in the range between 0 and full power may be the resonant frequency of some part of the tail. When the motor hits that frequency, the tail could shake violently.

By spacing out the pressure pulses from the prop, there won't be any speed where they all resonate with the tail.

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    $\begingroup$ Cars (and laptops, apparently) reduce objectionable noise similarly: mechanics.stackexchange.com/questions/24327/… $\endgroup$ Jul 2 '19 at 16:46
  • $\begingroup$ The engine on the Discus cannot be throttled in a conventional sense, it only has on and off modes. $\endgroup$ Jul 3 '19 at 10:42
  • $\begingroup$ @AEhere, but as it goes from Off to On and back, it goes through all the speeds in between. The control might be binary but the engine rpm is still analogue. $\endgroup$ Jul 3 '19 at 10:46
  • $\begingroup$ Yes, of course, I was merely pointing out that the control is a bit unusual. Also, the pilot must decompress the engine for it to start, so there's that. $\endgroup$ Jul 3 '19 at 10:53

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