I know scimitar propellers are used to reduce noise, but what is their downside? There has to be one, considering most propellers don't use that shape.

  • Good question. I'll be watching this one. I know that they only provide marginal performance improvements, if any at all, for GA aircraft. – acpilot May 22 '16 at 22:58
  • My guess would be that this kind of propellers is too costly relative to the performance and noise benefits. The performance increase may be noticeable only in the Mach > 0.8 region. But I don't have any real numbers to support. – Jordy May 24 '16 at 10:24
up vote 7 down vote accepted
  1. Molding of the intricate shape requires expensive carbon fiber

  2. For it to dampen the noise it's best with 5+ blades (see 3.)

  3. Which is hard to balance and too much weight for your run-of-the-mill 4-seater single engine plane

  4. By the time the benefits were realized, enter the jet engine for the big planes that would have benefited from it. You only see it in few turboprops nowadays.

Edit:

For more on the correlation between no. of blades and noise: it was mainly discovered from tests. Here's a long pdf with tons of info, and a snippet from it follows (T denotes Test, summary: more blades, less noise):

enter image description here

  • Thanks for the answer ymb1! Can you elaborate on why noise dampening is best achieved with 5+ blades? – Southbob Jun 7 '16 at 12:25
  • That's very interesting, thanks again! – Southbob Jun 7 '16 at 12:37

The biggest benefit of a scimitar propeller is the higher operating Mach number that can be achieved. Tip sweep delays the onset of shocks on the propeller and reduces drag at high subsonic speed. The noise reduction is achieved by reducing shocks on the blade tips at the same tip Mach number - supersonic props are known to be extremely loud.

The major disadvantage is the torsional moment created by the swept back tip. This can be somewhat mitigated by sweeping the inner blade forward which results in the typical scimitar shape. Still, in-blade torsion moments mean that the blade has to be built such that the deformation in flight will result in the desired blade twist. The use of stiff carbon fiber allows to keep this pre-twisting low.

That scimitar propellers have a high blade count is a result of their high operating speed: To keep the tip Mach number low, the diameter must be limited which increases the load on the single blade. Flying fast needs a lot of thrust, so the activity factor (power absorbed by the propeller per blade area) is high. To reduce the local Mach number on the blade, both chord and blade count are increased while the relative thickness of the blade's airfoil is reduced when compared to a conventional propeller. This results in a lower operating lift coefficient on the blades and lower supervelocities which again helps to increase the operating Mach number.

Your Answer

 
discard

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

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