6 ms·

Why are they only using odd numbers of blades?

JumpCrisscross 4y ago
Easier to balance; if everything is even, you need to balance two blades to counter one misbehaviour, and do so perfectly to avoid introducing new misbehaviours. (They’re prime for resonance reasons. I don’t remember what, specifically.)
- mhb 4y ago
  Why would they be easier to balance? The hand-waving about resonance also pervades that reddit "explanation" thread which doesn't explain anything about what prime or odd numbers of blades have to do with resonance in a single propeller.
  - lazide 4y ago
    “Prime numbers are generally used to reduce the magnitude of resonances. These occur in a non-linear multi-frequency system when two of the frequencies ω1:ω2 match at a ratio p:q, where p,q are comprime integers.” Per https://physics.stackexchange.com/questions/484288/why-choosing-for-prime-numbers-eliminates-vibration https://physics.stackexchange.com/questions/484288/why-choos... It’s also matched by practical experience - engines which have simple low integer multiple resonances in moving parts (like opposing 2 cylinders, or inline 4 cylinder) tend to have large vibrations/ + resonances that need special care and work to handle, or require running at much lower speeds to avoid self destruction. Most propellers and fans have 3 blades because of this, plus additional tradeoffs efficiency wise. More blades will move more air/media per rpm but with more friction (reduced efficiency). Fewer blades (2 or the absolute worst, 1) causes more vibration and requires more work to balance + cancel resonances.
    - mhb 4y ago
      Thanks. The explanation there makes sense. Still, this must be a secondary effect since there are plenty of examples of even-bladed and non-prime bladed propellers.
      - lazide 4y ago
        Sometimes, the constraints make the extra work worthwhile. It’s an engineering problem. Typically, you’ll see more blades when you have a limited area you can cover with the blades (aka, low wings or stubby wings, or lots of engines relative to the wing size) and need more power/thrust. Adding more engine power is easy enough, if the decrease in overall efficiency and stronger airframe required are ok. It’s not a super common trade off, but it’s not that uncommon. If the ‘more blades’ math means it’s an even number, then so be it. Balancing them more carefully, strengthening the airframe in problems areas more, etc. is all part of the equation. It is more work though. You’ll see similar tradeoffs between something like a jet ski water impeller and a cargo ship prop. Jet engines have similar type of trade offs - max power vs overall efficiency, or efficiency at cruise vs efficiency during variable condition use. Or noise vs power. The type of engine used in a fighter jet makes very different tradeoffs than in an airliner. Adding ducting around a prop or fan also allows much higher total thrust for the given real estate, at the cost of weight and a bit of efficiency. 3 is usually the cheap and easy answer. Sometimes it’s even the exhaustive and time consuming answer too!
  - JumpCrisscross 4y ago
    > Why would they be easier to balance? Have you balanced a fan? The wobbles aren’t solely at the blade points. Having a blade opposed by nothing helps avoid a problem where one blade balancing throws another off.
    - mhb 4y ago
      It's still not obvious why this would be different for even vs. odd blade numbers or why changing one blade throws the others "off". If you have two blades and change one, you need to be concerned with the other blade. If you have three blades and change one, you need to be concerned with the other two blades.
- mannykannot 4y ago
  Four years ago they were using a nine-bladed propeller, so while having a prime number may be desirable, it does not seem to be necessary.
Cerium 4y ago
Generally odd numbers of blades are better for vibration. I know that in wind turbines they are favored because it reduces the effect of the blade crossing the tower. When one of the blades in a three blade wind turbine crosses over the tower the force on it is reduced (since the wind is already slowed by the presence of the tower). The other two blades are in maximum loading, but balance much of their forces against each other reducing the bending moment on the turbine shaft.
- steve_gh 4y ago
  On wind turbines there are generally ground drag effects that reduce the wind speed closer to the ground. A 3 bladed setup minimises the wind speed (and therefore thrust) differences between blades, minimizing the off axis forces on the rotor bearings.

Easier to balance; if everything is even, you need to balance two blades to counter one misbehaviour, and do so perfectly to avoid introducing new misbehaviours. (They’re prime for resonance reasons. I don’t remember what, specifically.)

mhb 4y ago
Why would they be easier to balance? The hand-waving about resonance also pervades that reddit "explanation" thread which doesn't explain anything about what prime or odd numbers of blades have to do with resonance in a single propeller.
- lazide 4y ago
  “Prime numbers are generally used to reduce the magnitude of resonances. These occur in a non-linear multi-frequency system when two of the frequencies ω1:ω2 match at a ratio p:q, where p,q are comprime integers.” Per https://physics.stackexchange.com/questions/484288/why-choosing-for-prime-numbers-eliminates-vibration https://physics.stackexchange.com/questions/484288/why-choos... It’s also matched by practical experience - engines which have simple low integer multiple resonances in moving parts (like opposing 2 cylinders, or inline 4 cylinder) tend to have large vibrations/ + resonances that need special care and work to handle, or require running at much lower speeds to avoid self destruction. Most propellers and fans have 3 blades because of this, plus additional tradeoffs efficiency wise. More blades will move more air/media per rpm but with more friction (reduced efficiency). Fewer blades (2 or the absolute worst, 1) causes more vibration and requires more work to balance + cancel resonances.
  - mhb 4y ago
    Thanks. The explanation there makes sense. Still, this must be a secondary effect since there are plenty of examples of even-bladed and non-prime bladed propellers.
    - lazide 4y ago
      Sometimes, the constraints make the extra work worthwhile. It’s an engineering problem. Typically, you’ll see more blades when you have a limited area you can cover with the blades (aka, low wings or stubby wings, or lots of engines relative to the wing size) and need more power/thrust. Adding more engine power is easy enough, if the decrease in overall efficiency and stronger airframe required are ok. It’s not a super common trade off, but it’s not that uncommon. If the ‘more blades’ math means it’s an even number, then so be it. Balancing them more carefully, strengthening the airframe in problems areas more, etc. is all part of the equation. It is more work though. You’ll see similar tradeoffs between something like a jet ski water impeller and a cargo ship prop. Jet engines have similar type of trade offs - max power vs overall efficiency, or efficiency at cruise vs efficiency during variable condition use. Or noise vs power. The type of engine used in a fighter jet makes very different tradeoffs than in an airliner. Adding ducting around a prop or fan also allows much higher total thrust for the given real estate, at the cost of weight and a bit of efficiency. 3 is usually the cheap and easy answer. Sometimes it’s even the exhaustive and time consuming answer too!
- JumpCrisscross 4y ago
  > Why would they be easier to balance? Have you balanced a fan? The wobbles aren’t solely at the blade points. Having a blade opposed by nothing helps avoid a problem where one blade balancing throws another off.
  - mhb 4y ago
    It's still not obvious why this would be different for even vs. odd blade numbers or why changing one blade throws the others "off". If you have two blades and change one, you need to be concerned with the other blade. If you have three blades and change one, you need to be concerned with the other two blades.
mannykannot 4y ago
Four years ago they were using a nine-bladed propeller, so while having a prime number may be desirable, it does not seem to be necessary.

Cerium 4y ago

Generally odd numbers of blades are better for vibration. I know that in wind turbines they are favored because it reduces the effect of the blade crossing the tower. When one of the blades in a three blade wind turbine crosses over the tower the force on it is reduced (since the wind is already slowed by the presence of the tower). The other two blades are in maximum loading, but balance much of their forces against each other reducing the bending moment on the turbine shaft.

steve_gh 4y ago
On wind turbines there are generally ground drag effects that reduce the wind speed closer to the ground. A 3 bladed setup minimises the wind speed (and therefore thrust) differences between blades, minimizing the off axis forces on the rotor bearings.