for those wondering... https://www.reddit.com/r/AskEngineers/comments/g39s6s/is_the...
I read 80% of it and I'm still wondering.
MT-Propeller 11 blade propeller delivers 15% increase in static thrust
21–30 of 104 posts
Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#22For reference on more or less blades, aviation stack exchange has some great discussions on this subject. One of my favorites https://aviation.stackexchange.com/questions/23009/what-are-...
Interesting so basically the more blades, the less efficient it gets but you get more thrust
Sort of makes sense why drones have a couple blades so they can putter along for hours
Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#23At what point does a prop become a fan? A 777 engine has 22 fan blades, twice this thing.
> At what point does a prop become a fan?
Linguistically, a prop becomes a fan when you anchor it to something immovable, or attach it to something movable but without any intention of propelling it using the prop.
Fan - try to move the air around, but not the thing it's attached to.
Prop - try to propel the thing it's attached to
Edit: I realize that might sound ambiguous given terms like "turboprop" and "turbofan" both of which are on planes.
The turboprop uses air to speed up propellers to move the plane. A turbofan uses just accelerated exhaust from the engine, and the fans are just a means of feeding the engine combustion material (air)
Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#24At what point does a prop become a fan? A 777 engine has 22 fan blades, twice this thing.
> what point does a prop become a fan I don't know, because I think the whole purpose of a prop is to be a fan.
No quite. Fans == move air, relative to the thing the fan is mounted to.
So a jet engine has fans, because it is moving air into and around the engine, but not using them to move itself around directly (It uses the jet of air created by the thermal expansion from the burning fuel and compressed intake air to do that, mostly).
Propeller == moves something through air (or another media like water) directly.
Semantics, mostly.
Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#25At what point does a prop become a fan? A 777 engine has 22 fan blades, twice this thing.
Back in the 80s there were experiments run with propfans -- propellers with many stubby blades that offered near jet-like performance while consuming half the fuel. They never got anywhere because propfan engines were large and noisy and the dropping price of oil made the fuel economy concerns less urgent.
Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#26Why are they only using odd numbers of blades?
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.)
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.
Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#27Earlier quoted context omitted.
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.)
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.
“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-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.
Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#28I'd imagine that, just like in other industries, there are tons of metrics (or counter-metrics) that are also important. Some that come to mind are efficiency, noise, vibration, cost, maintenance, stall characteristics, RPM requirements, weight, etc etc. From some fiddling with RC quads, I remember that increasing the number of blades does increase thrust, but at the cost of efficiency and several other parameters.
The cost of this blade setup means no one will buy it. Very high cost to purchase and equally high cost to maintain for such a tiny benefit. I had a laugh at the noise reduction claim. Sure, it will reduce noise, but the open exhausts on planes are the loudest part unless some nitwit is screwing around with the pitch.
Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#29I'd imagine that, just like in other industries, there are tons of metrics (or counter-metrics) that are also important. Some that come to mind are efficiency, noise, vibration, cost, maintenance, stall characteristics, RPM requirements, weight, etc etc. From some fiddling with RC quads, I remember that increasing the number of blades does increase thrust, but at the cost of efficiency and several other parameters.
The cost of this blade setup means no one will buy it. Very high cost to purchase and equally high cost to maintain for such a tiny benefit. I had a laugh at the noise reduction claim. Sure, it will reduce noise, but the open exhausts on planes are the loudest part unless some nitwit is screwing around with the pitch.
Would that change when using an electric motor?
Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#30At what point does a prop become a fan? A 777 engine has 22 fan blades, twice this thing.
> At what point does a prop become a fan? Linguistically, a prop becomes a fan when you anchor it to something immovable, or attach it to something movable but without any intention of propelling it using the prop. Fan - try to move the air around, but not the thing it's attached to. Prop - try to propel the thing it's attached to Edit: I realize that might sound ambiguous given terms like "turboprop" and "turbofan"…
I am not sure this is really correct - turbojets (as in fighter jets) sound more like what you describe, turbofans if I remember correctly, get most of the thrust (>70%?) from bypass air around the combustion, and very little from actual combustion thrust. It just also feeds it's own combustion chamber air to keep the fan part going.
Perhaps it is a fan if enclosed? Vs. a prop if not?