What’s the weight?
their 7 blade is ~225lbs.. so I'd guess at a minimum it's ~265#
MT-Propeller 11 blade propeller delivers 15% increase in static thrust
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Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#42Earlier quoted context omitted.
“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 cyl…
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.
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!
Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#43Ok, how much better is 13 blades?
2 better than a prop that only goes to 11.
Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#44Somewhat related thread [0] from last month about the "tipless" Sharrow MX-1 prop [1] which achieves efficiency gains of between 9-15% over comparable 3-blade propeller designs. They started out working on an aircraft propeller but pivoted to a watercraft prop. 0: https://news.ycombinator.com/item?id=33949895 1: https://www.mby.com/gear/sharrow-mx-1-tipless-propeller-1101...
I immediately questioned cavitation and that link delivered. That's INCREDIBLE. The only issue is the price of course. That looks like a perfect candidate for 3D printing and then machining the final part vs billet.
Ultimately I suspect if they wanted to make these in significant volume, die casting or loss wax casting then machining would be correct route forward. 3D printing metal is expensive and the parts are likely to not be as strong. Most 3D printing (sintering) processes for metal result in a slightly porous part that needs sealing, those small gaps produce imperfections.
https://en.m.wikipedia.org/wiki/Die_casting
https://en.m.wikipedia.org/wiki/Lost-wax_casting
Take a look at turbine blades in jet engines, they are incredibly complex with tiny air holes. They are made using a loss wax then machining process.
Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#45I'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 big wins here are likely the increase in thrust and ultimately cruise speed, which makes twin props more competitive with very light jets, and the decrease in noise which may make more airports accessible to the aircraft in question due to local noise limitations. The big loss here is purchase price and maintenance price. But that's on a scale that's probably not bothering a person who can afford a brand new Beec…
If you trade efficiency for faster cruise speed, is your trip being shorter a net win for fuel costs?
Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#46I'm really not a big "fan" of MT's cheezy wooden propellers. They call them "Natural Composite" but it's really just densified/compressed wood with a thin fiberglass wrapping. Here's an example of what you can expect from in-service damage, a Jetstream 41 recently hit a bird, shed a blade, and it shot right through the passenger cabin. The hilarious thing is that the government investigation called it a "survivable accident" because no one happened to be seated in the row that it shot through. https://avherald.com/h?article=4f2a35e6
I was involved in the evaluation of proposed repairs to a Beechcraft Bonanza with a 3 bladed MT prop that taxied into a vinyl traffic cone at idle speed. The Beech shed two prop blades, bent the engine mount and firewall. There was a slight cut in the traffic cone, but it was returned directly to revenue service.
Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#47Earlier quoted context omitted.
I immediately questioned cavitation and that link delivered. That's INCREDIBLE. The only issue is the price of course. That looks like a perfect candidate for 3D printing and then machining the final part vs billet.
> That looks like a perfect candidate for 3D printing and then machining the final part vs billet. Now you need two expensive machines, not just one. Billet is not that expensive. I've seen people suggest this before and I looked on their site. They show a multi axis CNC machine making the parts. You would need exactly the same machine to post machine the 3D printed part, but now you have a second expensive and slow…
That makes it a great candidate for near-net casting. That way you save the 80% of tre billet you'd be turning into chips, and your fancy CNC gets more parts per shift.
Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#48Kind of related: Do the same efficiency dynamics that apply to wind turbines apply to a fan providing propulsion? I.e. if a one-bladed fan is the most efficient for capturing wind, would it also be the most efficient airplane propeller or boat propeller? Obviously efficiency isn't the only consideration, but I am curious and I expect there's going to be some fan experts in this thread...
Due to much higher rpm, balance is imporant, and your diameter is constrained (you want the propellor as small as possible because you want to A: keep the whole thing underwater and B: Not bang the tips on the bottom.
Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#49The Wright Bros achieved 90% efficiency with their propellers. Adding 15% to that gives 103.5% efficiency, which I'm not buying.
Re: MT-Propeller 11 blade propeller delivers 15% increase in static thrust
#50Earlier quoted context omitted.
The big wins here are likely the increase in thrust and ultimately cruise speed, which makes twin props more competitive with very light jets, and the decrease in noise which may make more airports accessible to the aircraft in question due to local noise limitations. The big loss here is purchase price and maintenance price. But that's on a scale that's probably not bothering a person who can afford a brand new Beec…
If you trade efficiency for faster cruise speed, is your trip being shorter a net win for fuel costs?
No, because drag scales with v^2. Going 10% faster requires 21% more power (and thus fuel), but only reduces trip time by 9%.