MT-Propeller 11 blade propeller delivers 15% increase in static thrust | Hacker News Reader

MT-Propeller 11 blade propeller delivers 15% increase in static thrust

mt-propeller.com

97 pointsvoidmain000198 comments

I'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.

Are the wooden construction essential to the new blade design? What are the Beechcraft Bonanza props made of?

(As a complete layman, not even a pilot, I love the sound profile of the proposed props)

Normally, the prop on a Bonanza is made of aluminum. Based on observation of prior incidents, I would expect no or negligible damage to an aluminum propeller in the same circumstance. However, even if no external damage is seen, some engine manufacturers (and insurers) require a complete engine teardown and inspection in the event the propeller contacts an object while the engine is running.

> 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.

No, that’s not why. See FAA definition below.

Survivable Accident - an accident in which the cockpit and/or structure remains relatively intact and the forces experienced by the occupants did not exceed or should not have exceeded the survivable limits of human G-tolerance. Such an accident is classified as survivable even if some or all occupants were fatally injured. (NOTE: the investigator makes his or her greatest contribution to air safety by documenting the reasons why aircraft occupants were fatally or seriously injured in survivable accidents.)

https://www.faa.gov/documentlibrary/media/order/8020.11c_.pd...

Trust me, if that propeller had hit someone in the head, they would have experienced forces that would have exceeded the survivable limits of human G-tolerance.

> Such an accident is classified as survivable even if some or all occupants were fatally injured.

> occupants did not exceed or should not have exceeded the survivable limits of human G-tolerance

I think it was a joke.

In other words, a survivable accident is approximately an accident in which one or more occupants could have theoretically survived, even if in fact they all died. One or more survivors would seem to prove the accident was survivable.

Since in enough quantities or the lack thereof everything is lethal, where does the threshold land?

I'm sure some people have survived explosions, I know of the one woman who fell out of a plane without a parachute and survived it, so falling off the empire state building is a survivable incident, right?

They mention limits on acceleration, etc. I presume there's a standard table somewhere. I'm not sure if they use median survivablility or some higher quantile.

I agree with what you're saying.

But to add a bit of colour to the incident: the Kori Bustard is the biggest flying bird in Africa, and it's mostly ground dwelling.

Seems like some bad luck was involved too, and I wonder if the same damage will occur with a smaller bird?

Somewhat 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.

> 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 process to manage (the metal 3D printing). With billet a huge bar of material shows up, you slice each piece in a few minutes, mount it in your CNC and go. 3D printing feed powders have their own expensive processes they need to go through, so they probably cost much more than billet, reducing any potential cost savings on the material side. And the second expensive machine and process adds a lot of new expenses. Plus, you have to validate the product all over again instead of just selling them.

I bet the high price is mostly due to patents and lack of competition. It is probably not inherently super expensive to make, beyond the need for a high end multi axis CNC machine.

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.

Came here to say the same, and want to add: And longer tool lifetime as well.

Alternately, you can do the rough machining on a cheaper 3-axis machine (even outsourcing it).

That’s a good point. Running two CNC machines is a lot easier than running one machine and dealing with a foundry. I mean I’ve only ever dealt with one foundry and they were flaky, and obviously there must be reliable foundries out there, but I don’t like relying on outside shops any more than necessary.

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.

> I immediately questioned cavitation and that link delivered.

of course, [super]cavitation can do incredible things for efficiency through water as well. maybe even more-so!

https://en.wikipedia.org/wiki/VA-111_Shkval

For 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

Props with high blade count substantially reduce noise for drones as well

https://youtu.be/1nk74KEIc2c?t=138s

Huh! Thanks for posting this. I haven't seen 5" builds with props like this before.

The opposite is definitely true for toothpick or smaller builds. Bi-blade is much quieter for something like a ~60g, 3" quad.

As a former aircraft owner, I immediately began to calculate the costs of doing on overhaul on those props. My three bladed props would need an overhaul at least every 2000 hours. There is also a time minimum as well if you don't reach the hours before the time interval. It seemed like the engines and the props would hit overhaul hours around the same time, usually hitting me with a bill well over $10,000.

I wonder what the cost will be on dual, 11-blade adjustable pitch props? It makes my eyes water and feel thankful I am not the owner/operator of such an aircraft.

Perhaps it all makes sense given the fuel consumption and speed improvements?

I'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.

My understanding as well. You increase on metric and make some other one worse. In the end it is complex process of finding some balance between these.

Not that there isn't some improvements that can be made like have been done with improved materials(less weight), computer design and so on in more complex engines.

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?

I'd say the most favorable arrangement for noise would be a pusher configuration, with the engines close to the back of the plane instead of a tractor config on the front. This is done on the Piaggio Avanti. Not sure how well it works, but it looks like a spaceship.

Eviation electrical plane recently changed their pusher design to a tractor, but kept the engines on the back (IIRC, they started with a central big one and two auxiliary ones on the wingtips for use during takeoff).

The problem with pusher configuration is that the prop operates in turbulent air, that has already ben disturbed by going around the rest of the airplane. Whereas in tractor configuration, the prop grabs "clean" air. This is traditionally cited as the reason why pusher prop planes are louder (from the outside). In fact, one of the noisiest GA planes is the Cessna 337, which is a pull-push twin configuration.

The advantage of the Piaggio Avanti is that you can do away with the prop heat completely, as the hot exhaust from the turboprop engine simply blows on the blades. Which makes for a very nice anti-ice system.

> Which makes for a very nice anti-ice system.

Didn’t know about that. Sadly, there isn’t much hot exhaust from electric motors.

This is for cabin noise, for overall noise tractor is still better. On some airports very close to the city the overall noise is the most concerning factor, in some places you cannot operate at night because of the noise, so in the end it depends what you are looking for.

To some extent, yes. Electric motors are not completely quiet, but there are some propellers that are relatively quiet, without hard numbers it is just informed guessing.

I can't say I have the numbers to dispute this. But the loudest thing on my Cessna is not the open, 1970s era exhaust. It is definitely the prop whipping through the wind.

Is it the external noise or the cabin noise? We changed the propeller on the plane I usually fly (Ikarus C42) a couple of years ago and the new one is extremely quiet as perceived from the ground, now the engine makes more noise than the propeller. In the cockpit the noise is lower overall, but I was never able to point to the engine or to the prop from that position.

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 Beechcraft King Air or to completely refurbished a used version of the plane in the video (they quit making the Piper Navajo 30 years ago).

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%.

AIUI, drag scales as v³, the reason doubling horsepower adds only a trivial amount of speed.

You are both right, there are v^2 and v^3 effects, and depending on the speed regime, one or the other will dominate.

In the pilot books in my country the v2 is the most significant factor, v3 is not even mentioned in General Aviation.

At 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.

> 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)

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?

Most modern fighter jets have turbofan engines as well.

>Perhaps it is a fan if enclosed? Vs. a prop if not?

Stipa Caproni

Dear Lord! Half of the lift is due to the repulsion Earth feels from it.

That's not true about a turbofan. A large part of the thrust is actually from the fan itself. The bypass air that never goes into the combustion chamber produces most of the thrust.

"A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the thrust.":

https://en.m.wikipedia.org/wiki/Turbofan

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.

A think a lot of the noise was because the blades were counter-rotating. The size limitation was to be compatible with tail-mounted engines like the ones used in DC-9's and 727's.

The relevant conceptual distinction seems to be whether the aerodynamics of the blade(s) are conceptually independent, or interdependent. Better explained here: https://en.wikipedia.org/wiki/Propeller_(aeronautics)#Aircra...

Kind 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...

With aircraft you can’t just say “efficient” you have to point out what you’re trying to optimize.

In rotor design it's pretty clear. Thrust/power ratio here, or FM (Figure of Merit).

It’s not at all. How much does it weigh? How does it perform at different speeds?

Thrust to weight ratio is very important to aircraft.

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.

Seems like 1 blade would theoretically be optimal, but due to requiring a counterweight, a two blade design is superior. [0]

[0]: https://aviation.stackexchange.com/questions/23009/what-are-...

Short answer is yes. But you have to compensate by making the rotor blades longer or spinning them faster to get the same thrust. You can't let the net velocity (vector sum of tip velocity + aircraft velocity) reach supersonic or bad things happen. So you have practical considerations that lead to tradeoffs that determine the number of blades.

The R22 and Cabri G2 are helicopters with basically the same mission, but one uses a 2-blade rotor, the other uses 3 blades. Different engineering choices, style, preferences.

Why 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.)

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.

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!

> 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.

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.

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.

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.

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.

> with an impressive noise and sound signature.

Not sure whether it means that it was loud or quiet. /s

Seems like this 11 blade propeller -- might make a great rear propeller for the Celera 500L:

https://ottoaviation.com/

I love how the stroboscopic effects in the video recording makes it look like the blade is spinning slowly like in videogame or anime show airships.

The Wright Bros achieved 90% efficiency with their propellers. Adding 15% to that gives 103.5% efficiency, which I'm not buying.

You don't add them up like this. I can make a car use 50% less fuel an infinite amount of times, not just twice.

Please rethink your statement. You cannot get more motive energy than the energy put into the propeller.

> I can make a car use 50% less fuel an infinite amount of times

No, you can't. Conservation of energy applies.

P.S. I didn't "add" them, either. 90% x 115% => 103.5%

That is not how it works.

1-((1-0.9)x0.85) = 0.915, 91.5%. 15% better.

(Your original point is not affected by this nitpicking, but:) A car, traveling any distance, between two points of constant height (in the gravity field), and neglecting work through friction (which is zero in the zero speed limit, albeit the movement is quite slow then, but a pumped out tunnel and mag lev gets you very close), requires no work, so no fuel. Technically.

> I can make a car use 50% less fuel an infinite amount of times,

At some point it'll be running on nuclear fission, as you won't be able to feed it a single whole atom. ;-)

Doesn't follow. One of the ways you can make a car use less fuel is by reducing drag and friction.

A propeller provides a certain thrust for a certain input power. If more power comes out than goes in, you can use it to construct a perpetual motion machine.

Your construction of what the number means doesn't make sense, and (therefore) you need to start over. It is generally a mistake to assume people quoting figures mean stupidly impossible things, even though sometimes they do.

Good luck with that.

> ...results in static thrust, 15% increase over the standard certified 5-bladed...

No mention of whether that higher static thrust is at some reference RPM, or when it's receiving some fixed X horsepower from the prop. shaft, or what.

From a quick search of the article, I see nothing connecting the 15% figure to efficiency.

Static thrust is plausibly measured with the airplane sitting still on the ground - which is helpful at the start of your take-off run, but probably not a metric that's worth any optimization.

They did not compare with the Wright Bros, but with their 5 blade prop. The Wright Brothers propeller was extremely inefficient at the speeds the test plane is flying, so don't take the 90% number at one speed and compare it blindly with a very different speed and behavior. Did the Wrights have cavitation or supersonic tip of the blade speeds?

No they did not. it was 66%

Prime number so no harmonics? Therefore no... vampiric eddys or vacuum bubbles...? Just my first guess.

How's 13? 17?

Obviously 13 would optimize for luck.

for those wondering...

https://www.reddit.com/r/AskEngineers/comments/g39s6s/is_the...

I read 80% of it and I'm still wondering.

More parts has a cost: the mtbf goes down geometrically.

What’s the weight?

their 7 blade is ~225lbs.. so I'd guess at a minimum it's ~265#

Ok, how much better is 13 blades?

2 better than a prop that only goes to 11.

It's luckier as well.

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