Lowest SFC turboprop/-shaft currently on the market?

[Pasoleati]

Definitely not any from the PT6-series...

 

[AeroFranz]

Well, it's a pretty open-ended question...i would say large engines rated for high power have an advantage because of scale-related effects. Other than that, the most recent ones benefit from improved materials and aerodynamics...so i don't know, maybe GE's catalyst? or large ones like the T408?

 

[Pasoleati]

Sfc figures for those? Last time I checked the GE T700 had substantially lower sfc than any PT6 variant.

 

[Pasoleati]

Manufacturer's sites are useless for detailed data. It is ridiculous that Jane's has more detailed tech data than makers' own sites. F*ck!

 

[YouCantbeCirrus]

Since most of the designs in service right now are at least a couple decades old (save for the Europrop TP400 and the Catalyst by GE) most are fairly inefficient. The PT6 and Catalyst are gas generating so waste lots of heat out the exhaust. Any turboshaft that is non-gas generating will be way more efficient since they can have 4 or more stages of power turbines whereas even the most powerful PT6 only has 2 free power turbines.

And as Franz stated the larger turboprops will be more efficient for SFC here's a short list:
PT6A-67D (948 SHP, Beech 1900D): 0.546 lb/(hp/hr)
TPE 331-10 (940 SHP, Rockwell Turbo Commander): 0.534 lb/(hp/h)
Walther M601D-1 (740 SHP max., Ayers Thrush): 0.62 lb/hp-h

These are all rather small but very common civilian turboprops, each is very comperable but you have to remember these are all non-FADEC manual control engines. Efficiency is hard to squeeze out in good old pneumatic-mechanical systems. GE claims the Catalyst is "30%" better than it's contemporaries but you don't see great gains in efficinecy until you get to much larger turboprops.

PT6C-67A (1,900 SHP, Bell 609): 0.47 lb/(hp/hr)
Allison T56 (5,900 SHP, Lockheed C-130) 0.4690 lb/(shp/h) at takeoff, 0.42 lb/(shp/h) cruise
EuroProp TP400 (11,000 SHP, Airbus A400M): 0.37 lb/(shp/h) at takeoff, 0.35 lb/(shp/h) cruise

As you get larger even the gas generating PT6 can be efficient when turning massive blades like it does on the Bell 609, and the EuroProp is a modern engine so the comparison is a bit unfair to the T56.

​

 

[Zoo Tycoon]

Although not presently on the market, one of the lowest SFC and high power engines ever made was the Napier Nomad (inter cooled, turbo compound diesel);-

Napier Nomad - Wikipedia

en.m.wikipedia.org

I recently saw a modern engine manufacturer revisiting the concept for a stationary power generation engine…… of course claiming to have invented it.

 

[YouCantbeCirrus]

Although not presently on the market, one of the lowest SFC and high power engines ever made was the Napier Nomad (inter cooled, turbo compound diesel);-

Napier Nomad - Wikipedia

en.m.wikipedia.org

I recently saw a modern engine manufacturer revisiting the concept for a stationary power generation engine…… of course claiming to have invented it.

Yeah, honestly some of the old diesel engines like the Junkers 2 Strokes and turbo compound engines like the Nomad are probably where the future of the ICE is headed as far as becoming more and more efficient to stay relevant in our ever electrified world.
I think the starter of the thread wants pure turbine engines though, but the Nomad is a very interesting design.

 

[Pasoleati]

I recall reading something like 0.46 lbs./hp/h for a GE T700...

But indeed, a compounded turbocharged diesel is probably the most efficient possible. And that achieves good efficiency over a larger range of power settings than a gas-turbine can...

 

[TomcatViP]

13GPH (g/hr) for 150hp @10000ft gives us 0.57lbs/hp/h sfc

A 200-hp Turboprop For Homebuilts

TurbAero is at AirVenture with a turboprop in development that might just give four and six cylinder homebuilt engines a run for their money.

www.planeandpilotmag.com

 

[YouCantbeCirrus]

13GPH (g/hr) for 150hp @10000ft gives us 0.57lbs/hp/h sfc

A 200-hp Turboprop For Homebuilts

TurbAero is at AirVenture with a turboprop in development that might just give four and six cylinder homebuilt engines a run for their money.

www.planeandpilotmag.com

This thing confuses me, that main draw of it is what exactly?
If you wanted to use Jet-A instead of 100LL for a homebuilt why not just go with some of the diesel ICEs that are on the market? And if it's single throttle control its the same thing since almost all the diesels on the market are FADEC and have single control throttle quadrants save for prop control.
Otherwise I am fascinated by it since it's a microturbine engine which are some of my favorites. I'd be interested to know how they handle prop strikes, cause if they don't have to be rebuilt like a normal Lycoming or Continental that'd be a niche benefit for flight schools.

 

[Pasoleati]

YouCantbeCirrus: Good questions. Honestly speaking, I feel that this turboprop "hoopla" is squarely based on the quite limited understanding among surprisingly many general aviation people; those who are used to believe their aerial Briggs & Strattons (=Lycoming/Continental) represent something excellent in aero-engine design. For example, there are, as far as I know, already on the market some quite good auto-engine conversions burning standard mogas, hence no 100 LL needed.

And as posted above, the diesels with turbocharging, whose fuel economy is far better than what this turbine provides. And with a single-lever controls (these are the same people to whom the idea of single-lever controlled aircraft mass-produced in WW2 is something they have never heard of; they are so ignorant).

 

[Grey Havoc]

On the other hand, given the (rather ironic) demonisation in recent times of diesel fuel in certain jurisdictions, a lot of operators simply may not want the likely regulatory hassles (not to mention potential PR pitfalls).

 

[TomcatViP]

Micro-turbines will be ideal in hybrid setups: the ratio of usable fuel energy converted into electricity will ensure a better economy and low storage weight.

Diesel engines rigorously are not suitable for light aircraft that fly lower and constantly have to alter their power settings.

 

[YouCantbeCirrus]

Micro-turbines will be ideal in hybrid setups: the ratio of usable fuel energy converted into electricity will ensure a better economy and low storage weight.

Diesel engines rigorously are not suitable for light aircraft that fly lower and constantly have to alter their power settings.

Indeed, in fact, microturbines that are used in hybrid electric setups are likely where most of the hybrid aircraft will go in the next 20-30 years. By burying the turbine in the fuselage or even wings you don't have to focus on the aerodynamics of the engine itself and more just feeding it enough air. There's been several companies proposing these types of aircraft with electric motors in pods which opens up so many new possibilities in aircraft design.
To me this is where the optimal electric aircraft will be in the near future, a bit like diesel electric locomotives where the engine is operating at it's highest efficiency power band at all times and substituting a turbine engine in there would give the best configuration for weight considerations.

 

[Pasoleati]

By the way, I read a test report on paraffinic diesel and the results were remarkable: all emissions were significantly reduced before paraffinic fuel oil burns so much cleaner.

 

[Pasoleati]

Diesel engines rigorously are not suitable for light aircraft that fly lower and constantly have to alter their power settings.

That's an odd statement considering that diesels work perfectly fine in cars, trucks, construction machinery where they have alter power settings many times more often than any engine in light aircraft would.

 

[AeroFranz]

The DA.42 and other Diamond aircraft have had reasonable success with the Austro 300 engine.

 

[TomcatViP]

Rigorously meant regarding emissions control. Diesel engines can match other reciprocating engines on that point only if they are at their design points (ex. MALE UAV). A small diesel engine constantly altering its power settings (just like any light aircraft engine do) wouldn't do any good to the industry aside from being a new design. This is exactly what the automotive industry did in Europe, shelving for decades any new engine design that was not diesel to push the market in that niche where they prevailed.
Let's not repeat the diabolicum.

 

[Trident]

Well, show me ANY aircraft engine with a catalytic converter - if emissions regs for aviation become so tough that diesels have a problem, ALL types of engines are going to have the same problem!

 

[TomcatViP]

Why would I carry an heavy passive Nox catching net* when I can use better refined fuel, plug a turbo and raise my compression ratio? Wouldn't that make more senses?

And even when we are discussing Turboprops and hybridization was mentioned?

*that would need a trustable recycling process...

 

[Trident]

Well, one attraction of the diesel quite unrelated to sfc is its ability to run on Jet-A1, which is ubiquitous in the aviation world, while the better refined fuel may not be as widely available. You can thank the dominance of the gas turbine for that

Hybridization has a non-trivial weight penalty - not enough to be consequential in a car, but it is surprisingly difficult to get an efficiency benefit out of it in an aircraft. Tightening emissions regulations on things like NOx could introduce a factor beyond fuel consumption which forces the manufacturers' hands here, but absent that, it has yet to deliver a compelling advantage.

 

[TomcatViP]

Obviously, with a more refined fuel, you can see improvement in at least one parameter:

The test results showed that combustion duration
increased with the usage of Jet-A1 in the test fuel due to lower
cetane number. Ignition delay was prolonged when Jet-A1
was used in the experiments. It was pointed that specific fuel
consumption increased with the usage of Jet-A1 in the test
fuel. Indicated thermal efficiency decreased with the addition
of Jet-A1 in the test fuel due to lower calorific value. This
situation can be realized via the variation of specific fuel
consumption. Indicated thermal efficiency was computed as
28.5 % and 27.8 % with diesel and Jet-A1 (A100) test fuels
respectively at full load. It was also found that CO and soot
emissions increased with the increase of Jet-A1 in the test
fuels. But NOx emissions decreased with the usage of Jet-A1
aviation fuel. It is clear to mention that the most important
influence of Jet-A1 was seen on NOx emission. It can be
concluded that Jet-A1 aviation fuel can be used via mixing
with diesel fuel.

But I would remind the reader that turbine efficiency with the same fuel is then well over the best achieved value...

Source

 

[Trident]

I recently saw a modern engine manufacturer revisiting the concept for a stationary power generation engine…… of course claiming to have invented it.

Turbo-compounding has been used in some modern truck diesel engines for a while (the example below dates from 2013, AFAIK):

Top-of-the-range engine with performance for the toughest requirements: The new heavy-duty Mercedes-Benz OM 473 in-line six-cylinder engine | marsMediaSite

Top-of-the-range engine with performance for the toughest requirements: The new heavy-duty Mercedes-Benz OM 473 in-line six

media.daimler.com

More-electric engine technology may give the concept a new lease of life, I've seen one project where a diesel with an integrated starter/generator was to be fitted with an electric turbocharger. That is to say, a turbocharger with a motor/generator on its shaft, which can alternately add (boost the compressor) or extract power (from the turbine). In conditions where the turbine can produce excess power (beyond the requirements of the compressor), this can be added to the crank shaft via the integrated starter-generator, effectively turbo-compounding the engine. And compared to the traditional implementation of turbo-compound, there is the added benefit of being able to buffer in a battery at times when excess power cannot be put to use.

This particular project did not explore the possibility, but there's some potential for cleaning up the sometimes circuitous charge air and exhaust gas piping, too. If you "split" the turbocharger into separate compressor and turbine, each with its own electric machine, they no longer need to be co-located. That gives you an additional degree of freedom in packaging the whole thing, the disadvantage being that you need two electric machines where one could do.

 

[Zoo Tycoon]

Turbo-compounding has been used in some modern truck diesel engines for a while (the example below dates from 2013, AFAIK):

Top-of-the-range engine with performance for the toughest requirements: The new heavy-duty Mercedes-Benz OM 473 in-line six-cylinder engine | marsMediaSite

Top-of-the-range engine with performance for the toughest requirements: The new heavy-duty Mercedes-Benz OM 473 in-line six

media.daimler.com

I agree straight forward turbo compounding has been around for a while, ie the R3350 being an early example, but the Nomad 1 high degree of recuperation and inter cooling is rather unique, with the possible exception of the similar vintage Dobrynin VD-4K.

 

[iverson]

Turbo-compounding has been used in some modern truck diesel engines for a while (the example below dates from 2013, AFAIK):

Top-of-the-range engine with performance for the toughest requirements: The new heavy-duty Mercedes-Benz OM 473 in-line six-cylinder engine | marsMediaSite

Top-of-the-range engine with performance for the toughest requirements: The new heavy-duty Mercedes-Benz OM 473 in-line six

media.daimler.com

I agree straight forward turbo compounding has been around for a while, ie the R3350 being an early example, but the Nomad 1 high degree of recuperation and inter cooling is rather unique, with the possible exception of the similar vintage Dobrynin VD-4K.

For an aircraft, a Nomad-style turbocompound engine is also complicated and heavy, compared to a gas turbine. I imagine that that is why they were more often used in boats than in airplanes. The Napier Deltic and the Zvezda M-503 (a descendant of the VD-4K?) are cases in point. If I may over simplify, power and light weight, not efficiency, have always been at a premium in aircraft, if only because flight is inherently uneconomical compared to surface transport.

 

[Zoo Tycoon]

For an aircraft, a Nomad-style turbocompound engine is also complicated and heavy, compared to a gas turbine. I imagine that that is why they were more often used in boats than in airplanes. The Napier Deltic and the Zvezda M-503 (a descendant of the VD-4K?) are cases in point. If I may over simplify, power and light weight, not efficiency, have always been at a premium in aircraft, if only because flight is inherently uneconomical compared to surface transport.

Well yes and no because it’s all a bit of a compromise. If I add a 100kg inter cooler which improves my thermal cycle efficiency, such I get more energy out of the fuel onboard, I don’t need to up lift so much. Hence plus 100kg hardware leads to several tons less fuel;- that’s happy days (there’s a “snow ball effect” to boot;- less wing area for the same preformance, less structural mass;- all of which counteract the 100kg! ). But if the intercooler clogs with ice every time the aeroplane passes through a condensation layer, then the whole concept is operationally pretty useless.

Now should someone figure out how to stop an intercooler icing, it could be game on again in these times of trying to reduce carbon emissions. All I’ve got to do is prevent any other operational issue from causing similar, such as a naughty bird from making a bird shaped hole in the rather fragile intercooler pipes.

 

[Pasoleati]

Fellas, I posted here some time ago a chart that compared the Ju 86 with diesels and SI engines. The latter had significantly better power/weight, but the overall efficiency of the aircraft in terms of fuel consumed per ton/kilometre was greatly better with the diesels. And that is a significant cost saving per ton/km. Hence the bottom line likes it.

In short, it is entirely dependent on the mission. If we are dealing with GA light aircraft. It is probably totally insignificant whether the power/weight is say 50 % better if at the same time the sfc is as it is with standard GA petrol-engines needing 100LL vs. diesel burning ordinary diesel fuel. Not too long ago I chatted with a spotter familiar with the local private flying club. He told me that the Thielert-powered plane is in much greater demand than its ordinary counterpart due to vastly lower fuel bill.

 

[Pasoleati]

Here's a T700 specification:

 

[iverson]

<snip>
This particular project did not explore the possibility, but there's some potential for cleaning up the sometimes circuitous charge air and exhaust gas piping, too. If you "split" the turbocharger into separate compressor and turbine, each with its own electric machine, they no longer need to be co-located. That gives you an additional degree of freedom in packaging the whole thing, the disadvantage being that you need two electric machines where one could do.

True. But each machine could be perhaps be optimized for its own operating regime, making it potentially more efficient and/or more of a standard component. My Prius has an electric motor dedicated to reverse, presumably because the specialized extra motor makes everything else simpler (no reverse gears? easier match of power/torque to the operating environment?).

I read somewhere that electric superchargers have caught on with amateur drag racers because they allow simpler installation and, as you say, packaging.

 

[YouCantbeCirrus]

<snip>
This particular project did not explore the possibility, but there's some potential for cleaning up the sometimes circuitous charge air and exhaust gas piping, too. If you "split" the turbocharger into separate compressor and turbine, each with its own electric machine, they no longer need to be co-located. That gives you an additional degree of freedom in packaging the whole thing, the disadvantage being that you need two electric machines where one could do.

True. But each machine could be perhaps be optimized for its own operating regime, making it potentially more efficient and/or more of a standard component. My Prius has an electric motor dedicated to reverse, presumably because the specialized extra motor makes everything else simpler (no reverse gears? easier match of power/torque to the operating environment?).

I read somewhere that electric superchargers have caught on with amateur drag racers because they allow simpler installation and, as you say, packaging.

The main area for recovery of energy with turbines and ICE's has always and will always be the exhaust; hence the fact that stationary power generation turbines have recuperaters/reheaters and heat recovery systems which make steam. The most efficiency is achieved by heating the air before it enters with less total energy expended and by having the most difference in heat energy between the combustion chamber and exhaust (essentially the cooler the exhaust the more energy has been extracted from the fuel.)

As far as electric super/turbochargers there's always a penalty behind them and most of them seem to be based off of a smaller turbocharger in-line with a regular turbocharger for less turbo lag no really for extra power. But still, there's no free lunch, that power has to come from somewhere and having a turbine in the exhaust which powers a generator would be the best instead of having it belt driven by the engine. The idea of having a separate exhaust driven generator would actually be amazing for recovering energy for any ICE aircraft looking to improve efficiency.

Also as far as your Prius having the electric motor drive the car in reverse, I suspect it's either a software thing to keep the engine from having to fire up just to reverse or that it's like the Honda Goldwing and that it has no reverse gear (on the motorcycle it has a sequential gearbox so it can't really, and in the car for simplicity/cost) and as such just uses the start in the Goldwing to reverse since it's a workaround.

 

[Aeroengineer1]

I find the discussion here interesting in that it diverged from the original question about fuel efficient turboshaft/prop engines to why not diesels. There is also some spurious mention of engine architectures being better than another. I currently work in the jet engine manufacturing industry, and have participated in the conversion of a light airplane from piston to turbine.

First about turbine engine configurations. An engine being single shaft all the way to three shaft, or do you put a gearbox in between? Except for single shaft turbojets, dual or three shaft engines have shown that they are all capable of meeting performance and weight targets. To do this, though, each architecture has other design solutions that enable these features. As to the PT6 and Catalyst having a "free turbine" the reality is that this is just a dual shaft design that allows for the design solution to not have a shaft running in the shaft. As such it allows for the decoupling of the compressor from the propeller. This is effectively what you have in large commercial medium and high bypass engines with the exception that the booster/lpc is not decoupled from the hpc. Because the engine is smaller there are solutions to deal with that change in a turboshaft engine. A recent competition that had a free turbine architecture and one engine with one engine having separate booster/lpc and hpc and the other having those integrated on a single shaft was the T900 vs the T901 engine competition. Each of these were able to meet the design requirements particularly around SFC, weight, and power. As I understand it, the major concern on the T900 was around the additional complexity without specific advantage. I would also guess that there could have been some concerns around the teaming approach with Pratt and Honeywell being teamed vs a single org. It would be interesting to come to an understanding on why Pratt and Honeywell teamed rather than each of them going it alone.

In the end a free turbine approach is not necessarily a detriment to the overall installation. The LPT can be sized appropriately and the number of stages contained in the LPT is not constrained because of the free turbine architecture, but rather other concerns in the engine design.

Why then do smaller engines tend to be less efficient over larger engines? There are a couple of factors that are in play here, but mostly due to leakage caused by tolerances goes up in smaller engines. The clearances are essentially the same regardless the size of the engine. This causes the smaller engines to have a larger percentage of their flow affected by the leakage. Another factor is that active controls are harder to add to smaller engines as they do not scale well. This is the main factor of why larger turbine engines tend to be more efficient than their significantly smaller counterparts.

As to diesels and small aircraft and why they are harder. Their power pulse tends to be at a higher impulse. This causes issues in structures, particularly structures that like to vibrate or ones that are sensitive to fatigue. Propellers generally do not like this at all. There are things that can be done, but usually involve very specific efforts to design blade frequencies and adding damping between the engine and the propeller. Without these efforts, there will tend to be large operating bands that you cannot operate the engine/propeller combination in due to resonant frequency crossings that are not well damped and like to excite. Other areas of challenge relates to the fact that diesel engines tend to be less power dense than their Otto cycle counterparts. The fuel also is substantially heavier than avgas. So while you can be more efficient, you tend to have lower payload. There are obviously examples that do not fit all these generalities, but these are the usual main challenges in any aero diesel installation.

Drawing this back to very small turboshafts/props and how this relates. There is a performance crossover point where you can get an overall more clean installation of a turboprop than a diesel and have the same amount of range. It depends on what your design point is. Are you designing for speed or are you designing for range and direct operating costs? It will indeed be interesting to see how electric and hybrid electric play in this space, but the challenge is going to be the development cost of such a system in the 100-300Hp range (where most of light general aviation is).

This also is a large differentiator between the technologies in the small engine space. Developing a certified (even a non-certified) engine takes a lot of money. Design side is likely to be in the 10's of millions of dollars particularly to get through certification and getting a production certificate. Then you need to look at market size and production rates. This leads to something that is hard to justify the investment in especially towards the lower end.

 

[Nicknick]

Hi there,

I'm currently working on an aero Diesel research project, so I see it from the other side than aeroengineer1...

At first, the fuel consumption of car engine based aero diesel (Continental CD 155, CD 300 etc.) is around 215- 220 g/kwh (about 0,36 lb/hp h) at full load and even a bit lower a cruise power. There is no difference between Diesel fuel or Jet a regarding the effeciency. Unfourtunally, car engines based aero engines are operated with high rpm which lowers their life expactation and increases fuel consumption, on the other hand, the gear box and clutch will reduce the impact of the torque fluctuations on the propeller (I guess it is even lower than on direct driven gazoline engines).

The CD 200 Engine (based on the SMA design) is a completly diffent approach with air/oil cooling, boxer configuration and direct drive. The SMA design (which came firt) was knowen to be troublesome for the propeller, they could have avoided a lot of difficulties by using six cylinders instead of four. The direct drive enables the engine to rund with an optimum speed of about 2200 rpm where the engine as well as the propeller are close to maximum efficientcy. Such a low speed Diesel engine with large cylinders should be able to archieve about 195 g/kwh, Truck engines of the same size are usually even better (ca. 186 g/kwh).

My approach is a water cooled six cylinder boxer engine with direct drive, with some tricks we will archieve a power output which is slightly better than that of classic turbo charged Lycomming/Continental engines with similar power (about 360 hp). The fuel consumption will be about 195 g/kwh or 0,32 lb/hp h.

It will be a long way to go to make it into production, but the first part of the prototype are allready there. I know about the develpment difficulties of Deltahawk and other (e.g. Zoche), but unlike their approach we are building something like a flying truck engine, all moving parts are fairly conventional (but slightly different and lighter).

 

[Pasoleati]

Direct-drive is utter stupidity. Given that today noise is a significant issue, at least psychologically, a propeller running at 2200 rpm is going to be noisy. Decades ago NACA tested a small aircraft with 2 major modifications: efficient exhaust muffler and a very wide-bladed much slower running propeller replacing the original direct-drivde one. The result was a massive reduction in noise. The difference was huge.

The experiment is described in NACA report 926.

 

[Nicknick]

I don't really believe that, when using a geared engine you are free to decide which prop speed you want to use and Continental is using 2300 rpm with an engine which turnes 3900 rpm. They could easiely choosed another prop speed if it would be more appropiate

 

[Aeroengineer1]

@Nicknick good to have your thoughts. Diesels in aviation have indeed had their challenges. I agree that the 4 cylinder, SMA approach was very hard on the prop. Higher cylinder counts at the pressures of a diesel get challenged for weight. The lower RPM that propellers want compared to what an engine wants to run for power to weight tend to dictate gearbox solutions. Similarly, as to RPM, to state a prop running at 2,200 ROM is a bad thing is a less than complete statement. It has to be decided with both the speed of the plane, the installation of the overall package for things such as ground clearance, vibes, etc, as well as the engine. This basic combination of factors will dictate the size of the propeller. One other factor to consider is that the more you drive down the RPM, the more complicated the gearbox becomes as well as the installation weight comes into play. Also the longer the blades get, the more you get into lower frequency vibration challenges, and propeller design becomes more challenging. Similarly, the longer the blades get, the longer the landing gear get. Landing gear contribute a significant portion of the empty weight of the vehicle.

This coupled with it makes the assumption that you have the freedom to change these things. If you are installing on a current airframe, you are much more limited. You have to maintain ground clearance. The landing gear size is generally fixed. You may be able to get away with the installation of larger tires, but again this is a drag penalty.

In the end, as with almost all engineering efforts, the ideal solution is the one where all the compromises are minimized to achieve the best solution in that trade space.

 

[Nicknick]

About the prop speed: When you are designing a new engine, you should design in it a way, that it can replace well established engines and you don’t need to built an entirely new plane around it. It was clear, that our prop speed (we choosed 2400 rpm for max. power) should be in the conventional range and not extraordinary high or low. I guess, in a single engine plane, the torque around the longitudal axis can be a problem too, for very low rpm/high torque propeller.

The outer dimensions, weight (including the cooler) and power will all stay in the envelope of established turbocharged six cylinder gasoline engines with comparable power output.

The number of cylinders will affect engine weight, but there is an optimum between very low numbers and very high numbers. Larger cylinders mean you can use casted wall thicknesses closer to what you need for mechanical reasons and not what you need for the casting process. The weight of the injector and piping depends on the number of the cylinders and, in most cases, not on the size of the cylinders. On the other hand, the front and backend of an engine will almost always weight the same, no matter how many cylinders you stuff between them, so more cylinders (and more power) will always help to improve the power to weight ratio. Bigger cylinders also mean higher gas forces and these forces are the main reason for the weight penalty of Diesel engines. The Crankcase has to be beefier for a four cylinder than for a six cylinder with similar displacement. If you want to keep the engine within the same width like conventional aero boxer engines, you need a short stroke design for the four cylinder (SMA SR 305) and can use a longer stroke for the six cylinders (same displacement), which also helps to reduce gas forces.

For water cooled engines, the surrounding structure of the cylinders will merge with the adjacent cylinders, so that the weight per cylinder is much better for the inner cylinders than for the front/back end cylinders. When using four cylinders in a boxer engine, all of your cylinders are at one end. Besides many other advantages (mass balancing, much lower torque fluctuations), we came to the conclusion that a six cylinder will be lighter than a four cylinder with the same displacement and power output.
We designed it for direct drive, this is the best approach for optimum fuel efficiency and gearboxes + clutches tend to be expensive and troublesome parts.

This sounds very simple, but the trick is to keep it light…

 

[Pasoleati]

Guys, read the NACA report. They experimented it on an otherwise standard Stinson L-5.

 

[Nicknick]

Do you believe the whole world is wrong about prop speed? The Stinson is about half as fast as anything powered with a 320-375 HP turbocharged Lycomming/Continental six cylinder. With 2500 Nm on the Prop shaft it will certainly be rolling very well (in one direction....)

 

[Aeroengineer1]

Guys, read the NACA report. They experimented it on an otherwise standard Stinson L-5.

I have read this report and many others on the topic that you have not listed. If you read through my comment, you will see that what you suggest is not the panacea of performance for an airplane. It is a single factor to be optimized amongst many other major factors. I speak from actual experience designing propellers, rotor blades, and jet engine components. This is not theory that I wrote about, but actual experience in working in the field.

In engineering all things need to be considered as a system, and in the end, it must meet the overall purpose of its existence. In the end, it must make money for the company that produced it. This means that the end consumer has to be willing to pay for the technologies, as well as the performance of the plane. Even experimental planes that are technology platforms have a price that the customer is willing to pay. So as you move through the engineering process to design a product, all these factors must be balanced.

To further put it into perspective, when you are in a weight out mode during the design phase, major technology insertions of the program can find their way into the scrap bin. It is not that the tech did not work, but it did not make a cost/weight/performance trade. Similarly long term maintenance costs have to be considered. It costs more to overhaul a propeller with more blades. It is the simple result that there is more work to be done to achieve this.

To further emphasize that things are all a tradeoff, this report measuring noise from various modifications of the propeller parameters across a wide variety of factors showed that there are times that when reducing a propeller's diameter may actually reduce noise. https://cafe.foundation/v2/pdf_tech/Noise.Technologies/AIAA.1980.Prop.Noise.Gregorek.pdf

So again, remember, engineering is all about compromises to achieve an end result that is the best set of compromises that can be achieved. Oh, and the X-57 is exploring propeller noise reduction through a completely different means, and their propellers are direct drive, just to electric motors.

The last caution that I give is that making absolute statements is almost absolutely wrong. There are almost always conditions that can show that the absolute statement is not correct. Before making an absolute statement, it is worth considering the outlying conditions that may invalidate the statement.

 

[Pasoleati]

The issue of a minor cost impact is pretty moot when the choices are either flying a quiet aircraft or not flying at all. It is very clear that public and political atmosphere, at least in the EU, is very much against private flying and one aspect is the noise.

And to be honest, in terms on total airplane operating and maintenance costs, how much is going from 2 to 5 blades to impact those? As much as elephant's vagina is dilated by an ant having a go at it?