Questions part 3: Another variety pack.

[AspiringHowardHughes]

A.H.H here again, back to pick the brains of this forum. There is no common topic here, nor much in the way of logical progression between questions.

Be warned - this is quite a wordy installment.
Same format as my previous posts, I am sure everyone knows the drill by now.

1: What factors influence the designer's choice of compression distribution between the pre-compression system (supercharger/turbocharger) and the engine cylinder itself? Do the different general balances selected by each nation (and where in that range each engine lies) represent different priorities with respect to the desired performance of the engine, or is there a truly ideal theoretical setup that each real-life effort tries to approach?

2: How does the above question apply to diesel engines? Similarly, are advancements in pre-compression systems (variable-speed drives, additional stages, turbocharger installations, inter/intracoolers....) more or less critical for diesel applications? Unfortunately, I do not have much data on the few diesel aero engines that saw service.

3: Would an engine design involving a tunnel crankcase (and thus likely a homogenous cylinder block for both banks) be viable in the context of WW2 aircraft? If not, why and by what margin?

4: What was the limiting factor on engine RPM, and how did it differ between engines? Candidates I can think of off the top of my head include piston/connecting rod/crankshaft stress, valve workload, the irrelevance of increased speed if proper breathing cannot be maintained, and diminishing returns as frictional losses increase. Were the limitations imposed by these "performance ceilings" (particularly the latter three) considered during the genesis of WW2 aero engines, or were they seen as not being of any relevance because speeds were nowhere near high enough to bring them into play?

5: What is the ideal configuration for cockpit controls and instruments? I suggest responding with a plane that you think executed things pretty well in this way (and why) or describing an ideal layout of your own design (and why it is ideal). Additionally, what gripes did pilots have about the layouts of certain planes or cock[it design trends within the industry? As someone who was nowhere close to being born at the time, I obviously have no dog in this long-finished race, but I particularly detest the "clock-type" altimeters I see in my various games and historical books. I am not clock-illiterate by any means, but I cannot stand the idea of a singular value being represented by two distinct pieces of information. Having watched a few videos on the subject recently, the design of the compasses of the era also bothers me - though this is likely more a result of technological limitations than anything. I am particularly interested to hear from those who are pilots of any sort, and see how preferences are formed and differ.

6: Being on fire tends to reduce the combat effectiveness of an aircraft - Me 163 and derivatives aside. Besides self-sealing tanks, what means of avoiding fire is preferable, and why? Options include neutral-gas pressurization systems, bladder-style contracting tanks, fire-suppression systems, or simply building things stoutly and hoping for the best.

7: Obviously there is no definite tipping point after which the aerodynamics of a plane are "good enough", but at what point are additional improvements not worth the trouble? For example, I recently thought of separating the landing gear bay fairings from the gear itself, so that they could be closed again once the gear is deployed so as to preserve airflow around the wing at an especially critical time for lift-generation and good stall characteristics (landing the plane). I am not aware of any WW2 aircraft that did this, but surely it could have reduced the required size of wing by improving wing performance during the "limiting factor" of winged flight. Do I overestimate the benefit this could have produced, or are there other reasons against this idea? I may make a separate post detailing a number of similar concepts I have come up with.

8: To what degree were the logistics of moving the aircraft around (without flying it) considered during the design process? As a designer I am always tempted to prescribe features like continuous wing spars and non-folding wings, even on aircraft destined for carriers - in fact I have had an idea of this sort which may also receive a post of its own in the coming days. Having watched videos of aircraft being unpacked and assembled after "crossing the pond", it seems there is a great deal of compartmentalization. This makes sense when intercontinental shipping is considered, but were aircraft produced in-theatre any less modular? It seems to me that there would be a great deal of performance to be gained by using monolithic parts rather than an assortment of material and fasteners. Alternatively, was this a concession to maintenance personnel?

9: I am sure we have all heard of the archaic construction of the Fairey Swordfish, and can agree that string and cloth are not the tools of the forward-thinking designer. That being said, just because a construction technique fell out of favour does not mean it is obsolete. A properly-designed monocoque fuselage is arguably the greatest option for performance, yet I am not aware of any planes (even wonder-weapon aircraft) to use this construction technique and eclipse the run-of-the-mill fighters of the day. Was stress analysis not up to the task? Similarly, is it true that the advanced geodetic airframe of the Wellington (and associated aircraft) only went unadopted because of the difficulty of tooling up for it? I so often hear that people were "trying everything" to increase the performance of their aircraft, going so far as to devise whole new engine systems in the process - why was airframe design left out of these desperate searches?

10: Last but not least, what would you all like to hear more of? I have far too much time on my hands and I am inclined to take things in a direction that will be to the enjoyment of everyone. In the coming weeks I hope to finish ironing out a hypothetical WW2 aircraft of my own design, and improve my sleeve-rotary valve concept that was the subject of an earlier post - it may be evident that some questions I ask with those goals in mind!

As always, explanatory images are available upon request.
-A.H.H

 

[z42]

7: Obviously there is no definite tipping point after which the aerodynamics of a plane are "good enough", but at what point are additional improvements not worth the trouble? For example, I recently thought of separating the landing gear bay fairings from the gear itself, so that they could be closed again once the gear is deployed so as to preserve airflow around the wing at an especially critical time for lift-generation and good stall characteristics (landing the plane). I am not aware of any WW2 aircraft that did this, but surely it could have reduced the required size of wing by improving wing performance during the "limiting factor" of winged flight. Do I overestimate the benefit this could have produced, or are there other reasons against this idea? I may make a separate post detailing a number of similar concepts I have come up with.

I'm quite sure the answer is that the extra complexity of such a thing is not warranted. You'll note, however, that at least some aircraft equipped with wheel well doors, such as the P-51, did close them again once the landing gear had extended.

9: I am sure we have all heard of the archaic construction of the Fairey Swordfish, and can agree that string and cloth are not the tools of the forward-thinking designer. That being said, just because a construction technique fell out of favour does not mean it is obsolete. A properly-designed monocoque fuselage is arguably the greatest option for performance, yet I am not aware of any planes (even wonder-weapon aircraft) to use this construction technique and eclipse the run-of-the-mill fighters of the day. Was stress analysis not up to the task? Similarly, is it true that the advanced geodetic airframe of the Wellington (and associated aircraft) only went unadopted because of the difficulty of tooling up for it? I so often hear that people were "trying everything" to increase the performance of their aircraft, going so far as to devise whole new engine systems in the process - why was airframe design left out of these desperate searches?

Hmm, as far as I understand the consensus at the time and to this day is that at least for aluminum construction, a semi-monocoque construction provides the least weight for a particular strength. So yes, the skin is load-bearing, but there is also additional framing to support the skin.

A pure monocoque construction might make sense where there is a requirement for significant skin puncture resistance. Or when using, say, composite construction and one can easily increase the skin strength in critical stress areas.

 

[AspiringHowardHughes]

In the case of question 7, what sort of performance penalty would open landing gear bays create?

A semi-monocoque structure may provide the greatest general strength, but aircraft are hardly subjected to equal stress in all directions. I imagine that thoughtfully-placed supporting ribs, if anything, would be all that is needed to improve the weak points of a monocoque structure.

Of course, this comparison depends a great deal on the shape of the aircraft. Round beasts like the P-47 would surely be better suited to a monocoque structure than a Ta 152.
A further advantage monocoque construction has over semi-monocoque is that even when additional bracing might provide better useful strength for a given weight of structure itself, there are inevitably weight increases due to fasteners, as well as the weak points produced where components meet. An ideal semi-monocoque (as if cast from a mold) may be structurally superior, but the realities of trying to actually assemble it may shift the balance.

Insufficiently-developed stress analysis was probably another limiting factor back then.

Maybe the ideal aircraft construction method would have been pure monocoque, with semi-monocoque in areas unsuitable for the former.
As someone without much formal engineering training (yet) it is hard for me to guess whether this would have improved things, or caused a worst-of-both-worlds situation.

 

[Bretoal2]

In the case of question 7, what sort of performance penalty would open landing gear bays create?

A semi-monocoque structure may provide the greatest general strength, but aircraft are hardly subjected to equal stress in all directions. I imagine that thoughtfully-placed supporting ribs, if anything, would be all that is needed to improve the weak points of a monocoque structure.

Of course, this comparison depends a great deal on the shape of the aircraft. Round beasts like the P-47 would surely be better suited to a monocoque structure than a Ta 152.
A further advantage monocoque construction has over semi-monocoque is that even when additional bracing might provide better useful strength for a given weight of structure itself, there are inevitably weight increases due to fasteners, as well as the weak points produced where components meet. An ideal semi-monocoque (as if cast from a mold) may be structurally superior, but the realities of trying to actually assemble it may shift the balance.

Insufficiently-developed stress analysis was probably another limiting factor back then.

Maybe the ideal aircraft construction method would have been pure monocoque, with semi-monocoque in areas unsuitable for the former.
As someone without much formal engineering training (yet) it is hard for me to guess whether this would have improved things, or caused a worst-of-both-worlds situation.

Difficult to analyze, understand, and judge 1935 technology with the eyes of 2026, isn't it?

 

[AspiringHowardHughes]

Unfortunately, yes.
I am sure everyone wishes their favourite part of history was more thoroughly dissected after the fact!

 

[z42]

In the case of question 7, what sort of performance penalty would open landing gear bays create?

For a definitive answer, which you seem to seek, you need wind tunnel tests or at the very least CFD analysis, not debates on an internet forum. (Also, the performance penalty goes both ways, in terms of added weight that the mechanism for closed landing gear bays would add, and that the plane would always carry with it, particularly also in flight with the landing gear retracted.)

A semi-monocoque structure may provide the greatest general strength, but aircraft are hardly subjected to equal stress in all directions. I imagine that thoughtfully-placed supporting ribs, if anything, would be all that is needed to improve the weak points of a monocoque structure.

If you look at post-WWII aluminum aircraft construction, semi-monocoque has remained the standard to this day.

Maybe the ideal aircraft construction method would have been pure monocoque, with semi-monocoque in areas unsuitable for the former.
As someone without much formal engineering training (yet) it is hard for me to guess whether this would have improved things, or caused a worst-of-both-worlds situation.

It's possible to have different types of construction in the same aircraft. For instance, the Typhoon and Tempest had semi-monocoque rear fuselages, but the forward fuselages where of a truss type, with the skin panels only being there for aerodynamics.

 

[AspiringHowardHughes]

You are correct that discussion can only go so far. What I was looking for by asking online was mention of any interesting factors that deserve consideration, rather than a universally applicable answer. For example, if cavities under the wing during landing created aerodynamic phenomena that aided handling or lift performance, I would love to be made aware (lest I waste time designing against an existing positive trait of aircraft). I considered the weight of the required mechanism, but perceived the possibility of having smaller wings for the same produced lift to outweigh (pun intended) the downside. Complexity is another matter of course, but increased complexity has been the trend of military aviation since its inception!

On the subject of construction type, I may have been overlooking the need to locate a bunch of equipment (like my proposed radiator design) within the aircraft. This internal payload can easily share support structures with the internal bracing, lessening the penalties I proposed it to have.

Do you know of any sort of "structural catalogue" where I might be able to examine and compare the internal frameworks (or lack thereof) of WW2 aircraft? It is interesting to hear of the differences between otherwise comparable planes.

When it comes to monocoque construction, I am only skeptical of the potential to hybridize it with another construction type because I feel it might be difficult to make a good transition between the two. If attachment points to the monocoque section are too sparse (suiting the design of the semi-monocoque section) I fear it would cause them too high a structural loading. On the other hand, if they are too frequent (suiting the design of the pure monocoque section) this would demand a heavy and complicated arrangement of structural elements.

I think this is another situation in which we have reached the practical limit of discussion, unless someone with experience in aircraft construction or an example in mind can chime in.

 

[ThomasP]

Possibly this would be of use?

"Aerospace Structures – Introduction to Aerospace Flight Vehicles"

It provides a very good (though somewhat general) summary of the various types of construction used in aerospace systems. The math gets a bit complex, but if you can understand the girder concept the rest is understandable (for the most part) without having to understand the math.

 

[AspiringHowardHughes]

Thanks for the suggestion, reading through it at the moment.
I am sure there will be plenty of helpful information.

 

[elbmc1969]

On question 1, look up "stroke ratio" if you haven't already.

 

[Shortround6]

1: What factors influence the designer's choice of compression distribution between the pre-compression system (supercharger/turbocharger) and the engine cylinder itself? Do the different general balances selected by each nation (and where in that range each engine lies) represent different priorities with respect to the desired performance of the engine, or is there a truly ideal theoretical setup that each real-life effort tries to approach?

A lot depends on the fuel being used. There is a real limit on total compression used (compression in the supercharger/s plus/times cylinder compression) and it also depends on what the goal is.
Peak power vs fuel economy and neither one may be the most efficient? Classic is the Allison vs Merlin. Merlin used a 6.0:1 compression ratio in the cylinders and the Allison used 6.65:1. Merlin could use a bit more boost and higher boost means more fuel/air in the cylinders for more power. The 6.65 compression in the Allison gave a bit more power for fuel burned (maybe 8%?) and had slightly better fuel economy but a bit max power. Carbs vs fuel injection can also play a role.
Heat if intake charge also plays a role in figuring out total compression. Using an intercooler can allow for either more manifold pressure or higher cylinder compression to hit the same limit. There are other limits, airflow was not well understood at the time compared to what is known now. Power was limited by the cylinder with either the poorest/leanest mixture of all the cylinders and/or the cylinder with highest temperature (worst cooling).
An R-2800 in a B-26 (or P-47 or F6F) was putting around a gallon (6lbs) of fuel a minute through each engine for cooling purposes at max power. They could make the power using a lot less fuel but the engine either detonated or melted down or both.

3: Would an engine design involving a tunnel crankcase (and thus likely a homogenous cylinder block for both banks) be viable in the context of WW2 aircraft? If not, why and by what margin?

You can draw a lot of different things, if you can't manufacture them in quantity at an acceptable cost and scrap rate they are no good practically. State of manufacture changed during WW II in many different countries.

4: What was the limiting factor on engine RPM, and how did it differ between engines? Candidates I can think of off the top of my head include piston/connecting rod/crankshaft stress, valve workload, the irrelevance of increased speed if proper breathing cannot be maintained, and diminishing returns as frictional losses increase. Were the limitations imposed by these "performance ceilings" (particularly the latter three) considered during the genesis of WW2 aero engines, or were they seen as not being of any relevance because speeds were nowhere near high enough to bring them into play?

A very good rule of thumb was piston speed. 3000ft per minute was pretty much the accepted limit in WW II. A few engines were higher but either they didn't make high power (high BMEP) or they broke a lot/had a low service life.
German 1939 F 1 racing engine (M154/163) was using 3370fpm (corrected) at 7500rpm by using 70mm stoke. Don't use modern engines for comparison. Modern materials, heat treatment, surface finishes and lubricants can make a big difference.
Everything you mentioned was at play. Many people do not realize how close WW II aircraft engines were to running on the edge. The 1939 Mercedes F1 engine was 3 liters making 480hp and weighed 603lbs
An Argus As 410 aircraft engine used in trainers and transports was 12 liters making 459hp and weighing 694lbs and was using 80/87 av gas not special racing fuel (86% methanol).
Nobody thought twice about flying behind aircraft engines making around 1 hp per 1.5lbs or less. High power aircraft engines were closer in power to weight to race car engines but were expected to last much longer (granted they were only expected to operate at full power about 10% of the time.

9: I am sure we have all heard of the archaic construction of the Fairey Swordfish, and can agree that string and cloth are not the tools of the forward-thinking designer. That being said, just because a construction technique fell out of favour does not mean it is obsolete. A properly-designed monocoque fuselage is arguably the greatest option for performance, yet I am not aware of any planes (even wonder-weapon aircraft) to use this construction technique and eclipse the run-of-the-mill fighters of the day.

Please describe the "string" used in the Swordfish?
I will note that the F4U Corsair used fabric on the outer wing panels behind the spar through the end of WW II.
I am not advocating for fabric except to say that designers of the time had reasons for using it. Swordfish had more wing area than some twin engine bombers. With a 690hp engine tasked with carrying a torpedo it was very, very important to keep the structural weight of the aircraft low. Fabic covering helps do that if the flight speed is low. Using metal skinning without an increase in engine power is going to hurt take-off and landing perfromance even if the plane is more durable as float plane/carrier plane.
Blackburn Shark used a lot more metal in it's construction, unfortunately the Armstrong Siddeley engine could have been considered a crime against the entire British commonwealth and so negated any and all advances the Shark showed in it's construction.

 

[AspiringHowardHughes]

Elbmc, I am aware that diesel engines generally have a longer stroke (compared to their bore) than their petrol counterparts.
What I was trying to get at are the reasons for performing compression inside versus before the cylinder.

 

[AspiringHowardHughes]

Shortround, thank you for your thorough (as usual) response.

1: I was not aware so much fuel was "wasted" on cooling. I can only assume this was foreseen by the designers, was this common practice outside the aircraft you have mentioned?

3: I have not been able to come up with any real reason why it would be impossible. I ask this question because for all I know there could have been some limiting factor to engine design that was so commonplace as to not be published prominently enough for me to be aware.
For example, it could have been impossible (at the time) to cast an external bearing race large enough for that style of engine if it were to be any more aggressively strained than an airship powerplant.
Generally, when I ask this sort of thing I am trying to draw upon the immense collective knowledge here for reasons why it would have been entirely out of the question, rather than outsourcing a thorough investigation that I ought to be doing myself.

4: It is nice to hear of a rule of thumb. While it makes sense that all those factors would be involved, have you heard of any cases in which an engine was being held back by one hurdle specifically? I imagine that designers would aspire to keep all elements of their engines simultaneously advancing as necessary, but of course nothing ever works out so smoothly.

9: I am under the impression that the doped fabric covering employed by the swordfish involved string hardened similarly as a way of providing some more rigidity to the cloth as it spanned between metal support structures. I could be wrong, a quick search has not yet been able to confirm or deny this.
(I am aware that the nickname of "Stringbag" does not actually allude to the materials used, I have read that particular Wikipedia page many times)

 

[Shortround6]

1: I was not aware so much fuel was "wasted" on cooling. I can only assume this was foreseen by the designers, was this common practice outside the aircraft you have mentioned?

This was the main difference between running rich and lean. Just about all aircraft engines used it to some extent. WW I Renault air cooled V-8s (and cousins) were noted as glowing faint red in dark unless running rich (or perhaps even when running rich). It was a quick and easy solution in WW I, the 1920s, 30s and 40s when demand for more power was out pacing the ability to either make more/deeper fins or mount water radiators of sufficient size to cool the engines at maxim output. That started the whole Prestone cooling 'thing' and higher coolant temperatures.

3: Would an engine design involving a tunnel crankcase (and thus likely a homogenous cylinder block for both banks) be viable in the context of WW2 aircraft? If not, why and by what margin?

3: I have not been able to come up with any real reason why it would be impossible. I ask this question because for all I know there could have been some limiting factor to engine design that was so commonplace as to not be published prominently enough for me to be aware.
For example, it could have been impossible (at the time) to cast an external bearing race large enough for that style of engine if it were to be any more aggressively strained than an airship powerplant.

The Tunnel crankcase was used by some heavy duty engines and some racing engines. Aircraft engine makers liked to make the engines as light as possible. Not saying there was never any crossover but again there were competing priorates. The American Miller/Offenhauser racing engines used the tunnel crankcase and was not the first. But it also shows some of the disadvantages. Crankshaft has to be pulled out from one end or the other and not dropped out the bottom (or top for inverted engines). It is sturdy but it also requires a heavy a heavy crankshaft.

Tunnel crankcase - Wikipedia

en.wikipedia.org

2nd picture.
This crank has failed but shows the large bearings used in some tunnel crankcases. maybe not problem in marine, stationary or RR locomotives. Not so good in high rpm engines, a lot of fly wheel effect. There is also a problem with the speed of the bearing surfaces and lubrication. At a given rpm the crank main bearings have a much higher surface speed than the rod bearings due to the larger diameter. This was sometimes solved by using ball or roller bearings. Some of the engines using ball/roller bearings also used multi-piece crankshafts. So each "piece" could be taken out through the side/bottom of the crankcase. A V-12 would use a 7 piece crankshaft, usually splined. Cost of such a crankshaft?

4: It is nice to hear of a rule of thumb. While it makes sense that all those factors would be involved, have you heard of any cases in which an engine was being held back by one hurdle specifically? I imagine that designers would aspire to keep all elements of their engines simultaneously advancing as necessary, but of course nothing ever works out so smoothly.

Engine cylinder design seems to have moved from decidedly under square (bore to stroke ratio of 1 to 2) pre WW I to nearly square just before WW II. Most of the oversquare engines were flat engines and they were trying to keep the engine width down as much as to keep piston speed down. Engine materials play a large part in this. Pre WW I even many racing cars used iron or steel pistons and using large, heavy chunks of iron on the ends of the piston rods was asking for trouble. Getting the right aluminum alloys and the right materials for compression and oil rings (and getting the right oil) to allow higher piston speeds took a while.

9: I am under the impression that the doped fabric covering employed by the swordfish involved string hardened similarly as a way of providing some more rigidity to the cloth as it spanned between metal support structures. I could be wrong, a quick search has not yet been able to confirm or deny this.

The 'dope' was the way fabric was tightened/made more rigid. There were different types (weights) of fabric and the airframe designers could use different spacing of the ribs and bracing. Any tension of the wings (and alignment of upper and lower wings) was done by steel wires and turnbuckles. RAF and RN had "riggers" whose job it was to keep the wings in alignment as any sag or warp could affect how the aircraft flew. They were using steel wire in WW I. A lot of the wire that was exposed even in the 1920s was aerodynamically profiled (oval or with a taper on the backside) to lessen wind resistance.

 

[Simon Thomas]

Please describe the "string" used in the Swordfish?

Rib stitching of the fabric required plenty of string.

 

[ThomasP]

From the Swordfish manual:

"69. The fuselage is covered in the normal manner by laced-on fabric, . . ."


Here is an illustration, from AP 1107 'Rigging For Aircraft", showing 3 different methods for attaching doped fabric to metal ribs. The method(s) used on the Swordfish would be similar.

 

[AspiringHowardHughes]

Shortround, did this strategy of using rich mixture for cooling exist in the second world war as a holdover from the first, or was there some additional benefit to using fuel compared to water or a mixture such as MW 50? Other than only needing one reservoir and associated delivery components, none immediately come to mind. I am not aware when water injection (or anything-other-than-fuel-injection) was first developed, but I imagine it must have been prior to even the first world war.

I have heard of the Offenhauser engine, particularly one viewpoint that the unique (and potent) design it used was partly a result of the designer not being familiar with common practice in the industry. I am curious about this prospect particularly because my sleeve-rotary valve design (which was the subject of a previous post of mine) has the potential to allow for no conventional head nor head gasket. As a result, the idea of eliminating the long structural bolts from the engine entirely is quite enticing. Where the flywheel effect is concerned, my intent is to design an H-24 layout like that of the Sabre in my profile picture. While the Sabre crankshafts rotate the same direction, these would rotate opposite directions. Hopefully gyroscopic effects could be eliminated this way. In a single-crank engine of this type, perhaps the torque of the propeller could be harnessed to counteract it?

I am interested to see how these multi-component crankshafts are sectioned. Do you have any idea where I might find explanatory images?

As for the hurdles that needed jumping in pursuit of faster speeds, the story of insufficient materials development seems to be a tale as old as time. I enjoy reading about how the specific military and economic circumstances faced by each country shaped the path of their engine development.

I was aware that stiff braces of all sorts were generally profiled so as not to cause too much drag, but hearing about aerodynamic wires is new. I cannot easily imagine the machines required to produce it.

 

[AspiringHowardHughes]

Thank you for the commentary on the role of string in aircraft, Thomases.
On a related note, is there any catalogue of aircraft structural designs that provide instructional views such as that one?

 

[Simon Thomas]

Rich mixture cooling chart below was developed by GAMI ( General Aviation Modifications, Inc. ) based on the Continental IO-550, but would be very similar for any aircooled engine. You are probably most interested in the CHT =Cylinder Heat Temperature. 50°F ROP (Rich of peak) is the old school typical cruise condition. Take off is typically around 120°F ROP. Liquid cooled engines can generally run leaner.

How do you plan on balancing the torque from your crankshafts? The Sabre had that brilliant helical/straight balance beam gearing system to perfectly balance the torque from each crank onto the prop shaft.

 

[Bretoal2]

I think the crankshaft's gyroscopic torque is a non-issue; the main source of this torque is the propeller, which combines a large diameter with significant mass.

Returning to the Sabre, among the parameters that led to the architecture designed by Halford was the desire to drive the sleeves of upper and lower banks using a common system, which takes the form of "cranks" placed between two sleeves of the same rank in these two banks.

The primary consequence of this architecture is that these two upper and lower sleeves are timed 180° apart, therefore the pistons they contain are timed 360° apart. The second consequence is that the crankshafts must rotate in the same direction. And a third consequence is that on this 24-cylinder engine, two cylinders fire simultaneously, and the explosions occur at 720/12 = 60°, not 720/24 as one might expect.