Pros and cons of engine types?

[Tupac]

The question is a bit vague, but some issues are addressed in what has become the following wall of text. Hope it's useful.

There's a little ambiguity in the world of aircraft about what an inline engine is as the term seems to be interchanged quite freely with V engines. The history of this might be to distinguish them from radial engines, while in the automotive industry they are distinctly different things. A V engine has essentially two banks of inline engines sharing a common crankshaft.

One thing you notice about many V engines is that the same angle keeps suspiciously popping up. The Rolls Royce Merlin and the Griffon, the Allison V-1710, Daimler-Benz DB 601 and 605 and many others all use a 60° angle between the banks. This is not insignificant, for a V12 engine the 60° bank gives a theoretically perfect primary and secondary balance: in other words the reciprocation of the pisons, con rods and the rotation of the crank cancel each other out harmoniously (A straight six also has this attribute and many WWI aircraft employed this format). This is a very different situation from a (Ford, I think it is) 90° V8 with a flat plane crank which produces a vertical oscillation twice the frequency of the revolutions, so the engine literally wants to bob up and down in its mountings. This has an obvious advantage in terms of reducing vibration and stress on the engine, and hence increasing reliability.

One restriction of  the 60° bank is that it does limit the space available for the fuelling system. This is why many Ferarri engines have a bank of 72°, the needs to improve aspiration overode the balance issue.

Additionally regarding perfect primary and secondary balance with twelve cylinder engines, there is another bank angle which also has this benefit: 180°. The Hawker Typhoon and Tempest essentially have two 180° V12s on top of each other in a shared casing. These engines are called horizontally opposed engines but not necessarily boxer engines. The way to distinguish is that in V engines (of any angle) two connecting rods typically share a big end bearing while in a true boxer engine each connecting rod has its own bearing.

Any boxer engine with six cylinders or more will also have perfect primary and secondary balance (VW Beetle for example does not, it has the same secondary balance characteristics as an inline 4). Many post war civil aircraft use a six cylinder boxer unit, which has an additional advantage regarding cooling which we will address later. Porsche has stood by the boxer engine from the first 356 right through all models of 911 and many of their racing cars including their 8-cylinder Grand Prix cars, however an interesting example is the vaunted Porsche 917 which in fact employed an air-ccoled 180° V12 and NOT a boxer engine.

This brings us to another interesting point about engines: positive net torque output. If you imagine a cyclist doing his thing, only part of the revolution of his pedals is delivering torque to the bicycle (and hence the road). As his pedal comes over the top only at about 20 degrees or so does the lever of the crank begin to deliver rotational force, at the top and bottom of the pedal rotation and all the way back up there is no positive torgque contribution, infact a negative or a subtraction from the torque is the situation. Only the momentum of the crank and his other leg keeps the whole thing rotating smoothly.

With a four-stroke engine this positive and negative torque is exasperated because each piston is only making a power stroke every other rotation. The magic number with piston engines is 7. 7 pistons never have a negative torque phase and are thus always making positive torque. Although a six cylinder engine comes very close!

All of the aircraft in Aces High have a positive torque output (including the Storch) but again consider what more firing strokes means. At low altitudes the Nakajima Homare engine with a displacement of only 32 litres and two valves per cylinder nearly matches the output of the Daimler Benz DB605 with over 35 litres of displacement and four valves per cylinder (and a superior supercharger). The 18 cylinder engine simply has 50% more power pulses per 720 degrees of revolution this explains the outputs (here is another difference between air and liquid-cooling: it is very difficult to employ a 4-valce head with an air-cooled engine and a bigger valve area makes for a more efficient output. This also weighs into the selection).

This brings us to a discussion about engine cooling. Of course a radial lends itself very well to air cooling while an inline or V engine does not. There is some merit to the famous quote that a liquid-cooled aircraft engine makes as much sense as an air-cooled submarine however it is not as simple as that. In a dive an air-cooled radial engine can actually become over-cooled which leads to temporary fuelling problems.

The biggest penalty for liquid cooling is of course the radiator which can produce an awful lot of drag. Arguably the P-51D had the best wartime solution for this in single engine aircraft. With a twin engine such as the Mosquito or the Whirlwind you have the opportunity to nest the rads in the inner wing section which is as close to an optimal solution you can find. This opportunity was missed by the Me410 design team who essentialy doubled the drag of a single engined fighter's radiators!

Many modern civil piston craft use a boxer-six which has many advantages of both the radial (air-cooled, good access) and of the V / inline: perfect primary and secondary balance.

In the end all design solutions have compromises and especially with aircraft it is often only at the flight testing phase that the true measure of the design as a holistic entity announces. The relative advantages and disadvantages of the different formats are core to these design choices.

In closing there are two interesting aircraft which switched between air-cooled radials and liquid-cooled V engines throughout their production run:

The Fw190 started as an air-cooled radial and employed a liquid-cooled V12 in the later stages of its development and the Ki-61 did the opposite, being adapted to carry a radial when the supply of V12s ran out!

This is an excellent post with excellent information, thank you.