There are two main ways of classifying reciprocating engines: a) by cylinder arrangement with respect to the crankshaft i.e. radial, rotary, in-line, V-type or horizontally-opposed, and b) by the method of cooling: liquid-cooled or air-cooled. 

Radial Engines

Radial engines were designed to have one or more rows of cylinders arranged in a circular pattern around the crankcase. They have one crank throw per row and a relatively small crankcase, resulting in a favorable power to weight ratio. The cylinder arrangement exposes a large amount of the engine's surfaces to the air and this air-cooled design provided for even cooling and smooth running since the reciprocating forces tended to cancel.

The lower cylinders under the crankcase tended to collect oil when the engine had been stopped for an extended period because oil could slowly seep passed the cylinder walls. Serious damage could occur if this oil was not cleared prior to engine start i.e. hydraulic lock.  

Rotary Engines

It became apparent circa. WWI, that inline engines were too heavy for the amount of power needed. At the time, the rotary engine met the goals of being lightweight, powerful, cheap, and easy to manufacture in large quantities. The entire engine rotates with the propeller, providing plenty of airflow for cooling, regardless of airspeed. The cylinders are arranged in a circle around the crankcase like a radial, but the crankshaft is attached to the airframe, and the propeller is bolted to the engine case.  

This provided a high power and power-to-weight ratio but the design suffered from severe gyroscopic effects from the heavy rotating engine, which in turn made for difficult flying. Very high oil consumption and a messy character left the engine with too many awkward limitations for effective widespread use.

In-line Engines

In-line engines have a comparatively small frontal area, but their power-to-weight ratios are relatively low. The rear cylinders of an air-cooled, in-line engine receive less cold air, so these engines were normally limited to 4/6-cylinders. This type of engine had cylinders lined up in one row. It typically had an even number of cylinders but there are instances of an odd number of cylinders being used like in the radials. The biggest advantage of an inline engine was that it permits a low drag from a design with a small frontal area.

When compared to equivalent radials, the inline engine delivered a lower power-to-weight ratio, because the crankcase and crankshaft were long and heavy. An inverted inline engine has its crankshaft above the cylinders, allowing the propeller to be mounted higher up for ground clearance. An inline engine may be either air-cooled or liquid-cooled, but liquid-cooling is more common because it is difficult to get enough air-flow to cool the rear cylinders directly. The inherent disadvantages of the design soon became apparent, and the inline design was abandoned, becoming a rarity in modern aviation.

V-type Engine

V-type engines provided more horsepower than the in-line engines and retained a small frontal area. They have the cylinders grouped into two and arranged in a V at a certain angle (e.g. 45, 60 and 90 degrees). An inline 6-cyl engine was comparable in size to a V12 with the V12 being slightly wider. The V-type also provided higher torque at lower RPM. Further improvements in engine design led to the development of the horizontally-opposed engine.

Horizontally-Opposed Engine

Horizontally-opposed engines are the most popular reciprocating engines used on small airplanes. They have two banks of cylinders on opposite sides of a centrally located crankcase. Opposed-type engines have high power-to-weight ratios because they have a comparatively small, lightweight crankcase. It could be either air-cooled or liquid-cooled, but air-cooled versions tended to predominate.  

Opposed engines are mounted with the crankshaft horizontal in airplanes. In addition, the compact cylinder arrangement reduces the engine’s frontal area and allows a streamlined installation that minimizes aerodynamic drag and led to smooth running because of reciprocating forces tending to cancel. Unlike with radials, an opposed engine did not experience any problems with hydraulic lock. Opposed, air-cooled four and six cylinder piston engines were by far the most common in small GA aircraft requiring <400hp.

The Advantages & Disadvantages of Radial Engines

Radial engine designs have been around since the earliest days of aviation. They were used in many aircraft - large and small - through the war years and conceded to the power, efficiency and reliability requirements of modern aviation. Many aircraft are still flying strong with their original, well maintained radials still mounted. One question that seems to pop-up from time-to-time is: What are the advantages and disadvantages of radial engines? A simple web search returns a plethora of opinions, mostly conveying similar but incomplete lists of advantages or disadvantages. 

Perhaps this question cannot be answered properly without comparing radials with all engine types and aircraft designs, for which there are many permutations. Since radials are not re-entering the mainstream market, this type of exhaustive comparison might be interesting but totally overkill. Most comparisons are against liquid-cooled inline engines, which, in fairness, were the only other major choice of engine at the time of peak popularity.   As a compromise, the objective is to highlight the general aspects of air cooled radials in respect of single and twin engine installations and categorise them as one or the other. We acknowledge that some early successful radial designs were also water-cooled. In the near future, we will provide the reasoning behind each categorisation. This will both shorten the content and aim to keep this comparison objective and succinct.

No doubt some of the items could be combined or split to reduce or increase the number of items in each category but the optimisation of the lists has not been a concern. The reader is left to place the relative importance on each item since their application or level of interest is mostly subjective.