14
$\begingroup$

Before WW2 if you wanted an air-cooled airplane engine you had a choice of radials, inverted inline engines (like the de Havilland Gipsy minor/major/queen/king) and a few small boxers.

But after WW2 boxers seem to have become the default choice. radials are an afterthought and the "inverted inline and air cooled" category disappeared entirely.

I get that a big part of what changed is "jet engines made all big piston aero engines obsolete" but what happened to the small 5/7/9 cylinder radials and 4/6/8 cylinder inverted air cooled inline engines?

Improve this question
$\endgroup$
5
  • $\begingroup$ The simple answer is: 500+hp applications all moved on to turbo props and piston engines doesn't need more than four cylinders. $\endgroup$ Commented Apr 13 at 6:11
  • $\begingroup$ @user3528438 But boxers are extremely rare on ground-vehicle engines, so why are they common in the air (if indeed they are)? $\endgroup$ Commented Apr 13 at 9:24
  • $\begingroup$ @MikeB Air cooled engines are rare on the ground too. On the ground you don't want to have fewer than 3 cylinders in a row because you want each cam shaft to at least drive three cylinders' valves to be cost-effective (most ground engines are water-cooled OHC now). So we have inline 3-6 and V6-V12. On airplanes you don't want more than 2 cylinders in a row because it makes air-cooling very difficult (airplane engines are mostly air-cooled OHV). So we have boxer 2 and 4. $\endgroup$ Commented Apr 13 at 11:26
  • $\begingroup$ @user3528438, what are you talking about? A lot of airplanes use 6-cylinder engines including the Cessna 182, the Cirrus SR-22, the Bonanza and the Mooney. $\endgroup$ Commented Apr 13 at 15:55
  • $\begingroup$ Aged 15, I had a moped with 25 km/h top speed and a boxer engine. $\endgroup$ Commented Apr 14 at 12:28

4 Answers 4

Reset to default
17
$\begingroup$

Horizontally opposed (i.e. boxer style) engine designs are more popular because they are better in many ways:

  • They have a relatively small frontal profile, allowing smaller cowlings with reduced drag, and better visibility in single engine airplanes. Radial engines are much bigger around, creating a lot of drag and hindering forward visibility.
  • Because the cylinders are opposed the engine is shorter than an in-line engine, reducing the length of the front cowling. This means better visibility.
  • Horizontally opposed engines are generally smoother running.
  • Air cooling in-line engines is problematic, they tend to get hotter farther away from the front of the cowling, requiring liquid cooling which adds weight, complexity and cost.
  • Radial engines suffer from a condition called hydraulic lock, where oil pools at the bottom cylinder. Radials need to be pulled through a few rotations before starting, which is extra preparation you don't have with inline or horizontally opposed engines.

I'm not saying the boxer style engine is the absolute best for light airplanes, but they are better than inline and radial engines. There are definitely better designs, they haven't been adopted due to a lack of investment in the technology, as well as difficult and expensive certification processes.

Improve this answer
$\endgroup$
7
$\begingroup$

The main decisions in determining aeroplane engine type are the required power and the engine weight, not the type of coolant. The invention of the turbo-prop meant: high power piston engines have no place anymore in aviation.

From Synthesis of Subsonic Airplane Design, Figure 4-13: enter image description here

  • Boxer piston engines have good balance and good air cooling for the cylinders in front.
  • In-line engines have a small frontal area (lower drag) but air cooling is a problem when more than 4 cylinders are installed.
  • The radial engine's large frontal area results in high drag and excellent cooling.

So the more cylinders visible in front, the better the air cooling and the worse the frontal drag, with the 2 cylinder boxer type apparently presenting the optimum, for low speed aeroplanes. Although even that configuration seems to be outdated, given the 4-cylinder, jet fuel propelled, water cooled engine of the Diamond DA40

From the Diamondaircraft site From the Diamondaircraft site

Improve this answer
$\endgroup$
6
  • $\begingroup$ Yes. The DA40 is a very interesting concept. It solves one important problem - the price and decreasing availability of Avgas fuel. As it can run on Jet Fuel which is generally available at larger airports it may be less costly to run. But on the grass fields Jet is normally not available. Historically this was not an issue. $\endgroup$ Commented Apr 13 at 21:36
  • 2
    $\begingroup$ Also notice that it uses a smaller displacement engine running at a higher RPM and a set of reduction gears (or chain). Developing working reduction gears for aeroplanes is a lot more complicated than expected. Many car engine conversions has fallen on the wayside due to problems with the reduction. $\endgroup$ Commented Apr 13 at 21:37
  • $\begingroup$ And add a third point - what about electrical engines. They seem to be just around the corner today. The problem with the answer as given is that it talks about the situation in a near future (or maybe slightly historical), but planes live for 50 years or more. $\endgroup$ Commented Apr 13 at 21:38
  • $\begingroup$ @ghellquist Fully agree with the advance in engine technology evident on the DA40. 21st century hooray. $\endgroup$ Commented Apr 14 at 2:57
  • $\begingroup$ @ghellquist electrical engines. Yes, high torque, low weight, modest dimensions. But the problem is the battery of course, and range is limited is using an extension cord... $\endgroup$ Commented Apr 14 at 2:59
6
$\begingroup$

It is as often difficult to give authoritative answers to the historical why-questions. Sometimes it can be pure chance and sometimes it is market pressure, sometimes a bit of a chicken and egg story.

The usage of aircraft engines for smaller aircraft with opposed cylinders, aircooled and non geared was in place before the WW2. Two exemples are Cessna that had the 120 and 140 that progressed into 150 and later 172. Piper had what became the L4 or later known as the Cub. Several other manufacturers used the same type of engines. Of course other types were in use, but these are some of the most successful ones. Successful because they were relatively cheap, available and good enough.

If we look at the environment these engines work in an airplane the aircooled boxer directly coupled to the propeller makes sense. Aircooling avoids all the extra complications from water cooling. It keeps the engine simple, reliable and easily serviceable. Not having reduction gears removes one great liability as to reliability. This requires that the engine should have a low RPM, say below 2500 RPM. The reason is that otherwise the tips of the propellers may move faster than the speed of sound in air which creates a lot of problems. The boxer setup additionally leads to a shorter crankshaft compared to an inline arrangement, again giving higher reliability. The shorter engine makes for a shorter cowling which adds to visibility compared to an inline engine. The flattened engine adds to visibility compared to radial engine.

But in the end it seems to partly go back to the chicken and egg situation. The smaller aircraft used boxer engines and hence after the war there was a market for smaller boxer engines. And as the boxer engines was on the market, new planes was designed for them. They were good enough for the purpose, available at reasonable prices and reliable. Not a bad combination of factors at the time.

Improve this answer
$\endgroup$
1
$\begingroup$

Parts.

Radials and in-line cylinder jobbers were, to a large extent, the outgrowth of a nascent aircraft industry suddenly swept into WW-II. After the war, these engines suddenly lost demand as the aircraft industry realigned with commercial and private aircraft needs. For the larger super radials, turboprops were initially the replacement while competitive aspects in the market adapted military aircraft to commercial needs. Hence, we see the jet-powered KC-135 emerge as the 707, giving Boeing an unsurpassed edge in the commercial marketplace.

Lighter 4-, 6-, and 8-cylinder aircraft engines were largely an outgrowth of the pre-war private aircraft industry centered in the midwest. These light-weight engines had cylinders that were horizontally opposed, and would easily accommodate modification regarding size and horsepower, something not easily achieved with a radial. Consequently, they could be fitted to almost any aircraft in this aviation market segment. Essentially standardized parts could be made for these engines so their maintenance and repair was easily accomplished. Given their service environment, the operation, maintenance, and manufacture of these engines was much more economical than could achieved with a similar radial.

Improve this answer
$\endgroup$

You must log in to answer this question.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.