Radial Engines - June 2009

[allardjd 1,853]

 

John’s Corner

by John Allard

 

Radial Engines

 

My view of aviation and the aspects of it that I find most interesting and attractive are typically slanted toward the technical side of things, either in the hardware itself or in the technical details of operating the aircraft. It’s why I prefer IFR to VFR flight in the simulator, as one example – it’s just because it’s more technical. So, bear with me here – I’m going to get technical. I’ll try to keep the numbers out of it, mostly, but you can hardly talk about engines without getting a little oil on you – you might want to change into some old clothes before continuing to read this.

 

When I consider individual aircraft types, one of the most obvious discriminators is the kind of engine – piston, turbo-prop or jet. (I know – there are a few exceptions, but darned few.) Like most, I have my favorites. If it has a propeller, or two or three or four, I put it on a higher plane (Shameless Pun Alert!!!) than if it has none. I don’t dislike jets exactly, but don’t like them as much as the others. I don’t fly jets frequently in Flight Simulator. I don’t find them more difficult or revolting or repulsive in any way, it’s just a matter of what I prefer.

 

Propeller driven aircraft, turbo-props included, are much more interesting to me. It may be partially a matter of appearance in many cases. You must admit that modern jet transport AC have all pretty much morphed into the same basic shape. Engineering and the laws of physics drove them to the current configuration. It is the optimal shape with current power plants and materials, but they’re all very much the same - only size and details separate them.

 

I see a parallel between jet aircraft engines and the diesel engines that have replaced steam in locomotives and in most of the world’s ships. As a practical person I cannot fault the improvements that are inherent in those new, more modern power plants. Aviation is much improved because of jet engines just as railroads and shipping are because of the widespread introduction of the large-scale diesel engine. I understand that and I acknowledge and appreciate the benefits. None the less, I’ve always felt that something intangible, but very worthwhile was lost to the cold, hard economic, operational and safety realities when steam powered ships, steam locomotives and piston-engined aircraft began to be supplanted by those new and improved things.

 

Though piston powered aircraft still exist in large numbers and are being manufactured yet today, it is mainly limited to the smaller types. They are mostly single-engine and almost exclusively utilize the horizontally opposed 4 and 6 cylinder air-cooled configuration. Like the configuration of those jet transport aircraft, the realities of the economic and physical boundaries that constrain them have caused the aviation piston engines of today to vary only in the small details – almost all that exist are much more alike than they are different.

 

It wasn’t always so. Before the time when jets and their cousins, the turbo-props, drove the large piston-powered aircraft from the sky there was another type that ruled the roost. I refer, of course to the radial engine, which culminated in the late 1940s and early 50s in 4-row, 28 and even 36-cylinder monsters rated in thousands of horsepower, mounted four at a time (or six in one case). I take nothing away from the water-cooled vee types, which rivaled the big radials during the war years. The Rolls-Royce, Allison, Daimler-Benz, Packard and other similar types were large, loud and lusty and contributed their fair share to the war efforts of their respective countries and the onward march of aviation technology. I give them their due, but here, today, for the purposes of this article, they’ll be relegated to the back seat in favor of the radials. The radials are my favorite, bar none.

 

What is a radial engine? Without getting overly technical, it is defined by the orientation of the cylinders. Their centerlines lie in a single plane and radiate outward from the axis of the crankshaft. Typically, they have an odd number of cylinders, most commonly seven or nine, though examples with five or even three have been built and flown. If you step outside aviation for a moment, possibly the most numerous example of all has only two cylinders. Take a close look at a Harley-Davidson motorcycle engine next time you see one. It’s a two cylinder radial engine.

 

The other defining feature is a crankshaft with a single throw – only one crankpin. One piston of the radial engine, almost invariably the one at top center, carries the “master rod”, the connecting rod which is tied in to all the other connecting rods. The “big end” of the master rod, the crankshaft end, typically carries a row of pins in a circular pattern for the other connecting rods to mate up with. Only the master rod encircles the crankshaft journal.

 

Multi-row radial engines are really no more than back-to-back single-row engines, arranged along a single crankshaft and sharing some common services; oil, fuel, ignition and the like. Each row, considered individually, is essentially a radial engine in itself.

 

The radial engine configuration brought some inherent advantages beloved of designers, and some almost equally dramatic disadvantages. Cooling was easy – every cylinder was right out in front and saw the same cooling flow as its neighbors. The engine geometry lent itself easily to high power output at low rotational speeds, easily matching the requirements of propellers, usually without having to resort to reduction gears. Where a gear reduction was needed, efficient, robust and easily designed gear trains with ratios near 1:1 were all that were required to match the engine to the prop. Radials were generally easier and cheaper to manufacture and maintain. The tolerances required for most parts were looser than for liquid cooled vee-types. Horsepower to weight ratios, all-mightily important in aviation, were usually better than for other engine types as well, though that does not hold universally true. The round engines were generally more tolerant of in-flight failures or combat damage. Many limped home with an entire cylinder shot away, but still producing some usable power that extended the glide and made the difference.

 

Of disadvantages, there were a few. If you look up “drag” in the dictionary, chances are there will be a photo of a radial engine, or perhaps one of Michael Jackson, but I digress... The frontal area was, of necessity, quite large when compared to the in-line types. That contributed to visibility problems in most single engine types, where a cowl the size of Rhode Island blocked most anything the pilot might wish to gaze upon, whether in the air or on the ground. Radials are epitomized by oil leaks – you can tell when a radial needs oil - when it stops leaking. They were typically somewhat less fuel efficient than other types because a significant portion of their cooling was from the evaporation of unburned fuel – they had to be set up to run a little richer than their liquid cooled brethren.

 

Radial engines were a mainstay of aviation as far back as WWI, when many of the power plants hung on military aircraft of the period were radials. A unique, but common type of that period, thankfully relegated to the scrapheap soon after the War to End All Wars, was the rotary engine. It was a radial whose crankshaft was bolted to the firewall and whose propeller was fastened to the crankcase. The entire engine rotated. Cooling was admirable – almost nothing else was. They had two speeds; fast and stop. There was no throttle, only an on/off switch for the ignition. You controlled the engine by turning the ignition on or off. When on, it ran full tilt – when off, well, it was off. Imagine flying that way, if you can. It was a bad idea, but saw much use for a while.

 

Between the wars, radial engine design made great leaps and the models that were the immediate predecessors of the WWII types began to appear. Aviation pioneers of the period between the two world wars flew radial-engined crates into history – Byrd, Lindbergh, Post, Earhart and a host of others were dragged into the annals of aviation behind a radial engine, or two, or three.

 

The immediate few years leading up to WWII, when the value of military aviation was dawning on all but the most hard-headed, brought about a great spike in the progress and refinement of radial engine technology. Superchargers and turbo-chargers became nearly universal; those air pumps which made an engine think it was near sea level even up in the rare air. Engine sizes and power outputs were steadily ramped up. Several of the pre-war and early war types were still designed with single-row engines – the ubiquitous B-17 is a good example, soldiering on with single-row radials for its entire production run.

 

Most of the WWII fighter types that used radials stepped right into double-row engines from the beginning. Later bombers and transport types too mostly sported 18-cylinder, 2-row engines. Horsepower and performance continued to increase throughout the war with most aircraft types steadily employing ever more powerful engines as each new variant was introduced.

 

The US Navy (and thus the Marines, who had no choice in the matter) loved radials like no one else. They would have no other during the war years and after, not until the jets and turbo-props finally pushed the Bearcats, Tigercats, Skyraiders, Neptunes and Traders, Tracers and Trackers right off the flight decks and Naval Air Station ramps and runways. As far as the Navy was concerned, salt air and engine coolant just didn’t mix and ne’er the twain shall meet, at least not on their aircraft.

 

The late war and post-war period saw the emergence of the three and four-row engines and the final stages of radial engine development. By that time they had largely surpassed the liquid cooled types and became the standard for peacetime civil aviation in those first few golden post-war years. The Boeing 377 Stratocruiser motored along behind four P&W R-4360 Wasp Majors, crossing the oceans at will and with high confidence, if not quite at the flight levels we are accustomed to today. A good many military types too were built with the large 3-row and 4-row post-war radial engines. If you’ve ever wondered about the numbers, that’s cubic inches of displacement, folks – yes that’s an engine with about 4,400 cubic inches, or in today’s automotive terms, just a little shy of 72 liters.

 

It can be argued that radial engines were close to their design limits at that point. It may be that those still making them might have been able to continue to build them bigger and more powerful yet, but it seems difficult to even imagine. Certainly the technology evolved no further once the jets took to the skies in commercial liveries. The military continued to use them for transports for a while, including in some new aircraft types, but the evolution of the radial engine froze dead in its tracks when the de Havilland Comets, Boeing 707s, Douglas DC-8s and Convair 880s began to roll out.

 

Today we see and hear radial engines mostly at airshows. I’m thankful every time I hear one or better yet, several of them. They’re making their thunder these days just a little more muted, limited to lower manifold pressures on lower octane fuel than they were originally designed to run with. Even constrained by that a little, it’s still better than good.

 

The coughing and shaking and smoke when they’re coaxed to life is something to be savored and then they settle down to that loping idle that the owners hear saying over and over again, “a-gallon-of-gas, a-gallon-of-gas, a-gallon-of-gas”. There’s nothing like it in the world.

 

Because there are folks out there who can afford the stewardship of the birds that bear them and who care enough to do so, we can go to a decent air show and see them, hear them, and maybe even touch them - all those fat, finned cylinders splayed out with the pushrod tubes and oil hoses and plug-wire harnesses pointing the way to the cylinder heads. If those guys who own them and maintain them and fly them are having even more fun than I am, that’s alright by me, as long as they keep bringing them for us to see and hear and experience.

 

Everyone loves a Mustang, but listen to one make a high speed pass at an air show and then listen to a Warbird with a radial go by – even the trainer types, and tell me which speaks aviation to you the best. Not the loudest; that wouldn’t be a fair comparison, just the best. I’ll pick the bird with the big round engine every time. I guess I get a little sappy about these kinds of things but of all the sounds in the world, radial engines barking though eight inch long exhaust stacks are all the music I ever hope to hear. It makes me smile and it makes me proud and it sometimes even brings just a bit of a tear to my eye. If anyone notices I blame it on the sun or the wind, but it’s really not.

 

[mutley 4,488]

Excellent article John, you are speaking my language.

I remember last year hearing the distant rumble of a radial engine, bit by bit it was getting closer and I was looking for the source of this wonderful sound when in my view appeared the C-47 from the Battle of Britain Memorial Flight. It was sporting a pair of Pratt & Whitney R-1830 Twin Wasp's the same as they use on the Liberator your Father in Law flew, the low rumbling just demands attention and was music to my ears!

The Dizzie, Connie and Catalina surely rate amongst my all time favourite aircraft and technically demanding enough to hold my attention too.

Thanks for posting John, I really enjoyed this one, 10/10 from me.

[simi_av8r 0]

Excellent article John, you are speaking my language.

........10/10 from me.

i really couldn't agree more, a superb post there John, and hearing a radial grumbling in the distance never fails to get my neck hairs stand on end!

sadly, few WW2, let alone WW1, examples of radial powered aircraft are found in the UK, and those that can are either owned by the BBMF (the C-47's as Mutters reffered to in his post) or are housed at Duxford and kept in tip-top airworthy condition by The Fighter Collection....although there are plans afoot to attempt a restoration of a Blenheim by ARC, also based at Duxford...oh the joy it would give if ever this bird returns to the air, the sound of its twin Bristol Mercury engines roar puts me on edge even now!

[wisemanp 0]

I have a free morning tomorrow John, so I shall have a good read of the newsletter, and comment then!

[Axel 0]

John if you like big system aircraft, you'll love this

[rob16584 42]

Excellent article John, a great read as ever!

I agree with you though, the radial engines sound the best. Being quite young i've only really ever known jet aircraft so it's great to hear something a little different every now and again!

[George 0]

Great topic - I wasn't alive back in the 50's, 60's 70's but I definitely wished I could have been! Living near the Fleet Air Arm Museum at Yeovilton (RNAS) we get all sorts of radials come over in formation - we had a Spitfire just this morning I believe with the Rolls-Royce (could have been a Hurricane). Of course the annual air show brings its jets but nothing excites me more than the radials!

[allardjd 1,853]

I recently encountered this gem, made from a radial engine master connecting rod. You don't find these all up and down the block...

 

John