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Radial vs "Inline"-Engines


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#1 azrael

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Posted 16 February 2005 - 06:55 PM

Hi there everyone, I have a few questions concerning the two basic concepts in piston-engine design. What were the advantages disadvantages of the two types?

To the best of my knowledge they are as follows:
A) Radial-Engines:
1) Can be air-cooled, making them less vulnerable to damage.
2) More compact than "inline" engines.

B) Inline-Engines
1) Lower cross-section and seperate radiator allows for better
aerodynamics

There must be some additional advantages to the "inline"-engine I assume or they would not have been used so widely. One thing I do not understand is why most high-altitude aircraft used "inline"-engines (Bf109, FW190-D+, Lightning, Mustang....), the FW190 even going as far as replacing their radial with an inline to make them better high-altitude fighters. What is the reason for this?

Looking at the aircraft mounting the engines I deduce that you used a radial if ruggedness was important and an "inline"-engine when pure performance was your main criterium. The above advantages do indicate the point for the radial, but not exactly the one for the "inline". Does anyone know more about this?

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#2 Mark J

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Posted 16 February 2005 - 07:18 PM

Thermal shock

I read somewhere that quick changes in altitude (diving) will rapidly cool a radial, sometimes to the extent of shrinking the outer metals ( I'm talking thousands of an inch ), possibly causing physical damage to the moving parts, slowing down the engine and maybe effecting the fuel/air ratio. Can't remember exactly.

The liquid cooled engines (inline), had their temperature controlled by thermostats and would maintain it at the extremes of altitude or during a dive.

Or maybe it was just what was available at the time. The FW 190-D used the Jumo 113 because it was not in as much demand as the BMW 801 or DB 603, plus the Jumo 113 had a better supercharger, similar to the DB engines.
cheers

#3 GregP

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Posted 17 February 2005 - 11:02 AM

Radials were compact and generally air-cooled, as has been observed above. If you wanted more power with the same diameter, you added another row of cylinders. You could increase bore and stroke and rows to get "more."

Inlines had a lower frontal area and, being liquid cooled, could take a slightly higher boost before reaching temperature limits, which is why they were used for high-altitude work. They could be boosted to a higher degree. You added a pair (or four in the case of an "H" layout like the Napier Sabre) of cylinders to get "more," but there was only so much length you could add.

The inline V-12 was about as long as was desirable in an inline. For just slightly more length, you could make a 4-row radial, like the R-4360 Wasp Major.

Of the two types, radials were more powerful but were also confined to lower altitudes, in general. Inlines made for fast airplanes with lower power than the radials, but they were more vulnerable to ground fire.

There were different skills required to tune the engines, too. The Japanese were very good at tuning radials, but never got the hang of tuning inlines very well.

My own opinion is that radials were more "robust" in general, but there were excpeptions.

The Soviet Union fielded many fighters with engines developed from the Hispano-Suiza 12Ycrs (the M-103/105 series) that ran very well in harsh climates on low-octane fuel and crude maintenance, so perhaps the inlines were "more finicky" when developed in Western hands with higher-octine fuels and with an eye toward "highly-tuned" engines that were developing the most power they could get with the fuels and blowers available at the time.

So, the Soviet Union was an example of both radials and inlines that were robust. Conversely, they didn't have much in the way of widely-available superchargers, so most Soviet WWII fighters and dive bombers were low-to-medium altitude creations.

The West, by contrast, had "high-strung" engines that could be pushed to "War Energency power" with methanol or water-methanol injections, higher compression ratios, and higher-octane fuels.

Consider this:

The Supermarine Spitfire, the North American P-51 Mustang, and the Yak-3/9 series were all inlines.

The Focke-Wulf Fw-190 (except the "D" and the Ta-152 series) , Republic P-47, Lavochkin La-5/7 series, and Grumman Hellcat were all radials.

While many in this forum seem to LOVE the Spitfire (for good reason), many also believe the La-5, particulalrly the La-5 FN, was the best fighter of the war. The Hawker Sea Fury was also a very good aircraft, though it was really post-war in actuality, though not by much.

So, The choice was often made along the lines of "what engine of the required power and reliability is available in quantities that can help the war effort," and they went from there ...

Witness the Beech A-38 Grizzly. It was a great plane, but used the same engines as the B-29 (R-3350), so it never got built in production.

#4 Tony Williams

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Posted 17 February 2005 - 04:45 PM

Hmm. I've responded to this already but my post seems to have disappeared.

However, I'll recap: while I agree that the air-cooled radial had advantages in some circumstances (especially in being far more resistant to battle damage) I believe that liquid cooling had technical advantages. It was more efficient than air-cooling, and permitted closer control of the engine's working temperature. So a good liquid-cooled engine was probably capable of producing a higher power output than an air-cooled one of equal size and quality, and would produce a lower fuel consumption. Of course, individual differences between engine designs make direct comparisons difficult.

Liquid cooling did of course have the practical advantage of permitting more slender engine designs (typically vee-type) to the benefit of aerodynamics, which also helped improve speed and range. It's difficult to separate that issue out from the technical one mentioned above. And the issue is further confused by the fact that the aircraft they were fitted to varied considerably in aerodynamic quality, quite apart from the engine installation.

Basically, I'd choose an air-cooled radial for naval planes, ground attack and long-endurance patrol planes (i.e. mostly lower altitudes), and liquid-cooled in-lines for interceptors, fast bombers etc (i.e. mostly higher altitudes).

Tony Williams
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#5 azrael

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Posted 17 February 2005 - 05:26 PM

Thanks for the replies, guys.
I wonder how the different designs (radial vs inline) influenced the lifetime of the engine. Nowadays air-cooled engines have fallen out of favour in the automobile industry for that very reason. Air-cooling can only take place on the surface of the engine and is thus less capable in cooling down the critical hotspots near the combustion chambers. Did this drawback shorten the lifetime of radial engines as well or is the aircooling on a fast-flying aircraft sufficient to prevent this from becoming a major problem.

#6 Kutscha

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Posted 17 February 2005 - 08:06 PM

It is claimed that the radials were high drag, but how much drag did the cooling system for the inline engines add? Also, how much weight did it add to the overall inline engine 'package'?

#7 azrael

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Posted 17 February 2005 - 09:13 PM

Kutscha,
looking at the construction of a radial engine I would say that the high drag was the result of two very distinctive factors:
1) Larger crosssection increasing the frontal area of the engine.
2) The way the actual airflow was guided over engine and cooling ridges.
The layout of the cylinders in a radial would cause a rather turbulent airflow, increasing the drag even further than their pure cooling surface would indicate, a problem that could largely be avoided with a radiator that you can design in any way you like.

Concerning the weight of the engines, no clue how much heavier an inline would be than a radial, although it can be deduced that the increase is significant. In addition to the radiator and coolant itself you have to build a compact, enclosed engine to allow the circulation of the coolant within the actual block. Aircooled engines are basically just bare cylinders with some ridges to increase the cooling surface. Compare that to the block of an inline engine (open the hood of your car). I would say that the additional dead weight of the block alone is far worse than the weight of the hoses, radiator and actual coolant.

#8 Tony Williams

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Posted 17 February 2005 - 09:46 PM

quote:Originally posted by Kutscha

It is claimed that the radials were high drag, but how much drag did the cooling system for the inline engines add? Also, how much weight did it add to the overall inline engine 'package'?


The cooling system drag depended very much on the design. The P-51 featured a carefully-shaped radiator which used the heated air coming out of the back to provide some thrust, more or less enough to counteract the drag.

TW


#9 Kutscha

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Posted 17 February 2005 - 10:31 PM

Yes Tony, but the P-51 was the exception. The P-40's radiator 'system' was continually being modified to decrease drag.

Compare the A-9 and D-9. Not much difference in top speed.

The big fat P-47 and the lean P-51 do not have much of a speed difference.

Dago Red and Rare Bear don't have much of a speed difference.

A radial needs a good flow of air or there will be no cooling. Azreal you mentioned auto engines. Look at motorcycle engines that are air and water cooled. The air engine is lighter. Air cooled engines have bad emissions, so are are out of favour.

To add, 1 American gallon of water weighs 8.34lb. How many gallons in a coolant system?

#10 azrael

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Posted 17 February 2005 - 11:57 PM

quote:Originally posted by Kutscha

Yes Tony, but the P-51 was the exception. The P-40's radiator 'system' was continually being modified to decrease drag.

Compare the A-9 and D-9. Not much difference in top speed.

The big fat P-47 and the lean P-51 do not have much of a speed difference.

Yes, but the P-47D also had close to 30% more power at 2300 bhp compared to the 1700 bhp of the Mustang. And the Mustang was still 20kph faster.

quote:Originally posted by Kutscha


Look at motorcycle engines that are air and water cooled. The air engine is lighter. Air cooled engines have bad emissions, so are are out of favour.


Not quite correct. There is a few aircooled engines still but first of all, they cheat as they use huge oil-coolers and second, they are generally not high-performance (mostly cruisers). They are still being used for two reasons: good looks and good sound. All high-performance, non-racing bikes are liquid-cooled, partially to prolong their lifespan and partially because there is no way to get rid of the heat otehrwise. I mean we are talking about 160+ bhp/litre here.

#11 ickysdad

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Posted 18 February 2005 - 08:08 AM

azrael,
Yes the P-51 had 30% less HP than the P-47 and was faster to boot BUT the P-51 also had much more of a high speed wing shape along with being much lighter.

#12 Tony Williams

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Posted 18 February 2005 - 11:50 AM

quote:Originally posted by ickysdad

azrael,
Yes the P-51 had 30% less HP than the P-47 and was faster to boot BUT the P-51 also had much more of a high speed wing shape along with being much lighter.


Which illustrates the point I meade before: that there are too many variables affecting aircraft performance to separate out the effects of the engine cooling system.

Tony Williams
Military gun and ammunition website: http://www.quarry.nildram.co.uk


#13 Ricky

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Posted 18 February 2005 - 04:41 PM

Unless you take one plane, and try it out with 2 different engines...

#14 Johnny G

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Posted 18 February 2005 - 06:16 PM

quote:Originally posted by Ricky

Unless you take one plane, and try it out with 2 different engines...


They did this with the Hawker Tempest. Both versions flew operationally. Does anyone know how they varied in performance?

#15 Tony Williams

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Posted 18 February 2005 - 07:09 PM

quote:Originally posted by Ricky

Unless you take one plane, and try it out with 2 different engines...


But that would still only be valid if the two engines were the same apart from their cooling systems.

TW

#16 Ricky

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Posted 18 February 2005 - 09:16 PM

Yes, using a radial engine and an inline engine of equal hp. They would not neccassarily be equal weight. This is wherecomparison becomes tricky. 2 inline engines can be of equal hp, but differernt weights. So how to guage if they are 'comparable'?

#17 PMN1

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Posted 18 February 2005 - 10:49 PM

quote:Originally posted by Johnny G

quote:Originally posted by Ricky

Unless you take one plane, and try it out with 2 different engines...


They did this with the Hawker Tempest. Both versions flew operationally. Does anyone know how they varied in performance?


Another question - the Sabre's Tempest cooling intake was in the nose wheras the proposed version with the RR Eagle appears, from the drawing I have seen in 'Interceptor' by James Goulding would have had its radiator arranged as in the Mustang (it looks like a Mustang on steroids) - anyone have any figures for an Eagle Tempest?

#18 Tony Williams

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Posted 18 February 2005 - 11:25 PM

quote:Originally posted by Ricky

Yes, using a radial engine and an inline engine of equal hp. They would not neccassarily be equal weight. This is wherecomparison becomes tricky. 2 inline engines can be of equal hp, but differernt weights. So how to guage if they are 'comparable'?


There is also the question of efficiency, in terms of both the power extracted from a given cubic capacity and the fuel consumption at steady speeds.

TW

#19 seppalar

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Posted 20 February 2005 - 04:50 AM

Does anybody have any opinions on the inline aircooled engines? AFAIK none of them got beyond the 900 h.p. class but that is surely just an issue of displacement. The inline aircooled seem to have offered the advantages of both types and featured in quite a number of early and pre-war projects.

Did anyone ever try an X aircooled engine? Also, pursuant to one of Tony's suggestions in another thread, did anyone ever mount a machinegune between the cylinder banks in such an engine?

On to the original topic: we should be able to compare the Centaurus and Sabre engined Hawkers to see which one was preferred. AFAIK the radial version was considered much better.

#20 Tony Williams

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Posted 20 February 2005 - 02:26 PM

The problem with the aircooled inlines was in getting enough cooling to the rear cylinders - by the time the air got there it had already been heated up. The more powerful the engine, the more heat is generated and the more difficult this becomes.

Tony Williams
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