Radial vs liquid cooled engines

[tyrodtom]

I didn't start driving till the early 60's, but in some mighty old cars. Most of the problems I had could be blamed on two things, high wear and poor maintenance, not something most aircraft would encounter.

But comparing the mass produced liguid cooled cars of the day with the liguid cooled aircraft of the same time is no different than making the same comparision today with modern cars and modern aircraft, there just is no comparision..

 

[renrich]

Cars during WW2 and afterward were powered mostly by liquid cooled reciprocating engines just like many of the AC of that time. Today most AC are powered by gas turbine engines except for general aviation types which are largely powered by air cooled reciprocating engines. There was a very valid comparison between car engines and AC engines in the 30s and 40s. Packard car engines were mass produced just like Packard RR engines and they were both liquid cooled. A great many of the AC engines used for US AC in WW2 were produced by car companies.

 

[tyrodtom]

Aircraft in 1940's were a quatum leap ahead in technology of the technology in MOST 1940's mass producted cars, just as they are today.
And Packard of the 30's and early 40's was not your usual American car manufactor.

I'm not saying automotive manufactors of the 40's weren't capable keeping precision tolerances, and advanced engineering when it came to producing engines, they were. What they could do, and what they put off the assembly line when it came to cars was just not the best they could do.

 

[tyrodtom]

Let's take for example the flathead Ford V8, built from 1932-53, in various displacements. Like most V8's it's valves were on the inside of the V, and since for economy Ford cast the block in one piece, the exhaust passages from the valves in the block went between the cylinders to the outside of the block to the exhaust manifold. It was noted for it's tendency to overheat from the first to the last, even dual waterpumps couldn't cool it enough.
Did it have cooling problems because it was a liquid cooled engine ?
No, it had cooling problems because the hot exhaust gasses went from one side of the block to the other, before exiting, transfering a lot of heat to the coolant.
That was state of the art at Ford for a liquid cooled engine in the 30's, but no aircraft manufacture would try such a dumb engineering.

 

[FLYBOYJ]

That was state of the art at Ford for a liquid cooled engine in the 30's, but no aircraft manufacture would try such a dumb engineering.

Not that simple - many times aircraft manufacturers (engine manufacturers) are restricted to design guidelines either set by the military or civilian aviation authorities. You're actually looking at "dumbing down" a design for either safety or design standardization. In the 30s and 40s it was less restrictive for obvious reasons than it is today, but it did exist.

 

[davparlr]

Also, in what way was the Tempest II superior to the Tempest V?

Other than being faster and climbing better?

 

[CobberKane]

I agree that the liquid cooled engine has another system to maintain and be susceptible to battle damage, so it is more vulnerable to battle damage. I, however, believe that the difference in vulnerability is overexaggerated. It is done in two ways - exaggerating the vulnerability of liquid cooled engines to battle damage ("just one shot from a small calibre gun will do the trick"), and exaggerating the ability of air cooled engines to survive battle damage (using one or two examples of a particular engine having survived after a cylinder was shot off).

Ease of mainetnance is a more difficult sell - because in both categories some are easier to maintain than others.

Also, in what way was the Tempest II superior to the Tempest V?

It was more restant to battle damage lol.

Seriously, I just back handed a library book on the Hawker Typhoon and Tempest (titled, with dazzling originality 'Hawker Typhoon and Tempest') which went into the developement of the Tempest II in some detail and there was an overall performance and relaibility increase, which I suppose is why the Tempest V was supplanted. I didn't take a note of the figures, sorry.

 

[wuzak]

Other than being faster and climbing better?

It did climb a little better, still nowhere neear as quickly as a Spitfire, but it was barely faster than the Tempest V, if at all.

 

[Jabberwocky]

It did climb a little better, still nowhere neear as quickly as a Spitfire, but it was barely faster than the Tempest V, if at all.

RAF didn't seem to think so.

From the Tempest II vs Tempest V tactical trials

Speeds

45. The Tempest II is 15 m.p.h. faster up to 20,000 ft., dropping to 10 m.p.h. from 4,000 – 7,000 ft., it then increases its advantage to 20 m.p.h. at 12,000 ft., dropping again to 10 m.p.h. at 15,000 ft. and holding this advantage up to its ceiling.

Acceleration in straight and level flight

46. The Tempest II is definitely superior when opened up from cruising or slow speeds to full throttle, and rapidly goes away from the Tempest V.

Climbs

47. The Tempest II has a better rate of climb at all heights than the Tempest V, being 350 ft/min. better up to 3,000 ft., increasing to 1,000 ft/min. from 7,500 to 8,500 feet, dropping to 400 ft/min. at 12,500 ft. and maintaining this advantage service ceiling.

Zoom Climbs

48. In the zoom climb at equal power settings, the two aircraft are very similar, but at full throttle the extra power of the Centaurus V gives the Tempest II a definite advantage.

Dive

49. The two aircraft are identical.

Turning Circles

50. There is very little to choose between the two aircraft, if anything the Tempest V appears to have a slight advantage.

Rates of Roll

51. During the trials carried out, the Tempest II proved definitely superior to the Tempest V at speeds up to 500 m.p.h. I.A.S. As the two aircraft have the same airframe there appears to be no aerodynamic reason for why the Tempest II should be better. It is therefore assumed that the ailerons on Tempest II MW.754 are above the average, and may not be truly representative of a production aircraft.

Conclusions

52. The Tempest II is superior to the Tempest V in every way, except in the turn where it is at a slight disadvantage.

 

[Shortround6]

Aircraft were also more subject to vibration than most cars. Aircraft were built as light as possible as were their engines. Some engines were notorious for vibrations, legend has it that you could tell what engines a DC-3 pilot flew ( Cyclones or Twin Wasps) by how long it took his hands to stop shaking after a flight. A huge number of aviation museums have Curtiss Challenger radial piston engines on display, again legend has it that the owners pulled the engines well before they wore out to put in other engines so they would have something, ANYTHING,that vibrated less ( it was a two row 6 cylinder radial, never tried again for a production engine). Some of the liquid cooled engines were no better.
It took a while for engineers to fit loops and bends into fuel, oil and coolant lines just for stress relieve and to provide cushioning for the vibration.
Some early 6, 8 and 12 cylinder engines had NO COUNTERWEIGHTS on the crankshaft, and some later ones only had enough weight to keep the vibrations/forces within certain limits up to a certain RPM.

Fatigue cracking was only beginning to be understood at the end of WW I and in the early 20s. They called it "crystallization", they knew it was happening, older parts failing and having a different grain structure than new parts, but they didn't know how or why.
Radiators were also predominantly soldered, by hand. Between human failures ( hundreds of solder joints in a single radiator) corrosion, vibration and dozens if not hundreds of heating cooling cycles the liquid cooling system was one more thing to go wrong.
The liquid cooled engine was pretty much dead in the commercial market in the 1930s. Jumo Diesels and post war use of the Merlin not withstanding. The extra performance wasn't worth the extra cost of maintenance for a commercial aircraft. For military planes it was another story, at least for another decade or so.

 

[CobberKane]

It seems to me that a good analogy for the air-cooled v liquid cooled aero-engine might be the development of off road motorcycles. Trail bikes, like combat aircraft, are inclined to get used hard and knocked around a bit, and even with the temptation of the extra power that can be wrung from an LC engine air cooled designs persisted quite a bit longer on trail bikes than road bikes. Today trail bikes seem to be mostly liquid cooled, no doubt as the manufacturers chase extra power at a given capacity, but anyone who does serious bush-bashing on a liquid cooled traily will be aware of the effects of a branch through a radiator or coolant hose. The same nasty stick would have been poking holes in thin air if the bike had been air-cooled

 

[61fury]

A question here, civilian airliners before the Jet Age all seemed to be radials. Was there a reason for this? Were there any liquid cooled airliners? I know fighting and flying and just flying are different goals but the radial engine era seemed to last until jets took over.

 

[buffnut453]

The reason for in-line engines becoming popular in the mid/late 1930s was to provide a smaller frontal area hence less drag and more speed. Those design drivers continued through the early war years (1939-1940) by which time radial engines started to produce sufficient power to compete against in-line engines in the overall performance stakes. However, by that time development of the jet engine was indicating that the future of high speed effectively sounding the death-knell for piston engines in high-speed implementations.

Radial engines survived in the civilian market because the drivers for those aircraft were long endurance, heavy load-carrying and (consequently) lower speeds. Given wing thickness and generally less aerodynamic designs of transport aircraft and airliners, there is no benefit to having a closely-cowled, in-line piston engine implementation for such applications.

 

[wuzak]

RAF didn't seem to think so.

From the Tempest II vs Tempest V tactical trials

Thanks for clearing that up.

From that sit the Tempest V with Hispano Mk IIs and Sabre IIB at +11psi boost (2400hp) had a top speed of 435mph. A Tempest V with Sabre IIA at +9psi boost (2200hp) and Hispano Mk Vs had a top speed of 236mph, and with Hispano Mk II and Sabre IIA a top speed of 432mph.

The Tempest II is shown to have a top speed of 447-448mph with the Centaurus using +12psi boost. This probably shows that the Sabre was behind where it should have been in development - at the same time Merlins were being rated at +18, +21 and +25psi boost.

Also of interest to me is that the Tempest I with Sabre IV (2400hp) and leading edge radiators was nearly 20mph faster than the Tempest II and 30mph faster than the Tempest V. The Tempest I flying before the II did.

A later development of the V with an annular radiator and more developed Sabre (c. 3000hp) was also faster than the Tempest II.

 

[61fury]

thanks Buffnut

 

[Ascent]

One of the troubles is that although we know about radial engine aircraft getting back to base with damaged engines what we don't know, and can't know, is how many crashed due to similar damage. What proportion of damaged engines made it home? Did the extra size of the engine mean that actually they were twice as likely to be hit, so a few getting home shouldn't be too suprising, or did it make no real difference?

Would it be remarked on in the same way if a liquid cooled engine got back to base with a damaged cooling system?

The trouble here is selective reporting, not as a deliberate thing but simply because those stories that come to us are those we find remarkable and we're too far removed in time from what happened to be able to go and collect the data we need.

 

[Elmas]

Did the U.S. Navy ever use a liquid cooled engine on the planes operating from carriers?
I'm not aware of a single one.

 

[CobberKane]

Sure, and if liquid cooled engines were ever getting back to base with a cylinder shot away it would be remarkable and we would know of instances. But we don't know of instances (or at least, no one supplied one) so by this logic its reasonable to believe it didn't happen.
This is about cooling. If you remove a cylinder from an air-cooled engine with a cannon shell you will completely deprive the engine of any cooling and in a matter of minutes it will stop. Even an bullet that will smash cooling fins of a radial will have a good chance of breaching the water jacket of an LC engine, depressurising the cooling system - and it will stop.

 

[wuzak]

Sure, and if liquid cooled engines were ever getting back to base with a cylinder shot away it would be remarkable and we would know of instances. But we don't know of instances (or at least, no one supplied one) so by this logic its reasonable to believe it didn't happen.
This is about cooling. If you remove a cylinder from an air-cooled engine with a cannon shell you will completely deprive the engine of any cooling and in a matter of minutes it will stop. Even an bullet that will smash cooling fins of a radial will have a good chance of breaching the water jacket of an LC engine, depressurising the cooling system - and it will stop.

Which production liquid cooled engines could have a cylinder shot away without having half the engine missing? Well, I supopose the Vulture could count, DB 606 DB 610, maybe the V-3420, in which case only a quarter of the cylinders were shot away.

Sure the LC engine will stop once it has lost its coolant - but it won't necessarily be immediately.

I reiterate, the very few occasions where pilots have returned to base with parts of their air cooled engines is undoubtedly the exception, rather than the rule. Of course there is little way to prove that since those that didn't return to base were probably dead or unaware as to why their engine stopped. The guys that did return to base would probably have bailed had the known what had happened.

 

[Shortround6]

It is not only what we think or what we think we know now but what was thought at the time. The US army in the 1930s bought the argument that air-cooled engines were less vulnerable and specified only air cooled engines for "attack" aircraft. Aircraft intended for ground support and more likely to be hit by ground fire.

While only a few radials may have made it home with cylinders missing many more made it home with damaged cylinders, dented or broken cylinder fins or damaged cylinder heads. Hits that would have caused leaks on a liquid cooled engine. While a liquid cooled engine won't stop immediately with a cooling hit and may even run at low power settings for 15-20 minutes getting "home" all depends on how far you are from "home" ( friendly air field or friendly farmer's field) when the damage occurs. Some pilots may have nursed an ailing liquid cooled engine even further.

There is little doubt that the size of the liquid cooling system offers more target area than an equivalent air-cooled engine.