Radial vs liquid cooled engines

[RCAFson]

The only liquid cooled airliners were a Few Canadian and British plane using Merlins. At the time the British (including the Commonwealth) had one heck of a trade deficit problem and foreign (meaning US) engines simple could not be afforded. That left the Merlin, Griffon, Sabre, Hercules and Centaurus engines. The Sabre and Griffon found no takers at all in the commercial field and Rolls, by expending a fair amount of effort and trading a bit on the Merlin reputation did make the above mentioned sales. Commercial Hercules ( even licence built in France) and Centaurus engines filled the British commercial aircraft until the jet and turbo prop.
The Americans, for a variety of reasons, dominated the post war commercial aircraft market and so did American engines, which after the Allison stopped production, meant air cooled radials.
One less system to fool with and maintain and with acceptably low operating costs ( and airline owners are a hard headed tight fisted bunch) the air cooled radial did the job.

The other factor was the incredibly low price of avgas in those days. I suspect that if ICE were still used today for airliners, that they would all be liquid cooled because of the lower specific consumption.

 

[tyrodtom]

It was "copied". Some Martin Mariner patrol bombers used fan cooled R-2600s. The P-47J used a fan cooled R-2800 ( dropped for the P-47M&N) I am not Sure if the Russians had a few fan cooled installations of the Ash-82. At least one Bristol Hercules commercial installation post war used fan cooling. I am not sure about the Hawker tempest II or Fury.

That is off the top of my head, there may certainly well be others without getting into buried or pusher installations.

What looks like a cooling fan in the cowling of the La-5 and La-7 is a controllable cooling veturi louvers, needed to limit cooling air during Russia's extreme cold.

I know helicopters powered by radials had a cooling boost, but wasn't aware of any other fixed wing aircraft using the FW solution.

 

[Shortround6]

Yes, by saying 'for the same horsepower' you are restricting yourself artificially in the same way a motorcyle manufacturor is by saying 'we must not go beyond 750cc'. In practice the designer of the figher aircraft is going to use the engine that gives the best balance of performance, fuel consumption etc, irrespective of whether it is defficient in one particular area like drag. The fact that radial engine fighters were matching the performance of the best inline engines right up to the end of the war indicates that their disadvantage in terms of drag was not enough to place the radial engine at a disadvantage compared to the inline V in practise .
I guess in theory it would be possible to build a mile-long V-100000 with no more frontal area than a V-12, whereas a radial of increasing capacity would have to get wider and wider. In practice all that was laid aside by the jet engine, and even if for some hypothetical reason the jet could not be made to work other alternatives eventually came along for driving airscrews.

Basically the air cooled and liquid cooled engines stayed pretty much neck and neck power wise for most of their history. One or the other would have an advantage for a few months or even a year or two but the pendulum usually swung back.

In the world of fighters I can think of only 3 examples that offer a good comparison. The P-36/P-40 combination. The Hawker Tempest and the FW 190.

Most other engine "swaps" entailed too much of a power change to make valid comparisons. Since these are generalizations I am doing some rounding up/down.
Italians replaced 900-1000hp radials with 1200hp German V-12s. gaining 20-33% in power makes it hard to access the change in drag alone.
Japanese replaced a 1200hp (or 1400 very unreliable hp) V-12 with a 1500hp radial. Again making it hard to figure out actual benefit/cost.
Russians replaced 1200hp V-12 with 1500-1700hp radial. This radial wasn't even a gleam in the designers eye when the particular V-12 was already several years old.

The P-36/40 offers the best possibilities for looking at the situation. It used 2 different radials (one in two different configurations) and two different liquid cooled engines and spanned 7-8 years in time. There are even a few more experimental/prototype engine installations. ALL of the engines varied in "nominal" output from 1000 to 1300hp with the more common spread being even closer. The American engines also had pretty much access to the same quality fuel at the same time.

The Wright Cyclone 9 and the P&W Twin Wasp 14 battled for top dog in the US since 1933-34 starting at around 800-850hp. by the late 30s they were over 1000hp (if not 1100hp) and while the P&W (R-1830) was a little smaller in diameter it was a little heavier than the Wright R-1820. Allison comes on the scene with the V-1710, it splits the difference weight wise (not including cooling system) but is a bit down on power. Things do get a bit confusing as Wright is the first to offer a two speed supercharger which improves power at both low altitude (take-off) and HIGH altitude. High at this point in time being anything much over 10,000ft. Allison offers an "altitude" rated engine, max power at around 11,000ft with it's single speed supercharger. P&W is working on a two stage supercharger.

Getting back to the nitty-gritty. During flight tests of the early P-40 with it's 1040hp engine at 11,000ft or so they figure that the P-36 with it's 1200hp (take-off) R-1830 had 22% more drag than the V-12 powered plane. Speeds for the Hawk 75 ( commercial P-36) were between 310-326 mph depending on source, engine fitted ( which particular r-1830 or R-1820) and armament fit. The Early P-40 went from 357 down to 345mph as more guns, armor and self sealing tanks were fitted. These speeds are between 10,000-15,000ft. A Hawk 75 demonstrator (P&W R-1830) with 2 stage supercharger was flown at the 1939 army fighter trials, performance is so far unpublished. Later P-40s got a lot heavier but also got more powerful V-1710 engines. The P-40F with the Merlin was supposed to be good for 364mph at 20,000ft where the air was thinner (less drag). P&W was battling back with things like the XP-42

Which was also flown at the 1939 trials. it was slower than the XP-40 and had cooling problems. Later a P-40 airframe was bailed back to P&W for engine development and in late 1942 P&W got this aircraft up to 386mph with an R-1830 engine making it one of the fastest P-40s ever. P&W had learned a lot in 3 1/2 years about two stage superchargers and radial engine installations. Please not that this aircraft may have done it's high speed more in the 20,000ft area than down lower with benefit of thinner air for less drag. It was also unarmed I believe for less drag and weight. It is this gain in knowledge than makes comparing planes built even a few years apart difficult in deciding which "engine" or plane was best. In about 4 years P&W using the same basic engine but a different supercharger set up improved the speed of the same basic airframe by by around 60mph. This is better than Rolls Royce managed to do with the Merlin and Spitfire but then the 1938/39 Spitfire was a lot further ahead of the 1938 P-36 to begin with
I would also note that in 1942 the R-1830 was strictly a 2nd class engine and the big effort was going into the R-2800 and R-4360. Still, what was learned about baffling, cowl design and scoops could be applied to the big engines.

 

[Shortround6]

The other factor was the incredibly low price of avgas in those days. I suspect that if ICE were still used today for airliners, that they would all be liquid cooled because of the lower specific consumption.

Some of those radials didn't do to bad for fuel consumption while cruising. Some of them were well within 10% or so of the contemporary liquid cooled engines if not even closer. Maybe the cost difference today would out-way the increased maintenance costs. The Wright turbo compound was noted for it's low fuel consumption but was unpopular for all but the longest range routes because of it's maintenance costs. The Candair North Star used two stage Merlins with inter-coolers that could also be used as charge heaters when cruising ( to prevent fuel puddling in the intakes.) An interesting comparison is the North Star 5 with P&W R-2800s which didn't use much more fuel for the same range. I will try to post numbers from the 1954-55 issue of Jane's later ( assuming that they are accurate)

 

[CobberKane]

There are practical limits to anything.

Adding length to a V engine will lead to diminishing returns because of crankshaft flexibility, and other associated issues. Adding cylinders to air-cooled radials can increase the diameter (more cylinders per row) and/or length (more rows). After a cerain number of rows it will be getting very difficult to cool the rear cylinders.

Note that liquid cooled engines can be radials too - R-2160 Tornado, Lycoming, XR-7755, BMW 803, for example. Granted, these never went beyond prototype stage.

You will notice that the air cooled engine powered fighters late in the war which matched liquid cooled ones were generally more powerful.

Yes, in practice six cylinders seems to have been the about where designers stopped making engines 'longer' and added another bank. And after they got to two banks of six they added another couple. These four bank engines seem to have been the way things were heading but I can only think of the RR Vulture and Napier Sabre that made it into production before the Jets came along. Any others?

 

[wuzak]

Not sure if you would call the small number of Eagle 22s prodction, or not, but that would be the closest.

The (air-cooled) Rolls-Royce Pennine was only at the prototype stage when cancelled infavour of jets. The same capacity as the R-2800 and max power about the same as the best, most developed, versions of the R-2800.

There was a French H-24 based on Jumo 213 components - but that wasn't production.

There were a couple of prototype V-16s - the Chrysler IV-2220, which had the power and accesories take off from the centre of the crank and the Daimler-Benz DB 609, a V-16 version of the DB 603. The DB 609 had the power take-off from the end of teh crank, like the DB 603. The Chryler IV-2220 was more than 120in (3048mm) long!

Engines like the DB 604 and Vulture could have worked had there been time to properly develop them. The Vulture was in production and in service before it was ready. The DB 604 was cancelled before it got to production - still not sure why.

 

[wuzak]

Forgot about the Allison V-3420. That was in some sort of limited production. Then it wasn't. Then it was. Then it wasn't....

And the Jumo 222.

 

[model299]

I'm just wondering,it maybe the US Navy had a good reason I'm unaware of. You wouldn't want your matelots drinking the stuff

Steve

As big as carriers are, they still grapple with the problem of space for "stuff" to this day. By choosing to rely on air cooled engines, they eliminated the problem of tankage, plumbing and such in the handling of glycol. They also eliminated the neccessity of having to keep a stock of spare parts for the cooling systems, the need to set up repair shops for the radiator assemblies, all of the associated hoses, clamps, fittings, etc, and all the tools associated with liquid cooling.

In 2004, I had the priviledge of spending some time aboard the Abraham Lincoln, and the Ronald Reagan (Which had just home ported, still have the t-shirt.) and learned some things about storing "stuff" on a carrier.

All vehicles aboard ship used jetfuel, there is no deisel fuel onboard. We had to make sure our machine (A self propeled deck surface scrubber.) would run on it, as there would be nothing else available. For all of our tests, both at the factory, and on the ship, we ran the vehicle on jet fuel. (and it ran just fine.)

While on board, I noticed the conspicuous lack of trash recepticals. I asked a Petty Officer about that. He told me that if they have them out, they get filled, and then they have to deal with it. That makes life difficult when you're at sea. That's why I noticed that whilst various crewmember came onboard, they carried no "bags of stuff." The "stuff" they did have was already out of any boxes or bags. Boomboxes, books and whatnot were all carried onboard without any packaging.

 

[johnbr]

On why the db-604 got the axe the designer of it said the RLM said it cost a little more to make then the JU-222.So it and the BMW 802 got killed.

 

[wuzak]

On why the db-604 got the axe the designer of it said the RLM said it cost a little more to make then the JU-222.So it and the BMW 802 got killed.

Kind ainteresting if it got axed for costing too much - considering it (probably) worked and the one it was axed for didn't!

 

[krieghund]

Not necessarily true. Me-109s had a radiator in each wing. They could be individually isolated in case of battle damage. As long as one wing remains undamaged you can limp home.

Annular radiators are another way to mitigate damage to liquid cooling systems. Hitting an annular radiator means hitting the prop and/or engine. Even without a radiator leak such damage is likely to seize the engine.

It appears to be difficult to isolate one radiator from the other. Another thought is you would be overtaking the plumbing and what would the chance be that a valve failure under normal operating conditions would knock you out as well? From automotive experience why does the bloody thermostat always fail in the closed position? (Ok the car companies need to sell parts)

Another issue with the radiator system is that it is pressurized which helps to evacuate the contents all the faster.

 

[krieghund]

(Of the early war planes you may compare Bf109E and A6M3, both with about the same power and same speed. ) The 109 had about 17% more power and 20mph more speed than the zero, so hard to draw conclusions from that.

Actually the better comparison would be the Bf109E (DB601A) to the A6M5 (Sakae 21). Both had similar speeds at critical altitude and horsepower (DB601A = 1005 HP) (Sakae21 = 985 HP)

 

[fastmongrel]

From automotive experience why does the bloody thermostat always fail in the closed position? (Ok the car companies need to sell parts)

From my experience Thermostats fail more often in the open position. We often get people coming into the garage at the start of the cold weather saying my heating isnt working, a classic sign of an open thermo. I always ask if they noticed the temp gauge wasnt going up to normal running temp and most people dont even know where the temp needle should be on the dial. Waste of time fitting temp gauges to cars which is probably why some new cars just have a warning light. Even then lots of people keep on driving with the dash flashing like a Xmas tree. Then when the ECU decides its time to shut down the engine and the car gets towed to the garage the owner starts ranting about modern cars having too many electronics despite the fact if it was an older car they would have kept on driving till the engine seized.

 

[CobberKane]

From my experience Thermostats fail more often in the open position. We often get people coming into the garage at the start of the cold weather saying my heating isnt working, a classic sign of an open thermo. I always ask if they noticed the temp gauge wasnt going up to normal running temp and most people dont even know where the temp needle should be on the dial. Waste of time fitting temp gauges to cars which is probably why some new cars just have a warning light. Even then lots of people keep on driving with the dash flashing like a Xmas tree. Then when the ECU decides its time to shut down the engine and the car gets towed to the garage the owner starts ranting about modern cars having too many electronics despite the fact if it was an older car they would have kept on driving till the engine seized.

I'm all for old Cars. I absolutely gaurantee that no pre 1998 Porsche 911 EVER had problems with thermostats radiators or any of that stuff

 

[CobberKane]

Great diagram Krieghung. Illustrates both the vulnerability of a liquid cooled engine and the way in which it allows the designer to pruduce a sleek, low drag profile

 

[krieghund]

Great diagram Krieghung. Illustrates both the vulnerability of a liquid cooled engine and the way in which it allows the designer to pruduce a sleek, low drag profile

I was trying to find a drawing of a late 'G' model but no use.

Anyway all my thermostat problems were seized in the closed position (Two in Fords, One in a Chevy, One in a Mercedes straight 6 and one in a Toyota land cruiser)

 

[Timppa]

Corect me if I'm wrong, but the LaGG 3 had susbtantially less power than the La 7.

And there were some liquid cooled installations that weren't well designed.

When using leading edge radiators the Tempest I was some 20mph faster than the Tempest II with less power. And a Tempest V with an annular radiator was also about 20mph faster than the Tempest II. One Fury prototype was also fitted with the Sabre with annular radiator, an was about 25mph faster than the Centaurus version. In both those examples the Sabre had a bit more power (about the same difference between Tempest II and standard Tempest V).

Ratio of speed is roughly the ratio of the cubic root of the power, so you can make estimates.

Do you have power and speed curves for Tempest I and this Fury prototype ?

 

[stona]

a sleek, low drag profile

Not words I'd normally assocciate with the Bf 109. I guess the F was about as good as it got

Cheers

Steve

 

[wuzak]

Ratio of speed is roughly the ratio of the cubic root of the power, so you can make estimates.

Do you have power and speed curves for Tempest I and this Fury prototype ?

No, I do not.

Just maximum speeds. I believe they are bboth on Wiki.

LA610 was eventually fitted with a Napier Sabre VII, which was capable of developing 3,400-4,000 hp (2,535-2,983 kW). As a result it became the fastest piston-engined Hawker aircraft, reaching a speed of around 485 mph (780 km/h)

Hawker Sea Fury - Wikipedia, the free encyclopedia

Elimination of the "chin" radiator did much to improve performance and the Tempest Mark I was the fastest aircraft Hawker had built to that time, attaining a speed of 466 mph (750 km/h).

Hawker Tempest - Wikipedia, the free encyclopedia

I have the Tempest Mk I's speed referenced in on eof my books, power said to be 2400hp from the Sabre IV. Lumsden has the Sabre IV with 2240hp normal power. Can't recall the altitude, but I think it was below 10,000ft. Max in FS gear was 1960hp normal at around 15,000ft, IIRC. Will check when I get home.

Also have a Flight Global document detailing Napier's work on the annular radiator. From memory after a quick glance this morning, the figures were:

Typical radial installation (presumably the Centaurus in the Tempest): 11.5
Standard Chin Radiator: 11.7
Leading Edge Radiator: 11.0
Annular radiator: 6.3

The lower the better. Can't recall what units were being used.

In the breakdown the Leading edge and chin radiator figures included the oil cooler drag, since it was incorporated with the coolant radiator, and extra was added for the radial install. None was added for the annualr installation, but I believe the oil radiator was in the same position as on the radial.

I think it would be fair to say that during the war more effort was expended trying to reduce drag for air-cooled engine than for liquid cooled engines.

 

[wuzak]

napier sabre | tempest | 1948 | 1659 | Flight Archive

And here is the Flight article with the drag table: tempest | 1946 | 1441 | Flight Archive (Not the first page of the article.)