Are Liquid Cooled Aircraft Engines Possible?

[Yukon]

Does anybody remember the nuclear industry saying that their generating stations would make electric power that was going to be too cheap to meter. (Free, with a service charge).

Does anybody remember that the Space Shuttle was to go into orbit every week like a space airliner?

Remember how the prop fan was going to be the next high-efficiency airline engine?

How about how the Wankel rotary was going to make pistons obsolete?

What about satellite relayed telephone calls. Wave of the future. Remember that annoying time delay?

My point is, technology doesn't always work the way we think it will. Inspite of 100 years of developement, liquid-cooled aircraft engines have yet to reach prime time.

You say, what about the P-51! Since they got shot down or crashed on a very regular basis, I'll bet nobody has any real idea of long-term reliability issues with these engines. They were also flown by young, intensively trained men wearing parachutes. When they failed, you would step over the side, and a new one was waiting for you when you got back to base. Merlins and Allisons also had 12 cylinders, so power pulses came much more often,
making power-train vibrations less of an issue. To my knowledge, no liquid cooled aircraft engine has ever been in US commercial service.

With the exception of the 100 hp Rotax 912, which employs a clutch and 2.43gearing, liquid-cooled, reduction-geared aircraft motors seem to be hard to perfect. They too have drive problems. Take a look at the reduction components and you decide it you would want to fly behind one. Looks like Swiss watch components.
http://www.rotax-aircraft-engines.co...kus/d01856.pdf

I suppose only time will tell if the aircraft PSRU will go the way of the dirigible. Maybe in another 100 years we will have that flying car they have been promising us!

[kcameron]

Liquid cooled aircraft engines are impossible. None have ever been built; let alone flown.

[TSwezey]

Originally Posted by kcameron

Liquid cooled aircraft engines are impossible. None have ever been built; let alone flown.

What? Did you not read the first post or hopefully you are kidding?

[SvingenB]

Originally Posted by kcameron

Liquid cooled aircraft engines are impossible. None have ever been built; let alone flown.

Actually one did fly. It came 300 feets into the air and self detonated in a huge explosion.

[Steve Mills]

Check your history...many of the first aircraft engines were liquid cooled. They switched to air cooled because the Germans kept shooting holes in the radiators.

[gmcjetpilot]

Sure liquid cooling is possible but is it efficient or the right thing to do:

1 - If the engine the right one
2 - If the the airframe is the right one
3 - If the heat exchanger'(s) is/are specifically designed & installed for the application (see item 1 & 2)


The issue is tractor engines on existing airframes are typically designed or optimized for air-cooled engines. Tractor airplanes like the RV have a big challenge fitting a heat exchanger in for good cooling & low drag. Yes it will fly but you either pay the price of drag and less efficiency or less speed. Since water cooled engines are not a quantum leap higher in efficiency, than the "radiators" are a drag, figuratively and literally. Fuel econ in cars comes from electronics that make a car's "mission" more efficient.

Pusher like a Cozy, LongEZE, Velocity have more room to duct & route radiator cooling air. Not saying it can't be done in a RV, just ad hoc installations are not ideal. Ross has the right idea with his RV-10. Make a super slick cowl with no inlets and move the radiator to the belly, aka P-51. Now to be fair the P-51 had bullets and cannons shot at it, which the RV-10 should not (unless he cuts me off in the pattern!).

The engine and airframe must match. Put the Pratt and Whitney R-2000-18W; 2,325 hp on the P-51 and the Packard/Merlin V-1650-7, 1,790 hp in the F4U Corsair, both airplanes would go from awesome to suck. The airframe needs to be made around the engine. Rutan's around the world Voyager had two engines, one of which was a liquid-cooled, a Teledyne Continental IOL-200. It was the aft engine and basically ran the whole time. The forward Teledyne Continental 0-240 was used as needed for 70 hours of the 9 day flight. This seems to be a good marriage of engine and airframe.

WITH SO MUCH AIR available to an aircraft engine, which never STOPS, air cooling makes a LOT OF SENSE. Plus there is less STUFF (to fail) and weight. There are no hoses, radiators, pumps or belts.

AIR + FIN's = HEAT EXCHANGER (with no liquid middle man).

The comments made about water cooling being "Modern" is funny, since the Wright Bros used a water cooled engine. Actually air-cooled is more advance in many ways and harder to do. The US War Dept and NACA spend Billions in today's money to develop and perfect air-cooled engines. The best of the best figured out a R2800 was pretty reliable and powerful. B-17's had their upper jugs blown off by gun or cannon fire, but the engine kept running, while the connecting rod flopped around all the way back to base.

Water cooling use for cars is two fold: One emission's, you can run tighter tolerances; Two noise, air cooled engines have more mechanical noise from valves clacking and so on. Frankly I'm an air-cooled Porsche 911 fan. I love the mechanical clanging. Porsche went to water cooled in 1999 for the reasons above, but its not a better car. In fact the air-cooled 911's are more popular and have higher resale. The new 911 is just another lounge lizard car to me now.

Air-cooled is a little bit of a misnomer. What people don't realize is both "air-cooled" Lyc and Porsche 911 are liquid cooled. They use oil for cooling. The Porsche as 16 qts of oils and two huge coolers under the front bumper. Same with a Lyc. If you have CHT issues and OT issues at the same time, you might need a better/bigger oil cooler or installation or both. If you are running "liquid around" like OIL why not use it to cool as well as lubricate. Air around the head of the Lyc is critical but only part of what carries the heat aways from the exhaust valves.

When it comes to ultimate capacity and better temp control, liquid cooling has it over air-cooling. You want your engine to get warm, the thermostat cuts liquid off, simple and very effective. A water cooled car engine can idle on the ground all day in traffic (well most cars) and drive all winter, while giving you heat for your feet. Not sure a Lyc can do that or you would want it to. A Lyc would suck as a car engine. Frankly if the temps are not high enough in a Lyc's combustion chamber, you don't activate lead scavenging compounds in the fuel, which can fowl the plugs. If oil temps are not hot enough, combustion byproducts and mosture does not burn off, staying in the oil and engine. Liquid cooling is king in temp control, which is important in a car. A plane operates in a limited envelope, WOT at 75% pwr most of the time.


Air-cooled aircraft engines, normally aspirated, are pretty tolerant of temp, but we all know CHT's and/or OT must be watched during climbs on HOT days. It's not near "thermal limits", but during extreme conditions you may have to watch and adjust your climb rate. In cold temps Lycs are over cooled, there's not a lot we can do, close cowl flap (if you have it), cover the oil cooler with a plate. Too cold can cause subtle problems. Turbo-charging can start to strain air-cooled aircraft engines "thermal capacity", necessitating bigger oil coolers, cowl flaps and more operational limits. It's not a big problem operationally but its a factor. It's not soccer Mom, mini-van simple, turn the key and drive with out a care in your head. On the other hand when car engines go into a plane, it no longer is turn key simple operational wise either. Many Wankel's and Subies have temp issues in-flight and need to be watched.

So what, you have to watch CHT and OT sometimes in the summer with a Lyc.

Experimental water cooled planes tend to uses little heat exchangers where they can fit, so temp is an issue, even with the water cooling. For some reason the translation from CAR to PLANE does not cross-over. You can afford a "big-Ol-radiator" on a wheeled vehicle, which travels at 65 mph but not at 200 mph; Does not mean it can't work on a plane, just that it takes things like variable air geometry (aka P-51). The design of engine, airframe, installation and mission need to match. What works on a P-51 (+400 mph) may not work on a 190 mph RV. There's some scale factor there.

I have faith that RV alternative guys will improve their water cooled bred. But it will take a nexus of right engine, right airframe and right heat exchangers. Other wise liquid cooling will just be DIFFERENT not BETTER.

[Yukon]

I should make a clarification. Maybe a reliable PSRU will prove technologically impossible, thus rendering the liquid-cooled aircraft engine undesireable. Not all engineering problems have cost-effective solutions.

[rtry9a]

Interesting question. Technically, water only transfers heat to heat exchangers- water/oil radiators are still air cooled.

Air cooling is efficient because the delta t is so high between ambient air and hot metal surfaces, but, that also assumes cylinder heads and valves stay hot, hopefully below temps where lubricants break down and detonation breaks parts.

Water cooling allows better control of localized hot spots (pistons/cylinder heads) and therefore, closer tolerances and better durability. Water cooling is probably better suited to high power/low drag applications, like the P-51 and most of the later WWII piston driven fighters.

[Mike S]

Originally Posted by Yukon

To my knowledge, no liquid cooled aircraft engine has ever been in US commercial service.

Curtis Jenny. Probably others, there were a lot of OX-5 engines sold to start up aircraft manufactures after the big war. Lincoln Paige comes to mind.

Current production aircraft--------at least recently produced, not sure if still in production--------Extra 400.

http://www.extraaircraft.com/ea400.asp

Here is a link to the newest version, with a kerosene burner on the nose.

http://www.extraaircraft.com/ea500.asp

[Ted Johns]

The aforementioned diesel is liquid cooled, and in commercial service in Diamond and Cessna aircraft. The reliability is not reputed to be very good yet. I imagine that if they stick around for a few decades it will improve.

Frankly, it's a miracle that any new aircraft engines are being designed. Thielert's was not a scratch design for aircraft, afaik, but an adaptation of a Mercedes design. Honda made a pretty prototype and then decided it was a waste of time.

I can easily imagine an engine similar to a Subaru but with a integral psru in an aviation specific design. Coupled to purpose designed radiators, in a purpose designed airframe, it would do quite well indeed. I just can't imagine any company bothering to do it, given the projected ROI.

Regarding PSRUs, if Tracy Crook can design a reliable one, with resources likely in the low 6 figures, I'd bet the farm that a multi billion dollar firm like Honda (Subaru, etc...) could design one in their sleep. And twice on Sunday.