enginehistory.org

[jjuutinen]

I definitely agree with David Birch (as per the latest TM) on the superiority of liquid cooling over air cooling in aircraft engines, especially in fighters. In his reply Kim McCutcheon mentioned installed weight and weight of fuel for a required mission. Well, I did compare some US installations and for all practical purposes the installation weight per hp is the same. Therefore the score is 1-1. So, the decisive factors are drag and the weight of fuel (see below). Here the liquid cooled engine wins an overwhelming victory. Therefore the final score is 3-1 to liquid cooling.

Let´s take the USN mission profile as example. It includes 5 minutes at WEP plus 15 minutes at non combat emergency (i.e. at TO equivalent for most aircraft). All in all, 20 minutes at high power. Now, let´s compare the Merln and the R-2800. At high power setting the sfc of the R-2800 is 20-40% higher than that of the Merlin. This means that e.g. for the USN mission profile an air cooled engine of similar power requires considerably more fuel than the liquid cooled engines. This means bigger and heavier airframe, and even more power to compensate for that. And thus we have a most vicious circle.

Let´s assume another fighter profile. This time a 1944 Defence of the Reich mission (Luftwaffe). This mission calls throughout operation at 70% plus power.In these conditions liquid cooled engine thrive. Now I hear that the FW 190A had an air cooled engine. True, but fortunately the BMW had much lower sfc than its Allied counterparts. BUT, it still had some 15% higher sfc at high powers than DB and Junkers liquid cooled engines.

[jrussell]

As far as civil aircraft, I will agree unequivocaly. As far as combat aircraft, the situation is not so clearly defined. The P-38 had double the loss rate of a P-47. The P-51 had a loss rate which was 50% higher than a P-47. One of my friends lost an engine due to cooling system damage on his P-38, when he got back he discovered one hole ( about a 1/4 of an inch wide ) was the cause.If this would have been the P-51 he flew later, he would have been a guest of the Luftwaffe for the next 2-1/2 years. He was later shot down in a P-38, and the cause of aircraft loss was engine failure due to cooling system damage.Most of his fellow pilots felt the cooling system on both P-38, and P-51's were by far the most vunerable part of the plane.A designer has to balance system fragility versus the performance advantage it gives. Even in civilian usage, liquid cooling is far from being a guarantee of superior performance. The Pond racer comes to mind immediately. Many race car programs have had severe and continuing cooling system problems which curtailed performance, and could not be solved in a timely manner.
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It runs best just before it blows!

[jjuutinen]

Well, be honest: e.g. how many air cooled Formula 1 engines have been produced during the last 20 years?

As for vulnerability, what is your source on the loss rates? Second, the P-51 cooling system is perhaps the most poorly designed cooling system reharding vulnerability. Think about the Il-2 attack plane. It had liquid cooling, yet the cooling system was never blamed for the loss rate. Granted, the loss rate was, but its mission profile was such that P-47s would have been simply obliterated on similar missions. Germans recommended to aim at the oil cooler located under the nose. Finns recommended aiming at the wing fuselage junction that apparently had a weak spot.

Many Bf 109s had the possibility of isolating the damaged radiator, i.e. if the left radiator was punctured, that could be isolated by pulling an emergency handle in the cockpit. Thus you did have ability to continue flight for awhile at lower power setting, a sort of limping home facility.

Finally, in comparing P-47 and P-51 vulnerability rates, I would also pay plenty of attention to their general construction. After all, all that plumbing for the turbo offers substantial additional protection for the P-47 engine.

[gryan]

I read that the Bugatti Veyron project got into trouble at one stage due to heat exchange problems (the w-16 engine sends 1030 bhp to the car's four wheel drive system for a top speed of 253 mph and 0-60mph is 2.9 seconds, with 0-100mph in 6 seconds). A redesign of the car was necessary.

The engine is water cooled. The designer mentioned he needed to shed one megawatt of waste heat from the engine compartment. The trouble was this is a small car, yet despite the need for all sorts of ducts and ventilation for heat exchangers of one sort or another, it needed to be slick enough to make its targetted top speed and remain stable while doing it. It appears he and his development team were successful. The car beats the 250 mph goal.

I wonder how this task would have been dealt with had the engine been air cooled?

On another subject. What was the trouble for the Pond Racer in relation to cooling?

[jrussell]

The Pond racer was designed for lowest possible drag, cooling being aparently designed for racing speeds, with too little capability at low speeds. At startup, they had to immediatly taxi a short distance to takeoff, after of course preheating the engine oil. On landing, they had a truck with a blower and hose with which to blow cooling air into the ducting, run up to the aircraft as soon as possible, as extended taxing ( which the other racers would tolerate) would overheat the engine. As I remember the aircraft was lost due to engine overheating and consequent failure. This being from magazine reports at the time, so if any of the details are incorrect, I apoligize.
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It runs best just before it blows!

[hfriedman]

jrussell writes "As far as civil aircraft, I will agree unequivocaly" that liquid cooled engines were superior. Once good radial engines became available (1925-27 in the US and considerably earlier in Britian) just about eveery successful airlliner was powered with air cooled engines.

[gryan]

That's really hairy. Having only enough cooling for race speeds is asking for trouble. The assumption that you'll always fly fast is a risky one to make, even for a racer. I guess the margin between safe flight and disaster for the Pond Racer was too thin after all. Perhaps they should have had an on board blower with extra ducts and hatches that could come into play whenever things got slow and hot.

Has there been any attempt to build a revised Pond Racer?

BTW, how fast did it go? Was it competitive?

[jrussell]

To hfriedman - To clarify my viewpoint, I believe liquid would be superior in an application where you are biasing design towards very high specific power levels, and fairly low sfc's. In a commercial application like an airliner, where total cost of operation was the dominant parameter, I think history clearly shows that the air cooled radial was very hard to beat.

To gryan - If I remember correctly, the speeds were marginally competive, even though they were going through the usual teething problems. As soon as I heard they were using turbocharged Nissan engines from a sports/racing prototype, putting out a claimed 1000hp, I was very concerned about the reliability of this engine in an aircraft application. I had experience modifying a production based engine to roughly the same multiple of design hp ( roughly 5x design ) and had constant failure of components one would usually not think of as "lifed" components. Things like valve cover bolts, valve covers, intake manifolds, and magneto's. The sad thing is, during this period, the Pond racer could have used the Ilmor designed Chevy Indy car engines that were available for a reasonable price ( for that type of engine) and I think would have been much more successful, being designed to put out about the same hp, with a use environment much closer to that of aircraft racing. Also being designed from the start with minimal packaging volume. A real race engine.
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It runs best just before it blows!

[gryan]

J Russell

What was the engine you were wroking on? Could you discuss some of the details? It sounds very interesting (and challenging).


Regards

G Ryan

[jrussell]

The engines we were using was a 484ci late model Hemi supercharged to about 30psi manifold burning nitromethane. This was in a AA/FC ( funny car) drag racer. If you ever want to see the ultimate way to abuse engine components, this is the way. Nitromethane is probably the worst possible fuel to use in an engine, but it's one sterling advantage is that it is about 26% Oxygen by weight. Because of it's poor combustion qualities we would run ignition timing at roughly 70 degrees btc! Due to the fact we ran NO cooling system ( we would fill the block and heads with water before the run ), at the end of the run the intake manifold temperature was about 300 degrees F. This with a fuel with an estimated octane of about 80. Do you think we had a problem with detonation? . But, it was the most exciting engine I have ever been around to run, by far. These things literally make the ground shake when you are standing next to them.
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It runs best just before it blows!

[raustin]

Interesting thread but how does that explain that it was a P-47 that was officially recorded as being the first piston engined aircraft to fly level at over 500 mph. I would have thought a P-51 or the MB5 or Spiteful would have easily done that, but no it was a great big P-47 that did it.

[gryan]

Drag racing is deceptive. On casual examination it looks easy. It isn't.

To really go fast and to win consistently takes a lot of thought, analysis, dedication and discipline. It's an excellent motorsport code to exercise the engine builder/developer. It was one of the last of the track based motorsports to have a formula libre element to the rules. I enjoyed the innovation (much of it home grown and often extremely cunning) that this code produced. Well worth the involvement.

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jrussell,

Can't say I have much so much experience with nitro engines. They sure are impressive though and I appreciate the engineering challenge.

The drag racing team I associated with employed a 602 cid naturally aspirated big block Pontiac (really a BB Chev with Big Chief heads). It ran high octane gasoline and plenty of nitrous oxide. We called that stuff the "chemical super charger." It won more often then not, even against supercharged competitors. I still think the nitrous option is a valid way to go.

I grew up tinkering around some interesting engines. Jaguar V-12 is a favourite of mine. American iron is excellent, as parts are cheap and power output impressive for a modest amount of work. Most useful in fast marine applications as well.

Too much fun!

BTW a friend of mine is building an two place aircraft out of carbon fibre. He is building his own prop (carbon fibre) and CSU. The aircraft is pressurised. It has a reduction box (carbon fibre case with Hewland gears) and is powered by a normalised Chevrolet small block V-8 (latest design- all aluminium). Speeds in the 400 mph region are expected.

He has been crew chief for various Indianapolis race car teams and still works for one so his choice of power unit and materials is not so surprising from that perspective. What surprises most people is the standard of the engineering, quality of fabrication and the design. This is a serious project which I expect will attain the high goals he set. If you are interested I can post some information about it.

The Chev Indy engine to which you refer was designed by Mario Illien of Ilmor in the UK. It started out looking remarkably like a turbocharged Cosworth HB, which isn't so surprising as the founders of Ilmor were both ex-Cosworth employees. It was a fragile engine at first but was developed into a formidable and reliable unit. The cylinder bank angle was around 75 degrees, not the usual 90 and it had a single plane crankshaft. The first design was the most radical (cam drives at rear of engine etc.), later series were more conservative (rules, contractual obligations, servicing etc.).

Returning to drag racing; I had thought about running a big diesel. that could be an interesting challenge and it wouldn't break the bank either (I hope).

Regards

Gerald

[jrussell]

I always thought that nitrous oxide was the way to go for drag racing. After doing some investigation, I found that nitrous could be held in solution in alchohol at a stoichiometric ratio. Think about that - an engine that needs no outside air! Unfortunatly, I couldn't talk anyone into funding that project. That was what I found to be so frustrating about the Nitro guys, no desire to think about using some basic engineering principles to change the way things were done. The parts that I was using to build engines in the late 70's, were all on the market in the mid 60's. No change in basic technology in that length of time. Compare that to what the Nascar guy's were doing at the same time. As far as drag racing being a whole lot harder than it looks, yes it is. An ex- drag racer named Jack Roush seems to be doing pretty good in Nascar these days.
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It runs best just before it blows!