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Most aircraft piston engines fall into two general categories:1

  • Radial engines (radials for short2) have their cylinders radiate out from the crankshaft in a star pattern; they tend to have a blunt, cylindrical nose shape, and are generally air-cooled.
  • Inline engines (inlines) have their cylinders all in a line along the crankshaft from front to back; they tend to adopt a more streamlined style of nose, and are almost always liquid-cooled.

As coolant, piping for same, and radiators tend to have positive mass[citation needed], and aircraft are typically operated in a positive gravity field[citation needed], the liquid-cooling system needed by most inlines imposes an unavoidable weight penalty; additionally, it makes these engines comparatively fragile compared to air-cooled radials of similar size, as the whole engine can easily be disabled by a leaky coolant line.3

However, even though the radiator, by its very nature, has to stick out into the airstream in order to lose heat to said ambient airstream, it does not (necessarily) impose a drag penalty; in the process of being cooled by the ambient air, it transfers thermal energy to said air, and some clever mad scientists figured out how to use this to generate thrust, essentially turning the radiator into a primitive jet engine. The thrust generated by this process (known as the Meredith effect) can easily exceed the drag produced by the radiator, turning the radiator into a net thrust producer; back when fighter aircraft still used pistons and props, this gave inline-powered fighters a performance advantage over their radial-powered counterparts, which is why most of the really successful piston fighters of World War II used liquid-cooled inline engines. (Today’s inline-powered aircraft, which generally do not have a need to outrun pursuing fighters, make do with simple non-thrust-producing radiators.)

There doesn’t seem to be any reason why a properly-designed radial couldn’t benefit from the Meredith effect as well; indeed, it should work somewhat better with an air-cooled radial, since the thrust-producing air is being heated by the cylinders directly, cutting out the liquid middleman and one of the two efficiency-robbing energy transfers necessary in a liquid-cooled system. Yet, despite this, and the fact that these big radials were used to power heavy aircraft that needed every bit of thrust they could possibly get, and the fact that the wide, flat, blunt nose of a radial engine makes the drag problem especially acute (and, thus, any possible way of reducing drag all the more desirable), none of the big radials ever was designed to take advantage of the Meredith effect.4

Why not?


1: In practice, so as to pack lots of cylinders into the smallest possible space, the larger engines from both groups tend to fall closer towards the middle; all but the smallest post-World-War-I radials have at least two to four rows of cylinders, stacked one in front of another, while most midsize-and-larger inlines have multiple parallel cylinder banks (again, usually from two to four).

2: No relation to the kind of radials you navigate with, except insofar as they both pertain to aviation.

3: This is one of the big reasons why all the really big World-War-II-and-later aircraft piston engines (the ones powering the big bombers of the war and the big propliners of the decade following it) were radials. Big piston engines are unreliable enough on their own; trying to keep a really big liquid-cooled inline running would be an absolute nightmare.

4: You sometimes hear that the NACA cowling design used on essentially all post-mid-1930s radials was designed to produce extra thrust from the heated cooling air. This is a misconception. Although the NACA cowling did indeed reduce (and quite dramatically, too) the drag penalty of a radial engine, it did this solely by smoothing the rough aerodynamic lines of the uncowled radial; no Meredith-effect wizardry was involved.

Note: Not a duplicate of this other question. That one asks whether radials used the Meredith effect; this one asks why they didn't.

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  • $\begingroup$ Use of "inline" is confusing, as inline ICEs are rarer in aircraft, what's common is V and Opposed -- the linked wiki article seems to have the answer: Many engineers did not understand the operating principles of the effect. A common mistake was the idea that the air-cooled radial engine would benefit most, because its fins ran hotter than the radiator of a liquid-cooled engine, with the mistake persisting even as late as 1949. Comes with a citation, are you looking for something more? $\endgroup$ – ymb1 Mar 27 at 1:08
  • $\begingroup$ @ymb1: The misconception, as it says right in your quotation, is that it would be more effective because the fins of an air-cooled radial are hotter than the radiator of a liquid-cooled inline, not that it would be more effective because of the reduced number of efficiency-draining energy-transfer steps (the justification that actually makes sense). Also, the page cited by that quote merely states the same information as is in the quote, without providing any explanation of any sort. $\endgroup$ – Sean Mar 27 at 1:12
  • $\begingroup$ I'm still confused, but ignore me -- if you can focus your question on that point, that would be great. $\endgroup$ – ymb1 Mar 27 at 1:18
  • $\begingroup$ I don't think you need a citation to back your assertion that liquid cooling components have positive mass. Most of us are smart enough to realized this intuitively... $\endgroup$ – Michael Hall Mar 27 at 4:15
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    $\begingroup$ I have to endure too much lawyer-speak already at work, so please try to write more in vernacular. $\endgroup$ – Peter Kämpf Mar 27 at 8:11
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Compared to a turbojet engine, the heating (from cylinder fins) of the working fluid (air) in an radial engine is relatively small, and so will be the possible thrust augmentation via the Meredith effect.

Better results can be had by dumping the engine exhaust into the cooling air stream exiting the engine, and directing this blend of spent engine cooling air and hot exhaust gas into a rearward-facing exhaust nozzle- as used in the Convair 440, which had radial engines.

The utility of the liquid-cooled vee engine was well-established in ground vehicle practice before migrating the design into aircraft use. The outstanding advantages in aircraft applications were 1) minimization of frontal area drag by stacking the cylinders behind each other, which was enabled by liquid cooling, and 2) superior cooling of the cylinder heads and exhaust valves by routing the coolant passages around the valve seats and combustion chambers, allowing higher power output without relying on a rich mixture to carry away engine heat, as radial engines were forced to do.

Whatever weight penalty might have come with liquid cooling was amply compensated for by profile drag minimization. If you have data on survivability differences between liquid cooled vee engines and air-cooled radials subject to aerial machine gun fire, please share it with us here.

As an aside, consider that for the purposes of a maximum-power, minimum drag engine installation, all the engine manufacturers (American, British, Italian, Russian, German) that built liquid-cooled aircraft engines in WWII converged on the same basic and robust (and very successful) design solution: 60 degree vee, 12 cylinders, liquid cooled, pushrod-actuated valves, 1700 to 2000 cubic inch displacement.

Regarding heat transfer, the efficiency of heat transfer using pumped water-based liquids far exceeds that of using air as a heat transfer medium. The most critical design issue in this realm is not heat transfer inefficiencies in the engine-to-coolant step or coolant-to radiator step but the aerodynamic design of the air intake and transport ducting for the radiator, to minimize drag across all flight regimes.

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  • $\begingroup$ Dont't forget that coolant pressurisation makes it possible to remove the same amount of heat even as air pressure drops with altitude. Of course, now the radiator has to be matched to the reduced air density. But the advantage is that supercharged, water-cooled engines can be cooled over the full altitude range whereas supercharged, air-cooled radials will develop cooling problems as altitude increases. $\endgroup$ – Peter Kämpf Mar 27 at 8:08
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1) For any thermal engine, energy efficiency is directly correlated to temperature, while power density is indirectly correlated to temperature as well. Because radiator temperature isn't very high, a jet engine using radiator heat is very inefficient and bulky.

2) For a ICE engine, majority of waste heat is carried away by the exhaust so amount of energy available at radiator is relatively small.

3) The reason why Meredith effect works better for liquid cooled engine is, it's easier to concentrate heat into a small flow, hence raising operating temperature. On a radial engine this is near impossible.

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The principle in general is known as the thermal ramjet: pass the air through a specially-shaped duct and heat it on its way through.

The Meredith effect applies specifically to intermediate circulatory systems which pump a special coolant fluid around in order to heat the air. Inline engines benefit from such systems because most cylinders cannot receive enough airflow and need a pumped coolant to collect the heat.

Radial engines have enough exposure and use the passing air directly as their coolant fluid. Their cowlings are indeed designed as thermal ramjets. If you look closely at the rear of the cowling, you can often see variable exhaust flaps similar to those on Meredith radiators.

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