What it says on the tin. Is the DC-6 supercharged? I thought all planes that fly above around 10,000-15,000 feet need forced induction, but wikipedia doesn't list the DC-6s engines as supercharged or Turbocharged. There was a variant for the P-47 which was "Turbosupercharged"(???? Is that like a Turbocharger and Supercharger in sequence, so air from teh turbo is rammed through the Supercharger?), but not the DC-6 variant. What gives?
Yes, the DC-6 is supercharged with mechanically driven centrifugal superchargers. A “turbosupercharger” is another name for “turbocharger” which is an exhaust driven centrifugal supercharger.
The DC-6 uses Pratt & Whitney R-2800 engines. The R-2800 has a mechanically driven centrifugal supercharger at the back of the engine. This is common for most WWII and later radial engines.
Some other WWII aircraft (B-17, B-24, B-29, P-47, etc.) also added an exhaust driven turbocharger (also called turbo supercharger) in addition to the centrifugal supercharger for additional boost and high altitude performance.
Wikipedia photo Pratt & Whitney R-2800 Double Wasp
From the Wikipedia entry on "turbocharger" (bolding added)1:
The current categorisation is that a turbocharger is powered by the kinetic energy of the exhaust gases, whereas a supercharger is mechanically powered (usually by a belt from the engine's crankshaft). However, up until the mid-20th century, a turbocharger was called a "turbosupercharger" and was considered a type of supercharger.
In actual fact, on an aviation engine, a supercharger that did not involve an exhaust gas turbine would almost always be gear-driven, not belt-driven.
Also, it appears that the usage of the term "supercharger" to refer to the larger category of all forced induction systems, whether gear-driven or driven by an exhaust gas turbine, has lingered longer in the aviation world than in the automotive world. Thus you still may encounter the terms "gear-driven supercharger" and "turbosupercharger". The latter term is synonymous with "turbocharger".
In the aviation context, even when consulting modern sources, you should generally not assume that the word "supercharger" always implies a device that is driven by gearing to the crankshaft rather than by a turbine in the engine exhaust gas flow. It may in fact refer to a turbosupercharger, i.e. a turbocharger.2
Some aircraft with turbosuperchargers (i.e. turbochargers) also have gear-driven superchargers. This was true of nearly all, or all, of the World-War 2-era combat aircraft with turbosuperchargers, such as all variants of the P-47 Thunderbolt.3 But the usage of the word "turbosupercharger" is not meant to specifically signify that a gear-driven supercharger is also present.4
In the diagram below of a World-War-2-era supercharger installation, the "internal blower" (item 3) is the gear-driven supercharger.
Another answer has already addressed the specifics of the (gear-driven) supercharging system used on the DC-6, so I'll end here.
This terminology is discussed in more detail in answers to this related ASE question: In the modern aviation context, is the word "supercharger" generally assumed to mean a gear-driven system rather than an exhaust-driven system?
See the related ASE question Did any WWII aircraft only use a turbocharger but not a supercharger? . Also, for more insight on the advantages of combining turbosuperchargers (turbochargers) with gear-driven superchargers in high-performance World-War-2 era aircraft engines, see this 1943 document from General Electric: "The Turbosupercharger and the Airplane Power Plant". And here's a "Quora" post that also gives some insight into the topic, rather concisely.
In other words, the word "turbosupercharger" should not be considered to be a "portmanteau" of "turbocharger" and "supercharger", signifying that both features are present; rather it is a simply the combination of "turbo" and "supercharger", describing the type of supercharging that is present.
Just to add a little more confusion (and amazing engineering)... Some piston engines were 'turbo-compounded'.
This is when they used a turbine in the exhaust gas to add shaft power back to the engine crankshaft. These engines also had a shaft-driven supercharger.
The TC18 variants of the Wright 3350 were the most widely produced and used turbo compound engines ever produced.
By the way, the centrifugal blower at the back of a radial engine served two purposes: it stirred up the fuel and air mixture to produce a uniform blend which was then presented to each intake runner tube in the engine, and it provided pressure boost to the runners as well.
The boost function could be tailored for the specific application by swapping out the drive gears; a higher gear ratio spun the impeller faster and furnished more boost, and better high-altitude performance.
Although not a radial, the Rolls-Royce Merlin engine had a mechanical boost with a two-speed gear drive, selectable by the pilot: low boost for lower altitude use, and high boost for high altitude use. This gave it superior high-altitude performance.