A synchropter (also known as an intermesher) is any helicopter rotor configuration with an intermeshing, side-by-side, two-rotor design; and no tail rotor. The two rotors mesh like gears because their axes are inclined away from each other in a bevel gear fashion. If the two rotors are to mesh so as not to hit each other, they must be synchronized and turning in opposite directions. Having a bevel gear connecting both rotors not only keeps the two rotors perfectly meshed but it also allows one motor to power both rotors simply and effectively, compared to coaxial rotor, or tandem rotor configurations.
The First Synchropter: Flettner FL-282
The first person to build a successful synchopter was Anton Flettner, a German inventor. Flettner, experienced with autogyros and helicopters of various other designs, designed the popular FL-282. The FL-282 was created in 1940, when Hitler's Kriegsmarine requested a naval helicopter. By 1942, 20 prototypes were in service. Although the prototypes saw military service, the 1000 more that were ordered were never finished because of allied bombing of BMW and Flettner factories. The FL-282's BMW engine of only 160 hp could lift around 2500 pounds (1134 kg). The FL-282 had an amazing lifting efficiency of 16 pounds per horsepower. Popular Rotorcraft's November/December 1968 issue says modern helicopters still haven't attained the lifting efficiency of the first-ever synchropter, the FL-282 (modern meaning pre-1969 in this context).
A Modern Machine: Kaman K-Max
The synchropter freshest in the minds of helicopter enthusiasts is the Kaman K-Max, used extensively for logging, fire fighting, construction, reforestation and oil/mineral exploration. The K-Max is widely respected for it's amazing lift efficiency; it can lift 5000 pounds (2268 kg) at altitudes up to 8000 feet (2446 meters) and 6000 pounds (2772 kg) at lesser altitudes. I saw a show on the Discovery Wings Channel on the K-Max, and it's simply jaw-dropping to watch this tiny helicopter lifting massive tree trunks like they're rag dolls. With a lift efficiency of a paltry 4 4/9 pounds per horsepower in comparison to the FL-282, it is still one of the world's most efficient helicopters when it comes to lifting massive objects.
The reason synchropters have such great lifting efficiency is the two counter-rotating blades means the helicopter doesn't need a tail-rotor to counteract the torque that normal single-rotor helicopters have to deal with. No tail-rotor means all the power produced by the helicopter's engine can be fed to the main rotors, and not wasted on the tail-rotor. Since the tail-rotor on a conventional helicopter doesn't contribute to either lift or acceleration, but only to counter-acting torque, there is an advantage in not needing one. I wondered why it was the two blades per rotor design for synchropters seemed universal (if you know of a synchropter that breaks from this generalization, please correct me). I tried to imagine each rotor with three blades apiece and couldn't imagine any reason why it wouldn't work. Imagining a three- or four-rotor synchropter design was too much for me to fathom with my limited knowledge of bevel gears and aeronautics in general (would the blades have enough room so to ensure they would not come in contact with each other, would it be less efficient, etc.). It frustrated me that no one had thought to try something different when it came to synchropter designs until I begun to look at regular helicopter design. I realized most 'normal' helicopters are designed to have two to four blades. Just as there are not many conventional choppers with more than four blades, there are no variations on the two-blade per rotor synchropter design (to the best of my knowledge). It has come to my attention that this is most likely based on diminishing returns as added blades means increased mass of the rotor, thus more strength required in the rotor, and increased turbulence from the blades passing through the wake of the blades in front of it1.
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