A Compound Coaxial Helicopter was a helicopter that replaced the tiltrotor. Unlike, the mono tiltrotor, it looked more like a conventional helicopter. The only difference was two rotors on top and no tail rotor.
(Note: The background section of this page uses most of the same words as the mono tiltrotor aircraft page of Terra Futura to save time.)
The concept of a helicopter originated with Leonardo da Vinci in the 1480s. Along with other flying machines, the helicopter took a while to be successful. The word helicopter was originally a French word derived from Greek. Helicopters were built throughout the early 20th century. However, it was not until 1942 that Igor Sikorsky built the first successful helicopter. With the advent of the R-4, helicopters entered mass production. The R-4 was the only helicopter used in World War II. With the advent of the turboshaft engine, more and more helicopters were being produced. They were larger, faster, and had a higher performance. But there was a problem. The rotor blades caused the helicopter to torque. A tail-rotor was used to stop that. Soon, however, more advanced designs would come.
Tiltrotors had been a concept since the 1930s. However, it was not until the 1980s that tiltrotors entered commercial use. The V-22 began development in 1981. Its first flight was in 1989. Testing began. The V-22 Osprey was finally brought into service in 2007. (Hence, the Tech Level.) There was a problem though. 6 accidents since introduction were caused by a hazardous transition phase. There were two ways to solve this problem. One of them was a mono tiltrotor aircraft.
The Mono Tiltrotor Aircraft was a unique aircraft. It was a helicopter, but it did not look like one. The design was pioneered by AeroCopter, Inc. in the early 21st century. AeroCopter created a manned version named Sarus and an unmanned version named Kestrel. The most prominent feature was a 21-foot encompassing ring that surrounded the main fuselage. It looked like a Vulcan starship from Star Trek, only horizontal. The ring contained two counter-rotating rotors. At take-off and at landing, the ring was parallel to the ground. At 1,000+ feet, the ring tilted up at an angle of 87 degrees. Power would then go to the rear engines for the purpose of moving forward. The Sarus could carry eight passengers and their baggage. The Kestrel could carry 400 lb of cargo. They would soon enter commercial production. The Mono Tiltrotor Aircraft was not replacement for conventional tiltrotors. There was also the compound coaxial helicopter.
Tech Level: 10
Compound Coaxial Helicopters were the most advanced helicopters ever. They had two sets of counter-rotating blades and a horizontal propellor. The counter-rotation was there to prevent torque. No tail rotor was needed. Therefore, a horizontal propellor took its place. Sikorsky Aircraft started working on this in 2008. Their demonstration model, the X2, also fly-by-wire controls. These controls slowed the blades down to prevent them from going supersonic. The top speed was 280 mph. Also, it did not go through a hazardous transition phase. Therefore, the X2 was safer than the V-22 Osprey. The first commercial compound coaxial helicopter was the S-97 Raider in 2014. Later models could lift 40 ton at 250 knots or better. This was the holy grail of helicopters.