I am wondering this because of the blades on the propellers on both of the planes. The Pilatus PC-12 can travel up to 360mph with its 4 blades on the propeller. The King Air C90 has 2 engines with 4 blades each, is around the same size and can travel at a slower speed of just 300mph. Why is this when the King Air has 2 engines instead of 1 with 4 blades and travels slower, and the Pilatus only needs one engine with 4 blades to go 360mph?
The speed of an aircraft is not only dependent on the number of propellers. It is a function of (among other things),
- Excess power available- The Pilatus PC-12 has more powerful engines compared to the King Air C90. The PW PT6 engine used in the Pilatus produces 895 kW, while (a different variant of) the same engine used in King Air produces 410 kW each. The power available for the PC-12 is more.
- Propeller efficiency- The PC-12 uses a different propeller compared to the C90, which may have more efficiency at high speeds. Also, the PC-12 propeller diameter is more compared to the C90.
- Aircraft design- The PC-12 flew around 30 years after the C90. The aircraft design has improved in many ways during the intervening years in aerodynamics, materials etc. These improvements can be used to increase the cruise speed.
- Power to weight ratio- The PC-12 has a better power-to-weight ratio compared to the C90, which translates into better speed.
It is difficult to pinpoint any single reason for increased speed. However, the increased (cruise) speed in PC-12 can be explained using a combination of the above reasons.
It's not the number of engines, but the power of them. A Boeing 777 has two engines, but can fly faster than a B707 with four engines. Similar to a four wheel drive vehicle not necessarily being faster than a 2-wheel drive - the contrary actually, since there are more drive train losses. More engines in an aeroplane also means more losses: engine nacelles, higher vertical tail to enable flying with one engine etc.
How the power of the engine is transferred to propulsive thrust is a matter of aerodynamics, which also puts a limit on the maximum speed of propellor planes. If the propellor tip approaches or breaks the speed of sound, the shock wave resistance consumes a lot of power which is then not available for generating thrust. No matter how many engines the propellor aeroplane has, it will always run into this speed limitation. Which can be improved by propellor technology, not number of engines, as visible on this photo of the Airbus A-400.