A basic understanding of vertical lift is very helpful in appreciating the value of the Mono Tiltrotor design. You may already know that efficient vertical lift is achieved with a large diameter rotor that generates a relatively low velocity wake underneath the rotor. This very useful bit of knowledge will now be examined a little further.
The velocity of the rotor wake is inversely proportional to the rotor diameter. So, if you reduce the rotor diameter by half then the rotor wake will double. Clearly ground erosion becomes a detrimental factor as the rotor wake increases, but the problem is bigger than that. The amount of power required to hover is directly proportional to the cube of the rotor wake. So, by reducing the rotor diameter by half the power required increases by a factor of eight (8). The engine must be eight times larger and eight times heavier to produce eight times the amount of power. However, the problem is even bigger because the amount of fuel consumed by the engine is proportional to the amount of power produced. So, the weight of the fuel required for hover is increased by eightfold, which in combination with the eightfold increase in engine weight takes away from useful payload. In summary, reducing the rotor diameter by half increases the engine weight by eightfold and increases the weight of the fuel burned in hover by eightfold. An advocate for smaller ducted fans could argue that this increase in weight is partially offset by having lighter blades and a smaller gearbox which is true, but the dramatic increase in fuel weight consumed due to a high wake velocity can only be offset by reduced useful payload weight. A graph showing the impact of wake velocity on engine weight and fuel weight consumed is shown here. These facts are derived from basic physics and define the performance of any powered vertical lift solution.
One other useful bit of knowledge is that a conventional helicopter provides the lightest practical aerodynamic structure for producing vertical lift, but it is less than half as efficient as a typical airplane in forward flight. For a typical helicopter its structure comprises 35% to 45% of the gross lifting capacity. The Mono Tiltrotor design essentially starts with the structural efficiency of a helicopter and simply adds 10% more structure for its fixed wing resulting in airplane levels of cruise efficiency. The weight of the wing pays for itself by dramatically reducing the weight of fuel burned in airplane mode cruise. All of our work on the Mono Tiltrotor design shows that its structural weight comprises about half of the gross lifting capacity, and the remaining lift capacity is comprised of 1/3rd fuel weight and 2/3rds payload weight for a 750 to 1000 nautical mile range mission.
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