There are performance pros / cons that roughly balance out. Perhaps surprising, performance is less influenced by blade count. If you blindfold an experienced pilot and take them for a ride, they can probably tell you if the main rotor has two versus four blades. Helicopter cost and vibration are particularly sensitive to the number of main rotor blades. For more detail on this see our article on momentum theory which derives the relevant equations. Hence a larger rotor, accelerating a larger volume of air by a smaller amount, is more efficient. The \(v^2\) term means that, when the same momentum is added to the air, more energy is required to do this with a smaller mass and more acceleration. The energy imparted to this air is proportional to the mass and the square of the velocity: \( mv^2\). This generates larger downwash velocities, albeit in a smaller area. To lift the same weight, a smaller rotor must impart more acceleration to a smaller volume (mass) of air. According to Newton, this thrust equals the product of the mass and acceleration of the downwash. Rotors create thrust by throwing air downward, producing downwash. Why? In simple terms, a larger rotor doesn’t have to work on the air as hard as a smaller one. Larger rotors can lift more weight per unit power / fuel. Larger main rotors are more efficient than smaller ones. The V-22 has not been used on several missions for this reason. This can prevent landing in certain areas due to risk of damaging structures and vegetation. When carrying the same weight, smaller rotors have larger downwash. Downwash can constrain main rotor size from the other end. see this video of a V-22), but with increased complexity and cost. Of course, this constraint may be avoided by designing folding rotor (e.g. For example, military helicopters may be required to fit in an elevator on an aircraft carrier. ![]() The size of the main rotor may be constrained so that it can fit in certain spaces. We focus on performance here, but briefly discuss some simple, practical matters first. The size of the main rotor has a widespread impact on design, affecting almost all criteria. In this article, we cover some of the basics of helicopter main rotor design. Examples include minimizing cost, maximizing performance, and keeping vibration and noise below thresholds. Main rotor design balances many contradictory goals and constraints. The main rotor is the most important and complex component of a helicopter. SpinningWing > Helicopters > Helicopter Rotors > Helicopter Main Rotor Design
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