Which term defines the ratio of blade area to rotor disk area in rotorcraft terminology?

• A. Solidity Ratio
• B. Pitch Ratio
• C. Aspect Ratio
• D. Diameter Ratio

Answer: (A)

Solidity describes how much blade area lies within the rotor disk. It is the ratio of the blade planform area to the rotor disk area. In practical terms, if you have N blades of chord c and radius R, the blade area is roughly N × c × R and the rotor disk area is πR², so solidity ≈ (N × c) / (πR). This parameter tells you how much of the rotor’s disk is actually covered by blade, which influences thrust capacity and aerodynamic efficiency. The other terms mentioned don’t define this area ratio—pitch relates to blade angle, aspect ratio is a wing term, and diameter ratio isn’t a standard rotorcraft term. Therefore, the term that defines the ratio of blade area to rotor disk area is solidity ratio.