Definition
In helicopter rotor design, the ratio of the total blade area to the total disc area swept by the rotor. It is calculated by dividing the combined area of all rotor blades by the area of the circle the blades trace as they rotate.
Plain English
How much of the rotor's spinning circle is actually filled with blade. A rotor with more blades, or wider blades, has higher solidity than one with fewer or thinner blades.
Context Anchor
Seen in helicopter rotor and airframe discussions, especially when comparing rotor designs or blade arrangements.
Derivation
From the Latin solidus, meaning 'whole' or 'complete.' In rotor terms, it expresses how 'filled in' the rotor disc is by blade material — a higher number means a more 'solid' disc.
Why Pilots Care
Solidity directly influences lift generation, drag, and the power required to turn the rotor.
Analogy
Think of looking up at a ceiling fan. A fan with wider or more blades fills more of its spinning circle than a fan with fewer narrow blades; that is the basic idea of higher solidity.
Intuition Check
Solidity does not mean the rotor is physically hard or unbreakable. Here it means the proportion of the rotor’s swept area that is made up of blades.
Example Sentence 1
Adding a fifth blade to the main rotor increased the system's solidity and improved lift capacity.
Example Sentence 2
The design team chose a solidity value that balanced hover performance with forward flight efficiency.