What is the ratio of the total rotor blade area to the total rotor disk area called?

• A. Solidity Ratio
• B. Aspect Ratio
• C. Lift-to-Drag Ratio
• D. Blade Ratio

Answer: (A)

Solidity describes how much blade area is packed into the rotor’s swept disk. It’s the ratio of the total blade area to the rotor disk area, a dimensionless quantity that helps predict lift and efficiency at a given rotor speed.

If the rotor has Nb blades each with chord c and the rotor radius is R, the total blade area is Nb × c × R (for a simple rectangular blade). The rotor disk area is πR². So solidity is sigma = (Nb × c × R) / (πR²) = (Nb × c) / (πR). This shows that for a given blade count and chord, bigger disks (larger R) reduce solidity, while wider blades or more blades increase it.

In practice, higher solidity means more thrust potential at the same RPM but more drag and potentially lower forward-flight efficiency; lower solidity reduces thrust capability but can improve efficiency.