FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsHard
A main rotor system has two blades, each with a chord of 1.1 feet, mounted on a rotor with a 17.5-foot radius. Using the formula solidity ratio = (number of blades × blade chord) / (π × rotor radius), what is the approximate rotor solidity ratio?
- A0.10
- B0.04
- C0.08
- D0.02
Show answer & explanationAnswer & explanation
Correct answer: B. 0.04
Solidity ratio = (2 × 1.1) / (π × 17.5) = 2.2 / 54.98 ≈ 0.04. This low solidity value is typical for a two-bladed helicopter, indicating the blades cover only about 4% of the rotor disk area.
Why the other options are wrong
- A. 0.10 is too high for a two-bladed rotor with this chord and radius.
- C. 0.08 would result from doubling the chord or blade count, not matching given values.
- D. 0.02 is too low; recalculating (2×1.1)/(π×17.5) gives approximately 0.04.
Rotor Solidity Ratio
The ratio of total blade area to the total rotor disk area, calculated as (number of blades × blade chord) divided by (π × rotor radius).
- Higher solidity means more blade area relative to disk area
- Two-bladed rotors typically have low solidity (~0.04-0.06)
- Affects rotor efficiency and power required characteristics
Memory trick: Solidity = how 'solid' (blade-filled) the disk really is