A mechanic compares two propeller designs of the same diameter intended for high-horsepower engines with limited tip clearance. One design uses wider blades with greater total blade area (higher solidity). What aerodynamic advantage does this design provide?
- AIt completely eliminates the need for blade counterweights
- BIt increases solidity, allowing the propeller to absorb more engine power without increasing diameter
- CIt reduces the centrifugal twisting moment on each blade to zero
- DIt decreases the tip speed of the propeller at a given RPM
Show answer & explanationAnswer & explanation
Correct answer: B. It increases solidity, allowing the propeller to absorb more engine power without increasing diameter
Increasing blade width (and thus total blade area relative to the disk area, or solidity) allows a propeller to absorb more horsepower at a given diameter and RPM, which is important when diameter is limited by ground clearance or installation constraints. It does not eliminate twisting moments, remove the need for counterweights, or reduce tip speed, since tip speed depends on diameter and RPM, not blade width.
Why the other options are wrong
- A. Counterweights may still be needed depending on propeller design.
- C. Centrifugal twisting moment still exists regardless of blade width.
- D. Tip speed is a function of diameter and RPM, not blade width.
Propeller Solidity (Blade Area)
Solidity is the ratio of total blade area to propeller disk area; increasing blade width raises solidity, allowing the propeller to absorb more power without increasing diameter.
- Solidity = total blade area / disk area
- Wider blades increase solidity
- Higher solidity absorbs more power at same diameter
- Useful when diameter is limited by clearance
Memory trick: Fatter blades, same circle, more horsepower packed in.