Prop Diameter

Reference to Diagram 14, Prop Diameter V. BHP, it can be seen what increase in diameter is required on a two-blade prop if the brake horsepower in increased. If the increase in diameter is unacceptable for any reason, then the next choice is to use a prop with three blades. The fictitious aircraft example with a 200 BHP engine, a two-blade prop of 74 inches could be used. If the 200 BHP engine was replaced with a 250 BHP engine, it would require a prop of 77 inches. But, if this diameter is too large the next choice is a three-blade prop of 74.5 inches . [Note this diagram is not a true one but hand drawn to illustrate the point being made].

The prop diameter on a single-engine aircraft may be limited by ground clearance or, on a multi-engine aircraft, clearance between the prop tips and the aircraft’s structure and the ground, which must be taken into consideration. Propeller tip clearance must comply with the certification regulations; for example, the American FAA regulations require a minimum clearance between the tips and the ground of seven inches (17.78 cm) for nose wheel aircraft and nine inches (22.86 cm) for tail-wheel aircraft