This calculator is designed to provide a rough estimate of the static thrust of a given airdrive setup. It is based on a simple conservation of energy equation, and so does not take into account details such as propeller geometry, number of blades, propeller rpm, etc. However, this can still be very useful in understanding the effect of simple changes such as using a larger diameter propeller or increasing horsepower.
The key parameter is efficiency, which takes into account all losses such as propeller induced drag, tip losses, swirl, and drag due to flow obstructions (engine, guard structure, rudders, etc). We find for typical air drive installations, efficiency varies between about 60-80%, with the higher efficiencies applicable for larger propellers (59" and up), and the lower efficiencies applicable for smaller propellers (under 36").
The "Direct Drive" checkbox eliminates an additional 5% power loss that is assumed due to losses in the belt reduction drive or gearbox.
NOTE: this calculator is applicable for STATIC THRUST only. This means an airdrive/pusher fan at rest with no wind. Thrust decreases as a function of forward speed, until at some speed the propeller begins to produce drag instead of thrust and is said to be 'windmilling.' For most boat installations (as well as hovercraft, powered chutes, some ultralights, and certainly bowfishing and flounder boats), forward speed is limited and static thrust is a reasonable estimate for thrust over a large range of operating conditions
DISCLAIMER: This tool is only to be used for rough estimation of performance and comparing different airdrive configurations. We make no claims or guarantees of its accuracy for any application.
ADDITIONAL NOTES: In some cases, increases in thrust obtained from increasing propeller diameter are larger than indicated by this calculator. This is because on larger propellers, the engine and belt reduction usually obstruct a smaller fraction of the propeller disk, resulting in cleaner airflow into the propeller. Larger propellers also move a larger quantity of air at slower speed, and lower speed air creates less drag moving past guards, rudders, support structures, etc. This is why larger propellers generally have higher efficiencies
ASSUMPTIONS This calculator is based on a simple conservation of energy equation which assumes that 100% of the mechanical energy produced by the engine is converted into kinetic energy of air flowing through the propeller in a purely axial direction. The 'Efficiency' term is a catch-all to account for the numerous losses that prevent us from achieving this peak performance in the real world. Typical losses are propeller induced loss, propeller tip losses, swirl in the propwash, and turbulence and drag generated by the engine, support structure, guards, rudders, etc.
The equation being solved in this calculator is T = (pi/2)*rho^(1/3)*P^(2/3)*D^(2/3), where T = static thrust, rho = density of air (at sea level), P = input power, and D = propeller diameter.
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