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Helicopter inflow ratio calculator

Inflow ratio is a non-dimensional measure of the flow speed normal to a rotor. It's defined as the ratio of the normal flow speed \(V_{N}\) to the rotor tip speed \(\Omega R\).

Helicopter Speed

\( KTAS \)

Forward Rotor Tilt

\( deg \)

Pitch, Positive Nose Up

\( deg \)

Helicopter Rate of Climb

\( ft/s \)

Main Rotor Radius

\( ft \)

Main Rotor Speed

\( RPM \)

Main Rotor Induced Velocity (+ down)

\( ft/s \)

Inflow Ratio


\( - \)


This calculator estimates the inflow ratio \(\lambda \) of a helicopter main rotor.


The following equations are used to estimate the inflow ratio.

\[ V_N = V\sin (\gamma - \theta ) + V_U \cos (\gamma - \theta ) + v_i \] \[ \lambda = \frac{V_N }{\Omega R} \]

Many symbols are defined in Helicopter Abbreviations and Symbols. In addition, we use the following symbols here.

\(\gamma \) main rotor forward tilt
\(V_N \) net downward flow speed, normal to the rotor
\(V_U \) helicopter rate of climb

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