This paper presents an attitude controller for a small helicopter Unmanned Aerial Vehicle (UAV) based on Incremental Nonlinear Dynamic Inversion (INDI). INDI is a sensor-based control method which responds quickly to the commanded input, but also to disturbances. While previous implementations of INDI used a control effectiveness matrix describing effects on rotational accelerations, the implementation presented in this paper uses rotational rates. This is possible with small hingeless-rotor helicopters since the rotational rates are achieved almost immediately, but also the transient is taken into account. By doing so, the matrix contains only constants and the control structure is much simpler. The proposed controller is implemented on a small helicopter which weighs less than 50 grams. The performance of the controller is demonstrated with step responses on roll and heading angles. Also disturbance rejection capabilities are demonstrated. Finally, the controller is deliberately configured incorrectly with wrong control effectiveness and actuator model parameters. A theoretical derivation is provided to predict the effect of incorrect parameters. With experiments, it is demonstrated that the helicopter can be stabilized over a wide range of incorrect values. It is concluded that the demonstrated controller is a suitable choice for small autonomous helicopters.