This Paper presents a new quasi-steady in-ground effect (IGE) model for small multirotor aerial vehicles such as quadcopter unmanned aerial vehicles. The model offers three major advances to the state of the art: 1) predicting the IGE thrust ratio without singularity, 2) relating the strength of the ground effect to the blade geometry (pitch angle, solidity, and radius), and 3) capturing the effect of multirotor configurations. Two IGE coefficients, based on blade element theory, are introduced and then analyzed using the method of images. Additionally, the thrust-loss phenomenon on multirotor IGE is observed and modeled empirically. Experiments are conducted to validate each component of the model, and the results suggest that the model can be used to predict the onset of the IGE for vehicle motion control and planning in the regime where IGE dominates.
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