Experimental and Computational Aerodynamics of Lilienthal’s Rundlaufapparat

Abstract

Otto Lilienthal’s contribution to the history of aerodynamics and flight is well established, yet the experimental foundation of his aerodynamic work remains less systematically investigated than his glider achievements. Beyond his pioneering glides, Lilienthal performed systematic rotating-arm experiments using the Rundlaufapparat, producing quantitative results that foreshadowed modern aerodynamic practice. This project reconstructs and operates a faithful Rundlaufapparat, formalizes the measurement chain converting timings, masses, and readings from the counterbalanced lever into lift and drag, and makes explicit the reduction from forces to coefficients to clarify Lilienthal’s methodology. Historical experiments are reproduced to reconstruct Lilienthal’s Plates, published tables interpretable as early aerodynamic polars, and the resulting data are analyzed to assess how normalization choices and contemporary theory shaped the reported magnitudes and trends. Rotating-arm measurements are compared with tests in DLR’s RTG open-jet wind tunnel and with two- and three-dimensional Reynolds-averaged Navier–Stokes (RANS) simulations under matched conditions. Findings show that the apparatus robustly resolves aerodynamic forces and captures trends, while systematic differences in coefficient magnitudes arise from normalization schemes, ranging from the Smeaton coefficient to normalization by the measured force at α = 90◦, and from kinematic differences between curved, accelerating Rundlaufapparat motion and rectilinear inflow in wind-tunnel and simulation setups. This study clarifies what Lilienthal’s Plates represent in modern terms, explains coefficients derivation and alternative formulations, identifies regimes of highest reliability, contrasts historical reductions with contemporary techniques, and establishes a reproducible basis for future rotating-arm aerodynamics.