Between 1955 and 1959 the French aircraft manufacturer S.N.C.A.S.O. (SUD-AVIATION) carried out, under the code name Project Montgolfier, a sustained experimental study of the electrokinetic thrust reported by T. Townsend Brown - the so-called Biefeld-Brown effect, in which an asymmetrically electroded high-voltage capacitor develops a net force directed toward its positive electrode. Working with carousels, torsion pendula, an evacuated bell jar and, finally, a purpose-built vacuum chamber, the experimenters repeatedly resolved the observed force into two superposed contributions: an unpolarised, geometry-controlled "torsion" force that dominates at atmospheric pressure and follows the applied high voltage from the sharp electrode toward ground, and a weaker polarised force, always directed from negative to positive, that reverses with polarity and survives evacuation. We show that this empirical decomposition is, mathematically, the projection of the thrust onto its even and odd parts under the polarity involution U->-U, and that the experimenters' "half-sum / half-difference of the positive- and negative-high-voltage curves" is exactly the even/odd estimator. We give the classical electromechanics of each component: the even part as a Maxwell-stress (edge/point) force plus an electrohydrodynamic (EHD) ion-drift thrust T I d/ whose pressure dependence reproduces the reported collapse of rotation in vacuum; and the odd part as a dielectric body force (Kelvin term) acting together with injected/frozen surface charge, which naturally accounts for the polarity reversal, the hysteresis, the strong barium-titanate enhancement, and the persistence of a small residual signal in vacuum. We reconstruct the torsion-balance calibration and the magnetic null test (a 6 G solenoid), and confirm the reported field to approx5.8 G. The synthesis is consistent with modern findings on EHD "lifters": the atmospheric thrust is ion wind and Maxwell stress, while the vacuum residual sits at or below the systematic floor set by dielectric charging, leakage currents and wall effects, and provides no evidence for an anomalous inertial or gravitational force.
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