This paper reports an experimental investigation of premixed propane and methane-air flames propagating freely downwards in tubes 1.5 m long and with diameters 54 and 94 mm. The tubes are opened at the upper end and closed at the bottom end with an acoustic damper that eliminates thermo-acoustic instability. Igniting the flame either on the tube axis or close to the wall, two regimes of propagation are distinguished by correlating the flame speed and the radius of curvature at the flame tip. The characteristic lengths are then related to the cut-off wavelengths estimated from linear theories and compared to previous results of Michelson-Sivashinsky simulations. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.