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The Fixed-\Phi (F\Phi) and Harmonic Mean (HM) fitting methods are two methodsto determine the average direction and velocity of coronal mass ejections(CMEs) from time-elongation tracks produced by Heliospheric Imagers (HIs), suchas the HIs onboard the STEREO spacecraft. Both methods assume a constantvelocity in their descriptions of the time-elongation profiles of CMEs, whichare used to fit the observed time-elongation data. Here, we analyze the effectof aerodynamic drag on CMEs propagating through interplanetary space, and howthis drag affects the result of the F\Phi and HM fitting methods. A simple dragmodel is used to analytically construct time-elongation profiles which are thenfitted with the two methods. It is found that higher angles and velocities giverise to greater error in both methods, reaching errors in the direction ofpropagation of up to 15 deg and 30 deg for the F\Phi and HM fitting methods,respectively. This is due to the physical accelerations of the CMEs beinginterpreted as geometrical accelerations by the fitting methods. Because of thegeometrical definition of the HM fitting method, it is affected by theacceleration more greatly than the F\Phi fitting method. Overall, we find thatboth techniques overestimate the initial (and final) velocity and direction forfast CMEs propagating beyond 90 deg from the Sun-spacecraft line, meaning thatarrival times at 1 AU would be predicted early (by up to 12 hours). We alsofind that the direction and arrival time of a wide and decelerating CME can bebetter reproduced by the F\Phi due to the cancellation of two errors:neglecting the CME width and neglecting the CME deceleration. Overall, theinaccuracies of the two fitting methods are expected to play an important rolein the prediction of CME hit and arrival times as we head towards solar maximumand the STEREO spacecraft further move behind the Sun.

Effect of Solar Wind Drag on the Determination of the Properties of Coronal Mass Ejections from Heliospheric Images