I hope your research is going well. Did you ever decide on an experiment proceedure? A slightly different approach to the one suggested by Jay is the one that proves that 2 objects of the same size but different mass still fall at the same rate.http://www.sciencebuddies.org/science-f ... p015.shtml
For the experiment suggested, since we already know that the objects, regardless of mass, will fall at the same rate, what you are testing is the effect of drag as the object moves through the air around it.
Assuming you are still conducting the experiment, I would suggest getting 2 (or more) balls of significant size difference (say a large beach ball and a tennis ball) and drop them from a known height and measure the time it takes to hit the ground. Since we know the accelleration due to gravity (9.8 m/s^2), you can calculate the average velocity. Any change would be due to drag (in large part) as a function of the cross sectional area of each ball. The larger the cross section area of the object, the larger the coefficient of drag, etc... This experiment can be extended to objects of different shapes to test the effects of aerodynamic properties of say a cone shape vs a blunt faced object, like a cube.
Please post back to let us know how you are doing wrt your experiment and if you need any additional advice.
I hope this helps.
"As the circle of light increases, so does the circumference of darkness around it."
~ Albert Einstein