If you don’t have a power meter Aerolab can interpret the data as having zero power.
If you are freewheeling, you have zero power.
So, the approach is to use is roll-down tests.
Ideally you need a sheltered hill that is long enough to reach a stable terminal speed safely.
Traffic and wind conditions can affect the result so low or no traffic and stable wind conditions help.
You can analyse the results in three ways:
Look at the maximum stable speed reached at the end of each run.
Analyse the file in Golden Cheetah in the normal way.
Use the MyWindSock method.
Bear in mind that speed sensors only report to three significant figures, so don’t expect the CdA to be precise to four figures.
Environmental conditions are variable day to day and on the day e.g. air density, temperature and rolling resistance. These changes need to be accounted for. Don’t expect absolute CdA values. What we are really interested in is comparisons. For example is helmet A better or worse than helmet B.