Difference of blade airspeed
Caption: Airflow in forward flight
United States Department of Transportation, Federal Aviation Administration
2012-01-01
<a rel="nofollow" class="external free" href="http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/">http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/</a> Chapter 02, page 2-18, Figure 2-33