The 4 forces on a helicopter in constant level flight: weight, lift, thrust, drag
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 2-2, Figure 2-1