Helicopter sound effects are a moderately popular sound effect sought after by film editors. They serve a very specific purpose, namely adding the 'real thing' to a scene in film or video that calls for a helicopter sound effect. Generally, if a Hollywood budget film is filming actual helicopters, they will almost certainly have professional and well paid sound recordists and designers on set to capture the helicopter sounds while the helicopter is being filmed.
However, for non-hollywood films, like the 99.99% of filmmakers and video producers out there, helicopter sound effects are not easy to come by. And, honestly, they are not generally easy to come by for sound effect recordists. A helicopter sound effect cannot be faked via foley, like footsteps or a window breaking. Helicopter sound effects can't be made with the human mouth or with any other piece of machinery or vehicle than an actual helicopter.
Therefore, a dedicated sound designer must place himself/herself in the path of a flying helicopter. Depending on where one lives, this could be an easy task or difficult. Adam Johnson, owner of SFXsource.com and sfx contributor to this site is an example of a sound designer who has easy access to helicopter sounds. This is because he lives in the path of the helicopters traveling between the White House in Washington DC and the CIA in McLean, VA. Some of the military helicopter sound effect clips on SFXsource.com were taken in his backyard. During the Bush administration, specifically, there were many helicopters flying over the suburbs of Northern Virginia making their rounds to various government agencies. Weirdly, or interestingly, since Obama has taken office the extremely loud low-flying helicopters of 2004-2008 have rarely made an appearance. Perhaps the flight path has changed.
Either way, who cares. The in and outs of helicopter sound effects remain. In sum, they must be made with an actual helicopter, as their sound wavs and sound forms are too intricate and complex to recreate.