I've observed helicopters being used often
Why aren't there any commercial passenger transport helicopter flights? Or are there any examples of this? Are airplanes simply that much more efficient?
Airplanes are much more efficient, much faster and scale better.
Helicopters are limited to around 150 knots because when flying forward the tip of the advancing blade must not exceed the speed of sound while the retreating blade must still move aft fast enough to produce lift.
Helicopters are also difficult to make large, because for efficiency the rotor disk needs to be large, but the diameter is again limited by the tip speed having to remain subsonic. And the material requirements to make the blades strong enough without making them too large to keep drag low.
Jan Hudec's answer has already detailed why helicopters are not widely used for mass transportation or have been scaled up to ferry passengers more often.
The few exceptions exist where landing space is limited and the helicopter's maneuverability is more advantageous.
Offshore Oil & Gas Rigs
(Image Source: www.wn.com)
Where transport by ship is not fast enough or not possible due to heavy swell, the use of helicopters allows quick access to the offshore platforms.
New York Helicopter Taxi
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The New York Helicopter Services allow quick access to the airports around New York for business travelers who would otherwise take more time to arrive in Manhattan.
Spending years being ferried off and on shore by helicopters, I'd say the biggest problem is noise. Even with a pair of decent ear defenders on it's not exactly quiet.
OK, so a commercial 'air bus' is never going to match a Mercedes or even a plush executive helicopter for levels of NVH, but there's nothing particularly attractive about the experience other than getting somewhere much quicker than you might otherwise.
The noise also means the 'go anywhere' ability of helicopters is severely curtailed if noise means they can't actually go many places.
Add to that the high cost and it's pretty much a non-starter except for edge cases as in SentryRaven's answer where an increase of speed of an order of magnitude or more, or the circumvention of otherwise impossible access outweigh these concerns.
There are a few commercial passenger flights, but not so many because of the costs.
In Sao Paulo, Brazil, there are about 500 registered helicopters, with about 700 flights per day, which include transporting CEOs and politicians from one part of the city to another. Several factors contribute to this:
Some might also cite safety concerns, but this is unlikely to play a major role.
There was regular helicopter service between Tallinn, Estonia and Helsinki, Finland during the mid-2000's. It was mainly used by business travellers between the two cities. I think the flight time was something like 20 minutes and the heliport was on both sides conveniently in the middle of the city. The regular plane service has a flight time of 30 minutes with additional time spent for airport procedures and taxi rides to and from the airport, so there definetely was a business case there.
However there was an unfortunate crash and the company never quite recovered from that. They use their copters for oil rig flights now, if I am not mistaken. See Wikipedia Copterline Flight 103.
When I took flying lessons, I looked into a helicopter certification. The rental cost was 2-3 times more than a fixed wing aircraft at the time. I don't know what it costs now, but I assume it's still really expensive.
There are passenger helicopters, usually used by corporations or very wealthy individuals. Some companies maintain their own fleet or charter when needed (which is probably cheaper).
One company I worked for early in my career maintained an intra-site fleet of helicopters (if your manager would approve it and the weather held, an hour drive became a 20 minute helicopter ride). I never got to use it because this was one of the first things to go when the company went into decline and needed to cut costs.
