Helicopters you'll actually see

Civil helicopter identification is easier than fixed-wing, because the fleet is far smaller and three features settle almost everything: what the tail does about torque, how many blades are on the main rotor, and whether it sits on skids or wheels.

Start at the tail

The main rotor tries to turn the fuselage the opposite way, and every helicopter answers that problem somehow. The answer is bolted to the back where you can see it, it doesn't change between examples of a type, and it immediately eliminates most of the fleet.

The main rotor tries to spin the fuselage the other way, and the tail is whatever the manufacturer did about it — which makes it the single most reliable thing to look at. A conventional tail rotor turns in the open at the end of the boom and throws a high-frequency whine sideways. A fenestron does the same job with a smaller, faster rotor inside a duct through a much broader fin: quieter from the ground, safer around people, and the reason city hospital helipads so often see the types that have one. NOTAR has no tail rotor at all — the boom is a plenum with slots along one side, and air blown through them plus a steerable jet at the tip does the work. If you can see straight through where the tail rotor should be, it is an MD.

Then count main-rotor blades. Two is a Robinson or an older Bell; three is the Airbus H125 family and a lot of light singles; four is common on medium twins; five and up means something large or recent. You can usually count them on a photograph, and often by ear.

Skids or wheels

Skids mean a light machine that lands where it is pointed — training, tourism, utility, news. Wheels mean something heavy enough to need ground handling, which in civil terms means offshore, VIP or air ambulance work out of prepared sites. A retractable undercarriage narrows it much further still: in the civil medium class, that is close to a Sikorsky S-76 giveaway.

Light singles

Robinson R22 / R44 / R66 — the world's training and private helicopters, and the ones you are statistically most likely to see. Thin two-blade main rotor, very slim tail boom, skids, and a distinctive slapping rotor noise. R22 is a two-seater and visibly small, R44 is the four-seat version, R66 swaps the piston for a turbine and gains a deeper cowling behind the cabin.
Airbus H125 (AS350 Écureuil) — the workhorse for utility, tourism, survey and firefighting, and the type that flies the sightseeing circuits over most famous landmarks. Three blades, a sleek sloping nose, often a cargo hook or a camera turret.
Bell 206 JetRanger / 407 — the classic news and charter machine. The 206's narrow cabin and slim tapered boom are instantly recognisable; the 407 is the four-blade development with a fatter body.
Bell 505 Jet Ranger X — the recent light single, with a notably large glass area in the cabin and a flat-panel cockpit. Increasingly common in training and private fleets.
Guimbal Cabri G2 — small, French, two seats, and unmistakable for a light piston because it has a fenestron, which almost nothing else that size does. Turning up in training fleets as a Robinson alternative.
MD 500 series — the egg. A small, round, almost spherical cabin with a thin boom, usually a four- or five-blade head, and in the 520N and 902 versions no tail rotor at all. Police, news and utility work.

Twins

Airbus H135 / H145 — the standard air-ambulance and police machines across much of Europe, and a fenestron on both. The H135 is the slimmer one; the H145 is noticeably bigger, with clamshell rear doors for loading a stretcher and, on the current version, a five-blade head.
Leonardo AW109 / AW169 — sleek, fast and retractable-geared on the 109, which is the VIP and air-ambulance machine you will see at city heliports.
Leonardo AW139 — the offshore and VIP standard. Five-blade main rotor, deep fuselage, wheels, and big enough that it reads as a different class from everything above.
Sikorsky S-76 — retractable gear, which almost nothing else in this class has, and a long low profile. Corporate shuttle and offshore work.
Bell 212 / 412 — the civil descendants of the Huey, and they still look it: two-blade on the 212 with that unmistakable slow heavy beat, four-blade on the 412.

Heavy and offshore

Fewer types, all big, nearly all working the oil and gas fields or search and rescue. Sikorsky S-92 and Airbus H225 Super Puma are the two you will see most, both with wheels, large sponsons on the fuselage sides and the bulk to make the class obvious at a distance; the Leonardo AW189 is the newer competitor. If it has a hoist above the door and a radome on the nose it is probably doing search and rescue rather than crew change.

A yellow and black EC135 helicopter on grass seen from the side, its tail rotor enclosed as a ducted fan inside a broad tail fin — An EC135's fenestron: the tail rotor is a ducted fan set inside a much broader fin, instead of turning in the open air at the end of the boom.Photo: James from Cheltenham, Gloucestershire · CC BY-SA 2.0 · source

Reading the job from the fit

Civil helicopters are usually working rather than travelling, and the equipment hung on them says what the work is — often more quickly than the type does.

Nose or chin camera turret — police, news, or survey. A gimbal under the nose is a very strong tell.
Searchlight on the skid or belly — police, usually alongside the turret.
Hoist above a side door — rescue, or offshore crew transfer.
Cargo hook underneath — lift work: construction, power lines, logging, firefighting. Often with a bucket slung beneath it.
Clamshell doors at the back — designed for loading a stretcher lengthways, so very likely air ambulance.
Floats on the skids — overwater work, and a common fit on tour machines operating along coasts.
A long boom ahead of the nose — survey, carrying magnetometers or a camera array.

Why they sound different

Blade count and tip speed set the character of the noise, which is why experienced spotters identify helicopters by ear before they see them. Two-blade heads slap — the Robinson's rhythm and the Bell 212's heavier beat are both unmistakable. Four- and five-blade heads produce a smoother, lower, more continuous sound. A fenestron or a NOTAR boom cuts the high-frequency whine a conventional tail rotor throws sideways, which is the real reason city hospital helipads so often operate those types: the complaint that closes a helipad is noise, not safety.

Fixed-wing types have their own pages: airliners, light aircraft, business jets and military.

Photographs and references

For photographs of the real thing, Wikimedia Commons' helicopters category is freely licensed and very large, and planespotters.net pairs its photos with each individual airframe's build date, operator history and fate. Helicopters are also the types most likely to be working rather than travelling — air ambulance, police, pipeline survey — so the operators themselves often publish the clearest photographs of the exact machine in your area.