As Robinson produces one of the most popular helicopters in the world, you are likely to encounter one. It all started with the two/three seat R22 in the 1970s, a simple piston engine powered helicopter with an exposed engine in the rear of the fuselage. In front of that is a cabin with large curved front windows and smaller side windows in the doors. The two blade main rotor is on a long mast that is surrounded by a streamline body. From the top of the rear fuselage, the narrow tail boom extends, holding a two blade tail rotor on the left. There are stabilising fins up, down and right, on most versions. Later the product line was expanded with the longer cabin R44 and R66. These have a covered engine with a slightly pointed rear fuselage, but is for the rest very similar to the R22.

To distinguish the different versions of the Robinson R22, R44 and R66 you'll have to look at, amongst others

• the length of the fuselage
• whether the engine is exposed of covered
• the shape of the horizontal stabiliser(s)
• the shape of the vertical fins
• the presence of air intakes on top of the fuselage, behind the rotor mast
• the presence of air intakes in the root of the tail boom
• the presence of an oval exhaust in the rear of the fuselage
• the length of the skids
• the visibility of the battery 

The R22 is the smallest of the family, with an open rear fuselage exposing the piston engine. Its vertical stabiliser, up and down, is slightly separated from the end of the tail boom, and attached to the horizontal stabiliser (see photo above). The original production version of the R22 has shorter skids than all later models: they stop at the attachment of the rear frame.

HP stands for high power, and indeed it the R22HP is powered by a different version of the O-320 piston engine. However, that is not visible from the outside. You can however see that the skids are longer, at the end. Also, no battery is visible.

This R22 has long skids and no visible battery, so it must be an R22HP. (photo: HawkeyeUK/WikiMedia)

Starting with the Alpha model, the battery of the R22 is relocated from the nose to the left side of the engine. This means that it is visible on the R22 Alpha & Beta.

Mutually, the R22 Alpha, R22 Beta and R22 Beta II only differ in the exact engine type. Although the engine is exposed, there seem to be no external differences. Late model Beta IIs have symmetrical horizontal stabilisers attached to the bottom of the tail boom, just in front of the tail rotor.

Robinson has a dedicated certified version of the float equipped R22, the Mariner. Apart by it giant floats, is can be recognised by the flat horizontal plate attached to the bottom of the vertical fin. These parts can be removed from the R22 Mariner though, and then it looks not different than the R22 Beta.

Huge floats are the main recognition point of the R22 Mariner. Additionally, it has a flat horizontal panel at the bottom of the vertical fin. (photo: Dale Coleman/WikiMedia)

While the basic appearance remains the same, the R44 is more than a stretched R22. Indeed the cabin is longer, with a second side window. More important, the engine is covered behind an enclosed cowling. The rear of that, and thus the fuselage, is flat. An exhaust protrudes it at the bottom. Cooling air for the piston engine comes from long, narrow slots underneath the root of the tail boom. Finally, the vertical fins are longer and directly attached to the tail boom, without space between. 

R44 Astro is the marketing name for the R44 with electrical controls, while the R44 Raven has hydraulic controls. The Raven II has a fuel-injection engine, but is indistiguishable from the Raven I on the outside. Similarly, the R44 Cadet is a two seat training version, but you'll have to look inside to see the number of seats.

These are the float equipped versions of the R44 Raven I and Raven II. These can be large and fixed, like on the R22 Mariner, or inflatable. In the latter case, the skids appear thicker.

The skids of this R44 Clipper II appear to be thicker than the regular ones. That is due to the inflatable floats that are located on top of them. (photo: Mike Burdett/WikiMedia)

The turboshaft powered version of the R44 is designated R66. It is the same as the R44, except for the air intakes and exhaust. The R66 has intakes at the root of the tail boom, just aft of the rotor mast. There is a grid intake below, at the side of the fuselage, and an additional intake at the base of the rotor mast. The oval exhaust at in the flat rear of the fuselage.

The R66 Turbine has different air intakes than the R44: the main in the root of the tail boom, and additional ones at the base of the rotor mast and at the side of the fuselage. (photo: Alan Barclay/WikiMedia)

From this angle you can see the oval exhaust of the R66 Turbine in the flat rear of the fuselage. (photo: Alec Wilson/WikiMedia).

The mix between the R44 Clipper and R66 Turbine is called the R66 Turbine Marine. Like the Clipper, it can be fitted with inflatable floats or fixed ones.