FlightRadar and similar websites use several sources for aircraft detection, most commonly they use ADS-B. Not all General Aviation aircraft, to which helicopters would count normally, are equipped with transponders that are capable of emitting ADS-B data, so they would not show via regular means on FlightRadar.
The primary technology that Flightradar24 use to receive flight information is called automatic dependent surveillance-broadcast (ADS-B).
ADS-B is a relatively new technology under development which means that today it's rarely used by Air Traffic Control (ATC). Our estimations show that roughly 65% of all commercial passenger aircraft (75% in Europe, 35% in the US) are equipped with an ADS-B transponder. For general aviation this number is probably below 20%. But this percentage is steadily increasing as ADS-B will become mandatory for most aircraft in most airspaces around the world, by year 2020. When mandatory, ADS-B will replace primary radar as the primary surveillance method used by ATC.
Flightradar24 has a network of more than 4,000 ADS-B receivers around the world that receives plane and flight information from aircraft with ADS-B transponders and sends this information to our servers. Due to the high frequency used (1090 MHz) the coverage from each receiver is limited to about 250-400 km (150-250 miles) in all directions depending on location. The farther away from the receiver an aircraft is flying, the higher it must fly to be covered by the receiver. The distance limit makes it very hard to get ADS-B coverage over oceans.
About 99% of Europe is covered with ADS-B receivers. There is also good ADS-B coverage in USA, Canada, Mexico, Caribbean, Brazil, Russia, Middle East, India, Japan, Thailand, Malaysia, Indonesia, Australia and New Zealand. In other parts of the world the ADS-B coverage varies.
The other option is to receive the regular Mode-S transponder signal by a technique called Multilateration (MLAT), however the FlightRadar website states that this is something that only works reliably at high altitude, at least 5 000 to 10 000 ft.
In some regions with coverage from several FR24-receivers we also calculate positions of non-ADS-B equipped aircraft with the help of Multilateration (MLAT), by using a method known as Time Difference of Arrival (TDOA). By measuring the difference in time to receive the signal from aircraft with an older ModeS-transponder, it's possible to calculate the position of these aircraft. Four FR24-receivers or more, receiving signals from the same aircraft, are needed to make MLAT work. That means that MLAT coverage can only be achieved above about 5,000-10,000 feet as the probability that signal can be received by four or more receivers increases with increased altitude.
Most parts of Europe are today covered with MLAT above about 5,000-10,000 feet. There is also some MLAT coverage in North America, Mexico, Australia and Brazil. More areas will get MLAT coverage during 2014 and 2015.
For more information, check FlightRadar's "How it works" page.