Image copyrightRTA/VOLOCOPTER

Tech companies are competing to develop the first viable passenger-carrying sky taxis, whether manned or pilotless, but how soon could these clever copters really be whizzing over our cities? And would you trust one?

Dubai is racing to be the first to put drone taxis in the air.

In June, its Roads and Transport Authority (RTA) signed an agreement with a German start-up Volocopter to test pilotless air taxis towards the end of this year.

The firm has received 25m euros (£22m; $30m) from investors, including German motor manufacturer Daimler, to develop the 18-rotor craft capable of transporting two passengers at a time.

The promotional video claims a top speed of 100km/h (60mph) and a maximum flight time of around 30 minutes, while nine independent battery systems ensure safety.

"You will never require" the onboard emergency parachute, Volocopter assures us.

Dubai's RTA has also teamed up with China's Ehang and is testing the drone maker's single passenger Ehang 184 "autonomous aerial vehicle".

Image copyrightRTA/EHANG

Image copyrightEHANG

But the largest city in the United Arab Emirates faces stiff competition. It seems the whole world has gone gaga for air-cabs.

In February, ride-sharing giant Uber poached Nasa chief technologist Mark Moore and set him to work heading their Project Elevate - "a future of on-demand urban air transportation".

Airbus, the French aircraft maker, is also working on a prototype air taxi, Vahana, saying it will begin testing at the end of 2017 and have one ready by 2020.

They all spy opportunities in the air because traffic is becoming increasingly clogged on the ground. To take an extreme example, in Brazil's Sao Paulo, the world's 10th richest city, traffic jams average 180km (112 miles) on Fridays, and sometimes stretch to a barely credible 295km.

Yet the world's megalopolises are continuing to grow. No wonder air taxis are capturing people's imaginations.

Image copyrightA3/AIRBUS

Ehang carries a single passenger, Volocopter two, while City Airbus is looking at four to six. And each of these companies is pursuing electric propulsion, seeing it as greener and quieter.

The preferred horizontal rotor technology allows for vertical take off and landing, which makes sense in densely built up urban spaces. And composite materials, such as carbon fibre, help keep weight to a minimum.

But how will they work in practice and will they be affordable?

Uber's Mr Moore says the cost, with three or four passengers sharing a pool, will be "very similar to what an UberX [car] costs today".

More seriously, given the trade-off between power and weight, how long will these things be able to stay up in the sky relying on battery power alone?

Because if you don't like your mobile going flat, you definitely won't like it when your air taxi does.