AREN’T QUITE READY YET!
300+ kph speed and it is also very quiet. Joby says it has an agreed pathway to certifica- tion with the Federal Aviation Authority, and expects to be cleared for piloted passenger carrying in 2023. Some of its pre-SPAC venture funding came from Toyota, so there is at-scale manufactur- ing expertise on tap.
LIFT AND CRUISE
There are other kinds of copter-plane transitioning craft out there. Some don’t tilt their propellers, instead having separate, dedicated vertical and horizontal thrusters. This is referred to in the air-taxi sector as “lift and cruise”. Drag from the lift fans when cruising is a problem here, as is the weight of two sets of equipment, but the approach does have the virtue of simplicity. Contenders in this area include the Cora from Wisk Aero, spun off from Google co-founder Larry Page’s Kitty Hawk venture, and the PAV from well-known aircraft firm Aurora Flight Sciences, now owned by Boeing.
DUCTED-FAN CRAFT
Yet another design is the use of ducted fans, where the propellers are inside round or tubular cases which can be separated from the aircraft or built into it. One company taking the built-in path is Lilium, which mounts the tubular ducts in rows along the wings of its aircraft. The company calls these “jets” and its aircraft is known as the Lilium Jet. The electric jets swivel to point down for takeoff and landing, and then point back in the cruise. Significantly, the planned Lilium Jet has maximum takeoff weight of more than three tonnes, triple that of the Volocopter and half again that of the current Joby 2.0. This means that it can carry six passengers plus pilot – and lift more battery.
The hefty aircraft and the jets’ small discs mean high disc loading and heavy energy drain in the hover, and Lilium admits this is a weak point. But the ducts act to cut down noise and once in the air, the Lilium Jet’s large battery and streamlined, closely integrated fans should mean better range and speed than its competitors - 250+ km and cruising speed of 280kph - about the same as Joby. Lilium also states that it’s not interested in flights of less than 20km.
JULY 2021
So there’s a spectrum, perhaps, among the electric vertical- lift startups. At one end there are short-hop companies such as Volocopter and at the other companies such as Lilium seeking to link different cities – but still from downtown pads rather than out-of-town airports. And in the middle there’s Joby, claiming to be the best of all worlds.
BATTERY CAPACITY OR LACK OF...
There is one huge issue hanging over all these companies, and that is the matter of battery capacity. It’s more or less openly acknowledged by most of them that their projected endurance, range and speed figures do not refer to the prototypes they have flying today. Rather, these figures are based on their current aircraft – but equipped with better batteries which they think they might be able to get in the near future.
Yes, an electric multicopter or tiltrotor or ducted-fan craft can lift off, fly, and land again. But it’s safe to say that with current battery technology, very few of the current aircraft have stayed up for long or gone very far. There isn’t, yet, a working flying car/taxi.
This can be stated because the mathematics of disc loading, weight and battery power are relatively well understood, and the sums have been done by people other than the elec- tric air taxi firms. NASA engineers, for instance, have recently said that battery packs capable of storing 400 Watt-hours of energy per kilogram of weight (400 Wh/kg) are a requirement for useful vertical-takeoff electric aircraft.
It’s important to note here that there’s a big difference between the specific energy a single lithium-ion cell can hold and the amount a whole lot of cells assembled in a pack can hold: cells can’t just be packed together in a mass, they need to be insulated from each other and usually provided with cooling systems and other additions. It’s not uncom- mon, when battery cells are either charging or discharging quickly, for them to overheat: if precautions aren’t taken, cells can catch fire and then set off other cells, in a phenomenon called “thermal runaway”.
All this means that assembled, ready-for-service battery packs have much worse specific energy figures than their individual cells, so that almost miraculous 600 Wh/kg cells would probably be needed to achieve NASA’s required 400 Wh/kg packs.
It therefore seems that we may be waiting for our flying taxi rides for quite some time yet.
This article is an abridged version of a feature written by Lewis Page for VERDICT
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