Supposedly, something big happened in the defence world on 12 December. Upstart startup Anduril announced a joint venture with Archer Aviation, one of the leading companies in the industry for making electric vertical take-off and landing (eVTOL) aircraft.

To eVTOL backers, the announcement pointed to replacement of many military helicopters by electrically powered, multi-rotor aircraft, which in cruise would fly as aeroplanes.

But we have every reason to be sceptical. The eVTOL craze has generated an awful lot of talk and minimal achievement even in its original, civilian market.

Archer said that its new defence division was targeting a ‘potential program of record’ from the Department of Defense (DoD). Never mind that there is no such thing: a program of record is not ‘potential’, because, by definition, it is an approved requirement backed by money.

The association with Anduril allowed Archer to raise money that it can use to fund the development of the Midnight, its civilian eVTOL product. Archer and its competitors are spending a lot of cash on building factories and testing prototypes, while working with the Federal Aviation Administration and other authorities to earn safety certification without which they cannot make revenue.

Bulls in the eVTOL business say that the sky is the limit. In a LinkedIn post, Mark Moore—eVTOL guru, long-time personal aviation advocate at NASA and now chief executive of Whisper Aero, developing electric aviation propulsion—notes that ‘Anduril’s deep pockets and DoD positioning enables Archer to have a clear path to impressive DoD products. … Archer will have a clear path to a dual-use military-civil market strategy that directly competes with helicopters in these markets.’

The eVTOL craze started with 2013 remark about Twitter by the entrepreneur Peter Thiel: ‘We wanted flying cars; instead they gave us 140 characters.’ But it was really got underway by Moore as co-founder of Uber Elevate and by the company’s 2016 report that was the first of many to predict that eVTOLs in their thousands would provide zero-emission point-to-point urban air travel at a price comparable with a ground limo.

Not so fast.

You can read eVTOL media all day without running across one awkward fact: after six years and billions spent, no eVTOL has performed a basic VTOL mission, flying 40km out, landing, taking off and returning, all while carrying a representative payload on one charge. The best available batteries are not up to the job.

Archer and Anduril mitigate this problem for defence users by switching to hybrid power, with both batteries and a turbo-generator onboard. But that adds complexity to a system that’s already complex, because the philosophy behind eVTOL aircraft is safety through redundancy. The Midnight ties the 95-year-old record for most engines installed on one aircraft, with 12 motor-prop units in tandem pairs on the wing. A Midnight should be able to lose two engines and land safely.

But six fixed-pitch lift-only rotors and six variable-pitch tilting rotors, generating high energy airflow streams that interact and change with airspeed and tilt angle, add up to a flight control challenge.

Crewed eVTOLs depend on a level of automation that goes beyond the common fly-by-wire technology through which pilots control modern aircraft. Pros from the traditional rotorcraft world point to the long time it takes to get certification for even a conventional helicopter with fly-by-wire. They doubt eVTOL leaders can achieve certification by their 2026-27 target dates.

Still, defence customers might accept a higher level of risk.

But, even so, what will a hybrid eVTOL bring to defence missions?

Designed for very short flights, the Midnight has a cruising speed, 240km/h, no better than that of a helicopter. Multi-rotor advocates claim higher efficiency than a helicopter in cruise, but any such benefits will be offset by a heavier propulsion system.

There are more technical reasons for doubting the military value of eVTOLs. The Midnight’s rotors are about 2 metres in diameter—but, even with 12 of them, the discs they form when rotating have well under half the area of the Airbus H135 helicopter, which has about the same gross weight (2900kg). Higher disc loading means more power is needed to hover (1 to 1.2MW for the Midnight, versus 940kW for the H135) and consequently a powerful, complex downwash field that will kick up much more debris. Who wants that in a military operation?

The Midnight spans 15 metres tip-to-tip, compared with the H135’s 10-metre rotor diameter. That is hardly an advantage in most tactical operations and distinctly not great for operating on warships.

Scaling multi-rotor eVTOL designs up, perhaps to act as airborne 2-tonne trucks, will exacerbate all these issues. Enlarging rotorcraft increases disk loading unless you change the configuration, which is why the CH-47 with its two 18.3-metre rotors rules the heavy-lift helicopter segment.

Going lighter and unmanned? Well, another company, Griffon Aerospace, is already there, with the neat and simple (only four motors) Valiant drone.

It’s early days, and neither sceptics nor boosters are infallible. Let’s see what the mystery ‘program of record’ turns out to be.