On Flying Cars

Kicking around some thoughts on flying cars: Their stupid. The idea that you can have both a car that drives, and car that flies is silly. And its' the wrong model. The proper model is the 'personal helicopter', based off of drones. The little quad-rotor radio-controlled things keep getting bigger, and more able to handle larger payloads. At TRB this year, I saw one with a 45 lb payload (IIRC). How much harder can it be to carry three or four times that amount (ie, an adult human). I assume that would be associated with an increase in the number of rotors, and the weight of battery packs. And, as with rockets, the more 'fuel' you carry, the more of your 'lift' you have to devote to compensating for that weight.

Last night, my wife suggested that the first personal helicopters are simply going to be a swing beneath a drone. And it makes so much sense: Why put the center of gravity above the propellers, as opposed to beneath them, as every 'flying car' supposes?

There are already cities where traffic congestion is so awful that the rich commute by helicopter. And indeed, a generation or so ago (before the oil crisis) we were starting to see just that--helicopter 'bridges' between the airport and the CBD.

Why not helicopter taxis? The previous limitation was cost and safety. Helicopters are far more energy intensive than planes, and far more dangerous. It's part of their essential nature: A plane can glide, a helicopter can only fall. Hence historic limitations in their operations.

Quad-rotors are totally different than helicopters. Helicopters are complex: It's not just a whirling propeller on top and back: In addition to the speed of the rotor, there is the 'pitch of the blades attached to the rotor, and the 'tilt' of the rotor itself to be able to move.

Rather than all the complexity of a helicopter single axial, a quad-rotor has four: And it moves up and down, forward and back, just by running those rotors at different speeds. It's a far simpler design, with fewer ways in which to fail. It is hence, far less likely to crash.

A helicopter is a complex machine to operate: It can both move and rotate along three axes. Hence, when something goes wrong, and the mechanics holding it balance fail, it starts twisting, dropping and yawing all at once. A disorienting experience likely to end in crashing.

With a quad-rotor, the 'balance' between forces is software control. As I reason it, a quadrotor could fly with one 'engine out'. An oct-rotor (8 engines around 4 axes) seems like it could sustain as many as three engines out. Far safer, and more fault tolerant.

The fault tolerance of multi-rotor aircraft suggest they would be far safer for urban operations: They are less likely to experience a major malfunction, and less likely to crash when they do. They are also more robust, as they benefit from over a decade of 'trial by fire' as RC.

Using the metaphor of a flying car, we get a lot of baggage-the idea that the 'flying car' will enable us to do all the 'car things' we already do: Starting from home, starting and stopping where we like with zero notice, with almost zero range anxiety. Military history is pretty clear that any vehicle that tries to do two things typically winds up doing neither of them well. 'Cars that fly' have a long history of failure, and will likely continue to exist only as 'sport aircraft' costing a half million each.

But for urban mobility, a 'copter', could be it: A way to provide rapid, point to point transportation for time-sensitive commuters, free from congestion. Owning and operating such a vehicle would be expensive. The rich would be the early adopters. Adoption of quad-rotors capable of human transportation would soon spread to commercial providers: auto-owners were not slow to realize the opportunity to act as cabs. And as technology improves (battery life, operating efficiency, reliability), an costs fall, adoption rises.

In America, providing a flying taxi (air mobility for a paying passenger) requires special training, maintenance and licensing. The FAA is very safety oriented, perhaps obsessively so. Development of personal copters may be pioneered outside America as a consequence.

Given the technical limitations of batteries (and the safety limitations of flying within urban areas) it seems likely that personal copters will be limited to short journeys for the foreseeable future: No exurban commuting possible. Flight times now max at about 20-30 minutes. Given a reasonable safety buffer, this suggests flights no longer than 10-20 minutes. I'm uncertain of speeds, but even at 20 mph, that's a radius larger than the core of most urban areas, where most Uber/Lyft rides currently take place.

The real advantage of personal copters over cars is congestion: Cars operate in two dimensions, copters in three. As a pilot once noted to me: "When you run out of capacity at one elevation, you just add another at a higher elevation". Unlike roadways, a copter-based urban transportation system could add capacity, almost infinitely, as zero cost. It makes investments in roads look almost foolish.