Are flying cars a dumb idea?
It would be shocking if they weren't, because almost every idea is a dumb idea before it's a successful innovation. Early automobiles were not much more than a collection of dumb ideas. When Kipling published his story They in 1904, he opened with narrator on a cross-country "motor car" tour, in which: "I was on the point of reversing and working my way back on the second speed ere I ended in some swamp, when I saw sunshine through the tangle ahead and lifted the brake."
There isn't much "lifting the brake" these days, as we've found that operating them via a pedal on the floor works better, just as we've found that steering wheels work better than the tillers found on some early experiments. We've also discovered technological fixes that replaced the manual spark advance, the choke, and a host of other inconveniences. My father once told me that in the 1920s people would drive from Vancouver to Seattle and brag of "only getting one flat tire". Crank starting was unreliable and dangerous enough that there was a medical condition named after a common consequence of using it: the chauffeur's fracture, which could be avoided by holding the crank such that the fingers didn't wrap fully around it, so if the engine backfired the crank would slip off the heal of the hand rather crunch the end of the radius.
If you scroll far enough down this page you'll see that there was even some confusion as to the optimal number of wheels in the early days. Four or six? Time will tell!
In the modern world, we no longer wait until after we've started to manufacture and sell a new technology to figure out how to reduce failure, injury, and death to a bare minimum. Sometimes we even decide not to do things because it turns out to be impossible to make them safe and reliable!
There are obvious advantages to this--fewer people suffer breakdowns, are injured, or die--but there are hidden downsides as well, because the human imagination is not very good, and figuring out how to fit a collection of moving parts and complex sub-systems together into a coherent whole takes time, if it's even possible at all.
When considering the problem of possibility, we can start off in Aristotelian mode and point out that "flying cars are said in many ways." What different people mean by "flying car" is very different, and some concepts are almost certainly more practical than others.
"A safe and legal roadable vehicle that flies" is almost certainly not what the first practical "flying car" will look like, because the engineering constraints are almost diametrically opposed due to completely different operating environments.
Things that fly need to be light, and don't need to worry too much about bumping into things at relatively low speed. Because they don't need to maintain continuous contact with a relatively flat surface they don't need much in the way of suspension, and it doesn't matter if their frame undergoes fairly extreme deformation due to what is referred to in aeronautical engineering circles as "gust", which almost always comes in the form of forces that are fairly smoothly distributed across the airframe.
Road vehicles need to be robust against banging into sharp pointy things--trees, curbs, other cars, pedestrians with umbrellas, elk, and each other, for a start--which means everything about them has to be heavier. Aircraft skins are thin and easily dented, and they don't have five-mile-an-hour bumpers adorning their front and rear.
There are two basic design moves in engineering: take two functions that used to be combined and separate them, and take two functions that used to be separate and combine them. The former increases robustness and expands the available design space. The latter increases efficiency and contracts the available design space.
In the case of flying cars, combining the "roadable" and "flyable" functions decreases the design space to (almost) zero: the Aeromobil is currently the leading contender, and when it fails to deliver in 2023 as promised I will not be in the least surprised.
There are other ways of saying "flying car" that look considerably more hopeful, because they focus more on what a car does for us than how it does it. Henry Ford famously said that if he'd asked his customers what they wanted they would have said, "A faster horse." But a car isn't a faster horse. It's a different animal--as it were--and to a great extent does a different job. A horse provided medium-to-long-distance transport for the rich. Cars provide short-to-very-long-distance transport for all but the very poor, and required a complete redesign of urban environments as well as the construction of nation-spanning highways to realize their promise.
Personal flying vehicles will require the same kind of revolutionary infrastructure. Detroit in 1900 had no parking spaces, few commuters, and no suburbs in the modern sense of the term. People lived and died close to where they worked.
So the question of flying cars is one of systems and infrastructure as much as technology, and finding a sweet spot that allows integration of some kind of personal flying vehicle with existing transportation systems seems the most likely way forward.
My personal bet is on "separating functions that used to be integrated" as the most important design move. Currently both aircraft and automobiles have integrated passenger/driver compartments. Disaggregating the part that holds the people from the part that does the moving would solve a host of hard engineering design problems. That is, I'm expecting some kind of modular hybrid: a passenger compartment that can be easily unlatched from a flying system, be it wings or a drone-like multi-copter frame, or something vaguely in between like the gloriously misnamed "Blackfly".
That might require "transfer ports" where exchangable, shared, flying and driving modules would be swapped between personal, private, passenger modules.
Or maybe the future holds something else entirely: