A number of us can thank a cartoon character from the future, George Jetson, for instilling our longing. Students of aviation history might look for inspiration to the Autoplane prototype built in 1917 by the flight pioneer Glenn Curtiss. And tens of millions of motorists who have been stuck in traffic jams stretching toward the horizon must also feel a need to know: Where are the flying cars?
It's a dream that has reduced many would-be inventors to despair as they grasped the immensity of the engineering and design challenges rooted in the widely divergent natures of airplanes and cars. Cars must provide occupants with comfort, decent handling and braking and protection in the event of an accident - while complying with government air-pollution and fuel-economy standards. Keeping weight to a minimum is important, but a few extra pounds here and there can be tolerated.
Airplanes are quite a different matter. Weight is everything in a flying machine; weight determines the engine power and the wingspan required to get off the ground. Thus, aircraft make extensive use of lightweight materials that their designers fashion into the most efficient structures they can dream up.
Trying to reconcile the conflicting requirements of the two types of vehicles invariably results in a boatload of compromises that, some say, make the flying car a nonstarter. Yet, the dream lives on.
Ray Morgan, a consulting aerospace engineer in Simi Valley, California, knows a lot about efficiency and lightweight structures, having led a group that developed a series of prototype aircraft, cars and blimps at AeroVironment Inc. Morgan has also studied flying-car design.
"In general, the idea that you are going to use an airplane for a car is just not realistic," he said in a recent phone interview. "You are very likely to end up with both a bad airplane and a bad car."
Over the years, flying-car proponents have explored all sorts of propulsion systems, including propellers, downward-facing lift fans and helicopter-style rotors. Several small companies are working on flying-car designs that they think will be the ones to finally crack the nut.
One of these, Terrafugia of Woburn, Massachusetts, has flown a prototype with self-folding wings and a pusher propeller nestled between two tail booms. The company is working on an advanced hybrid design capable of vertical takeoff and highway driving using electric motors powered by batteries, along with a piston engine turning a pusher propeller during forward flight.
A startup in Bratislava, Slovakia, called Aeromobil has a prototype with a pusher propeller for flight and drive shafts turning the front wheels when driving. Its wings pivot backward parallel to the fuselage when the craft is used in automobile mode.
It helps to put on an imaginary engineer's hat when pondering the systems that would need to be on board a viable flying car. For starters, there must be an engine. And for weight and complexity reasons, it would be ideal for a single engine to provide power for flying and driving. Therefore, some kind of transmission must be devised to send power either to a propeller or to the driving wheels.
Wings, and how to stow them for highway travel, are another big piece of the puzzle. "The heavier your vehicle is, the more wingspan will be needed to make the aerodynamic lift to fly it," Morgan said. "Every time your weight goes up, due to adding systems peculiar to the car function, those wings must get longer. Or else you will need a more powerful engine, and your takeoff and landing speeds get faster."
The longer the wings become, the greater the challenge of getting them out of the way, so the flying car will fit the width of a highway lane. Building hinges or pivoting mechanisms into the wings reduces their structural efficiency and adds weight.
Even a flying car's suspension system would require a fair amount of engineering. An automobile suspension is designed to withstand forces of about 2 Gs as it smooths out the bumps in the road. Airplanes sometimes have bad landings, however, and their suspension systems must be built to tolerate 4-G forces.
When Morgan prepared a study for the Defense Advanced Research Projects Agency on a rough-country "flying Jeep" concept, he envisioned adapting the long-travel suspension from a sand buggy. DARPA rejected the idea.
"I can't say I've ever met a real military guy who thinks a flying-car-type vehicle is a good idea," Morgan said.
Another looming challenge for a prospective manufacturer is clearing the hurdles presented by multiple federal regulatory agencies. A new aircraft must receive from the U.S. Federal Aviation Administration what's known as a type certification, which approves the plane's design as safe and airworthy. Also mandatory is a production certification, which attests that the manufacturer has exacting quality standards in place and can build multiple airplanes identical to the approved prototype.
"It can take the better part of a decade to go through both of these approval processes for a new airplane," said Dick Knapinski of the Experimental Aircraft Association in Oshkosh, Wisconsin. "When the vehicle you're building is also a car, then you have to deal with the U.S. Department of Transportation and the National Highway Traffic Safety Administration.
"The things you do to get automotive approvals add weight. Now, are you still within limits for the aeronautical side of the program?"
Enthusiasts have a think or two coming if they assume that one need only buy a flying car and point the nose skyward to soar above the dreary highways. Nobody gets off the ground without training and a pilot's license, and no government agency will sign off on citizens' routinely taking off and landing on public roads. That's what airports are for.
A final impediment to swarms of flying cars filling the skies is the existing air traffic control system, which isn't set up to keep track of low-flying aircraft that don't have a flight plan and may impulsively change course.
"If this situation became reality, how many of those people would live to see dinner?" Morgan asked.
Despite this mountain of obstacles, there's no reason to expect the allure of flying cars to wither. What child hasn't imagined floating around the house a few feet above the floor, looking down on things from a new perspective? It's a small leap from there to the great outdoors. And the march of progress may yet bring flying cars within reach.
NASA and the U.S. Federal Aviation Administration are working on a GPS-based technology called an Automatic Dependent Surveillance-Broadcast, or ADS-B, receiver that would help multiple light aircraft coordinate flight paths and request changes. The proliferation of gasoline-electric hybrid drive componentry in the auto industry could also help get weight and complexity out of powertrains. Terrafugia is working on that idea.
© 2014, The New York Times News Service