Much effort has gone into the development of roadable airplanes or flying cars.  Among others there was the 1917 Curtiss Autoplane, the 1938 Waterman Arrowbile, the 1939 Pitcairn Roadable Autogyro, the author’s 1939 sketches of AeroAuto, the 1946 Convair Skycar IV, the 1946 Fulton Airphibian, the 1947 ConVairCar, and the 1947 Plane-Mobile.  Molton Taylor’s Aerocar was one of the most promising.  It was granted full FAA certification in 1956, but only seven were produced.  The author is indebted to Peter Bowers, and his book, UNCONVENTIONAL AIRCRAFT, for much of the above information. 

Graduating from two-dimensional travel on the ground to three-dimensional travel in the air, and adding the limitless number of free ready-built as-the-crow-flies paths the air provides, would seem like a tremendous forward (and upward) leap.  In reality, the use of roadable airplanes as a dualmode transportation system has great disadvantages.  Our fixed paths on the ground (roads and rails) not only avoid the danger of falling but also provide a huge amount of protection from collisions.  An automobile driver usually only needs to look ahead, while a safe airplane pilot must constantly look in all directions, including up and down.  And in the air there are additional fallible humans involved—the air-traffic controllers. 

        Many airports are already operating at capacity; the traffic jams resulting from putting most of our cars into the air would be unthinkable.  Surface dualmode offers huge capacity in a single lane because the cars can travel safely extremely close together.  Highway capacity is many times less per lane because there we must allow a hundred to two hundred feet between cars for safety.  In the infinite and unmarked paths of the air we must maintain not just feet but often miles between vehicles for adequate safety.  Even with that third dimension, safe capacity in the air may be less than it is on the highways, depending upon the collision-avoidance equipment we have and the rules we adopt. 

If a high percentage of the population were to use roadable airplanes for daily transportation we would have to have a huge number of new airports, and the roads to the airports would be jammed with vehicles.  The airspace would also be jammed and very unsafe, especially under conditions of poor visibility.  Vertical-takeoff-and-landing hybrids would reduce the problems only slightly.  Midair collisions would become more common than ground-traffic collisions, and the fatality rates would be much higher. 

        The requirements for airplanes are so different from the requirements for cars that hybrids are always poor airplanes, poor cars, or both.  To date the only satisfactory power plants for airplanes burn fossil fuels, so roadable airplanes would still be polluting, noisy, and on the ground when the oil is gone.  Also, roadable airplanes would have to meet vehicle and operator licensing requirements for both the highways and the airways—a bureaucratic nightmare. 

“The Jetsons.” use appealing-looking personal air cars in their comic strip, but to meet the requirements of the real world these would have to be automatically navigated and controlled, provide vertical take-off and landing, be non-polluting quiet safe and affordable, use no fossil fuels, fly in all weather, and somehow solve the difficult traffic problems we have mentioned.  All of these requirements won’t be met for fifty years, if ever. 

  Amphibian automobiles, a few of which have been built, are also dualmode vehicles, but they would be even less of a solution for our traffic problems than air cars would be.


“An alternator may be used to generate electrical power when driven mechanically, or it may be used as a motor to develop mechanical power when driven electrically.  An alternating-current generator and a synchronous motor may be one and the same machine.” --E.A.Loew, in DIRECT AND ALTERNATING CURRENTS, McGraw-Hill.

Professor Loew was thinking in terms of rotating synchronous machines, but his statement also applies to linear synchronous machines.  The linear magnet coils in the guideways, interacting with the magnets under REV cars, will act as motors to propel the cars whenever thrust is required to keep them at synchronous speed.  But if the cars are going down hill these machines will act as AC generators (alternators) and pump electrical energy back into the system. The cars will run at exactly the speed established by the frequency of the applied power in either case, but the phase of the sine wave of voltage generated by a synchronous machine will lag that of the applied voltage slightly when the machine is acting as a motor, and lead it slightly when the machine is acting as a generator. 

If we have more than one car in a string on an exit-ramp they will have to be synchronized in order to keep the spacing between the cars constant.  All of the linear motors in an exiting string must remain locked into the ramp frequency, and that frequency must gradually decrease in order to decelerate the string of cars as a unit.  They would then all come to a stop at the same time on the final portion of a single exit ramp.  Their drivers would then drive off onto the streets one by one, much as cars in line at a traffic light start up when the light turns green.

An exit ramp will be provided with the AC guideway power until the last car of an exit train entering it has demerged from the guideway.  At that point the guideway power to the ramp will be automatically switched off, but the cars will still be electromagnetically connected to each other through the ramp.  If any car in the train then tends to travel faster than the others, its “motor” will start to act as an alternator, and phase shift will produce a regenerative braking effort, which will keep that car from closing the gap in front of it.

Likewise the motor of any car in the train with a tendency to slow down faster than its mates will draw enough generated power from the other cars in the train to hold it to exactly the speed of the other cars at that moment.  No cars could ever lead or lag by more than a small percentage of one cycle of the alternating current frequency being generated at that moment; they will decelerate precisely together; the distances between cars will remain constant.  They will act like a railroad train coasting to a stop, but their couplings will be electromagnetic instead of mechanical.

For a while after an exit train has demerged from a guideway it will decelerate naturally, since the aerodynamic drag due to its yet high speed will rapidly absorb kinetic energy from the cars.  As the velocity of the train decreases, however, the drag will become insufficient to produce deceleration rates consistent with affordable exit-ramp lengths.  Remember that the cars are still levitated and without ground or wheel friction.  They would coast and coast; and the brakes in the cars can’t be used with the road wheels off the ground.  Therefore, to supplement the rapidly dissipating aerodynamic drag, we will put an electronically controlled “brake” on the frequency of the alternating current that is being generated in the exit-ramp coils by the cars passing over them. 

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Last modified: August 02, 2006