I don’t quite understand why the AvroCar didn’t reach more than 3 feet? Was it a ground effect keeping it at that height and it didn’t have the capability to fly higher?
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From what can be gleaned from the Net, it was instability. The Avrocar had plenty of power but was unstable. The resulting behavior made it uncontrollable when leaving ground effect. In this publication I found an illustration which tries to explain what went on:
Any asymmetry in flow would shift the center of pressure, resulting in a wobbling motion called hubcapping (because it resembled the motion of a hubcap when dropped to the ground).
From the linked document:
On September 29, the first attempt of sustained flight was made with the Avrocar pinned to the ground by cables. After that the vehicle took off, an uncontrollable oscillation occurred with each wheel alternately bouncing on the ground. […] These first captive flight[s] revealed then a new problem, called “hubcapping” which has never been fully resolved. The hubcapping was rapid and unpredictable swings in pitch and roll axes.
answered Jun 11, 2019 at 19:32
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3$\begingroup$ I once had a teacher who worked on the program as a technician at Avro Canada. If it was designed today with modern synthetic stability they might have something. But even then I expect the overall efficiency would be way less than a regular helicopter from a lift perspective. A lot of it seemed to just be that 50's infatuation with "flying saucers". $\endgroup$– John KCommented Jun 11, 2019 at 21:15
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$\begingroup$ @JohnK: Conventional aerodynamics tells us that the center of pressure on a circular wing is at 24% of the mid chord. If you want to change direction without rotating the disc, half the lift must be added by vertical blowing over the rear half to balance the craft. Even with fast, digital flight controls the amount of control power needed makes a flying saucer simply not worth it. $\endgroup$ Commented Jun 11, 2019 at 22:14
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3$\begingroup$ Your point is taken but in the Avrocar's case the body was never intended to function as a circular wing, as there was no means to propel it along so that the body could operate at an angle of attack like an airfoil. The center fan driven by the exhaust of the 3 tangential turbojets buried inside produced almost all of the lifting force, blowing straight down, and the air diverted directed out around the D shaped ring and duct was for supplemental lift and attitude control. To gain speed it had to tilt the body to tilt rigid fan's thrust vector. In effect a kind of bizzaro-world helicopter. $\endgroup$– John KCommented Jun 12, 2019 at 1:08
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6$\begingroup$ @JohnK: Even if it is not intended to produce aerodynamic lift, it will do so if given the angle of attack. So the slightest misalignment with the zero-lift angle will produce highly destabilising forces. What a concept! $\endgroup$ Commented Jun 12, 2019 at 4:57
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$\begingroup$ It might have worked if they have made it completely unaerodynamic like this one, which predated any computer control system: youtube.com/watch?v=f8W2SI4c93s. $\endgroup$ Commented Jun 12, 2019 at 12:37