The purpose of the Portable Flyer is to travel fast at long distances, not to loiter hovering for fun. The hovering is to be used for special cases (like rescue).
With the big (~15ft diameter) propellers you suggest,
it would be good to hover but not to travel fast,
like the Chinook CH47 which is better for hovering / auto-rotation than the OSPREY V22, but worse for covering at high speed / high mileage long distances.
As for the “pendulum stability” you talk about, it is wrong.
Intuitively it seems correct, however it is wrong.
Like you, Mayman (JetPack manufacturer and pilot) thinks the same:
(the following is from another forum)
Quote from Mayman’s interview with New Atlas’ Loz Blain (https://newatlas.com...-speeder/58822/ ) :
“Jetpack Aviation's David Mayman on his upcoming Speeder flying motorcycle
. . .
Indeed, he's become one of the four superhero horsemen of a new personal flight revolution. Mayman, with his calm, methodical aviator's approach and multi-turbine jetpack, plays Buck Rogers. On the other end of the scale is Frenchman Franky Zapata, an extreme sports nut and former jet-ski champion whose turbine-powered Flyboard Air has been seen thundering across air and water throughout Europe and the US. Zapata makes a natural Green Goblin.
Then there's Richard Browning, a British martial arts master with a multi-turbine jet suit and a company called Gravity. Browning's suit places jets not only on his back, but on his arms, making him a shoo-in for Iron Man. And standing slightly apart from them all is ex-Swiss military pilot Yves Rossi, whose extraordinary Jetwing literally lets him dance with aeroplanes in the sky. It needs to be launched out of a plane, though, so while it certainly looks like an incredible experience to fly, it can't lift straight off the ground like the others.
. . .
But the pilot has to be on top. So the thing is literally dynamically unstable. Inherently unstable. And it has to be flown by computer. So that's what we're building. And the prototype is exactly that. The engines are clustered together, we purposely put the weight above that, and then we try to fly it.”
End of Quote.
In the specifications of the Speeder of Mayman, while the take-off weight is unknown, the price is US700,000$.
“This is nonsense. In a hover situation with an axial thruster, the stability is the same no matter how low or high the CG might be relative to the thruster. An axial thruster always exerts its force along the same axis as that axis tilts. Although somewhat non-intuitive the situation is not the same as a parachute where the thrust remains upwards as the system tilts. (A parachute needs to be above the CG)”
For most people the situation is “heavily non-intuitive”.
Despite his long flying (and manufacturing) experience, Mayman can’t get it.
So let me help with a drawing:
It is the “Broom Flyer” of a previous post modified by adding another OPRE Tilting propulsion unit at the bottom of the (yellow) pipe.
Suppose for a moment that the pilot is aerodynamically “transparent”.
Case 1: upper engine running, lower engine stopped.
Case 2: lower engine running, upper engine stopped.
In either case the pilot feels the same, and controls his flight the same way.
The force from the spinning propellers is along the yellow pipe in both cases.
In the one case the power unit is above the pilot, in the other case the power unit is below the pilot.
But the pilot is not aerodynamically “transparent”, which brings a significant difference of the two cases.
With the running engine above the pilot (i.e. with the pilot inside the downstream of the propellers), he has not only “weight displacement control”, but he has also aerodynamic control over his flight: his head and limbs act as flaps / ailerons / fins: deflecting a part of the downwash, they receiving reaction forces which give full control over the flight (yaw, pitch and roll).
With the running engine under the pilot (say, as in Zapata’s Jetpack) the pilot at take-off, at landing, at hovering and at low / medium speeds is based exclusively on “weight displacement” to control his flight. And this is not a full control.