“Now for this 100kg of thrust in 4 foot circle, it is gonna be a hurricane under that thing.
My very rough calculation gives 30 m/s air velocity to get 1000N with the 4 foot diameter.
That's a 70 mph / 100 kph wind.”
Have you ever ride a motorcycle cruising at 100Km/h?
Or at 100mph (160Km/h) ?
Or at 150mph (240Km/h) ?
It is not a punishment.
The Harley Davidson riders:
may spend all day cruising at 80-100mph with their bodies’ long axes normal to the relative air wind.
In the following image:
the third from left posture is more or less the posture of a Harley Davidson rider. The frontal area is 6ft2
The right posture shown is the posture of the Portable Flyer pilot in the downstream of the propellers (what matters is the relative speed of the air and pilot). The frontal area is 1.2ft2, i.e. 5 times smaller.
Based on the frontal areas, and supposing the drag coefficient is the same for the above two postures, the Portable Flyer pilot receives 5 times less drag force for a given air speed.
But the drag coefficient changes heavily with the body posture:
When a guy is driving a Harley (sitting posture) the aerodynamic drag coefficient is three times larger than when the same guy is driving the Portable Flyer (supine posture).
And because the drag force is linearly proportional to both, to the frontal are and to the aerodynamic drag coefficient, when riding the Harley at 100mph the guy receives 5*3 =15 times more air drag force than the air drag force his receives being inside the 100mph downstream of the propellers of the Portable Flyer.
In the following image, at left it is shown a vertical take-off, or a vertical landing, or a hovering, at right it is shown a high speed cruising:
- if riding the Harley at 100mph, the drag force on the rider is 50Kg / 500N (this force (in N) multiplied by the speed (45m/sec) gives 22kW or 30bhp, which is the power consumed to move the rider alone, not the motorcycle),
the drag force reduces 15 times (from 50Kg to 3.3Kg) when the same guy rides the Portable Flyer being into a 100mph downstream of the propellers.
Now think of a sport motorcycle cruising not at 100mph but at 150mph (240Km/h) with the rider suffering from the blasting air (we always talk for relative speeds: the air is standstill, the rider moves in the standstill air) and compare to the case wherein the same guy rides a Portable Flyer cruising at 150mph, with the pilot lying on an air mattress and receiving a more than ten times weaker drag force (above image, at right).
You also write:
“we can estimate that onlookers must stay back about 4 meters from the portable for safety during take off. God help the pilot though, he has no protection from that gale force.”
As explained, the drag force on the pilot is several times weaker than what you think.
And why the onlookers to stay close to a Portable Flyer at take-off and landing?
You also write:
“They say you cannot fight the laws of physics right...”
The good with the laws of physics is that they apply without exceptions.
According the physical laws:
- When you take off with a fuel having low energy density, your range will be shorter. No exceptions.
The sooner one gets this simple thing, the better.
By the way,
wrote, he is a engineer.
and he is a “neighbor” of Uniflow
He can easily explain to Uniflow why ethanol fuel is not a good choice for flying devices.
To put it simply:
If you ask Zapata to “cross” again (not cross, “cross”; real cross means without refueling) the English Channel with his JetPack, but this time using ethanol as fuel, he would ask for one or two additional refueling stops (to cover the 22miles of distance).
This is what the physical laws command, even “If you live in America and have the most expensive lawyers”.