"Jim Logajan" wrote in message
.. .
"Capt. Geoffrey Thorpe" The Sea Hawk @See My Sig.com wrote:
"John" wrote in message
...
...
It's supposed to be fairly stable because the thrust reaction point
is well above the CG, so there is a strong pendulum effect. They
claim it's better than a helicopter.

An often made, completely wrong assumption - "pendulm effect" - ain't
no such thing for an object in free flight.

Technically speaking, I don't know under what assumptions one could ever
claim an object moving in a fluid is ever moving "freely". More below on
the importance of this point....

Yes, aerodynamics play a big part in real life.


Early rocket experimenters often attempted use "tractor" engines
assuming that it would provide stability - Dr. Robert Goddard's first
liquid rocket is an example. It didn't take them long to figure out
that they were wrong.

But a rocket and an rotorcraft aren't equivalent under all cases of
interest. For example, if your rotorcraft's engine fails, then because it
is traveling through a fluid the craft will rotate so the center of
aerodynamic pressure is above the center of gravity.

The "jet pack" that is the topic of this thread has two ducted fans. When
they quit, it's game over.

You are correct the center of gravity will align with the aerodymanic
center of effort. But, where are the Cp and Cg on the "jet pack" with
ducted fans in the "tractor position" - and how much stability will it add
in a hover?

--
Geoff
The Sea Hawk at Wow Way d0t Com
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