> Zik Saleeba wrote:
>> providing approximately 6kg of thrust.

> Not likely since Kg is not a measure of force.

PICy picky :-).
While that is strictly true, there are many exceptions to
such general usage that allow such nominally "lazy"
statements to be made in polite and even informed company.

eg the pound mass is not a valid unit but is embedded in the
psyche of the US and UK community. What Y'All think is a
pound is actually 1/g th of a slug, but slugs never caught
on. Slugs are to pounds force what Kilograms are to Newtons,
which is Olin's point.

And, properly, one should ask for sold-by-weight material by
it's force value rather than it's mass. Which the lbf is.
But I am not aware of anywhere in the world (though such
perverse places may exist) where eg dead fish is sold by the
Newton.

And even steely eyed rocket men have haggled for countless
thousands of hours over many many decades over the
proprietary of mixing force and mass units in certain
classical instances. eg the unit of merit of a rocket motor
and propellant combination is Isp = "specific impulse".
Some, indeed many, declare the units thereof to be "seconds"
as it is eg the number of seconds that a pound mass of
propellant will produce a pound force of thrust, OR,
pertinent here, the amount of time for which a kgm or
propellant will produce a kgf of thrust. Cancel the units of
eg kgm/kgf and you get seconds. Many argue volubly, as
would, perhaps, Olin,  about the impropriety of such a
cancellation of mixed units. But seconds is "easier" than
saying lbm/lbf or kgm/kgf.

So, mass units as force units is a time honoured aberration
in the thrust using community and, interestingly, more
understood by the general public than the use of the
"correct" units. If Zik had said that his UAV would produce
about 60 Newton of thrust fewer would have understood the
order of force involved. Or, that to be VTOL capable that it
would probably need to "weigh" less than 6 kilogram (even
though kilogram is NOT a valid weight unit :-) * ).

* "It masses about xxx kg" is one way around this.
I haven't met anything being said to eg "mass under 0.4
slug", as Zik's UAV will.

        Russell




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