Originally Posted by
dook
We might return to Sir Isaac. A derivation of his second law of motion is F=Ma.
F = force
M = mass (quantity of matter per unit volume.
a = acceleration (rate of change of velocity)
Thus: for a given mass and g being gravitational acceleration the derived force is called weight.
Therefore if g were zero the aeroplane would be weightless.
Actually, as alluded to by a previous poster, defining straight and level flight as 1g is just a convention.
When I was first studying aeromechanics some 40+ years ago, in the UK, the convention on this side of the water was to define S+L flight as 0g. Apart from anything else, that makes concepts like "stick force per g" much more intuitive, since of course stick force in straight and level flight is zero.
We were, however cautioned that other less enlightened nations used the 1g convention, so we should be prepared to meet that in our careers and now, like many other transatlantic imports, it's pretty well universal.
But to return to the topic - whichever convention you choose, the proposition that you can fly a loop while pulling no more g than in level flight is clearly ludicrous.