Myth: 1 G barrel rolls are impossible.

On Jun 11, 1:09 pm, Jim Logajan wrote:
Myth:

It is impossible to perform a barrel roll such that the pilot feels exactly
1 gee of force perpendicular to the floor of the cockpit. (Barrel roll is
defined here as the maneuver depicted by the definitions and diagrams on
these website:http://en.wikipedia.org/wiki/Barrel_...arrel_roll.jpg)

Fact:

The aspect that I think appears to mislead people is the presence of a
gravitational field and an implied requirement that the axis of the helix
must remain straight and parallel with the (flat) ground. But the latter
requirement can be dispensed with and still yield a recognizable helical
flight path - and that is enough to make a 1 gee barrel roll possible. The
"trick" is accomplished by superimposing two equations of motion:

(1) Start with a "zero gee" parabolic trajectory. So basically the plane
travels laterally over the ground while first traveling up (and then down)
such that the pilot would feel weightless absent any other motions. The arc
is a classic parabola.

(2) Superimpose by vector addition the centrifugal force of the plane
"flying" a circle around (and along) the moving center established by the
parabolic trajectory in (1).

(3) Set the radius and angular speed of the circle in (2) to yield one gee
equivalent force and rotate plane's attitude to keep the centrifugal force
vector perpendicular to the floor. End of procedure.

A reasonable nit pick is that the axis of the helix of the barrel roll
doesn't remain "straight and level." But none of the definitions explicitly
state that requirement. And in any case, it is possible to end the 1 G
barrel roll at the same altitude at which it began.

So there. :-)

(If there is a demand (and I can find more time) I can work out and post
the complete set of equations of motion.)

The answer to your question as you ask it is no. You can not perform
a "Barrel roll" and maintain 1 G. We all have 1 G pressing on us as
we are sitting at our desks, or flying straight and level in an
airplane. To perform a barrel roll, you pick a point 20 degrees off
heading (usually to the left in aircraft with US engines). You then
must execute the beginings of a loop by applying back pressure on the
stick. You can not do this without adding additional G forces. You
should be at 90 degrees bank when you are just over the point you
selected 20 degrees off the origional heading. As you continue the
roll, you will be at a point 40 degrees off the origional heading when
you have completed 180 degrees of roll and your wings should be level
with the horizion in the inverted position. As you continue the roll
the nose of the aircraft will be 20 degrees below the horizion and at
a 90 degree bank when you are back at the point 20 degrees off the
origional heading. You now continue the last quarter of the roll
while "pulling" to wings level - again you can not do this without
adding G.

I have done thousands of barrel rolls - and have done them with open
bottles of water on the dash - same principle as swinging a bucket of
water over your head and not spilling any. As long as you keep
positive "G" (not gee) force on the plane - the water will not spill -
let it go negative and you will have a mess.

If the question you are asking is can this maneuver be done by adding
1 additional G unit (now you would be at 2 G's) the answer is you
could rotate around and probably not spill the water, but you would
not execute what is considered a "Barrel Roll" - it would be more of a
sloppy aileron roll where you end up lowing altitude from your
origional position.

A "slow roll" is one where the aircraft follows a straight line and if
you are doing these on a horizontal line you will not keep "positive"
G's on you and the aircraft.