Barrel roll And g's Quest.

In article ,
says...
Seems to me that at the top of the roll, he would have had to be rolling at
a rate
sufficient to have centripital force equal to 2 g; such that when you
subtract the normal downward
1 g, there's a resultant 1 g left acting in the conventional direction

You're mixing force and acceleration. First of all, it is not possible to do a
barrel roll with a constant 1g, simply because an airplane is "pulling" 1g in
level flight. A barrel roll involves a climb, so at the instant of departure
from level flight, the "g" increases a bit, regardless of how gentle the
manouver might be. Similarly, near the finish of the barrel roll, the nose will
be down and must be brought back to level flight; that takes more than 1g,
since level flight itself is 1g.

That being said, the "g" the pilot talks about is what he sees on the "g"
meter, which measures the force resulting from acceleration in the direction of
the "z" axis. If the plane is inverted (and level) and showing 1g positive, the
actual acceleration toward the earth is, as you've surmised, 2g's, or 64 ft.
per second per second. But the resulting apparent force on the pilot and on the
g meter is (the result of) 1g. So, the g meter really measures force resulting
from acceleration, rather than acceleration itself. You clearly know how to
analyze this, so now that you know how a g meter works, it'll be easy.
Obviously, an airplane in level flight, at which time its g meter indicates 1g,
is NOT accelerating toward the ground at 32 ft. per second per second. Nor is a
g meter sitting on my desk, indicating 1 g. BTW, the g meter reads zero when
flying a vertical line up or down, and it reads -1 when flying straight and
level inverted.

This brings up an interesting observation that I made an evening or two ago
while practicing snap rolls and trying to snap with the minimum possible stick
pull (thus, minimum energy loss). When snapping on a 45 degree up line, more
stick movement is needed than when snapping on a level line. Why? Well, the
stick is pulled to load the wings to a critical angle of attack close to a
stall, so that when the rudder is kicked, one wing stalls (or nearly so) and
the other grabs a chunk of lift to autorotate the airplane. When on a 45 degree
upline at a constant airspeed (it's almost possible), the g meter reads only
.707 g (got my bifocals tuned up for that observation). Hence, it takes more
stick pull (motion, not force) to load the wing to the critical angle of
attack, since the angle of attack was less to start with.

Nuff said, maybe someone who is actually an expert will chime in...

Doug Sowder

I went out once and tried to do a barrel roll with a constant 1g
(obviously, as you indicate, a little more at the the initial pull-up and
again at recovery); it was pretty scary cause I lost so much altitude and
gained so much airspeed.

As to your snap roll observation... in level schnapps do you start with
full power? It takes more pull and g's to snap with full power, as you
would in a climbing line, than it does with reduced power. I know myself
that I keep to 120 mph or less in a level or diving snap, while I kind of
throw away the book going up because I need all the airspeed I can get.
Higher snap speed also will mean greater stick forces and g's.

You probably know all this, unless you think I'm totally blowing smoke.
--
Dennis Yugo
http://www.worldpassage.net/~dyugo

As to your snap roll observation... in level schnapps do you start with
full power? It takes more pull and g's to snap with full power, as you
would in a climbing line, than it does with reduced power. I know myself
that I keep to 120 mph or less in a level or diving snap,

I, too, try to keep level or down snaps at 120 or below, and whatever power it
takes, depending on what came before.If the snap follows a half loop up or some
other speed-losing stunt, I just leave the throttle open till I get my 120.
Unless, of course, it's a combination with, say, a half loop up with half snap
attached. If following, say, a push 5/8 loop as in this year's Advanced known,
I reduce power on the 45 down to make the line before the snap a bit longer, so
I can lengthen the line after for more speed out. There's a difference between
constant speed and fixed pitch prop, too. With the CS prop, the gyro is always
at full speed, whereas with a fixed pitch, it varies.

You're right about the barrel roll and the altitude loss if trying to maintain
approx. 1g. I think that the most customer-friendly roll starts out as a barrel
roll, but with an increased roll rate at the highest point of the arc, and
going to about 1/2 g when inverted. Or even 1/4 g, just enough positive so
they're not hanging on the belts.

I've been told that a "military" barrel roll, as taught to guys like my dad who
took PT in Stearmans, by definition has a 90 degree heading change at, shall we
call it, the apogee. Finishes on original heading, of course. I taught myself
these...they are very fun, but can't stay below about 3 g's in the Pitts. Then
I went out and demo'd them to a friend in his stock 220 Stearman. Those were
REALLY fun! A plane that actually uses wings flys so much differently...

Doug