Andrey Serbinenko wrote

I'm slightly disappointed by how quickly you've assumed my complete
brainlessness. Yes I know what FARs say, and what POH says, and no,
I'm not going to read this newsgroup and go break my neck the next
morning. What I was hoping to hear was an opinion of someone who's
familiar with what is involved in test-flying for normal/utility
category and how much it covers the type of stress imposed on the
airframe and systems in a barrel roll.

First Andrey, you will probably find much disagreement on this group
as to what actually constitues a "barrell roll". If conducted as
normally accepted in the aerobatic community, it consists of a
combination loop and roll which at most, places about 3g's on the
airframe. The g loading is not the problem in the utility category C-
172, its lack of power to complete the loop portion is what makes it
particularly difficult.
Below, I have copied a description of a "barrell roll". This may or may
not be the maneuver that you have in mind. It would obviously be much
easier in one of the higher powered 172s than in the O-300, 145hp
powered version that I fly. :-)

After reading the following, you might find that you really didn't mean
to say "barrell roll". :-) BTW the two following descriptions are how
I was taught to perform the "barrell roll" in Navy flight training. :-)

From:
http://acro.harvard.edu

The Barrel Roll is a not competition maneuver. The barrel roll is a
combination between a loop and a roll. You complete one loop while
completing one roll at the same time.
The flight path during a barrel roll has the shape of a horizontal cork
screw. Imagine a big barrel, with the airplanes wheels rolling along the
inside of the barrel in a cork screw path.
During a barrel roll, the pilot experiences always positive G's. The
maximum is about 2.5 to 3 G, the minimum about 0.5 G.

And another description from Kershner's Flight Instructor's Manual:

THE BARREL ROLL
• Preparation. It's almost impossible to draw a barrel roll
on the chalkboard, but a model will give the desired results.
Have the trainee study the references.

•Explanation. The barrel roll is a precise maneuver in which
the airplane is rolled around an imaginary point 45° to
the original flight path. A positive-g level is maintained
throughout the maneuver, and the ball in the turn indicator
should stay in the middle.

You may wonder why the barrel roll is taught this late,
since it appears to be so simple. Well, it is a precise maneuver
requiring particular airplane attitudes at particular reference
points, which is difficult for the average trainee to do properly
at first.

This maneuver might be considered an exaggeration of the
wingover, but instead of starting to shallow the bank at the 90°
position, the pilot must steepen it continually until the airplane
has rolled 360° and is back on the original heading. The rate of
roll must be much greater than that used for the wingover
because the airplane must be in a vertical bank at 45° of turn,
and it must be inverted at 90° of turn. The roll and turn is
continued until the airplane is headed in the original direction with
the wings level. Compare the barrel roll in Fig. 23-12 with the
same view of the wingover in Fig. 20-7.

From behind the maneuver looks as though the airplane is being
flown around the outside of a barrel. This is a very good maneuver
for gaining confidence and keeping oriented while flying inverted in
balanced flight.
Good coordination is required to do the barrel roll properly and
the trainee will show an improvement in that area after a session of
barrel rolls.

The barrel roll is generally more difficult and precise than the
aileron roll, and he may have to work on this one awhile.

Why-
The barrel roll is one of the best maneuvers for improving orientation.

Unlike the other acrobatic maneuvers covered thus far, the barrel
roll requires a constantly changing bank and pitch (with attendant
changing airspeed) and a radical change in heading (90°) while the
airplane is rolling. The average trainee probably will be looking at the
wing tip at a time when he should be checking the nose, or vice versa.
When he is able to stay well oriented in the barrel roll, he is ready to
move on to the reverse Cuban eight or reverse cloverleaf.

How-
You might use the following explanation, or develop your own:
(1) Make sure the area is clear, then pick a reference on the horizon
off the wing tip as in the wingover and lazy eight.
(2) Set the throttle to low cruise rpm and ease the nose over to pick-
up about 10 K more than used for the wingover or set up the
airspeed used for a loop, whichever is higher. Power adjustment
should not be necessary during the maneuver. You might have
some of your sharper trainees apply full power as the airplane
approaches inverted and then remind them to throttle back as the
airspeed picks up in the last part of the maneuver.
(3) Smoothly pull the nose up and start a coordinated climbing turn
(note that it will have to be at a much faster rate than was used
for the wingover) toward the reference point. (Assume that at first
the roll will be to the left.)
(4) When the nose is 45° from the original heading, it should be at its
highest pitch attitude and the left bank should be vertical.
(5) When the nose is at 90° from the original heading, you should be
looking directly at the reference point that was originally off
the
wing tipfrom a completely inverted position (momentarily).
(6) When the airplane heading is again 45° from the original, the bank
is vertical but you will be in a right bank as far as the ground
is
concerned; that is, the right wing is pointing straight down at
this
instant of roll. The nose will be at its lowest pitch attitude at
this
point.
(7) The roll is continued to wings-level flight as the nose is raised
back
to the cruise attitude.

The maneuver must be symmetrical; the nose must go as far above the
horizon as below. The barrel roll requires definite checkpoints to
ensure
that the airplane is at the correct attitude throughout. It is
interesting
to note that if the barrel roll is to the left, all of the airplane's
path is to
the left of the original line of flight and the airplane's nose is
always
pointed to the left of the original flight line (until it merges again
at the
completion of the maneuver). The opposite occurs, naturally, for the
barrel roll to the right.

Another method of doing a barrel roll is to pick a reference on the
horizon, turn the airplane 45° to the reference point, and proceed to
make a wide roll around this real point. One disadvantage of this
method for the newcomer is that it depends on the pilot's own
judgment of how large the orbit around the point should be. For an
introduction to the maneuver, the first method is usually better, but
you
may prefer the second and work out your own techniques of instructing
it.

Demonstration.
Try not to lose the reference point yourself while demonstrating
this one. You may find your explanation is not keeping up with the
airplane, which usually results in sputtering and stuttering while the
maneuver proceeds to its foregone conclusion -and then you have
to do a new demonstration. Don't worry, this will happen plenty of
times during your career of instructing aerobatics -when your mouth
can't keep up with your brain or the maneuver-and it can ease tension
if you react to it with humor.

Usually the trainee is surprised to see the same wing tip back on the
reference point and may confess that, like the first snap roll, the
earth
and sky were blurred and he had no idea where the reference was
during the maneuver.

Practice.
You may rest assured the trainee will "lose" the reference point during
the first couple of barrel rolls. He'll usually stare over the nose,
seeing
nothing but blue sky or ground and not really seeing the point at all.

Common errors during barrel rolls include these:
1. Not pulling the nose high enough in the first 45° of the maneuver,
which means that the highest and lowest nose positions are not
symmetrical to the horizon.
2. Not maintaining a constant rate of roll. Usually things are fine at
the
45° position; the nose is at its highest pitch and the bank is
vertical.
As you approach the position of 90° of turn you will probably find
that he is not going to be completely inverted at that point and
will
have to rush things a bit to make it. The usual reason is that he
did
not maintain a constant rate of roll. Remember that the nose is up
and the airspeed is slower in this segment of the maneuver, so the
controls must be deflected more to get the same rate. This is where
coordination comes in. Watch for it in particular.
3. Letting the nose drop after passing the 90° point; losing too much
altitude and gaining excess airspeed.
4. Failure to roll out on the original heading; having the wing tip well
ahead, or well behind, the reference when the maneuver is completed.

Evaluation and Review.
Review each barrel roll briefly in the air, and have the trainee use
the model on the ground. This one can be hard to "see," so go over it
again as necessary after getting on the ground.

By the time a half-dozen barrel rolls have been practiced, the
average
trainee should be oriented throughout the maneuver even though he may
still have minor problems of heading and symmetry. After a dozen rolls
he should be starting to work on a constant roll rate and starting to
ease
his heading problems. After several hundred, he may begin to be
satisfied
with his barrel rolls but will realize that constant practice is
required.