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Barrel roll And g's Quest.



 
 
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  #1  
Old July 9th 03, 06:38 PM
Robert11
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Default Barrel roll And g's Quest.

Hello:

I'm not a pilot.

Saw a show on the first 707 on the WINGS TV channel regarding the barrel
roll
the test pilot did (unexpectedly) on the first test flight of the dash-

They said that he was able to maintain a constant 1g during the maneuver.

Here's where i'ma bit confused.

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
(tending to pull the engines away from the wings-the same
as if in a normal level flight)

If so. what happens to the 2g when the plane is at the 3 and 9 o'clock
positions?
The resultant there would be over 1g, wouldn't it ?

What am I misssing, or mis-interpreting ?

If someone could walk me thru the maneuver, and the resultant g's at the
various
positions, would be most appreciative.

Also, could exactly the same result(s) be obtained via an aileron roll ?

Thanks,
Bob


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  #2  
Old July 9th 03, 10:09 PM
DSowder
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Default

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
  #3  
Old July 10th 03, 02:05 AM
Spencer Suderman
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Default

I *think* Doug has just admitted to performing aerobatics while looking
at the instruments.....I have been using a technique that's been around
since man started flying. It involves using the gluteous maximus and
it's relationship to the ergonomic interface installed within the
cockpit of the airplane....

I like to perform barrel rolls when I take people for rides. It is not
very physiologically demanding while being very visually exciting. I
have gotten the g meter to read as high as +1.5 and as low as +/- 0
after much practice at my stick technique. I usually do the manuever at
+2 g's and +.5 g and it "feels" right.

-Spencer



DSowder wrote:
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


  #4  
Old July 10th 03, 02:45 PM
Dennis N. Yugo
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Posts: n/a
Default

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
  #5  
Old July 10th 03, 04:22 PM
DSowder
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Default

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
  #6  
Old July 14th 03, 09:13 PM
Andrew Boyd
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Posts: n/a
Default

wrote:

first 707 - barrel roll


To be picky, the 367-80 did an aileron (aka ballistic)
roll, not a barrel roll.

A barrel roll is actually best thought of as a spiralled
loop, which by necessity is going to pull 3 to 4 G's.

An aileron roll is much more gentle: get some speed,
pitch up, neutralize in pitch, then full aileron, and
wait 'til it's blue at the top again. The reason you
pitch up is to avoid an excessive nose-down pitch angle
at the end, which results in loss of altitude and excessive
airspeed.

The slower the roll rate, the greater the initial pitch
angle required, and the greater the initial airspeed required.

If you reset the G-meter, you should see +2 and +0.5 as the
telltale values after the aileron roll. +2 during the pullup,
and +0.5 while inverted.

Then there is the completely misnamed slow roll, in
which the aircraft rotates around it's longitudinal
axis, at any rate of roll - faster is easier. The
slow roll can be done in the horizontal or vertical,
or on any angle inbetween.

And then we have the point (aka hesitation) roll,
which has pauses during the rotation of the slow roll.

Then there is the snap roll, which is a violent, high-G,
stalled, yawing maneuver, which is a cousin of the spin.
A snap roll can be performed with either positive
or negative G. High torsional loads are imposed on
the airframe. Gyroscopic precession of the propeller
can be very hard on the crankshaft, too.

Then there is dutch roll, which has to do with
the roll/yaw coupling of swept-wing aircraft - this
oscillation was especially noticeable in aircraft
with undersized vertical fins such as the 707. IIRC
the initial "small-tail" variants of the F-100 and
MiG-21 and many others were later upgraded with
larger vertical fins.

There's a funny story behind why the F-86 didn't
suffer from dutch roll, but that's getting way
off-topic.

--
ATP www.pittspecials.com
  #7  
Old July 16th 03, 02:51 PM
Andrew Boyd
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Andrew Boyd wrote (about kinds of rolls):

Sigh. I forget to mention probably my favorite kind
of roll, the torque roll:

The aircraft is pulled (or pushed) to the vertical with
full power and full aileron deflection opposite the torque
of the propeller: left stick with a Lycoming, right stick
with an M14P.

If the aircraft is kept balanced perfectly vertical - no
barrelling - it will stop, then start to tailslide backwards,
and the aircraft will continue to rotate opposite the torque
of the propeller. Remember, when the IAS goes to zero, the
flight controls (at least the onew outside the slipstream)
have no air to push against.

Here is one of the finest pilots in the world - a Lithuanian
by the name of Jurgis Kairys - demonstrating a torque roll.
You can tell from the direction of rotation he's behind an
M14P (or M14PF):

http://www.jkairys.com/common/movies...002_2nd_ex.mpg

The trick with a torque roll is that the more rolls you
do backwards, the faster you will end up going, and the
more violent the ensuing pivot will be. Recovery is power
off, both feet firmly on the rudders, both hands on the stick
and arms locked to avoid damaging the flight controls.

Also, one needs to think a bit about the use of smoke during
the torque roll.

--
ATP www.pittspecials.com
 




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