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>Since a properly executed barrel roll can be a 1-G maneuver, it's technically possible in any aircraft. Although it's a low positive G maneuver it's not 1G, o

by pedrocr 10y ago

>Since a properly executed barrel roll can be a 1-G maneuver, it's technically possible in any aircraft. Although it's a low positive G maneuver it's not 1G, only straight and level flight is 1G, everything else requires a deviation from that to change direction. Wikipedia says it will vary between 0.5 to 3G throughout the maneuver which should be possible in almost all aircraft as it keeps fuel flowing and isn't a major stress on the airframe. It's probably even possible to do it at no more than 1.5G if you do it large enough so that it's even comfortable and not particularly noticeable for the passengers that don't look out the window.

throwaway91111 10y ago
I don't think this is correct. There's a constant 1G force; you can adjust for the difference necessary in angular momentum as you turn. Sure you won't be perfectly level in the same way have a constant rotational velocity would be but it's still a barrel roll. Of course, it would point in a different direction, but the magnitude would be 1G. Hell, i think you could easily develop an intuition for it.
- falcolas 10y ago
  The 1G is from the frame of reference of the passenger. Gravity's effects are essentially nil for the passengers (and the aircraft itself), since everything in the aircraft is accelerating in the same way at the same time. You only really feel the force of the lift generated by the wings.
kobeya 10y ago
Physicist here. There is no law of the universe that prevents flying an absolutely perfect 1G barrel roll. Maybe those numbers are assuming perfectly steady thrust and a constant angle of attack, or true circular motion? That's the best I can think of.
- LeifCarrotson 10y ago
  How do you ever ascend or descend without accelerating up or down?
  - falcolas 10y ago
    It depends on your frame of reference. If you're on the space station, are you falling down with 1G of force? Yes, but so is everything else around you, so we consider this to be effectively weightlessness. An aircraft which is freefalling will be experiencing 0G's in that downward direction, but if the aircraft's wings are creating 1G of force perpendicular to the force of gravity, the only forces experienced within the frame of reference of the aircraft is that 1G sideways.
- mikeash 10y ago
  If you maintain exactly 1 gee, then the moment your lift vector deviates from vertical you'll begin to accelerate downward, since the vertical component of your lift vector will no longer cancel out all gravitational acceleration. At the end of the maneuver you're going straight and level again, which means the vertical velocity you built up needs to be eliminated. The only way to do this is by accelerating at more than one gee for some period of time. You can stay arbitrarily close to 1 gee, given unlimited time and altitude, but you can't stay exactly at one gee throughout a barrel roll.
  - comex 10y ago
    edit: I'm wrong, leaving this for posterity. You're neglecting two potential sources of upward acceleration. One, the turn itself, or in other words air resistance: if you stop turning when you're pointed straight up, clearly you're going to go up, not down (at least to start with), which means the turn accelerated you upward. And two, any forward acceleration provided by the engine while "forward" isn't horizontal. (I don't know enough about aerodynamics to actually determine how a barrel roll actually works, though, only enough to contradict your post :)
    - mikeash 10y ago
      I'm not addressing any sources of acceleration. I'm only looking at the final acceleration vector. If the magnitude of that vector is 1 gee, then there are only two possibilities. One is that it perfectly opposes gravity, resulting in zero net acceleration. The other is that it doesn't perfectly oppose gravity, resulting in downward vertical acceleration. It is not possible for a 1 gee acceleration vector to result in upward vertical acceleration, regardless of what causes the acceleration.
  - falcolas 10y ago
    > At the end of the maneuver Here's the difference, once the plane's wings are level with the horizon, the roll is considered to have been completed. The rest (regaining a stable pitch) is recovery. Yes, you are correct that the aircraft's velocity is not maintainable after the maneuver has been completed, and must incur positive G forces to regain level flight, but it's not technically part of the barrel roll. EDIT: As I noted in another response (in which I go into a lot more detail), the pilot probably doesn't even have to take any action to negate the downward velocity component; the change in the angle of attack (the angle at which the wing intersects with the airflow) would naturally increase the amount of lift being generated by the wing, at the cost of more drag.
- pedrocr 10y ago
  You know of a path that keeps the acceleration 1G and pointing downwards at all times? Or you mean 1G in total acceleration that can point in any direction? I'd be curious to see a reasonable path for the second and don't see how the first can be possible (and 1G usually means more than just an acceleration vector of length 1 otherwise we wouldn't talk about negative G).
  - deleted 10y ago
    [deleted]
  - falcolas 10y ago
    The second is an aileron roll done purely with the ailerons, engine and tail, not using the lift generated by the wings. Not all planes can do it because the engine needs to be very powerful to support horizontal flight, and the wings need a neutral airfoil shape to provide lift when upside down. Trying to come up with a better explanation for the first. EDIT: Try this on for size: Given - the force imparted by the wings is 1G (enough to cancel the force of gravity), and will always be pointed straight through the roof of the aircraft. The pilot takes no action to increase the amount of lift. The G's are measured from the frame of reference of the passengers in respect to the aircraft. Rotational forces are not considered - most people are unable to kinesthetically perceive any rotation which occurs at less than 5 degrees per second, and don't realize that they could be upside down and still feeling like they're right side up. It's why instrument training involves so much instruction and reminders to trust the instruments, not your body. The maneuver is initiated by rolling the plane (clockwise, with respect to the pilot) with the ailerons. Since no effort is made to change the amount of force being generated by the wings, the downward component of that thrust (as measured from an external reference point) will lessen, allowing the aircraft to start accelerating downwards while also accelerating to the right. The force felt by the passengers is still exactly 1G - the force created by the wings, and it is still pointed vertically through the plane. Since the downward component affects the passengers and plane equally, there is no measurable effect on the G forces with respect to the passenger's frame of reference. The aircraft reaches 90 degrees, and is accelerating to the right at 1G, and downwards at 1G. The downward force is not felt by the passengers, again because their entire frame of reference is accelerating at the same speed, only the force pushing them into their seats. The aircraft reaches 180 degrees, and is now accelerating at 2Gs downwards. Passengers are still feeling only 1G of pressure from the seat. 270 degrees - the acceleration to the left cancels out the previous acceleration to the right, passengers are still being just pushed into their seats. 360 degrees - the maneuver is complete. The plane is significantly lower and some distance to the "right" of its original position. Their velocity now includes a significant downwards component. Now here's where things perhaps become a matter of semantics - pilots would consider the barrel roll to be completed at this point, and they simply need to recover from their new orientation. Of course, this will probably require little to no input from the pilot, it will simply happen naturally due to the changed angle of attack induced by the downward motion increasing the lift generated by the wing.
- taneq 10y ago
  Given that "1G" is here defined as "in the vertical plane of the aircraft", no it's not, unless you're willing to accept a permanent vertical delta-V (which, as a pilot, you aren't.)
falcolas 10y ago
> everything else requires a deviation from that to change direction Well, kinda. The trick is that you have 1G downwards constantly, which can be reduced by initiating a descent at the same time you introduce other directional changes. Rotation (the major component of a barrel roll) does not impart G forces, as recognized by the term. I'm sure few to zero pilots who can keep it at 1G exactly throughout the maneuver, but it is at least theoretically possible (so long as you don't mind losing altitude via the maneuver).
- pedrocr 10y ago
  >Well, kinda. The trick is that you have 1G downwards constantly, which can be reduced by initiating a descent at the same time you introduce other directional changes. This isn't possible. In straight and level flight you are experiencing 1G acceleration. To be able to change the plane's orientation in any way you need to impart some acceleration to it and thus deviate from the 1G. For example initiating a descent requires an acceleration downwards which will be felt by the passengers as less than 1G. If you finesse it enough and do it big and wide enough you can probably maintain it within a tolerance for what we consider to be 1G but it's not possible to stay at 1G exactly.
  - wongarsu 10y ago
    In any realistic scenario it's of course only possible to stay within some threshold around 1G. But why would it be theoretically impossible? If I drop the nose down slightly I reduce downward forces to say 0.95G. In the same manouver I accelerate left with 0.31G. Since 0.95^2 + 0.31^2 = 1^2, I still have a net force of 1G on the aircraft. In theory it should be possible to compute a path that keeps me at exactly 1G at all times.
    - pedrocr 10y ago
      I understand 1G as also requiring the vector to be pointing down on the aircraft. If you allow it to point in any direction as long as the vector has length 1 passengers will notice and not all mechanical parts will be guaranteed to work so the fact that it's technically 1G is not enough to guarantee any airplane can do it. I think that's not the usual definition of 1G either as we even talk about negative Gs (and airframes are particularly sensitive to them) which wouldn't make sense in that case. But it would be cool to see if there's a theoretical path that we can recognize as a barrel roll than keeps exactly 1G of acceleration in any direction needed.
Nightshaxx 10y ago
I am not at all familiar with aircraft, but in the video were Tex Johnson talks about doing the barrel roll, he literally says it is a 1G maneuver. https://youtu.be/Ra_khhzuFlE?t=86 https://youtu.be/Ra_khhzuFlE?t=86
- pedrocr 10y ago
  I read something similar in John Boyd's biography (highly recommended) about the barrel roll as well. I assume they mean it can be done while keeping the plane in a 0.9-1.1G window or something along those lines. In other words in can be done in a way that if you closed the blinds on all the windows and did it in a commercial flight full of passengers no one would complain or even be able to tell.
  - TylerE 10y ago
    That's not true. There will still be rotation that can be felt.
    - falcolas 10y ago
      Below a certain threshold, somewhere near 5 degrees a second, your inner ear can not sense the rotation. It's why you need instruments when flying without visual cues. Oh, it can also tell you you're tumbling wildly when you are actually going completely straight. Really disorienting.
    - cyberferret 10y ago
      I had a beer with a B-1 Lancer pilot who was here on exercises many years ago. He said that on low level flight infiltration exercises, his normal method to go over ridge lines and small hills is to pull the aircraft gently up, roll it inverted and pull back while cresting the ridge, then roll upright once over. He said he is experienced enough to maintain 1.0 to 1.5G throughout the manoeuvre, and the most of the time, the weapons guys in the back of the plane who are usually 'heads down' and don't have big windows to look out of, don't even realise they were inverted for 10 to 15 seconds. He said he has a standing wager with them when they land, and if the back seaters can accurately tell him how many ridgelines they crested inverted correctly, he buys them a beer. He told me he hardly ever has to buy.
Merad 10y ago
A gentleman by the name of Bob Hoover would like a word with you: https://www.youtube.com/watch?v=V9pvG_ZSnCc https://www.youtube.com/watch?v=V9pvG_ZSnCc
- ChuckMcM 10y ago
  I love that video. I've got an autographed pencil sketch of Bob's P-51 (autographed by both the artist and Bob!). And while watching him fly, the only explanation I could come up with that explains how effortless he makes it look is that he was actually a bird in an earlier lifetime.
- slavak 10y ago
  First, this video is amazing. But as the video demonstrates quite well, the first phase of a barrel roll requires pitching up into a gentle climb. This is physically impossible without exceeding 1g at any point.
  - FabHK 10y ago
    True. Needn't be much though: to put it into a 2 m/s climb (400 ft/min), you'd just need to hold 1.1 G over two seconds.
- _ao789 10y ago
  That's definitely a very cool video
- IIIIIIIIIIII 10y ago
  That demonstrates that there are no negative G forces. It doesn't demonstrate that it's 1G. I'm a private pilot who has done aerobatics, exactly 1G doesn't work - and it's completely unnecessary (though you can stay pretty close to it, so that someone with their eyes closed would not know they were rolled). But just about the video as "proof of 1G": As long as you stay positive you are fine, even psychologically with passengers not used to it. The real turning point, in real effects as well as psychologically, is when you approach 0G, the feeling only starts at less than 0.5G when you begin to feel more and more weightless. When you go even slightly negative it becomes a completely different matter, both in terms of real effects (fuel, oil, lose stuff flying around the cabin) as well as psychologically: Even though you made the harness extra tight with as much force as you could muster in preparation for an aerobatic flight with negative G forces, when you get here it feels as if you hang upside down in the harness and the seat is miles away from you, as if you dropped a few centimeters and now literally just hang in the airplane. I (in my small and somewhat underpowered aircraft) go below 1G when I get close to the top, because if I tried to maintain 1G the nose of the airplane would have to drop (towards the earth in that position), and I want to keep that at a minimum, so that I don't end the roll with the nose in too much of a nose-down attitude from which I will have to pull out. Here is an image from my flying some years ago, in a Grob 115C Acro (rented from Attitude Aviation, Livermore, CA, ca. 2002): http://i.imgur.com/Rd5VW3R.jpg http://i.imgur.com/Rd5VW3R.jpg (the inside of this airplane: http://i.imgur.com/4bwJn13.jpg http://i.imgur.com/4bwJn13.jpg) EDIT: (after reading some comments) To me a roll is over when the airplane is back to straight and level, after 360 degrees. So my statements are for that interpretation of the word "roll". And my frame of reference for g-forces are the people on board the aircraft.

by pedrocr 10y ago

throwaway91111 10y ago

I don't think this is correct. There's a constant 1G force; you can adjust for the difference necessary in angular momentum as you turn. Sure you won't be perfectly level in the same way have a constant rotational velocity would be but it's still a barrel roll. Of course, it would point in a different direction, but the magnitude would be 1G. Hell, i think you could easily develop an intuition for it.

falcolas 10y ago
The 1G is from the frame of reference of the passenger. Gravity's effects are essentially nil for the passengers (and the aircraft itself), since everything in the aircraft is accelerating in the same way at the same time. You only really feel the force of the lift generated by the wings.

kobeya 10y ago

Physicist here. There is no law of the universe that prevents flying an absolutely perfect 1G barrel roll. Maybe those numbers are assuming perfectly steady thrust and a constant angle of attack, or true circular motion? That's the best I can think of.

LeifCarrotson 10y ago
How do you ever ascend or descend without accelerating up or down?
- falcolas 10y ago
  It depends on your frame of reference. If you're on the space station, are you falling down with 1G of force? Yes, but so is everything else around you, so we consider this to be effectively weightlessness. An aircraft which is freefalling will be experiencing 0G's in that downward direction, but if the aircraft's wings are creating 1G of force perpendicular to the force of gravity, the only forces experienced within the frame of reference of the aircraft is that 1G sideways.
mikeash 10y ago
If you maintain exactly 1 gee, then the moment your lift vector deviates from vertical you'll begin to accelerate downward, since the vertical component of your lift vector will no longer cancel out all gravitational acceleration. At the end of the maneuver you're going straight and level again, which means the vertical velocity you built up needs to be eliminated. The only way to do this is by accelerating at more than one gee for some period of time. You can stay arbitrarily close to 1 gee, given unlimited time and altitude, but you can't stay exactly at one gee throughout a barrel roll.
- comex 10y ago
  edit: I'm wrong, leaving this for posterity. You're neglecting two potential sources of upward acceleration. One, the turn itself, or in other words air resistance: if you stop turning when you're pointed straight up, clearly you're going to go up, not down (at least to start with), which means the turn accelerated you upward. And two, any forward acceleration provided by the engine while "forward" isn't horizontal. (I don't know enough about aerodynamics to actually determine how a barrel roll actually works, though, only enough to contradict your post :)
  - mikeash 10y ago
    I'm not addressing any sources of acceleration. I'm only looking at the final acceleration vector. If the magnitude of that vector is 1 gee, then there are only two possibilities. One is that it perfectly opposes gravity, resulting in zero net acceleration. The other is that it doesn't perfectly oppose gravity, resulting in downward vertical acceleration. It is not possible for a 1 gee acceleration vector to result in upward vertical acceleration, regardless of what causes the acceleration.
- falcolas 10y ago
  > At the end of the maneuver Here's the difference, once the plane's wings are level with the horizon, the roll is considered to have been completed. The rest (regaining a stable pitch) is recovery. Yes, you are correct that the aircraft's velocity is not maintainable after the maneuver has been completed, and must incur positive G forces to regain level flight, but it's not technically part of the barrel roll. EDIT: As I noted in another response (in which I go into a lot more detail), the pilot probably doesn't even have to take any action to negate the downward velocity component; the change in the angle of attack (the angle at which the wing intersects with the airflow) would naturally increase the amount of lift being generated by the wing, at the cost of more drag.
pedrocr 10y ago
You know of a path that keeps the acceleration 1G and pointing downwards at all times? Or you mean 1G in total acceleration that can point in any direction? I'd be curious to see a reasonable path for the second and don't see how the first can be possible (and 1G usually means more than just an acceleration vector of length 1 otherwise we wouldn't talk about negative G).
- deleted 10y ago
  [deleted]
- falcolas 10y ago
  The second is an aileron roll done purely with the ailerons, engine and tail, not using the lift generated by the wings. Not all planes can do it because the engine needs to be very powerful to support horizontal flight, and the wings need a neutral airfoil shape to provide lift when upside down. Trying to come up with a better explanation for the first. EDIT: Try this on for size: Given - the force imparted by the wings is 1G (enough to cancel the force of gravity), and will always be pointed straight through the roof of the aircraft. The pilot takes no action to increase the amount of lift. The G's are measured from the frame of reference of the passengers in respect to the aircraft. Rotational forces are not considered - most people are unable to kinesthetically perceive any rotation which occurs at less than 5 degrees per second, and don't realize that they could be upside down and still feeling like they're right side up. It's why instrument training involves so much instruction and reminders to trust the instruments, not your body. The maneuver is initiated by rolling the plane (clockwise, with respect to the pilot) with the ailerons. Since no effort is made to change the amount of force being generated by the wings, the downward component of that thrust (as measured from an external reference point) will lessen, allowing the aircraft to start accelerating downwards while also accelerating to the right. The force felt by the passengers is still exactly 1G - the force created by the wings, and it is still pointed vertically through the plane. Since the downward component affects the passengers and plane equally, there is no measurable effect on the G forces with respect to the passenger's frame of reference. The aircraft reaches 90 degrees, and is accelerating to the right at 1G, and downwards at 1G. The downward force is not felt by the passengers, again because their entire frame of reference is accelerating at the same speed, only the force pushing them into their seats. The aircraft reaches 180 degrees, and is now accelerating at 2Gs downwards. Passengers are still feeling only 1G of pressure from the seat. 270 degrees - the acceleration to the left cancels out the previous acceleration to the right, passengers are still being just pushed into their seats. 360 degrees - the maneuver is complete. The plane is significantly lower and some distance to the "right" of its original position. Their velocity now includes a significant downwards component. Now here's where things perhaps become a matter of semantics - pilots would consider the barrel roll to be completed at this point, and they simply need to recover from their new orientation. Of course, this will probably require little to no input from the pilot, it will simply happen naturally due to the changed angle of attack induced by the downward motion increasing the lift generated by the wing.
taneq 10y ago
Given that "1G" is here defined as "in the vertical plane of the aircraft", no it's not, unless you're willing to accept a permanent vertical delta-V (which, as a pilot, you aren't.)

falcolas 10y ago

> everything else requires a deviation from that to change direction Well, kinda. The trick is that you have 1G downwards constantly, which can be reduced by initiating a descent at the same time you introduce other directional changes. Rotation (the major component of a barrel roll) does not impart G forces, as recognized by the term. I'm sure few to zero pilots who can keep it at 1G exactly throughout the maneuver, but it is at least theoretically possible (so long as you don't mind losing altitude via the maneuver).

pedrocr 10y ago
>Well, kinda. The trick is that you have 1G downwards constantly, which can be reduced by initiating a descent at the same time you introduce other directional changes. This isn't possible. In straight and level flight you are experiencing 1G acceleration. To be able to change the plane's orientation in any way you need to impart some acceleration to it and thus deviate from the 1G. For example initiating a descent requires an acceleration downwards which will be felt by the passengers as less than 1G. If you finesse it enough and do it big and wide enough you can probably maintain it within a tolerance for what we consider to be 1G but it's not possible to stay at 1G exactly.
- wongarsu 10y ago
  In any realistic scenario it's of course only possible to stay within some threshold around 1G. But why would it be theoretically impossible? If I drop the nose down slightly I reduce downward forces to say 0.95G. In the same manouver I accelerate left with 0.31G. Since 0.95^2 + 0.31^2 = 1^2, I still have a net force of 1G on the aircraft. In theory it should be possible to compute a path that keeps me at exactly 1G at all times.
  - pedrocr 10y ago
    I understand 1G as also requiring the vector to be pointing down on the aircraft. If you allow it to point in any direction as long as the vector has length 1 passengers will notice and not all mechanical parts will be guaranteed to work so the fact that it's technically 1G is not enough to guarantee any airplane can do it. I think that's not the usual definition of 1G either as we even talk about negative Gs (and airframes are particularly sensitive to them) which wouldn't make sense in that case. But it would be cool to see if there's a theoretical path that we can recognize as a barrel roll than keeps exactly 1G of acceleration in any direction needed.

Nightshaxx 10y ago

I am not at all familiar with aircraft, but in the video were Tex Johnson talks about doing the barrel roll, he literally says it is a 1G maneuver. https://youtu.be/Ra_khhzuFlE?t=86 https://youtu.be/Ra_khhzuFlE?t=86

pedrocr 10y ago
I read something similar in John Boyd's biography (highly recommended) about the barrel roll as well. I assume they mean it can be done while keeping the plane in a 0.9-1.1G window or something along those lines. In other words in can be done in a way that if you closed the blinds on all the windows and did it in a commercial flight full of passengers no one would complain or even be able to tell.
- TylerE 10y ago
  That's not true. There will still be rotation that can be felt.
  - falcolas 10y ago
    Below a certain threshold, somewhere near 5 degrees a second, your inner ear can not sense the rotation. It's why you need instruments when flying without visual cues. Oh, it can also tell you you're tumbling wildly when you are actually going completely straight. Really disorienting.
  - cyberferret 10y ago
    I had a beer with a B-1 Lancer pilot who was here on exercises many years ago. He said that on low level flight infiltration exercises, his normal method to go over ridge lines and small hills is to pull the aircraft gently up, roll it inverted and pull back while cresting the ridge, then roll upright once over. He said he is experienced enough to maintain 1.0 to 1.5G throughout the manoeuvre, and the most of the time, the weapons guys in the back of the plane who are usually 'heads down' and don't have big windows to look out of, don't even realise they were inverted for 10 to 15 seconds. He said he has a standing wager with them when they land, and if the back seaters can accurately tell him how many ridgelines they crested inverted correctly, he buys them a beer. He told me he hardly ever has to buy.

Merad 10y ago

A gentleman by the name of Bob Hoover would like a word with you: https://www.youtube.com/watch?v=V9pvG_ZSnCc https://www.youtube.com/watch?v=V9pvG_ZSnCc

ChuckMcM 10y ago
I love that video. I've got an autographed pencil sketch of Bob's P-51 (autographed by both the artist and Bob!). And while watching him fly, the only explanation I could come up with that explains how effortless he makes it look is that he was actually a bird in an earlier lifetime.
slavak 10y ago
First, this video is amazing. But as the video demonstrates quite well, the first phase of a barrel roll requires pitching up into a gentle climb. This is physically impossible without exceeding 1g at any point.
- FabHK 10y ago
  True. Needn't be much though: to put it into a 2 m/s climb (400 ft/min), you'd just need to hold 1.1 G over two seconds.
_ao789 10y ago
That's definitely a very cool video
IIIIIIIIIIII 10y ago
That demonstrates that there are no negative G forces. It doesn't demonstrate that it's 1G. I'm a private pilot who has done aerobatics, exactly 1G doesn't work - and it's completely unnecessary (though you can stay pretty close to it, so that someone with their eyes closed would not know they were rolled). But just about the video as "proof of 1G": As long as you stay positive you are fine, even psychologically with passengers not used to it. The real turning point, in real effects as well as psychologically, is when you approach 0G, the feeling only starts at less than 0.5G when you begin to feel more and more weightless. When you go even slightly negative it becomes a completely different matter, both in terms of real effects (fuel, oil, lose stuff flying around the cabin) as well as psychologically: Even though you made the harness extra tight with as much force as you could muster in preparation for an aerobatic flight with negative G forces, when you get here it feels as if you hang upside down in the harness and the seat is miles away from you, as if you dropped a few centimeters and now literally just hang in the airplane. I (in my small and somewhat underpowered aircraft) go below 1G when I get close to the top, because if I tried to maintain 1G the nose of the airplane would have to drop (towards the earth in that position), and I want to keep that at a minimum, so that I don't end the roll with the nose in too much of a nose-down attitude from which I will have to pull out. Here is an image from my flying some years ago, in a Grob 115C Acro (rented from Attitude Aviation, Livermore, CA, ca. 2002): http://i.imgur.com/Rd5VW3R.jpg http://i.imgur.com/Rd5VW3R.jpg (the inside of this airplane: http://i.imgur.com/4bwJn13.jpg http://i.imgur.com/4bwJn13.jpg) EDIT: (after reading some comments) To me a roll is over when the airplane is back to straight and level, after 360 degrees. So my statements are for that interpretation of the word "roll". And my frame of reference for g-forces are the people on board the aircraft.