[kobeya]

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.

[mikeash]

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.

[falcolas]

> 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.

[mikeash]

I don't think that's quite right. A barrel roll is supposed to be entered and exited in level flight. But I think we both understand what's going on, so that's just a dispute over where to draw an arbitrary line!

[bigiain]

Well actually...

I suppose it's high-school-physics possible to fly a 1 gee helical path (the "barrel roll") centered around an orbital zero-gee trajectory...

Imagine for a moment a zero gee orbital trajectory at a low enough altitude that you can still generate aerodynamic lift from the atmosphere. (Here we handwave away all the pesky frictional heating, because we're deep in the thought experiment world of perfectly spherical cows of uniform density.) Now imagine a helical "coil spring" shaped path with that orbital trajectory running through the center. "All" you need to do is get the diameter and spacing of those helical coils right so your acceleration around the coils needs to be 1G, while your averaged out path coincides with the orbital zero gee trajectory.

<grin>

(An aircraft with sufficient speed, fuel capacity, heat shielding, and whatever else I've glossed over - is left as an exercise for the reader...)

[bigiain]

I just thought of an easier (and probably achievable) thought experiment.

Imagine a fighter jet flying circles around an airliner as it follows it along - with the fighter pilot flying at just the right radius and speed that it's accelerating at 1 gee for the turn (so they'd be "feeling" 2 gee as they pass under the airliner, and zero gee as they loop over the top of the airliner) using a helical "barrel roll" path - and at the same time "keeping up" with the airliner along it's path, so if you were sitting in the airliner looking out it'd look like the fighter was flying circles around the long axis of the fuselage.

Now imagine the fighter pilot does the same trick following the vomit comet - as it flies its parabolic arcs which gives it's occupants 20-30 secs or so of "zero gee".

https://en.wikipedia.org/wiki/Reduced-gravity_aircraft

[mhandley]

However, if you keep flying 1G after exiting that parabolic barrel roll, you're going to make a hole in the ground. The vomit comet usually does a 3G pull up afterwards. You've got to exceed 1G, either to enter the parabolic arc from level flight, or to leave it to regain level flight, or more likely, both.

[comex]

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]

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.

[taneq]

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.)

[pedrocr]

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).

[kobeya]

Nobody is talking about a gravity vector pointing towards the center of the earth during the entire maneuver. The implied assumption, I believe, is 1G towards the floor of the plane at all times.

This is quite easy to do when you realize that the plane can (and will) lose altitude and transfer ground-relative horizontal momentum for the vertical.

[pedrocr]

>Nobody is talking about a gravity vector pointing towards the center of the earth during the entire maneuver. The implied assumption, I believe, is 1G towards the floor of the plane at all times.

I mean downwards towards the floor of the plane of course, not the center of the earth. mikeash has already explained much better than I could why you can't keep 1G from the point of view of the passenger and do a normal barrel roll from/to level flight. It seems easy to have a path that does it if you allow it to finish in descending flight.

[jacquesm]

> You know of a path that keeps the acceleration 1G and pointing downwards at all times?

Yes, but you're not going to like the outcome.

Freefall.

[shalmanese]

Freefall is a 0G maneuver.

[jacquesm]

From your perspective, yes. But you're accelerating with 1G right up until you reach terminal velocity (or impact) unless you're in orbit.

[shalmanese]

Maneuvers are always from the reference frame of the passenger. If a passenger is feeling subjectively to be in 0G, it doesn't matter what their acceleration is compared to any fixed reference frame.

[falcolas]

Or if you are just far enough away, with just enough lateral velocity, you keep missing the planet. Silly astronauts, can't even fall properly.

[falcolas]

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.

[LeifCarrotson]

How do you ever ascend or descend without accelerating up or down?

[falcolas]

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.