[falcolas]

Since a properly executed barrel roll can be a 1-G maneuver, it's technically possible in any aircraft.

The reason most aircraft are incapable of flying upside down isn't usually due to stresses on the airframe, it's because the fueling system relies on some form of force pushing the fuel through the bottom of the aircraft. A 1-G barrel roll does exactly that.

There's some interesting trickery done with carbureted aerobatic aircraft to let them fly upside down for an extended period of time, including special valves which shut off fuel flow to the bowls when gravity isn't in the direction they expect it to be, so the aircraft won't lose that fuel back into the fueling system (or out the top of the engine) and can run for a bit on that fuel in the carburetor bowls until the plane is righted.

[pedrocr]

>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.

[Merad]

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

[IIIIIIIIIIII]

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 (the inside of this airplane: 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.

[slavak]

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]

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.

[ChuckMcM]

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.

[_ao789]

That's definitely a very cool video

[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.

[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.

[Nightshaxx]

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

[pedrocr]

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]

That's not true. There will still be rotation that can be felt.

[cyberferret]

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.

[Moru]

We humans are pretty funny. The sensitivity comes a lot from what the eyes see, the hands feel and what we expect to happen from what the experience tells us (stick movement and such). Only a small ammount of it comes from the inner ear balance sense.

Try standing on one leg. Then close your eyes. Or try walking in a very dark but not completely black room. And then close your eyes. Your balance sense might tell you very strange things and if you haven't trained for this, you might fall.

[falcolas]

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.

[falcolas]

> 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]

>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]

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.

[mikeash]

You can indeed, in theory, start a descent while maintaining exactly 1 gee. But you can't stop that descent without exceeding 1 gee. The best you can do is to maintain your current vertical velocity.

[pedrocr]

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.

[Sohcahtoa82]

What he's saying is possible, but it wouldn't be a barrel roll anymore.

You can experience 1G while banked 90 degrees by pulling up slightly. However, the plane will be losing altitude as the wings are not generating lift.

[pedrocr]

Once you are banked 90 degrees you can do that but how do you get to that bank while maintaining 1G pointing towards the floor at all times?

[falcolas]

By smoothly transitioning into the turn as you roll. The simple act of rolling the aircraft doesn't change the amount of lift being generated towards the roof of the aircraft. In fact, if you don't increase that force intentionally, you will turn and lose altitude.

[throwaway91111]

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]

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.

[elmar]

electrical planes don't have this problem.

[falcolas]

Nor do fuel injected engines, nor jet engines.

Electrical planes have a weight issue with the need for batteries (or humongous wings covered with solar panels). Petroleum products, for better or worse, still have a huge advantage in the energy/weight department.

[sathackr]

How does the fuel get from the tank to the jet or fuel injected engine when the pickup is on the bottom of the now upside-down tank?

There are ways to ensure fuel delivery, but the problem certainly isn't unique to carbureted engines, only the particular issue of managing the fuel in the carburetor bowl.

[bigiain]

In "toy planes" we (used to) use "clunk tanks", where the fuel pickup is a piece of flexible tube with a weight on the end, so no matter what orientation the airframe is in, the "sloshed fuel" and the fuel pickup end up in the same place (near enough). For aggressively aerobatic model planes (like control line combat wings) a pressurised fuel bladder was sometimes used (made, back in my childhood, we made these from baby pacifiers!)

Note: this is a couple or more decades old knowledge... Modern electric powered rc planes and builders don't have this problem to deal with.

[Neliquat]

The issue is a non issue. For ages in planes, race cars, etc we have had fuel cells. A few types exist, from just adding more baffles and pickups with vapor locks, to fuel bags (no air to make it to pickup) or the more modern foam matrices that use capilary effect.

[sathackr]

As I said. There are ways to combat the problem, but the above comment that it did not apply to jet engines or fuel injected engines did not take into consideration the requirement that the entire fuel system be able to operate at low or negative G's