# How do planes respond aerodynamically when bringing them into inverted flight by rolling?

For the sake of the question the plane is fully capable of inverted flight, no problems with engines etc.

Background of the question is the observation that the airflow around the wing must also invert the direction. If I look from the end of the right wing in direction of the fuselage, I see the air moving around the wing counterclockwise, while inverted it must spin clockwise. So at one point the relative air flow must cease completely on average, there is no difference anymore between top and bottom of the wing.

While the airflow is not turbulent, this should feel for the pilot like stalling because there is no lift, also the wingtip vortices should stop. The "no lift" position should be somewhere near the point when the plane has its wings vertical.

So once a pilot brings a plane into a roll, what exactly happens? Is it a smooth transition from full lift to no lift to full lift back or is there a sudden loss of lift which must be overcome by spin inertia? I have also the suspicion that trying a roll with a too high angle of attack risks a real stall because the restart of the circulation around the wing is hampered.

I fly such a plane. My Laser has full inverted systems (oil, gas) and symmetrical airfoils.

You're right that when in upright flight the wings are producing lift in the direction away from the wheels and when in inverted flight the lift vector points in the opposite direction.

You're also correct that the wings are producing zero lift at 90 degrees of bank.

As for what happens during the roll, there's a smooth transition. Assuming you're maintaining a constant heading, the lift provided by the wings will decrease to zero by the 90 degree of bank point, then resume in full at 180 degrees of bank in the opposite direction relative to the wing. As lift decreases, the nose of the aircraft will typically decrease precipitously. It's not an aerodynamic stall and there's no sudden onset. It's just like rolling into a diving turn with the turn.

This is why trying to roll an airplane with no previous aerobatic training can be fatal. That "simple roll" you tried can easily result in a >Vne dive.

There are 3 ways to not lose altitude during the roll:

1. As you suggested, roll fast enough. My plane will roll a full 360 degrees in a bit over 1 second. There's not much time to lose altitude :-)
2. Use "top rudder" (the foot closest to the sky) to lift the nose up. At the 90 degree point you're using the side of the fuselage as a crude wing. It works in the right airplane. It requires an unusually large rudder and a lot of practice.
3. Anticipate the loss by pitching up a LOT before rolling. You'll get more of a barrel roll, but it's still a roll.

Please don't try this without competent training. Stupid people die this way.

• I've always wondered what an airplane with spoilers only for roll control, like a Northrop Black Widow or Mitsubishi MU-2, would do if you were slow rolling it and got to the inverted flight part. Would the extended spoiler, now on the "bottom" of the wing, act like a split flap and help with negative lift and continue the roll? Or would the opposite happen? Or some kind of in-between effect? Sep 11 '18 at 19:58
• Sage advice. Thanks. Sep 11 '18 at 20:07
• @JohnK - You should post that as a new question - I have no idea. Sep 11 '18 at 20:14
• @JohnK spoilerons increase drag and decrease angle of attack. Unlike ailerons they're only used on one wing, in one direction (towards upper wing). Flying inverted they still produce negative lift + drag. Roll rate slows down or stops when aoa becomes more and more negative, because adverse yaw affects relative lift dissymmetry between both wings. Until something that looks like roll inversion leads to imminent inverted flat spin entry Sep 11 '18 at 23:47
• "Stupid people die this way." +1 for that alone! Sep 12 '18 at 15:04

If I look from the end of the right wing in direction of the fuselage, I see the air moving around the wing counterclockwise, while inverted it must spin clockwise.

Note you could do the same thing simply by putting the aircraft into a dive. All you need is for the angle of attack to be negative. As it passes through zero angle of attack, the lift falls to zero.

While lift goes to zero, air is still moving over the surfaces. So as long as you stay away from stalling, it remains controllable.

• "If I look from the end of the right wing in direction of the fuselage, I see the air moving around the wing counterclockwise, while inverted it must spin clockwise, while inverted it must spin clockwise." What does this even mean? The air flows from the leading edge to the trailing edge along the top and the bottom of the wing, not clockwise or counterclockwise. Rolling inverted, whether maintaining +1, zero, or -1 G does nothing to change the direction of the flow over the wing. Sep 14 '18 at 1:38
• @MichaelHall, see large.stanford.edu/courses/2007/ph210/glownia2 or spsnational.org/sites/default/files/files/publications/observer/…. The circulation is a result of the lift. In inverted flight (or a steep dive), the angle of attack is negative, and the circulation is reversed. Sep 14 '18 at 4:06
• OK, so there is a pressure gradient component here that I think is misleading. I haven't fully digested what the authors are trying to convey, but Neuenschwander's figure 5, and Laughlin's figure 4 can be misleading. Air does not flow circularly around a wing, it flows from the leading edge to the trailing edge. Just like the other figures depict. Just like you would see it in a wind tunnel. Also, any discussion of "inverted" flight should caveat whether it is sustained negative 1G or not. Because a roll can be made while maintaining positive Gs and AOA. Sep 14 '18 at 19:05

If you are positively loaded all the way around the barrel roll, the circulation around the airfoil never stops, the plane just brings the circulation along with it. Like if you had an airfoil section making lift in a wind tunnel and you rotated the entire tunnel through 360 degrees of rotation.

On the other hand if you "push" to shift into negative-loaded flight as you approach inverted then yes, there must indeed be a reversal of the direction of circulation around the airfoil.