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

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.