Let’s revisit some fundamentals of what is happening in an aileron roll. First how do ailerons cause an airplane to roll about it’s longitudinal axis? Ailerons are nothing more than plain flaps along a local section of a wing. Just like a flap when deflected they change the lift slope curve.
As the above diagram shows, the lift curve and range of lift coefficient increases with flap deflection. With one aileron deflected down and the other deflected up in coordination these aileron deflections create changes in the lift curve slopes of the wings. The net effect is to create an imbalance between the lift produced by left and right wings. This imbalance in lift forces causes the airplane to roll. If we could see the net lift distribution due to aileron deflection across the span of the wings it would look something like this:
Notice how the greater lift distribution on the right compared to the lower lift distribution on the left results in the airplane rolling left. This is the rolling moment created by aileron deflection.
Of course we’ve by now also concluded in our above observations that there is another rolling moment that is produced which acts in the opposite direction of aileron rolling moment. This force is known as the roll damping moment. Conventional airplanes are designed to be laterally statically stable which means there is a roll damping force that naturally brings rolling moments back into equilibrium.
So what’s the main source of this roll damping moment? Just like the lift distribution differences of the wings cause a roll, the wing is also the main contributor in providing roll damping force. Colmbo pointed out that this occurs due to differences in angle of attack between up-going and down-going wings. What’s going on here?
Relative airflow with respect to a wing is not only a function of wing’s angle of attack to forward velocity but also a function of it’s rotational velocity as well. That means angle of attack of oncoming airflow to a wing is also affected by how fast an airplane is rolling as well.
As the above diagram demonstrates as an airplane rolls the down-going wing’s attack of attack actually increases while the up-going wing’s angle of attack decreases. The faster an airplane rolls, the greater the magnitude of these changes in angle of attack. We know that lift is also a function of angle of attack. This means that the faster an airplane rolls, the down-going wing produces more and more lift. The opposite is true for the up-going wing. The faster we roll the less lift the up-going wing produces. At some rotational velocity this change in lift between the down-going and up-going wing produces an imbalance in lift forces that equals the imbalance created by aileron deflection.
Simply put, roll damping moment increases the faster an airplane rolls until at some point it equals the opposite rolling aileron moment.
When ailerons are brought back to neutral, the airplane continues to roll. Because of the roll the down-going wing continues to produce more lift than the up-going wing (roll damping) which now causes the roll to decelerate to the point it stops when there is no longer any rolling velocity to create an imbalance in lift distribution between the wings. You can see this is in the following roll time history diagram. As aileron input is brought to neutral, it's the roll damping moment that causes the roll to quickly subside.
So the wings are key to both the aileron rolling moment and the opposite wing roll damping moment. This is a key concept.
All the above discussion is a based on the idea that neither left or right wings are stalled. What happens if the down-going wing stalls but not the other during a roll? As we’ve seen roll damping moment to counter-act aileron rolling moment is a function of the lift of the down-going wing. If the down-going wing is stalled then it produces less lift resulting in lower roll damping. The deeper the stall the less the roll damping produced. The lift curve slope of a stalled wing has a steep downward slope which means roll damping moment falls off rapidly at stall. The effect is that there isn’t much force to counteract the rolling acceleration due to ailerons which means the airplane rolls around much faster. This is what’s happening in a snap roll.
So there you have it moot. The reason your third roll was so much faster is because the down-going wing was stalled and there was little roll damping moment to counteract aileron rolling moment
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Now my fingers are worn out so I have to go give them a break!
Tango
412th FS Braunco Mustangs
