Slips and skids

The lift that each wing generates is given by the formula (air density)(wing area)(velocity)2(lift coefficient)/2. It’s long been one of my favorites because the expression not only spells out lift’s ingredients but betrays their relative importance as well. Dense air—as experienced at sea-level, on a cold winter day, and with a high altimeter setting—maximizes performance such as takeoff distance and climb rate. And the bigger the wing area, the more lift the wing generates. (Of course, drag increases too after all there is rarely a free lunch in aviation.) In fact, doubling each of these quantities means that lift doubles too. If a pilot is flying on a given day and not shedding wing parts, they really have little control over these two factors.

But a pilot has control over the last two terms—velocity and lift coefficient. That velocity is squared in the formula means that it has a profound effect on lift. Doubling the velocity means lift goes up by a factor of four. Or cutting the velocity in half reduces lift to a quarter. (See “The Power of Two: Why Math Matters,” November 2022 AOPA Pilot) Finally, the lift coefficient is basically a linear function of angle of attack—doubling the (absolute) angle of attack means the lift doubles if the angle of attack doesn’t become critical.

An aircraft that stays at a fixed bank angle must produce the same lift on each wing, since an imbalance would cause the aircraft to roll. There is essentially no difference in air density or wing area between the two wings, so the product of the velocity squared times the lift coefficient coincides for each wing. For the aircraft to remain at a fixed bank angle, V2leftCL,left= V2rightCL,right.

In uncoordinated flight, this equation becomes interesting. Slipping flight, which comprises both slips and skids, occurs whenever, viewed from above, the flow across the wings is not parallel to the longitudinal axis of the airplane. Let’s consider an aircraft that enters a left turn that features a constant bank angle but fails to maintain coordination.

During a turn, a proper slip occurs when the pilot fails to apply sufficient inside rudder for coordinated flight. In this case, the outside, right wing moves too slowly through the air and the inclinometer ball moves to the left as a sign that the pilot should step on the ball. In this case, since the velocity of the right wing has been reduced, the coefficient of lift on the right needs to increase for the equation to hold.

Thus, the outside/right wing flies at a greater angle of attack than the inside/left wing so, at the stall, it tends to drop first, and the airplane can enter a spin by first going over the top. A slip is a useful maneuver—in compensating for a crosswind on landing or for losing unwanted altitude while maintaining a constant airspeed—but it should be used with care.

In contrast, there is no sticky flight situation for which a skid is the answer. During a skid, the outside wing moves too quickly for the given bank angle and the inclinometer ball slides to the outside of the turn. So, for the equation to hold, the inside/left wing flies with a higher angle of attack to make up for the decreased velocity. If a stall occurs in this configuration, the low wing stalls first and the aircraft becomes inverted quickly and enters a spin under the bottom. In the pattern, there might be insufficient altitude for even the most advanced aerobatic pilot to recover. Skidding flight should always be avoided.

Now, as discussed in “It’s Complicated: The Wing’s On-Again, Off-Again Relationship with Lift” (June 2024 AOPA Pilot), sections of a wing can simultaneously be at varying angles of attack. When we refer to the angle of attack for a wing, as we have above, it’s understood in an average sense. Still, this simplified presentation can give insight into which wing will likely drop when stalling an airplane during uncoordinated flight.