The main cause of adverse yaw during rolling motions: the "twist" in the relative wind
This page was last modified on September 14, 2006
The simplest explanation of adverse yaw goes something like this: "When an aircraft is rolling toward the right, the left wing is creating 'more lift' than the right wing. This means that the left wing is also creating 'more drag' than the right wing, so the aircraft's nose will tend to swing toward the left."
Here's the flaw in this "explanation": the left wing only creates more lift than the right wing while the roll rate (toward the right) is increasing rather than constant. Yet in any aircraft that experiences marked adverse yaw, the adverse yaw is quite evident even when the roll rate is constant rather than increasing.
To get a better understanding of adverse yaw, let's start with a peek at this figure from John S. Denker's superb "See How it Flies" website. The diagram illustrates a rolling motion toward the right. (Examine the diagram carefully: at first glance the aircraft may appear to be rolling toward the left, but this not really the case!) The left wingtip is rising and the right wingtip is descending. At each wingtip, the relative wind (not illustrated in the figure) blows exactly opposite in direction to the green "motion" arrows. The rolling motion has caused the relative wind to "twist" to blow downward (in relation to the horizon) at the rising wingtip and blow upward (in relation to the horizon) at the descending wingtip. Since the lift vectors act perpendicular to the relative wind, the lift vectors are twisted too. The lift vector at the rising wingtip tilts aft, and the lift vector at the descending wingtip tilts forward. This creates a yaw torque toward the rising wingtip--toward the left in the above diagram. This is an "adverse yaw" torque--as the bank angle increases and the flight path begins curving toward the right, the nose needs to begin swinging to the left, not to the right, if the nose is to remain aligned with the actual direction of the flight path and relative wind at any given moment.
This "twisted lift" effect is one of the most important causes of adverse yaw. It will be present even in an aircraft that uses spoilerons rather than ailerons for roll torque, unless--as is often the case--the drag from the open spoileron generates enough "proverse" yaw torque to overcome the "twisted lift" effect.
This twist in the relative wind also decreases the angle-of-attack of the rising wingtip, and increases the angle-of-attack of the descending wingtip. This counteracts the pilot's roll input: this effect is called "aerodynamic damping" in the roll axis. This damping effect is why the roll rate does not continue increasing indefinitely as long as the pilot maintains a constant roll control input.
In the above diagram, the aileron deflection is depicted as a "twist" in the wing chord line that counteracts the "twist" in the relative wind and allows the net roll torque to be zero (meaning that the rolling motion can be sustained indefinitely with no increase or decrease in roll rate) at some useful, non-zero rate of roll. The details of roll control via spoilerons or weight-shift are slightly different but the end result is the same--as the roll rate increases, eventually the unfavorable roll torque from the "twist" in the relative wind counteracts the favorable roll torque from the spoilerons or weight-shift, and the roll rate cannot increase any further.
When a flex-wing hang glider is rolling to a steeper bank angle, the trailing edge tension changes in a way that partially counteracts this "aerodynamic damping in the roll axis" and allows the pilot to achieve a much higher roll rate than he could with the same weight-shift control input if the wing were completely rigid. The trailing edge of the mid-span part of the descending wing billows upward, reducing camber, angle-of-attack, and lift at this part of the wing. The washout near the tip of the descending wing increases, decreasing the angle-of-attack and lift at this part of the wing. The trailing edge of the mid-span part of the rising wing is pulled downward, increasing camber, angle-of-attack, and lift at this part of the wing. The washout near the tip of the rising wing decreases, increasing the angle-of-attack and lift at this part of the wing. These effects are somewhat analogous to the effects of ailerons. But it's worth noting that these changes in the shape of the wing are direct evidence that the lift-per-unit-area is higher on the descending wing than on the rising wing--that's the nature of a "passive" flexible wing with no active aerodynamic controls. In this situation, the reason that the roll rate is constant rather than decreasing is that the pilot is keeping his body to the low side of the aircraft centerline. This means that in relation to the CG of the whole system, the area of the descending wing is smaller than the area of the rising wing. This allows the total lift--or more accurately, the total roll torque--created by each wing to be equal. Naturally, if the flex in the wing shape were driven by an "active" wing warping system rather than a "passive" system, this zero-net-roll torque, constant-roll-rate scenario could be achieved even if the pilot's body was kept on the aircraft centerline.
Our discussion up to this point really has been aimed at cases where the pilot is making a strong roll input and achieving a brisk roll rate, so that other aerodynamic effects are relatively unimportant in comparison to the pilot's roll input and the resulting roll damping due to the "twist in the relative wind". But we'll see the same twist in the relative wind, which will lead to the same adverse yaw effect and the same roll damping effect, when a glider is rolling due to some other cause, such as a difference in airspeed and lift between the two wingtips. The rolling motion of the aircraft with respect to the airmass is the key factor here, not whether or not the pilot is making a rolling-in control input or rolling-out control input. The situation is more complex when one wing is struck by a vertical gust--here the twist in the relative wind may be opposite to the direction shown in the above link, because at the instant the vertical gust is striking one wing, the airflow is forcing, rather than resisting, the rolling motion. In this case we may not see "adverse yaw" in the usual sense, at least initially.
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