Airplanes have inertia and therefore take time to react to changes in relative wind. As pilots we tend to be impatient about such upsets and delays; we start moving yokes and pedals almost immediately to restore the airplane to its pre-disturbance situation. Sometimes this is a good idea, as when a downdraft drops your airplane below glideslope on final approach. Sometimes, however, immediate action is not necessary or helpful, such as when you're in for an hour of mild turbulence during a cross-country flight.
Let's take a look at how a sudden change in relative wind affects an airplane. First we'll review the basic ways airplanes can move. Then we'll set up the relative wind change and explore the airplane's reaction. Finally, we'll examine a flying scenario, including common pilot responses to the change in relative wind and how those responses affect the piloting task at hand.
Six Degrees of Freedom
Airplanes can move in six directions: three rotational and three translational. All pilots are very familiar with rotational movements; these are pitch (nose up or nose down), roll (right wing or left wing down), and yaw (nose left or nose right). The other three movements are called translational because they cause the airplane to move, or translate, without rotating. They are called heave (up or down), sway (left or right), and surge (fore or aft).
We have direct control over the three rotational motions through the airplane's flight controls. The elevator or stabilator controls pitch, ailerons control roll, and rudder controls yaw. There is some surge control available to us through the use of the throttle. We can't directly control heave or sway, but we do have indirect control. For example, pulling the yoke back not only pitches the airplane nose up but also causes heave and surge. The heave is first downward when the elevator deflects trailing edge up, pushing the airplane downward. Then the airplane is pushed upward after the upward-deflected elevator rotates the airplane nose up, creating additional lift. The indirect surge effect is caused by the increased drag of the deflected elevator.
Eating the Elephant
You can see how this little exercise can quickly become jumbled if we try to consider all of the possibilities. So, let's limit our discussion to something a little simpler. Your airplane is flying along in a steady air mass at a steady airspeed and heading when it encounters a right sideslip. A sideslip is a condition where the relative wind is coming from the right or left of the airplane's nose. We don't care whether the sideslip condition was caused by a gust or wake turbulence or your six-foot-four-inch copilot who inadvertently hit the left rudder pedal as he tried to stretch his legs. All we care about is that the airplane is suddenly in a right sideslip.
The relative wind is now approaching your airplane from slightly right of the airplane's nose; let's say the one o'clock position. It's not marching directly down the wing from tip to root, but, just as in a wind triangle problem, there is a component of the wind that is broadside to your fuselage. This crosswind component applies pressure to all of the vertical surfaces on the right side of your airplane, pushing it to the left. Unless your airplane is somehow exempt from Newton's laws of physics, it responds to that force by translating, or swaying, to the left.
You'll recall that an airplane rotates around its center of gravity (CG). When an airplane pitches, rolls, or yaws, the center of these rotations is its CG. Some of that sideward component of wind pushes on the airplane's vertical surfaces forward of the CG. In the right sideslip the force acting on these forward vertical surfaces tries to yaw the airplane nose left.
Similarly, the force of this sideward component acting on the vertical surfaces aft of the CG yaws the airplane nose right, which is what we want because this action swings the airplane's nose back into the relative wind. This is positive directional stability. The vertical stabilizer exists to provide a large area of vertical surface far behind the CG, improving the airplane's directional stability. With the vertical stabilizer, there's usually a lot more vertical surface behind the CG than in front of it, so the airplane has positive directional stability.
So far our right sideslip has caused a translation to the left and a nose-right yaw.
Does your airplane have geometric dihedral? Geometric dihedral, which is generally more obvious in low-wing airplanes, is where the wing tips are higher than the root. The relative wind from the right "sees" more of the bottom of the right wing than it would if there were no sideslip and the wind approached the wing from directly in front of the airplane. Because the wind from the side approaches the right wing from below, that wing has a higher angle of attack (AOA) than it does when there's no sideslip.
This is the same set of physical laws that causes your airplane to climb when you pull back the yoke. Pulling the yoke aft rotates the airplane nose up, increasing the AOA of both wings as the relative wind shifts downward. Higher AOA causes more lift, and the airplane climbs. That's what happens to the right wing of an airplane with positive geometric dihedral when it experiences a right sideslip. This increased lift of the right wing causes the airplane to roll left. Similarly, the relative wind contacts more of the top of the left wing during this right sideslip. The result is a lower AOA and less lift for the left wing, and this also causes the airplane to roll left. The fuselage also blocks, or at least disturbs, the wind approaching the left wing from the right side of the airplane. This makes the left wing less effective at producing lift and also contributes to this left roll.
This positive dihedral effect, where the airplane rolls away from the sideslip, also occurs in high-wing airplanes. Although the geometric dihedral is usually much smaller in high- wing airplanes, the dihedral effect is typically just as strong. To observe the effect, displace the left rudder pedal a small amount. You'll see your airplane yaw nose left, creating a right sideslip. Immediately after it yaws, you'll notice it begin to roll left without any aileron deflection.
Here's where we are in our right sideslip or relative wind from the right scenario. Your airplane sways to the left, yaws nose right, and rolls to the left.
Now let's go back to that vertical tail. The relative wind during the right sideslip approaches the vertical stabilizer from its right. The same increased AOA scenario applies here. The vertical tail is nothing more than a wing installed vertically on the airplane, something like a sail on a sailboat. Instead of the relative wind approaching the wing from below, we have the relative wind approaching the vertical tail from the right side. Instead of increased upward lift, we have increased sideward lift to the left. The sideslip angle, or the angle that the relative wind makes with the chord line of the vertical tail, is analogous to the AOA, or the angle that the relative wind makes with the chord line of the wing when you pulled back the yoke. The only difference is that this time the whole picture is rotated 90 degrees.
We've explained how the right sideslip causes the vertical tail to create additional lift to the left. We know that the airplane rotates around its CG in flight, and we know most of the vertical tail is located above and behind the airplane's CG. So, most of this lifting force to the left acts above the CG, and that makes the airplane roll left. If you think about it, it's no different from the way deflected ailerons roll the airplane. When the ailerons are deflected, one wing creates more lift than the other. The lift occurs or acts at some distance from the airplane's CG, causing the airplane to roll. In this case, instead of that distance being along the wing, it's along the vertical tail.
The tally so far for your airplane in a right sideslip:
- A sway to the left resulting from the force of the sideward wind component from the right pushing on all of the airplane's vertical surfaces.
- A nose-right yaw caused by the airplane's built-in positive directional stability attempting to orient it back into the relative wind.
- A left roll caused by the airplane's positive dihedral effect.
- Additional-although much smaller magnitude-left roll from the sideward lift to the left of the vertical tail.
Propeller Effects
The right sideslip causes the airplane to pitch nose down. Whoa, how can that be? The effect is identical to the effect that makes your airplane want to yaw nose left while climbing. The difference is that the relative wind approaches the propeller disk from the right in our right sideslip example instead of from below as it does during climbs.
The concept requires a little mental imagery, so here's the setup. Picture the relative wind approaching the propeller disk a little from the right of the airplane's nose. The propeller on most airplanes rotates clockwise when viewed from the cockpit. As the prop blades rotate through the propeller's 12 o'clock position, they are moving more into the relative wind than when they are passing through the prop's six o'clock position. These 12 o'clock blades make a larger angle of attack with the relative wind than the six o'clock blades. More angle of attack means more lift, or, in the case of the propeller, more thrust. So the top half of the propeller creates more thrust than the bottom half, and this tends to rotate the propeller-and the entire airplane-nose down. Notice that a left sideslip would cause a nose-up pitching moment.
There's also a reaction effect of the propeller in the right sideslip. The relative wind approaches the propeller disk from the right and exits pretty much perpendicular to the disk or parallel to the fuselage. Newton said that every action has an equal and opposite reaction. He may not have been thinking about you and your airplane at the time, but his words apply anyway. Because the propeller turns the air flow, a force is exerted on the prop and airplane that is opposite to the direction of the turn. In our example that's a nose-left yaw. The force of this reaction depends on the speed and direction of the relative wind and how much the prop can turn that wind. The effect of the reaction is to yaw the airplane away from the relative wind, or away from the sideslip, and that fights against the vertical stabilizer's effort to turn the airplane into the sideslip. In other words, the propeller's reaction to the sideslip is destabilizing-it has a negative directional stability effect that the vertical tail and vertical surfaces aft of the CG must overcome to give the airplane overall positive directional stability.
Although we said that the propeller turns the air flow more parallel to the airplane's fuselage, it's not completely parallel. It's likely that the flow downstream of the propeller will be slightly to the left of the fuselage because the relative wind coming from the right blows on the propwash just as it blows on the entire airplane. The result is that the propeller-blown air flows slightly to the left of the fuselage. Picture this by imagining a view of the airplane from above. In balanced flight, the propeller pushes the swirl of air straight back. In the right sideslip, the air from the propeller flows from the prop toward the left horizontal tail. How much it "angles" depends on the speed of the air exhausted through the propeller, the sideslip angle, and the airplane's forward speed.
Whether the airplane is in a sideslip doesn't change the fact that the propwash is a swirl of air. The propeller imparts a rotation to the air passing through it. This helical airflow wraps around the airplane as it travels from prop to tail. Like the propeller, this flow is clockwise when viewed from behind the airplane.
OK, time for more mental imagery. Let's say that you are behind the airplane as it travels along in straight-and-level, sideslip-free flight. The air leaves the propeller in imaginary ribbons of color so you can watch it swirl around the fuselage as it travels aft. Notice how the swirl sweeps up from below as it encounters the leading edge of the left wing near its root and sweeps down from above as it encounters the leading edge of the right wing near its root. The angle of attack of the inboard portion of the left wing is higher than the corresponding portion of the right wing because of this difference in the direction of approaching air. Higher AOA means more lift is generated by the left wing because of the prop-induced swirl. More lift from one wing means a tendency to roll.
There's a similar effect on the horizontal stabilizer. The propwash is still swirling clockwise way back at the tail. It approaches the leading edge of the left horizontal tail more from below than it does the right tail. Again, there's a lift differential and a consequent rolling moment. And then there's the vertical tail. The swirling propwash approaches the vertical tail from its left.
Now conjure again that image of the airplane from above-the one where we talked about the propwash traveling toward the left of the airplane as it moved aft when the airplane is in a right sideslip. Got it? Now imagine that flow with swirl. Keep those ribbons of color in mind and see where the swirling air encounters the airplane when the propwash angles to the left. Less of that downward flow approaches the leading edge of the right wing near its root. Do you see more of the left wing bathed in upward propwash flow? Do you think that the left wing creates substantially more lift because of the right sideslip? It's hard to say with the limited information in our hypothetical example. What's happening to the helical flow at the airplane's tail? Maybe the propwash has shifted so far left by the time it passes the airplane's tail that the upward flow is way off to the left, and the left horizontal tail is immersed in the downward flow. In this case the vertical tail would be outside of the swirling flow altogether and would just be exposed to the sideslip flow from the right.
OK, let's rein all this propeller effect and sideslip back into the perspective corral. The point is simply this: Sideslip can have a profound effect on the airplane's pitching, rolling, and yawing moments through changes in propeller thrust line, action/reaction, and the shifting of the propwash. The magnitudes and relative strengths of each component depend on the airplane design, propeller, power setting, configuration, and flight condition.
There I Was?
Some good-and all bogus-first-person pilot stories seem to begin with, "There I was?," so that's how we'll start this one. There you were, entering the roundout for landing following a flawless approach in your single-engine airplane. The tower warned you that the wind was gusting, but it's been steady all the way around the pattern-until now. Just as you raise the nose above the horizon to begin your flare, a gust from the right hits your airplane. If you do nothing, you can probably expect that your airplane will:
Sway to the left as a result of the force of the sideward wind component from the right pushing on the airplane's vertical surfaces.
Yaw nose right because of the positive directional stability attempting to orient the airplane into the relative wind.
Roll left-wing-down thanks to the airplane's positive dihedral effect.
Roll left-wing-down as a result of the sideward lift to the left of the vertical tail.
Yaw nose left in reaction to the propeller turning the slightly-from-the-right airflow more parallel to the fuselage.
Experience a change in pitch, roll, and yaw because the swirl of propwash shifted to the left. Which way each of these rotations is affected depends on your airplane and flight condition.
What? You've never noticed all this during a gusty approach? Good. That's because you didn't sit back and watch the airplane react. As soon as you recognized the upset, you turned and pushed or pulled the yoke, stepped on the appropriate rudder pedal, and may have adjusted the throttle. You applied a little clockwise yoke, which not only prevented the dihedral effect from raising the right wing, but actually lowered the right wing slightly to fly the airplane back toward the runway centerline. While making this downwind drift correction you also kicked in a little left rudder. You knew this would change the vertical tail's lift to the right, overpowering the left lift from the right sideslip. This smart action overcame the vertical tail's left-rolling tendency while simultaneously countering the nose-right yaw associated with your airplane's positive directional stability, keeping your airplane aligned with the runway. Your natural response to the gust-induced sideslip is to apply exactly the control inputs needed to cope with the crosswind landing that this gusting situation has created.
Because you were in the process of rounding out your glidepath, you probably had the engine at idle already. This convenience saved you from having to deal with the nose-down pitching moment resulting from the asymmetric thrust from the upper and lower halves of the propeller disk. At idle, there isn't enough thrust to cause a noticeable effect. Having idle power established also minimized the propeller reaction yawing moment along with the shifted propwash swirl. Chances are that your handling of the drift, wing rise, and yaw incorporated whatever small corrections might have been necessary to counteract the propeller effects.
Nice landing.
