Sideslips and forward slips

Sideslips and forward slips

Steve Seibel
www.aeroexperiments.org

This page is still under construction!
This page was last modified on August 13, 2006

 

In "More detailed definitions of slips and skids", we noted that in a 3-axis "conventional" aircraft, when a pilot uses the rudder to yaw the aircraft in one direction while simultaneously using the ailerons to bank the aircraft in the opposite direction, the flight path can follow a straight line rather than a curve. In "Aerodynamic sideforce during slips and skids", we noted that in this situation, the airflow impacts the side of the fuselage and other surfaces of the aircraft, which generates an "aerodynamic sideforce" that opposes and cancels out the horizontal component of the lift vector from the banked wing, so that there is no net horizontal force acting to curve the flight path either to the left or to the right.

Throughout the Aeroexperiments website, we often use "sideslip" and "slip" as synonyms. But when speaking specifically of linear, non-turning slips, where the flight path follows a straight line rather than a curve, pilots of 3-axis aircraft often distinguish between "forward" slips and "sideslips". The main purpose of a "forward" slip is to yaw the fuselage sideways in relation to the flight path through the airmass so that drag increases and the descent rate steepens. The main purpose of a "sideslip" is to yaw the fuselage sideways in relation to the flight path through the airmass so that the fuselage (and landing gear) will be aligned with the aircraft's ground track as it lands in a crosswind. Aerodynamically, these two maneuvers are identical--after all, the aircraft doesn't "know" which way the wind is blowing, or in which direction the ground track is running. (Diagrams to be added.)

Here's one series of control inputs a pilot could use to slip an aircraft to increase drag during the approach to landing. We'll start by assuming the aircraft is on final approach, in "coordinated" non-slipping flight, following whatever flight path through the airmass happens to be required to keep the ground track aligned with the runway. (If there is no crosswind, the flight path through the airmass and the ground track will both run in the same direction, and the aircraft's heading will be the same as the runway heading.) To begin the slip, the pilot could apply and hold heavy pressure on the rudder (for example, to the right). The amount of rudder input should be in proportion to the amount of extra drag the pilot wants to create. The aircraft's nose will yaw to point to the right of the actual direction of travel though the airmass. The airflow will strike the left side of the fuselage and create drag, as well as a horizontal sideforce component that will tend to make the flight path curve to the left. To prevent the flight path from curving in response to the right rudder, the pilot should lower the left wing. With the right "balance" of yaw and bank, the flight path through the air will continue to follow a straight line, as will the ground track. If the pilot drifts off course to the left and needs to make the flight path curve to the right, he should increase the right yaw or decrease the left bank. If the pilot drifts off course to the right and needs to make the flight path curve to the left, he should decrease the right yaw or increase the left bank.

In actual practice (in part to avoid complications from adverse yaw), it often actually works best to first bank the wing in proportion to the amount of drag that is desired, and then make steering adjustments with the rudder. Fundamentally though, it is the fact that the aircraft is yawed to one side in relation to the actual direction of the flight path and airmass, not the fact that the aircraft is banked, that is causing the increase in drag.

Here's one series of control inputs a pilot could use to slip an aircraft to align the fuselage with the runway heading in the presence of a crosswind during the approach to landing. We'll start by assuming the aircraft is on final approach, in "coordinated" non-slipping flight, following whatever flight path through the airmass happens to be required to keep the ground track aligned with the runway. (The aircraft will be "crabbed" into the wind, which sounds like an unnatural state of affairs, but actually only means that ground track runs in a different direction than the flight path through the airmass.) To begin the slip, the pilot could apply and hold however much rudder is needed to swing the nose of the aircraft into alignment with the runway heading. We'll assume in this example that the wind is blowing from left to right across the runway, which means that the flight path through the air is aimed to the left of the runway heading, which means that the pilot must apply and hold right rudder to swing the nose into alignment with the runway heading. To prevent the flight path from curving in response to the right rudder, the pilot should lower the left wing. With the right "balance" of yaw and bank, the flight path through the air will continue to follow a straight line, as will the ground track. If the pilot drifts off course to the left and needs to make the flight path curve to the right, he should increase the right yaw or decrease the left bank. If the pilot drifts off course to the right and needs to make the flight path curve to the left, he should decrease the right yaw or increase the left bank.

When the pilot is compensating for a strong crosswind, in actual practice it often works best to use the rudder to keep the aircraft's heading aligned with the runway heading, and to adjust the bank angle as needed to "slide" the ground track to the left or right so that the aircraft remains over the extended centerline of the runway. Fundamentally though, both the horizontal component of lift from the banked wing and the horizontal force from the airflow striking the side of the fuselage are "trying" to produce a curvature in the flight path and ground track (in opposing directions), not to "slide" the ground track to one side or the other. In fact, there is really no such thing as "sliding" the ground track to the side. For example if the ground track is heading in the correct direction but is displaced to the left of the extended runway centerline, to get back "on track" the flight path and ground track actually must first curve to the right, and then must continue in a straight line until the aircraft is back over the extended runway centerline, and then must curve to the left so that ground track again parallels the runway heading. As the pilot manipulates the rudder and ailerons as described above, the curvatures in the flight path and ground track happen when the horizontal force component from the banked wing and the horizontal force component from the airflow impacting the side of the fuselage are not exactly in balance, and the straight-line segments of the flight path and ground track happen when the horizontal force component from the banked wing and the horizontal force component from the airflow impacting the side of the fuselage are exactly in balance.

When a pilot is slipping on final approach to compensate for a crosswind, if he finds himself falling short of the runway he can relax his rudder input to reduce the slip angle, which also reduces drag. As long as he also reduces the bank angle, there will be no change in the direction of the flight path and ground track. However, assuming that the wind remains constant (more on this later), the pilot will need to re-apply the original rudder input, and the original bank angle, sometime before the actual moment of touchdown. Otherwise the fuselage (and landing gear) will not be aligned with the ground track at the moment of touchdown.

Similarly, when a pilot is slipping on final approach to compensate for a crosswind, if he finds himself needing to steepen his glide path he can increase his rudder input to increase the slip angle. As long as he also increase the bank angle, there will be no change in the direction of the flight path and ground track, even though the nose of the aircraft will no longer aligned with the runway heading. However, the pilot will need to go back to the original rudder input, and the original bank angle, to re-align the fuselage (and the landing gear) with the ground track and the runway sometime before the moment of touchdown.

If there is a crosswind, and the pilot wants to slip the aircraft to increase drag, it makes sense for the pilot to bank the aircraft in the upwind direction and apply rudder in the downwind direction. Slipping the other direction would be produce just as much drag (after all, the aircraft does not "feel" which way the wind is blowing), and would not make the flight path or ground track curve, so the ground track would continue to parallel the runway. But the pilot would have to reverse the direction of the slip sometime before the moment of touchdown, if he wants the fuselage (and landing gear) to be aligned with the ground track and the runway.

Of course, it goes without saying that when there is no crosswind, and a pilot is slipping an aircraft to increase drag during the final approach, the slip must be ended before the moment of touchdown, or the fuselage (and landing gear) will not be aligned with the ground track at the moment of touchdown.

When an aircraft is slipping to compensate for a crosswind, it appears at first glance that the wind is trying to "push" the aircraft in the downwind direction, and the banked wing is compensating for the force of the wind by trying to "push" the aircraft back in the upwind direction. In reality, of course, an aircraft does not "feel" the wind; instead an aircraft operates "within" the wind. Remember that the real purpose of the slip is not to stop the aircraft's ground track from drifting sideways in the downwind direction--this can easily be accomplished simply by aiming the flight path in the upwind direction and flying the aircraft in a coordinated ("crabbing") manner. The real purpose of the slip is to align the fuselage (and landing gear) with the ground track and runway, even though the flight path through the airmass is running in a different direction. When an aircraft is slipping to compensate for a crosswind, it is actually the pilot's steady pressure on the downwind rudder pedal, not the external, meteorological wind, that causes the relative wind or airflow to strike the upwind side of the fuselage. The resulting aerodynamic sideforce would cause the flight path to curve in the downwind direction, if the pilot didn't bank the wing in the upwind direction to create an opposing horizontal force component. Fundamentally, the external, meteorological wind is not contributing to these forces in any way. It is the case however, that whenever a pilot is applying just enough pressure on the downwind rudder to keep the aircraft's heading exactly aligned with the ground track, then the sideways component in the relative wind or airflow over the aircraft will happen to be exactly equal to the crosswind component in the external, meteorological wind.

We'll close with a few practical notes--

* With most aircraft, the rudder is the limiting control in a slip: even with a full-deflection rudder input, the pilot typically still has plenty of aileron authority to counteract the rolling-out torque created by the interaction between the sideways airflow and the wing's dihedral geometry, and other related effects. However with a low-winged aircraft with minimal dihedral and a narrow-tracked landing gear, the limiting factor in a slip near the ground may be the clearance between the ground and the low wingtip: with full rudder deflection, the amount of bank required to prevent the flight path from curving might not allow enough ground clearance for the low wingtip.

* The amount of slip required to compensate for a crosswind usually decreases as the aircraft descends though the last few tens of feet above the ground, because there is usually less wind near the ground. In some cases this may allow the pilot to bring the nose into alignment with the runway heading shortly before touchdown, even if a full-deflection rudder input was inadequate to yaw the nose into alignment with the runway heading at a higher point in the final approach. Of course, a pilot shouldn't allow an aircraft to touch down if a full rudder input just barely keeps the nose aligned with the runway heading, with no extra margin of control authority left over for additional steering inputs.

 

For more practical reading on crosswind landing techniques, see Harvey S. Plourde's superb book "The Compleat Taildragger Pilot" (1991).

 

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