Roll torque created by dihedral during slips and skids

Roll torque created by dihedral during slips and skids

Steve Seibel
www.aeroexperiments.org

This page is still under construction!
This page was last modified on August 4, 2006

 

In "Definition of a 'slipping' turn", we noted the following: an aircraft is engaged in a "slipping" turn if the aircraft's nose is allowed or forced to yaw to point to the "high side" or "outside" of the turn in relation to the aircraft's actual direction of travel through the airmass at any given moment. In other words, an aircraft is "slipping" whenever the nose of the aircraft is allowed or forced to yaw to point to the "high side" or "outside" of the turn in relation to the actual direction from which the relative wind is blowing at any given moment. In a left turn, an aircraft is slipping if the aircraft's nose is pointing to the right of the actual direction of the flight path through the airmass at any given moment, i.e. to the right of the direction from which the relative wind is blowing at any given moment. (Diagram to be added.)

In "Definition of a 'slipping' turn", we also noted the following: when an aircraft is "slipping", there is a sideways component in the relative wind or airflow over the aircraft. This sideways component blows from the "leading" wing to the "following" wing, i.e. from the "inside" wingtip to the "outside" wingtip, i.e. from the low wingtip to the high wingtip. For example, in a left turn, if the aircraft is "slipping", i.e. if the nose is pointing too far toward the right in relation to the actual direction of the flight path and relative wind at any given moment, there will be a left-to-right component in the airflow over the aircraft. (Diagram to be added.)

In "Oblique views and side views of aircraft with dihedral", we explored how dihedral creates a roll torque in the "downwind" direction whenever there is a sideways component in the relative wind or airflow over an aircraft.

Putting all this together, we can see that when an aircraft with dihedral is turning to the left, if any sideslip is present, the resulting left-to-right sideways component in the relative wind will interact with the dihedral to create a roll torque toward the right. (Diagram to be added.) This roll torque will tend to roll the aircraft back toward wings-level. As a general rule, during a slipping turn, the sideways component in the relative wind will interact with any dihedral that is present to create a roll torque that will tend to return the aircraft to wings-level.

In "The rudder as a roll control--aircraft with dihedral", we noted that some aircraft use the rudder as the sole means of roll control. In such an aircraft, all rolling motions will be initiated by first yawing the nose to point in the desired direction of roll, in relation to the actual direction of the flight path and relative wind at that moment. In particular, to roll the aircraft from a banked attitude back to wings-level, the pilot will apply "outside" or "top" rudder to yaw the nose to point toward the outside or high side of the turn, in relation to the actual direction of the flight path and airflow at any given moment. In other words, the pilot forces the aircraft to slip as it turns. The resulting sideways component in the relative wind interacts with the dihedral geometry of the wing to create a roll torque in the "downwind" direction, i.e. toward wings-level. As the aircraft rolls from the banked attitude back toward wings-level, the curvature in the flight path will decrease and then stop completely. So to exit a turn in an aircraft that uses the rudder as the sole means of roll control, the pilot simply yaws (slips) the nose to point away from the direction of the turn.

**As a side note, pilots of "conventional" aircraft are very familiar with this rolling-out torque created by dihedral and other related aspects of an aircraft's geometry during a slip. Non-turning (straight-line) slips are often used in "conventional" aircraft to steepen the glide path on final approach, and/or to allow the fuselage (and landing gear) to be aligned with the runway at the moment of touchdown even when a crosswind is present, which means that the aircraft's direction of travel through the airmass must be different than the runway heading. (Diagram to be inserted.) Elsewhere in these tutorial pages we'll examine the linear forces that allow a "conventional" airplane to travel in a straight line rather than a curving path during this non-turning (straight-line) slip maneuver, even though the wing is banked. But for now, the point of interest is this: during a sustained slipping maneuver (including a slipping turn as well as a non-turning or straight-line slip) in a "conventional" aircraft where the wing is banked to the left and the nose is aimed to the right of the actual direction of the flight path and relative wind, the pilot typically must hold a strong right rudder input to keep the yaw or slip angle from decreasing, and the pilot typically must also hold a strong left aileron input to keep the bank angle from decreasing. (During a non-turning (straight-line) slip, both of these control inputs work together to keep the aircraft's heading constant: a change in either the yaw (slip) angle or the bank angle would also cause the aircraft's heading to start changing to the left or right as the flight path began to curve). All this is very different from the normal state of a affairs in a steady, constant-banked, non-slipping turn, where minimal roll (aileron) input is usually required to keep the bank angle constant. During a sustained slipping maneuver in a "conventional" aircraft, where the nose is pointing to the right of the flight path and relative wind and the wing is banked to the left as described above, the reason that the pilot must maintain the strong right rudder input is that the aircraft's inherent yaw stability or "weathervane" effect is constantly trying to yaw the nose to the left, back into alignment with the actual direction of the flight path and relative wind. And the reason the pilot must hold the strong left aileron input to prevent the bank angle from decreasing is that the sideways airflow from the slip is interacting with the aircraft's dihedral geometry to create a roll torque in the "downwind" direction that tries to roll the aircraft back to wings-level. In the case of high-winged aircraft with little dihedral, a dihedral-like effect is usually created by the way that the fuselage interferes with the airflow over the wing, and/or by the "pendulum effect" created by the fact that the aircraft's CG is located well below the wing, and/or by wing sweep--we'll explore these effects in detail elsewhere in these tutorial pages. In the case of a mid-winged aircraft with no dihedral or sweep, a sustained slipping maneuver in the direction described above would still require steady right rudder input to keep the nose from yawing back into alignment with the actual direction of the flight path and relative wind, but little or no aileron input might be needed to hold the bank angle constant.

 

In "Definition of a 'skidding' turn", we noted the following: an aircraft is engaged in a "skidding" turn if the aircraft's nose is allowed or forced to yaw to point to the "low side" or "inside" of the turn in relation to the aircraft's actual direction of travel through the airmass at any given moment. In other words, an aircraft is "skidding" whenever the nose of the aircraft is allowed or forced to yaw to point to the "low side" or "inside" of the turn in relation to the the actual direction from which the relative wind is blowing at any given moment. In a left turn, an aircraft is skidding if the aircraft's nose is pointing to the left of the actual direction of the flight path through the airmass at any given moment, i.e. to the left of the direction from which the relative wind is blowing at any given moment. (Diagram to be added.)

In "Definition of a 'skidding' turn", we also noted the following: when an aircraft is "skidding", there is a sideways component in the relative wind or airflow over the aircraft. This sideways component blows from the "leading" wing to the "following" wing, i.e. from the "outside" wingtip to the "inside" wingtip, i.e. from the high wingtip to the low wingtip. For example, in a left turn, if the aircraft is "skidding", i.e. if the nose is pointing too far toward the left in relation to the actual direction of the flight path and relative wind at any given moment, there will be a right-to-left component in the airflow over the aircraft. (Diagram to be added.)

Bearing in mind again that dihedral interacts with a sideways component in the relative wind to create a roll torque in the "downwind" direction, we can see that when an aircraft with dihedral is turning to the left, if any skid is present, the resulting right-to-left sideways component in the relative wind will interact with the dihedral to create a roll torque toward the left. (Diagram to be added.) This roll torque will tend to roll the aircraft toward a steeper bank angle. As a general rule, during a skidding turn, the sideways component in the relative wind will interact with any dihedral that is present to create a roll torque that will tend to roll the aircraft toward a steeper bank angle.

In "The rudder as a roll control--aircraft with dihedral", we noted that some aircraft use the rudder as the sole means of roll control. In such an aircraft, all rolling motions will be initiated by first yawing the nose to point in the desired direction of roll, in relation to the actual direction of the flight path and relative wind at that moment. In particular, to roll the aircraft from wings-level (or from a shallow bank) into a more steeply banked attitude, the pilot will hold "inside" rudder to yaw the nose to point in the intended direction of roll. In other words, the pilot "skids" the nose to point in the direction that he wants the aircraft to roll, so that the nose is no longer aligned with the actual direction of the flight path and airflow at any given moment. The resulting sideways component in the relative wind interacts with the dihedral geometry of the wing to create a roll torque in the "downwind" direction, i.e. away from wings-level and toward the intended direction of roll. As the aircraft rolls from wings-level (or from a shallow bank) into a more steeply-banked attitude, the flight path will curve (or the curvature in the flight path will increase.) So to intitiate (or tighten) a turn in an aircraft that uses the rudder as the sole means of roll control, the pilot simply yaws (skids) the nose to point in the direction that he wants to turn.

 

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