Stability and Moments (Part Two)

Roll Stability and Moments

As described in the Pilot’s Handbook of Aeronautical Knowledge, more dihedral or less anhedral in a WSC wing creates more roll stability. More roll stability might be helpful for a training wing or a fast wing made for long cross-country straight flight, but most pilots want a balance between roll stability and the ability to make quicker turns and a sport car feel for banking/turning. Therefore, a balance between the stability and the instability is achieved through anhedral plus other important wing design features such as nose angle, twist, and airfoil shape from root to tip.

An aerodynamic characteristic of swept wings is an “effective dihedral” based on the sweep of the wing and angle of attack. The combination of the physical anhedral in the wing and the effective dihedral due to wing sweep provides the balance of stability and rolling moments for a particular wing design.

The design of the wing can have actual dihedral or anhedral in the wing. Even with anhedral designed in the inboard section of the wing, the outboard sections of the wing could have some dihedral because of the flex in the outboard leading edges. As the wing is loaded up from additional weight or during a turn, the tips flex up more creating more dihedral and a roll stabilizing effect when loaded. [Figure 2-30]

Figure 2-30. Wing front view example showing anhedral in the middle of the wing and dihedral at the outboard section of the wing because of leading edge flex.

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Generally, it is thought that the wing remains level and the weight shifts to the side to initiate a turn. Another way to look at how the WSC wing rolls is to examine the carriage and the wing moment from the carriage point of view. For example, the CG hangs far below a wing weighing ⅛ of the carriage weight. When the control bar is moved to the side, creating a moment about the carriage/wing hang point, the carriage stays vertical and the wing rotates around the carriage. Therefore, there are two rolling moments that both contribute to the WSC rolling into a bank:

The pilot creating the force on the control bar rotating the wing about the wing/carriage hang point.
Shifting weight to one side of the wing, thus warping the wing to aerodynamically change the lift on each side, as in airplane roll control. [Figure 2-31]

Figure 2-31. Pilot induced moments about wing/carriage hang point and resultant CG rolling moment.

Carriage Moments

Carriage weight and resultant CG are the main factors that contribute toward increasing the roll moment for the carriage. Carriage aerodynamic forces are not typically a factor for rolling moments.

Roll Stability Summary

Overall, roll stability and moments are a manufacturer/ make/model balance between dihedral/anhedral, wing twist, nose angle, airfoil shape from root to tip, and leading edge stiffness. Some designs are stable, others neutral, and others can be designed to be slightly unstable for quick side-to-side rolling.

Yaw Stability and Moments

There is no significant turning about the vertical axis because the WSC wing is designed to fly directly into the relative wind. Any sideways skidding or yaw is automatically corrected to fly straight with the swept wing design. An airplane uses the vertical tail to stabilize it to fly directly into the relative wind like a dart. The unique design of the WSC aircraft performs the same function through the swept wing design, but also the wing twist and airfoil shape from root to tip assists in the correction about the vertical axis. A simple way to understand the yaw stability is to see that any yawing motion is reduced simply through the increased area of the wing as it rotates about its vertical axis. [Figure 2-32]

Figure 2-32. Yaw correction about the vertical axis.

There is a slight amount of adverse yaw similar to an airplane that can be noticed when a roll is first initiated. The amount varies with the specific manufacturer’s design and make/model. In addition, the wing can yaw side to side to some degree, with some different manufacturer’s make/ model more than others. The higher performance wings with less twist and a greater nose angle are noted for less yaw stability to gain performance. These wings also require more pilot input and skill to minimize yaw instability through pitch input. An addition to the wing planform, twist, and airfoil shapes to minimize yaw, some wings utilize vertical stabilizers similar to these in airplanes and others use tip fi ns. [Figure 2-33] Generally, the WSC wing is yaw stable with minor variations that are different for each wing and can be controlled by pilot input, if needed.

Figure 2-33. Keel pockets and vertical stabilizers are additional tools designers use for yaw stability on the wing.

Carriage Moments

The wing is a significant factor in the design of yaw stability, but the carriage can be a large factor also. If the area in front of the CG is greater than the area in back of the CG, and the wing yaws to the side, then the front would have more drag and create a moment to yaw the WSC aircraft further from the straight flight. Therefore, fins are sometimes put on the carriage as needed so the carriage also has a yawing aerodynamic force to track the WSC aircraft directly into the wind. [Figure 2-34]

Figure 2-34. Wheel fins for carriage yaw stability.

Since the carriage has such a large effect on yaw stability, the carriage is matched to the wing for overall compatibility. Each manufacturer designs the carriage to match the wing and takes into account these unique factors of each design.

Yaw Stability Summary

These factors make the WSC aircraft track directly into the relative wind and eliminate the need for a rudder to make coordinated turns. Designs and methods vary with manufacturer and wing type, but all WSC wings are designed to track directly into the relative wind.

Thrust Moments

WSC aircraft designs can have different moments caused by thrust based on where the thrust line is compared to the CG. This is similar to an airplane except the WSC aircraft has no horizontal stabilizer that is affected by propeller blast.

If the propeller thrust is below the CG [Figure 2-35, top], this creates a pitch-up moment about the CG when thrust is applied and a resultant decrease in speed. When reducing the throttle, it reduces this moment and a nose pitch down results with an increase in speed.

If the propeller thrust is above the CG [Figure 2-35, bottom], this creates a pitch-down moment about the CG when thrust is applied and a resultant increase in speed. When reducing the throttle, it reduces this moment and a nose pitch up results with a decrease in speed.

With the thrust line above or below the CG producing these minor pitch and speed changes, they are usually minor for most popular designs. Larger thrust moments about the CG may require pilot input to minimize the pitch and speed effects. Most manufacturers strive to keep the thrust as close as possible to the vertical CG while also balancing the drag of the carriage and the wing for its speed range. This is why the carriage must be matched to the wing so these characteristics provide a safe and easy to fly WSC aircraft.

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