The wings of a Stearman are constructed like a majority of wooden wing biplanes. Stearman wings use wood, aluminum and steel to form a system that is capable of producing lift and transferring that lift into the fuselage of the aircraft to carry payload. The various components of the wing system, or Cellule, are made of materials selected by the designers and engineers to meet the required loads while keeping as light as possible. Strong enough for the mission required but light. That’s the goal in designing an airplane.
Let’s look at the components in a Model 75 Stearman wing and discuss the purpose of each. Most folks who have seen a Stearman wing uncovered have a good idea of what all the various parts are and why they are there. In addition to the components that make up a single wing, you have the other wings and center section, struts and streamline tie rods (flying wires) completing the wing Cellule system. Fabric covers the outer surface of the wing creating the surface area needed to produce lift as the wing passes through the air. The Rib lacing (rib stitching) attaches the fabric to the ribs like a fabric version of a rivet in the wing of a span can airplane. The ribs form the airfoil shape and transfer the lifting loads from the fabric to the spars. The spars are the main span-wise load-carrying members that are the backbones of the wing and attach the wings to the rest of the airplane. Compression members are located between the spars to maintain the position of the spars relative to each other. The compression members work in conjunction with the drag and anti-drag wires that “X” between the spars and compression members.
It is these drag and anti-drag wires that seem to confuse some builders/restorers more than the other parts of a wing. With this confusion in mind, lets discuss the reasons for the size, location, number, etc of drag and anti-drag wires in a wing.
The names of “Drag Wire” and Anti-Drag Wire” are fairly self-explanatory. Drag wires run from a point inboard and forward in a bay to a point outboard and aft. In this position, the Drag wire is there to RESIST drag forces that would tend to pull the wing tips aft as the airplane travels through the air. Anti-Drag Wires oppose the drag wires running from an inboard aft location to an outboard forward point in its bay. As the name indicates, these anti-drag wires oppose forces in the reverse to the direction of flight. Maybe the wires should be named, anti-drag and anti-anti-drag. Nah, that’s too hard to type…………
Now, in a Stearman wing, the Anti-Drag Wires are larger diameter (or bigger square) than the drag wires. This is questioned often. Are drawings and part numbers wrong? The first line of thinking would be that the Drag forces on the wing would be greater than the anti-drag forces. Well, as it turns out, the part numbers are correct and the drag wires are the small ones because the anti-drag forces are higher than the drag forces.
So why are the anti-drag wires larger? What causes the loads to be higher in these wires? Simple. The wing tips try to move forward when g loads are applied and the angle of attack increases. FORWARD?!?! Yep that’s right. The wing tips are trying to get there first! A condition known as PHAA, Positive High Angle of Attack is the point where the wing is at its greatest AOA and producing lift. Like pulling the nose up to vertical or performing a loop.
The wing’s airfoil is cambered on the upper surface. The lift vector for a wing can be thought of as an arrow pointing up. Thrust is an arrow pointing forward. Drag aft, gravity down. The lift vector and thrust vector can be combined into a single arrow that points up and forward. This resultant vector represents level flight. Quite often in texts, the lift vector is represented as point straight up. As it turns out, it actually points slightly forward and is located at the center of lift of the wing. As the angle of attack increases, the wing begins to stall from the trailing edge migrating forward. So, the lift vector moves forward. As it moves forward, the vector points more and more forward as it curves around the camber of the airfoil. The vector is typically perpendicular to the wings surface. It is easy to see that the lift is pulling in the direction of the leading edge of the wing. The lift “sucks” the wing forward. Gravity pulls the fuselage and payload aft. This tug of war causes the wing tips to move forward and loads the anti-drag wires to an extreme.
A 3/16” drag wire is rated at 2100 lb pull load. A ¼” anti-drag wire at 3400 lbs. If, for example, the wings are designed to break at 10g, the anti-drag wire would fail when it reaches 3400 lb. If the airplane is flown to 6g, the anti-drag wire might see 2040 lb. That is far too close to 2100 lb for a 3/16” anti-wire. This is why the anti wires are larger.
Likewise, this is why the rear flying wires are larger than the front ones. As the wing tips move forward, the struts tend to rotate the wing tips nose down reducing tip incidence. The rear flying wires are subjected to a higher load as they transfer the lift loads to the fuselage frame.
This can be seen in the center section stagger wires (the one where the fuel lines run along). The anti-drag stagger wire is larger to hold the wings back. All these wires are part of the wing Cellule system and are designed to maintain wing position relative to the fuselage
This picture shows the 3/8 in. anti drag wire and the 5/16 in. drag wire. The rubber grommet was placed between the two wires, just to show the gap. If there is no gap, one wire is incorrectly placed on the wrong side of the other.