The assembly and rigging of a biplane require careful attention to alignment, incidence angles, wire tension, and overall structural symmetry. Because biplanes rely on multiple wings, struts, and bracing wires to achieve their strength and aerodynamic efficiency, even small rigging errors can affect aircraft performance, stability, and safety. Proper assembly and rigging procedures ensure that the aircraft conforms to its design specifications and is maintained in an airworthy condition.
Biplanes were some of the very first aircraft designs. The first powered heavier-than-air aircraft, the Wright Brothers’ Wright Flyer, which successfully flew on December 17, 1903, was a biplane.
The first biplanes were designed with very thin wing sections and, consequently, the wing structure needed to be strengthened by external bracing wires. The biplane configuration allowed the two wings to be braced against one another, increasing the structural strength. When the assembly and rigging of a biplane is accomplished in accordance with the approved instructions, the result is a stable, airworthy aircraft.
Whether assembling an early-model vintage aircraft that may have been disassembled for repair and restoration, or constructing and assembling a new aircraft, the following are some basic alignment procedures to follow.
To start, the fuselage must be level, fore-and-aft and laterally. The aircraft usually has specific leveling points designated by the manufacturer or indicated on the plans. The fuselage should be supported independently of the landing gear to ensure stability. A centerline should be drawn on the floor the length of the fuselage and another line perpendicular to it at the firewall, for use as an additional alignment reference.
With the horizontal and vertical tail surfaces installed, the incident angle for the horizontal stabilizer should be set. The tail brace wires should be connected and tightened until the slack is removed. Alignment measurements should be checked as shown in Figure 1.
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| Figure 1. Checking aircraft symmetry |
Install the elevator and rudder and clamp them in a neutral position. Verify the neutral position of the control stick and rudder pedals in the flight deck and secure them to simplify the connection and final tensioning of the control cables.
If the biplane has a center section for the upper wing, it must be aligned as accurately as possible because even the smallest error is compounded at the wingtip. Applicable cables and turnbuckles should be connected and the tension set as specified. [Figure 2]
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| Figure 2. Center section alignment |
The stagger measurement can be checked as shown in Figure 3.
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| Figure 3. Measuring stagger |
The lower wing sections should be individually attached to the fuselage and blocked up for support while the landing wires are connected and adjusted to obtain the dihedral called for in the specifications or plans. [Figure 4]
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| Figure 4. Measuring dihedral |
Next, connect the outer “N” struts to the left and right sections of the lower wing. Now the upper wing can be attached and the flying wires installed. The slave struts can be installed and the ailerons connected using the same alignment and adjustment procedures used for the elevator and rudder. The incidence angle can be checked as shown in Figure 5.
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| Figure 5. Checking incidence |
Once this point is reached, it is a matter of measuring, checking angles, and adjusting the various components to obtain the overall aircraft symmetry and desired alignment, as shown in Figure 1.
Also, care should be taken when tightening the wing wires because extra stress can be inadvertently induced into the wings. Always loosen one wire before tightening the opposite wire. Flying and landing wires are typically set at about 600 pounds and tail brace wires at about 300 pounds of tension.
Once the aircraft is properly rigged, step back and inspect the finished assembly. Are the wings symmetrical? Does the dihedral look even? Is the tail section square with the fuselage? Are the wing attaching hardware, flying wires, and control cables safetied? And the final task, before the first flight, is to complete the maintenance record entries.
As with any aircraft maintenance or repair, the manufacturer's instructions and specifications, or the procedures and recommendations contained in the construction plans, should always be followed when assembling and rigging the aircraft.