CRAFTSMAN'S CORNER

RIGGING THE BIPLANE PARTI BY BEN OWEN The early airmail pilots had occasional problems with breaking wires due to ice build-up in flight. We rarely have this problem, but biplane rigging and wire tightness . . . that's another story! These hints and tips for rigging a biplane have been developed over a period of years. Many of the basic suggestions will also work for other aircraft.

First you need to block the airplane up. Since most biplanes are of steel tube, the top longerons are generally used to level the airplane fore and aft

and side-to-side. The axles themselves should be blocked up sturdily. The airplane should not be on the wheels as tires may flex and cause an erroneous reading. The tail is usually put on a sawhorse and also sturdily blocked in place and tied down. A nice trick that has been used by many builders is to buy the particular type of bubble level with the concentric circles and the bubble in the middle. Epoxy it to a cockpit floor where it can be easily read from the outside. The concentric circle round bubble

can then be read easily and used over the life of the airplane. Incidence tools (see diagram) do not need to be elaborate. As much as possible the top bar should be level with the chord line. This constant chord wing incidence tool is courtesy of Curtis Pitts. It can be used on all four wings. An adjustable level can be placed in the corner as shown or placed on top of the tool. The new electronic levels are accurate to within 1/1 Oth of a degree and are very useful in rigging — would be strongly suggested.

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WICKS AIRCRAFT SUPPLY

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118 NOVEMBER 1998

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right, landing wires can then be inline and you can place a rod through stalled while the top wing is brought the front and rear fitting holes. For up to level on one side. The other most biplanes the rod angle will be side follows suit. At this time it's a equal to the lower wing incidence. good idea to measure from the wing Corrections necessary to the fuselage tips to the rudder post to make sure fittings or wing fittings should be that the wings are not skewed and to sight carefully down the leading made at this time. The next order of business is to edges to make sure that the fittings position the wing center section. If were properly built. Then you start you are working with a one piece up- the whole process of sighting down, per wing, a team of helpers may be checking incidence and shimming needed. The incidence of the upper where necessary. Famed racing pilot wing is set using the cabane struts Johnny Livingston was an expert at and the roll wires, if applicable. If this type of rigging. You could loan there is a small center section, boards Johnny your airplane, he would fly it across the longeron with an ad- for a week and it would come back justable camera tripod on top may flying much better than before. You help to position its incidence prop- can do it too, with a little thought. erly. Many cabane struts use Piper lift You may need to release tension on strut forks on the ends. They are gen- some wires to place shims here and erally used only once in the setting there as necessary to maintain inciup of the wing. You may be able to dence. Usually after you've got the establish the length of the cabane airplane set up it will fly well, but it struts without using lift strut forks by may need other minor adjustments. using other scrap tubing to determine Most ailerons do tend to float up in the exact center to center distance flight. Some people like to set them from the center section to the fuse- down and others like to set them in lage cabane. These test pieces holes line with the wings for appearance can then be transferred over to the when on the flight line. As you adjust, check level and streamline tubing, hopefully establishing the center section or top wing measure, you should gradually exactly. The nice thing about the lift tighten up flying wires, landing wires strut forks is that they do allow you and the tail brace wires. Wire tightness comes under the heading of art, to adjust for possible wing sweep offcenter, one way or the other, but once science and opinion. Keep a close done, the built-in forks are rarely eye on the incidence . . . move the inused and do add weight. cidence tool to the surface you are The next move is to place the up- working on! per wing in place and the more help Next month: On to the final rigthe better. One lower wing is put in ging in Part 2. place and the interplane strut is tackwelded in place, checked again and (Thanks to Brett Clowes and Hale final welded off the aircraft and rein- Wallace for their assistance on this stalled. When the interplane strut is article.)

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SPORT AVIATION 119