Test of Rib No. 1. There are 5 lightening holes between spars, with 2 inches of material left between the holes, and 1/2 inch between first hole and front spar opening. With 95 pounds concentrated load at the center, the first rupture appeared as a split between the first hole and spar opening. At 119 pounds, the flanges had pulled away from one side of the spar, and 1/8 inch away from the web. Full failure at 127.5 pounds. Web was split between each lightening hole with a complete cross break at center of web, the latter being caused by a brad hole in the web.

Test of Rib No. 2. Laminated web, with no brads driven opposite lightening holes. At 140 pounds rib deflected 5/16 inch, and when relieved sprang back only 3/16 inch. With 175 pounds, the deflection again was 5/16 inch, but the rib continued to bend slowly, the flanges pulling away from the web and spars. The wood was not broken anywhere, the failure being in the brads and glue.

Test of Rib No. 3. Same materials as No. 2, but web was fitted inside of the "I" beam spar and the rib flanges were screwed to the spar. At 175 pounds there was no sign of rupture anywhere, and the deflection was 5/16 inch. At 185 pounds the rib broke very suddenly and cleanly, and in such a way as to indicate that this was the true strength of the rib. The normal loading on the rib in flight was 17.5 pounds, uniformly distributed, so that with a concentrated load of 370 pounds equivalent, the safety factor was 21.1.

The conclusions to be arrived at from this test are as follows:

  1. When a solid soft wood web is used, there should be at least 2 1/2 to 3 inches between lightening holes.
  2. A laminated or three-ply web is the best.
  3. No brads should be driven opposite lightening holes.
  4. The web should fit closely to the spar sides and the flange of the rib should be tightly screwed to the top and bottom of spar.

The above gives an idea as to the strength of the usual form of wood rib, and can be used comparatively for other cases if the reader is not familiar with strength calculations.

Fig. 7. Rib Bending Press for Curving the Rib Flanges.

Making the Rib. Wooden webs are cut out on the band saw, and the webs are so simple that there is not much more to be said on the subject. The flanges, however, must be steamed and bent to the nearly correct form before assembly. After planing to size and cutting the groove for the reception of the web, the ribs are placed in the steamer and thoroughly steamed for at least an hour. A rib flange press shown by Fig. 7 consists of two heavy blocks with the inner faces cut approximately to the rib outline. The steamed ribs are then placed between the blocks, the bolts are screwed down tight, and is left for 24 hours so that the strips have ample time to cool and dry. For the amateur or small builder, the steamer can be made of a galvanized "down-spout" connected with an opening cut in the top of an ordinary wash boiler. One end of the spout is permanently sealed, while the other is provided with a removable cover so that the strips can be inserted. A hole cut near the center of the spout is connected to the opening in the boiler cover by a short length of spouting or pipe. The spout should be made large enough in diameter to contain all of the ribs that can be pressed at one time, and should be long enough to accommodate longer pieces such as the fuselage longerons, etc.

When removed from the press, the rib flanges can be glued to the webs taking care that the glue is hot, and that it thoroughly covers the groove surface. The rib must now be held accurately in place in a second form, so that the true rib outline will be retained until the glue drys. A great deal depends upon the accuracy of the second form, and the accuracy with which the web outline is cut. The larger manufacturers use metal rib forms or "jigs," but the small builder must be content with a wooden form consisting of a board fitted with suitable retaining cleats, or lugs. The outline of the aerofoil is drawn on the board, the tips of the cleats are brought to the line, and are screwed to the board so that they can be turned back and forth for the admission and release of the ribs. The strip bending press in Fig. 7 is only intended to bend the flanges approximately to form, and hence two layers may be put in the press at one time without much error.