Aeroplane Construction and Operation

by John B. Rathbun

Rib Material. In American aeroplanes, the flanges of the ribs are generally made of spruce. The webs are of poplar, whitewood, cottonwood or similar light material. There is not a great deal of stress on a rib, and the strongest material is not necessary, but as there are a great many ribs in a wing assembly lightness is a primary consideration. A few ounces difference on each rib makes a great deal of difference in the total weight, especially when there are 80 or more ribs in a complete machine. Exception to the above materials will be found in the Curtiss "Super-American" Flying Cruiser which has ribs with pine webs and birch flanges. European aeroplane practice makes use of hardwood in the ribs.

Web Stiffeners. The webs being thin and deep, and cut for lightening as well, need bracing at the points where concentrated loads are placed, such as at the front and rear spars, and at points between the lightening holes. By gluing thin strips to the webs (in a vertical direction), and so that the tops and bottoms of the strips come tight against the upper and lower flanges, a great deal of the strain on the web can be avoided. The stiffening blocks are shown by (x) in Fig. 4, and are placed on both sides of the front and rear spars F and S, and also between the lightening holes H.

Fig. 4. Details of "I" Type Rib, Showing Lightening Holes and Stiffeners.

Flange Fastenings. In the section at the right of Fig. 4 it will be seen that the web is inserted into a groove cut in the flanges and is then glued into place. It would be unsafe to trust entirely to the glue, owing to the effects of aging, moisture and heat, and consequently some additional means of fastening must be had. It has been customary to nail through the flange into the web, but as the web is only about 3/16 inch thick it is likely to split.

An approved method is shown in Fig. 4 in which Irish linen thread wraps (d) are passed through the lightening holes (H), and over the flanges (G). The thread is coated with glue before wrapping and after, and when dry it is thoroughly varnished for protection against moisture. The bands are spaced from 3 to 4 inches apart. If nails are used they should be brass nails—never steel or iron.

At the points (F) and (S) where the spars pass through the web, the web is entirely cut out so that the flanges ordinarily lie directly on the spars. In this case it is necessary to bevel the spar so that it at least approximately fits the curve of the flange. Sometimes when a full size spar is impossible, as in cases where the spar tapers toward the tips, wood packing pieces may be placed between the flange and the spar; tapered to make up for the curve. The flanges in any case must be securely fastened to the spar by brass wood screws as at (e), and the edges of the web should fit tightly against the sides of the spars.

Wing Battens. The wing battens run along the length of the wings, from end to end, and between the spars, and serve to brace the ribs sideways as shown in some of the general views of the wing assembly. To accommodate the battens, the openings (f) are cut directly under the flange. Usually the battens are thin spruce strips from 3/16 to 1/4 inch thick and 1/2 inch wide, and should be run through the web at a point near the stiffeners. The thickness from the top of the flange to the under side of the lightening hole is from 1/2 to 3/4 inch as indicated by (C).

Strength of Wood Ribs. The strength of a rib for any individual case can be found by the method used in computing beams, the rib usually being assumed to have a uniformly distributed load, although this is not actually the case, as before explained. The greater part of the load in normal flight is near the front spar, but this shifts back and forth with the angle of incidence so that there is no real stationary point of application, and the rib must be figured for the maximum condition. The total load carried by one of the intermediate ribs is due to the area between the ribs, or to the unit loading multiplied by the rib spacing and chord. The portion of the rib between the spars can be calculated as a uniformly loaded beam, supported at both ends. The entering edge in front of the spar, and the trailing edge to the rear may be taken as uniformly loaded beams supported at one end. The proportion of the loads coming on the ends and center position can be taken from the pressure distribution diagrams as shown under "Aerofoils."

A number of tests were made on ribs by Mr. Heinrich of the Heinrich Aeroplane Company, the ribs being built up on short pieces of spar so that actual conditions were approached. Instead of using a distributed load, such as usually comes on the rib, a concentrated load was placed at the center. If the rib were uniform in section the equivalent uniformly distributed load could be taken as one-half the concentrated load, but because of the lightening holes this would not be very exact. It would be on the safe side, however, as such a test would be more severe than with a uniform load. The ribs were of the same type as shown in Fig. 5, and were placed 32.5 inches apart. The front spar was 2 7/16 inches deep, the rear spar 2 1/16 inches deep, and the overall depth of the rib at the center was 3 1/8 inches. The rib flanges were of white wood 3/4 inch wide, and 3/16 inch thick. In rib No. 1 the web was solid whitewood, 3/16 inch thick, and in ribs Nos. 2 and 3, the webs were mahogany three-ply veneer (5/32 inch thick).