AC Spark Plug's . . . "Tip Of The Month Contest
55
Suggestions On Metal Ribs By Paul E. Best, EAA 2441
he customary method of making wing ribs and parts T like fuselage formers of sheet metal is by means of form blocks and a hammer. Two hardwood boards are sawed to the outline of the desired part and the sheet metal blank is clamped between them. Then the projecting flange of sheet metal is hammered over, crimping where needed to remove kinks from the flange. The process was fully described on pages 5 and 6 of the July, 1957 issue of this magazine. However, three years later amateurs still seem to prefer wood truss ribs, and it seems to me that the reason for their reluctance to change over to metal ribs may be due to a lack of awareness of the difference in cost and fabrication time between wood and metal. Many
was the important one of cost . . . the cost of the material and the amount of labor required to form it.
The average wooden truss wing rib using spruce strips and mahogany plywood gussets calls for about $2.00 worth of material and takes 75 to 90 minutes to assemble. To cite a common example, take the Baby Ace rib. It has a total of 38 small gussets, one on each side of every joint. With 26 ribs in the wing this means 988 gussets per plane. Each gusset has an average of six tiny nails in it, or a total of 5,928 nails in the whole wing! It is no wonder that even in the early C-3, the Aeronca people tried to get away from the cost of mahogany plywood and the labor of driving endless tiny nails by adopting fiber gussets, glued on in a jig designed to keep them from shifting as pressure was applied. On page 10 of the August, 1958 issue of this magazine is shown the Jurca type rib, in which strips of veneer are used in place of gussets. The required width of veneer could be homemade by slicing material off a board of the proper thickness with a table saw, and doubtless a jig can easily be made to position the truss members and veneer strips accurately to eliminate the need for nails. However, my interest in metal ribs was such that I have studied them carefully and would like to share my discoveries with others. A rib made of .020 ga. 2024 ST aluminum requires 2.5 sq. ft. of metal costing $1.50 as
compared to the $2.00 average for wood. If a small plane needed 24 ribs, a savings of $12.00 could be made on material alone, and about six hours of labor would also
be saved. Utility grades of sheet aluminum available years ago the lightplane factories gave up wood ribs, and planes such as the Cub, Taylorcraft and Aeronca all had metal ribs even though wood spars and fabric covering were retained. As you might suppose, the reason 28
NOVEMBER
1960
from building supply houses and mail order stores are even cheaper and while at present the use of non-aeronautical materials is frowned on, I feel that with sufficient
testing and investigation to establish their reliability,
ribs made of this aluminum could work out well and be the cheapest one could imagine. In the Air Force I came in contact with a $40,000
machine in a sheet metal shop. This little gadget forms ribs, bulkheads and other parts rapidly in a rather simple way. A metal blank for the part is cut to shape, with the desired flange width added to the circumference. This is put into a holding die and held in place by an overhead clamp arm fitted with fingers. The hammer, in principle, amounts to an upward-swinging "trap door", the arc of motion of which can be set at any desired angle. The operation is very rapid, there being a choice of 60 or 120 cycles per minute. The hold-down and the
is to get a piece of 3/16 in. steel rod. The end is given a few wraps of tape to avoid scratching. Open a vise about % in. and lay the flange over the end of this
opening. Lay the rod on top of the flange and tap down with a hammer to put the flute in. Space the flutes as needed to remove the twisting, usually one each several inches will do. Flute all ribs the same using the first
as a guide.
For making the stiffening flanges in the lightening holes, simple male and female dies turned from hardwood can be used. The male die can have a pilot pin in its center which fits a hole in the female die. This will
keep the two in alignment while they are squeezed to-
hammer are synchronized; the hold-down grips the sheet
gether with a vise, an arbor press, a hydraulic jack or even just a bolt and nut. For most ribs it is necessary
to the part being formed lets it be moved along so that, in what resembles sewing-machine fashion, the flange is rapidly bent up. To minimize warping and distortion, it is customary to pass the work around the machine three times, putting a successively greater bend into the flange until the 90-degree bend is attained.
Finish off the ribs by burring the raw edge of the flange so it will not chafe the wing fabric.
metal, the hammer comes up, drops down, then the holddown releases its grip slightly so that pressure applied
Inspired by this, I eventually hit upon a method of reproducing the forming action with simple hand equipment. My tool is nothing more than a maple or oak stick about a foot long, % in. thick and \Vz in. wide. One end has a slit cut in it with a thin-blade saw, this slit being of the same depth as the desired flange width. You lay out the part on the metal, being sure to add the flange width to its circumference. Hold the metal flat on the edge of a smooth table, push the tool over the metal's edge, and bend up about 20 degrees. Move the tool along on the metal a distance about half the tool's width and bend up again, and just keep going all around the part. On the second time around bend the flange to 40 or 50 degrees, and on the last time to 90 degrees. The first pass puts in the bends which establish the contour of the part. Moving along half the tool width at a time assures a smooth bend and uniform contour. It is even possible to work at flange-bending while watching TV or baby-sitting!
When the work with this tool is done, the rib or bulkhead will probably be twisted due to the strains in the flange metal. Fluting or crimping the flange will take this out. A pair of cheap pliers can be modified by brazing a pair of small shaped blocks to their jaws after this fashion: ((. Another way I have tried successfully
to make two or three dies for holes of varying size.
The advantage of my method of flanging is that troublesome, time-consuming distortion due to hammering is eliminated, and an accurate form block need not be made for each part in the airplane.
And here are a few extra tips. The approved manner of attaching metal ribs to wood spars is through a flange bent 90 degrees at the rib hole. At least three aircraft nails should be used in each flange. If ribstitching cord is used to attach fabric in normal manner, when it is drawn up tight after each loop the unsupported edge of the flange could be pulled down, possibly weakening or warping the rib. Also, the metal's edge could cut the cord in time. Partly for this reason and partly to speed up work, production planes use a variety of clips and strips to attach fabric to ribs. For the amateur, common self-tapping sheet metal screws are probably the best. A tiny washer is used under the head of each screw, partly to distribute pressure over the rib tape and partly to prevent the tape from developing unsightly twists and lumps when the screws are snugged down. In the November, 1959 issue of Popular Science Magazine a simple home-made sheet metal brake was described; I made one 28 inches long and found that for straight bends it works well and would recommend it to other EAA members.
Just wait until my friends see this in print . . . they thought I was "ribbing" them when I sat down to write an article!
SPORT AVIATION
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