And now we show the wing details for the vest-pocket biplane to which I have given the name, "Knight Twister." Together with the fuselage details, which were published in the November issue of POPULAR AVIATION, you will now have a complete layout of this snappy little job.
Attached are two drawings, one of which shows the wings while the other drawing displays the ailerons. The wing drawing shows only half of the upper and half of the lower wing because the two halves are identical except for the "hand."
The top line of the spars is straight while the bottom is tapered, the taper starting at the root rib, runs out to the last rib. There is no taper where they run through the center-section and fuselage. I have made the top spar double thick, from the 27-inch rib to the inner end of this spar which is located in the center of the fuselage. In other words, the spar must be two-inches thick at the 27-inch, the 28-1/2-inch, the 30-inch and, in the top wing, at the 31-1/2-inch ribs.
The rear spar is more than strong enough at the one-inch thickness, and there will be no danger of flutter. The inside distance between the spars is 11 inches. All of the ribs are spaced 12-inches, center to center.
To eliminate any danger of flutter, the upper wing is made of longer span than the lower wing. When the two wings are tied together, each of the wings contribute a different natural period of vibration, hence this connection dampens out vibration that would be likely to arise in exactly similar upper and lower wings. Again, with the extra strong spars, we need have no danger of excessive flexibility.
We are building this latest type of Knight Twister at Cicero, Ill., and she is a honey. This one is equipped for airwheels and possibly for brakes. It will not be long before we will have some photos to show you, and I know that you fellows will appreciate her fine qualities as well as we do.
I cover the wings with 1/32-inch plywood, for this not only makes a fine covering but it also eliminates the necessity of internal wire bracing in the wings. The complete wing is then covered with cloth which gives additional strength, because the cloth ties the whole thing together and protects the plywood from being dented or marred while in service. It is possible to get a very fine finish with the cloth in the usual way by first doping with clear dope and later with pigmented dope.
Some lightplane builders use two coats of clear dope and two coats of pigmented dope, but some of the dopes do not properly fill the pores of the cloth with only four coats and therefore the finished job does not have the highly desired glass finish. If the structure is strong enough to withstand the pressure due to the shrinking of the dope, then six or even eight coats can be used to insure a smooth finish.
In general, there are two kinds of dope. One is the cellulose acetate dope while the other is the cellulose nitrate type. The former is expensive but it does not burn quickly -- it only smoulders -- and it does not shrink the fabric as much as the nitrate type. The nitrate dope is the more popular for it is cheaper, but it burns quickly when ignited and does not give quite as good a finish. The regular practice of using six coats, including the pigmented dope, adds about 4-1/2 ounces of weight per square yard of fabric. One square yard of fabric also weighs 4-1/2 ounces so that the total weight of the doped fabric is 9 ounces per square yard.
I make the ribs of 1/4-inch plywood, using the plywood cemented with waterproof glue. The ribs are cut out of the solid and holes are cut out only where the aileron control tube passes through. If you wish to wire the upper wing for wing lights, another set of holes will be needed in the upper wing ribs for running the light wires. The nose pieces of the ribs are cut in 3/4-inch to allow a 1/4x3/4-inch spruce strip to be inserted. This acts as the leading edge strip.
The trailing tips of the ribs are cut off 3/4-inch to allow for the application of a spruce trailing edge strip. The strip is 1/4x3/4-inch and is planed down to the proper form, the tips cut off the ribs being used as patterns for the section of the trailing edge. Both the T. E. and the L. E. spruce strips are nailed and glued in, using aircraft cement coated nails and cold-water casein glue. This casein glue is waterproofed and should be made to the Army Air Service specifications.
In the upper wing we have seven different ribs, all of the same shape but tapering in the length of the chord. We make full size rib drawings on thin plywood and cut them out with tin snips. These are used as patterns, layed out on the 1/4-inch plywood and then the curve is transferred by drawing a pencil around the edges of the pattern. This is an easy way of making as many ribs as we may wish of each size of rib.
At the point where the wing tip begins to curve, we used some 1/8-inch plywood. This forms a part of the trailing edge, part of the leading edge and the bow end or wing tip as well. Where the wing tip curves, we place a balsa wood block.
When covering with 1/32-inch plywood, the joints should be lapped and the joint tapered by shaving with a knife and smoothing with sandpaper.
Notice that the spars are parallel, and when cutting out the ribs for mounting on the spars, be sure that lines on the ribs are square with the datum line or the line drawn through the leading and trailing edges. While not shown on the drawing, there are 1/4x1/4-inch spruce strips running the full length of the wing. They are underneath the plywood covering (between the spars) to reinforce the covering. This will prevent warping and twisting with changes in the weather.
| Aileron Details for the Knight-Twister: This aileron is made of steel tubing and is shaped to the contour of the M-6 wing section. The hardware for this aileron is shown by the details below. |
Click for larger image (1200 x 779 pixels)
The aileron is made of 3/16-inch outside diameter tubing with 0.028-inch wall thickness while the trailing edge of the aileron is of 1/4-inch outside diameter tubing. The entire aileron is oxy-acetylene welded, but not with an electric arc since the latter often results in crystallization and breakage.
The drawing of the aileron shows how the spar of 3/16-inch tubing is braced and connected to the ribs and trailing edge so that it forms one well trussed structure that can stand a lot of twisting without going to pieces. Take particular notice how the aileron spar-tip is bent back 3/8-inch because, when a wing-tip is tapered and the aileron is not built with the aileron spar-bent back, the aileron spar will hit or bump the rear spar-tip of the wing. It often simplifies matters, when building an aileron to build it on the wing, using the wing as a jig for the aileron.
The wing "I" struts are cut out of 3/8-inch plywood. On the dotted line, running from top to bottom of the strut, as shown, is a 3/8-inch square strip glued on each side of the plywood strut to stiffen the strut sidewise. The strut is faired with balsa wood to a streamline section, then wrapped with cloth and thoroughly doped to keep out moisture. The fairing and finishing of the strut is best done when the strut is on the wings and when assembled on the ship.
Note that there is only one flying-wire fitting. This flying-wire is not actually needed, for the spars are stiff enough when used as cantilevers, but it is placed there for appearance and for a sense of added safety as much as anything else. It is run from the upper wing at the front spar "I" strut, down to the front landing gear fitting on the lower part of the fuselage, or else to the No. 7 lower station.
A jig is needed for assembling the wing. We used a table made of 2x4-inch lumber and on the top of the table we marked centerlines with chalk. Then the positions of the spar and rib centerlines were marked. When the wing is assembled on this jig it is upside down for the top surface is a straight line from wing tip to wing tip. Next, take some one-inch boards and out of one edge cut the shape of the upper surface of each rib. There should be one of these boards for every rib in the wing.
These one-inch pieces were nailed on the table at the points and on the centerlines of the positions marked on the ribs. They acted as part of the jig and held the ribs in place when applying the spars. The table must be straight and level and the one-inch strips must also be leveled so that the L. E. and the T. E. are straight lines through each rib. The spars are held in place on the jig by small angle-irons while applying the ribs.
Much depends upon the accuracy with which the ribs are assembled, their alignment upon the spars and the straightness of the leading and trailing edges. In the first place, it will be difficult to maintain the covering without bags and wrinkles when the ribs are out of line, and second, the proper function of the wing section will be diminished.
Any sagging or bagging in the covering will always cause a loss of power and speed, for such hollows set up turbulent eddies that interfere with the flow of air around the airfoil and destroy the performance. In one small ship, where the veneer was not properly secured so that it wrinkled up in damp weather, the loss of speed was 17 m.p.h.
And another important point is the location of the center of pressure in regard to the center of gravity. In one of my former articles, I showed how the wing was adjusted in respect to the center of gravity, and in my next issue, or before you have the wing completed, I will show you how to locate the center of gravity itself.
Lack of the proper balance, due to improper adjustments of the center of pressure and the center of gravity, not only increase the difficulty of control, but also increase the danger of stalling and causes a loss of power. If the ship is nose heavy or tail heavy, then it will be necessary to apply the elevators continually.
There must be some adjustment of the wings, either back or forth, to make up for variations in weight of different types, makes and sizes of engines; for variations in the weight of the pilot and so forth, and this adjustment will be explained in the following article.