There have been so many inquiries for further details on the vest-pocket pursuit ship, called the "Knight Twister," that I have laid out the drawings for the fuselage structure in such form that they can be presented in magazine form. It will be remembered that the general outline drawings for this little biplane were run in the July issue of POPULAR AVIATION, together with general data on the dimensions and performance.
During the past year, there has been much activity with miniature low-powered racers of this general type and the boys who have been fortunate enough to own them have been cleaning up good money at the various races and air-meets around the country. There is no doubt but what the miniature planes will be still more popular in the future for they make a great hit with everyone who sees them.
Now to get back to the subject of the fuselage for the "Knight Twister" which is a steel tube welded type with wood fairing strips to maintain the required circular sectional form. The turtle back consists of plywood supported on spruce fairing strips as shown. The section is circular at all stations which is very attractive and presents as nice an appearance as a monocoque fuselage.
The fairing of the fuselage sides is carried out on the rudder, making the fuselage wide at this point so that a stronger connection is possible for the stabilizer than would be the case without the fairing. This is very important for safety.
Now, many of you, after examining the accompanying drawings, will remark that it seems too heavy and unnecessarily strong for this size of ship -- at least in places. However, in reply to this criticism, I wish to call your attention to the requirements of the Department of Commerce in respect to welded tubing. They consider that the strength of the tubing after welding is only 80 per cent of the original strength, and again, that it is a good idea to have 20 per cent additional strength as a safety factor against weak welds and softened material.
Our ship is designed with a load factor of 6.5, but after adding 20 per cent to make up for loss in strength due to the welding and then 20 per cent more for safety, it is almost the same thing as designing with a load factor of 8.5. Even with these large factors, the tubing could be made smaller than shown but we must also consider the stresses encountered when handling the ship at the airport. Handling often produces stresses in excess of the landing or flying stresses.
While the drawings do not show the actual fairing strips, the outline of the faired body is shown here so that the fairing can be applied to attain this form. The strips must be close enough together to insure a smooth application of the fabric or plywood without bagging or pockets, for any unevenness of this sort will most certainly increase the resistance and will correspondingly reduce the speed.
Usually, the sides are built first by drawing the outlines on the floor or some similar flat surface. A "jig" can then be erected for holding the parts in place. Write the size of the tubing with chalk on this drawing and then nail cleats or small blocks of wood (1/2"x2"x3") along the lines in locations proper for holding the tubing in place while tack-welding. To protect the floor from burning, slip a 16 gauge steel sheet between the floor and the welded joints.
All tubes should be well fitted together, so that there will be no large gaps to be filled with welding material. If too much metal is used for making the weld, it will cause excessive shrinkage when cooling and may distort the frame or even pull it apart when cold. Crinkled tubing is generally due to this fault.
| Note that the faired outline, as shown by the thin single line, is truly circular at all points. The small marks on the circle indicate the location of the fairing strips. |
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The longerons are spliced with a "fish-mouth" splice, cut to an angle of 30 degrees. The smaller tube is slipped into the larger tube by at least 4-inches according to regulations, and welding should be done by moving the torch toward the larger tube. If the large tube end is welded first, we are likely to anneal the small tube more than necessary. The engine mount is welded to the fuselage after the fuselage is completed.
Build the two sides of the fuselage in the jig as described, but without the engine mount or the tail-post. If the tail-post is put in at this time it will be impossible to square up the members and the engine mount will also interfere with making the alignment. After the two sides are tack-welded, they can be laid on a couple of saw-horses and then the joints can be completely welded. The longerons of this ship are straight so that the fuselage can be made in a jig having a flat surface for the longerons to rest on. In this way, the fuselage is built "bottom-side-up" in the jig.
Some of the boys use a sort of scaffolding for a jig to hold the pieces straight while the cross-tubes are being cut, fitted and tack welded. I have also seen fuselage members assembled without any jig at all, but this makes it difficult to keep the job square while proceeding from station to station.
When building, with the floor as the flat surface of the jig, first draw a straight line with chalk to represent the centerline of the top truss. This is nearest the floor. Now draw the top truss members on the floor, keeping the centerline in the center. Draw lines where the diagonal tubes are to go, marking the sizes with the chalk. Next, nail some cleats around the lines to locate the tubes when they are cut and ready.
The bottom truss, which includes the bottom longerons, has a few bends in the sides and must be held at or near each joint by scaffolding. This scaffold is built up the sides and then tied over the top of the fuselage. The "top" in this case is the top of the assembly job which is in reality the bottom of the fuselage when it is in flying position.
One way to build the scaffolding is to use 1"x4" lumber for the side scaffolding and a piece of steel angle iron across the top of the jig at each station. Notches are cut in the angle irons where the longerons pass through and the angles are bolted to the 1"x4" wood uprights. This is so that they can be removed and the fuselage lifted out of the jig when it is completely tack-welded.
The 1"x4" wood uprights are at each joint on the bottom truss, but are not close enough to touch the tubing so that they will be burnt while you are using the torch in welding. The uprights have to be braced outward from the sides as well as forward and back.
Leave the diagonal tubes to be put in last. Put in the cross-tubes first and tack weld. Now we must check our jig and that part of the fuselage now built, to see if it is square, before welding in the diagonals and internal bracing.
With a ruler and chalk, mark on the cross tubes the exact center of the fuselage, measured across the fuselage from side to side. The centerlines of the cross tubes of top truss, which is nearest the floor, should be directly over that chalked centerline we made on the floor at the beginning.
Now, if the cross-tubes in the lower truss, which is near the top of the jig, have not their centerlines above that chalked centerline on the fuselage we must put them there.
A good way is to stretch two lines of cord, one about two inches above the floor and exactly above the chalked centerline on the floor. The second line is above the jig and also exactly parallel and above the chalked centerline on the floor.
Now, standing either at the front or tail of the jig, you can sight along these two lines and see if the chalked center marks on the cross-tubes are in line with these two cords. If not, push it over at the top of the jig or where it is out of place, hold it there with a shim of steel, or so it will stay till a diagonal is welded in place. When all diagonals and internal bracing are tack welded, the fuselage should still be square and in line with these two cords.
Remember, be careful in fitting these tubes. After all this trouble you do not want it to distort out of shape when you start finishing up the welding at the joints. This is done after removing the fuselage from the jig.
With the fuselage in an upright position on a couple of saw horses, and the joints all welded, you are ready to put in the tail-post. In handling, you may have brushed off the chalk marks on the centers of the cross-tubes and if so, remark them. Tie a string or cord to the center of the cross-tube, in the top truss at station No. 1, run the string forward and tie to the center of the cross-tube at station No. 7.
This string should lay on the centers of all the cross-tubes in the top truss if the fuselage did not distort. A second string is stretched through the fuselage from the center of the crosstube at station No. 2 to the center of the cross-tube at station No. 7 in the lower truss.
Placing the tail-post is a delicate operation, you will cut and fit many times before welding. The tail-post should be held in place with wire so that you can go to the front and sight along the strings to see if in line with the tail-post. In sighting along these strings you should see the tail-post not on one side or to the other side, but smack in the middle of both top and bottom trusses.
Now lay a straight edge across the fuselage at the forward end, and have some one hold a 2 foot square alongside the tail-post, with the square sticking out to one side, at the top longeron. If you stand back, you will be able to sight over the square and the straight edge to see if the tail-post is square with the top longerons. It will then be square with the fuselage.
The fin (vertical) may now be welded onto the tail-post. The small tubing and fittings for the wings may also be put on now. The wing fittings are placed so that the wings are inclined at 3 degrees to the top longerons. This means 3 degree angle of incidence when the ship is flying with the top longerons level.
The fuselage drawing showing the top and bottom trusses, has also drawings of cross-sections of the fuselage at stations No. 5, No. 6 and No. 7. This shows the size of tubing.
Stations No. 5 and No. 6 are made so that the pilot's legs can go through without too much crowding. Station No. 7 has the front of the landing gear and the landing gear shock struts fastened to it. The shock struts come inside of the fuselage and connect to the center of the cross-brace at the top truss.
The top wing fittings are built like the bottom wing fittings. All are of 16 gauge cold rolled sheet steel. The fittings look like channel irons, that is the sides are bent up to reinforce them. All bolts in the wing fittings that pass through the spruce spars of the wings are 1/4" diameter, nickel steel with castellated nuts.
The landing gear fittings on the fuselage at stations No. 6 and No. 7 will be shown in the chassis drawing. The stabilizer fittings on the fuselage are to be shown in the tail surface control drawings. The tail skid is a 4 leaf spring, each leaf 1/8" thick and 1" wide with a cast steel shoe bolted to the end.