Aeroplane Construction and Operation

by John B. Rathbun

Fig. 10. Complete Framing Plan of Typical Monoplane Structure. (A) Pilot. (B) Passenger. (M) Motor. (S) Stabilizer. (E) Elevator. (R) Vertical Rudder.

Doped surfaces have from 10 to 25 per cent greater tensile strength and resistance to tearing than the undoped linen, and increases the weight of the fabric by about 0.7 ounce per square yard for each coat applied. Under ordinary weather conditions, dope will require from 20 to 40 minutes per coat for drying, and at least one-half hour should be allowed between each coat. Weather conditions have a great effect on the action of dope, and with cellulose compounds the best results are obtained in a clean dry room, well warmed, and without drafts. On rainy days the linen is damp and the dope does not set well, and this trouble is not greatly helped by artificial heat. Drafts cause white spots and streaks, especially if cold air is allowed to enter directly upon the warm wing surface. To prevent drafts the ventilating ducts should be near the floor, and as the vapor is heavier than the air, and flows downwards, this means of ventilation is entirely practicable.

Applying the Dope. The number of coats depend upon the character of the job, but at least five coats should be applied, and preferably seven. On the best grade of work, the dope is generally covered with three or more final coats of spar varnish, although this is not absolutely necessary. For ordinary work, dope alone on Irish linen has proved very satisfactory for land machines, five coats being the usual amount applied on exhibition aeroplanes and planes for amateur use. Seven coats of dope with three coats of spar varnish are specified for military machines that are to be used on salt water. Seaplanes are subjected to conditions that are particularly hard on fabric and must be protected accordingly.

In applying the dope, at least one-half hour should be allowed for drying between each coat, and more if possible. The first two coats should be painted on lightly, the purpose being simply to fill the pores of the fabric and to prevent the succeeding coats from sinking through. If the first two coats are too heavy, the dope filters through the mesh of the linen and drops on the lower surface, causing spots and a waste of a very expensive material. Dope is expensive even with the greatest care exercised in its application, and the writer has seen cases where the first two coats were so heavily applied that fully 50 per cent of the fluid ran through and caked in among the structural parts of the machine. This ran the doping expense up to a terrific figure. The cloth should be dry, and the work performed, if possible, on a dry day. To save dope, never take out of the supply drum more than can be used for one coat, for the dope soon becomes tacky on exposure to the air, and a satisfactory job is hard to obtain if it gets in this condition.

Fig. 10a. Testing the Wing Structure of a Curtiss Biplane by Means of Sand Bag Loading. The Wings Are Turned Upside Down and a Sand Load Is Laid Uniformly Over the Wings. So That They Produce a Load Equal to, or Greater Than, the Flight Load.

In placing the fabric on the wings, particular care must be taken in stretching so as not to have it too tight when cellulose dope is used. The dope shrinks the linen to a very considerable extent, and if it is too tight to begin with, the stress due to shrinkage will place an excessive stress on both the fabric and the structure. When of the proper tautness, the fabric should sound like a drum when snapped with the finger. Any less tension than this will permit the fabric to sag badly when under the air pressure and reduce the efficiency of the wings. In fastening the cloth it should be just stretched taut and no more. In damp weather the cloth can be stretched a little tighter than in dry weather.

Transparent Coverings. In some types of battle-planes and scouts, a part of the wing section directly above the pilot is covered with a transparent fireproof cellulose sheet, much resembling celluloid. This permits the pilot to see above him through the overhanging wing, and is of great value in action. In some cases, a strip is placed on the lower wings along the sides of the body so that the ground is also easily visible. These cellulose sheets will not crack nor splinter, and are nearly as flexible as rubber. Celluloid of the ordinary variety must not be used, for this is easily ignited and is likely to start a disastrous fire.

Placing the Fabric on the Wings. In some aeroplanes the seams of the fabric are run parallel to the ribs, and are tacked or sewed directly to them, while in other cases the seams are run diagonally across the plane or on the "bias." Diagonal seams are most satisfactory, and if care is taken there is no more waste of linen than with the straight seam. The seams should be of the double-lapped or "English welt" type, and this of course necessitates sewing before the fabric is placed on the wings. The seams used on overcoats are satisfactory for this purpose, and give a covering that will not stretch nor bag. Some use linen thread and others use silk, but the linen is preferable, since dope often causes silk to rot. The seams should be covered with linen to protect them from the weather and to prevent the entrance of water to the interior woodwork.