The One Chief Object.
An even distribution of the load so as to a.s.sist in maintaining the equilibrium of the machine, should be the one chief object in deciding upon the location of the motor. It matters little what particular spot is selected so long as the weight does not tend to overbalance the machine, or to "throw it off an even keel." It is just like loading a vessel, an operation in which the expert seeks to so distribute the weight of the cargo as to keep the vessel in a perfectly upright position, and prevent a "list" or leaning to one side. The more evenly the cargo is distributed the more perfect will be the equilibrium of the vessel and the better it can be handled. Sometimes, when not properly stowed, the cargo shifts, and this at once affects the position of the craft. When a ship "lists" to starboard or port a preponderating weight of the cargo has shifted sideways; if bow or stern is unduly depressed it is a sure indication that the cargo has shifted accordingly. In either event the handling of the craft becomes not only difficult, but extremely hazardous. Exactly the same conditions prevail in the handling of a flying machine.
Shape of Machine a Factor.
In placing the motor you must be governed largely by the shape and construction of the flying machine frame. If the bulk of the weight of the machine and auxiliaries is toward the rear, then the natural location for the motor will be well to the front so as to counterbalance the excess in rear weight. In the same way if the preponderance of the weight is forward, then the motor should be placed back of the center.
As the propeller blade is really an integral part of the motor, the latter being useless without it, its placing naturally depends upon the location selected for the motor.
Rudders and Auxiliary Planes.
Here again there is great diversity of opinion among aviators as to size, location and form. The striking difference of ideas in this respect is well ill.u.s.trated in the choice made by prominent makers as follows:
Voisin--horizontal rudder, with two wing-like planes, in front; box-like longitudinal stability plane in rear, inside of which is a vertical rudder.
Wright--large biplane horizontal rudder in front at considerable distance--about 10 feet--from the main planes; vertical biplane rudder in rear; ends of upper and lower main planes made flexible so they may be moved.
Curtiss--horizontal biplane rudder, with vertical damping plane between the rudder planes about 10 feet in front of main planes; vertical rudder in rear; stabilizing planes at each end of upper main plane.
Bleriot--V-shaped stabilizing fin, projecting from rear of plane, with broad end outward; to the broad end of this fin is hinged a vertical rudder; horizontal biplane rudder, also in rear, under the fin.
These instances show forcefully the wide diversity of opinion existing among experienced aviators as to the best manner of placing the rudders and stabilizing, or auxiliary planes, and make manifest how hopeless would be the task of attempting to select any one form and advise its exclusive use.
Rudder and Auxiliary Construction.
The material used in the construction of the rudders and auxiliary planes is the same as that used in the main planes--spruce for the framework and some kind of rubberized or varnished cloth for the covering. The frames are joined and wired in exactly the same manner as the frames of the main planes, the purpose being to secure the same strength and rigidity. Dimensions of the various parts depend upon the plan adopted and the size of the main plane.
No details as to exact dimensions of these rudders and auxiliary planes are obtainable. The various builders, while willing enough to supply data as to the general measurements, weight, power, etc., of their machines, appear to have overlooked the details of the auxiliary parts, thinking, perhaps, that these were of no particular import to the general public. In the Wright machine, the rear horizontal and front vertical rudders may be set down as being about one-quarter (probably a little less) the size of the main supporting planes.
Arrangement of Alighting Gear.
Most modern machines are equipped with an alighting gear, which not only serves to protect the machine and aviator from shock or injury in touching the ground, but also aids in getting under headway. All the leading makes, with the exception of the Wright, are furnished with a frame carrying from two to five pneumatic rubber-tired bicycle wheels.
In the Curtiss and Voisin machines one wheel is placed in front and two in the rear. In the Bleriot and other prominent machines the reverse is the rule--two wheels in front and one in the rear. Farman makes use of five wheels, one in the extreme rear, and four, arranged in pairs, a little to the front of the center of the main lower plane.
In place of wheels the Wright machine is equipped with a skid-like device consisting of two long beams attached to the lower plane by stanchions and curving up far in front, so as to act as supports to the horizontal rudder.
Why Wood Is Favored.
A frequently asked question is: "Why is not aluminum, or some similar metal, subst.i.tuted for wood." Wood, particularly spruce, is preferred because, weight considered, it is much stronger than aluminum, and this is the lightest of all metals. In this connection the following table will be of interest:
Compressive Weight Tensile Strength Strength per cubic foot per sq. inch per sq. inch Material in lbs. in lbs. in lbs.
Spruce.... 25 8,000 5,000 Aluminum 162 16,000 ......
Bra.s.s (sheet) 510 23,000 12,000 Steel (tool) 490 100,000 40,000 Copper (sheet) 548 30,000 40,000
As extreme lightness, combined with strength, especially tensile strength, is the great essential in flying-machine construction, it can be readily seen that the use of metal, even aluminum, for the framework, is prohibited by its weight. While aluminum has double the strength of spruce wood it is vastly heavier, and thus the advantage it has in strength is overbalanced many times by its weight. The specific gravity of aluminum is 2.50; that of spruce is only 0.403.
Things to Be Considered.
In laying out plans for a flying machine there are five important points which should be settled upon before the actual work of construction is started. These are:
First--Approximate weight of the machine when finished and equipped.
Second--Area of the supporting surface required.
Third--Amount of power that will be necessary to secure the desired speed and lifting capacity.
Fourth--Exact dimensions of the main framework and of the auxiliary parts.
Fifth--Size, speed and character of the propeller.
In deciding upon these it will be well to take into consideration the experience of expert aviators regarding these features as given elsewhere. (See Chapter X.)
Estimating the Weights Involved.
In fixing upon the probable approximate weight in advance of construction much, of course, must be a.s.sumed. This means that it will be a matter of advance estimating. If a two-pa.s.senger machine is to be built we will start by a.s.suming the maximum combined weight of the two people to be 350 pounds. Most of the professional aviators are lighter than this. Taking the medium between the weights of the Curtiss and Wright machines we have a net average of 850 pounds for the framework, motor, propeller, etc. This, with the two pa.s.sengers, amounts to 1,190 pounds. As the machines quoted are in successful operation it will be reasonable to a.s.sume that this will be a safe basis to operate on.
What the Novice Must Avoid.
This does not mean, however, that it will be safe to follow these weights exactly in construction, but that they will serve merely as a basis to start from. Because an expert can turn out a machine, thoroughly equipped, of 850 pounds weight, it does not follow that a novice can do the same thing. The expert"s work is the result of years of experience, and he has learned how to construct frames and motor plants of the utmost lightness and strength.
It will be safer for the novice to a.s.sume that he can not duplicate the work of such men as Wright and Curtiss without adding materially to the gross weight of the framework and equipment minus pa.s.sengers.
How to Distribute the Weight.
Let us take 1,030 pounds as the net weight of the machine as against the same average in the Wright and Curtiss machines. Now comes the question of distributing this weight between the framework, motor, and other equipment. As a general proposition the framework should weigh about twice as much as the complete power plant (this is for amateur work).
The word "framework" indicates not only the wooden frames of the main planes, auxiliary planes, rudders, etc., but the cloth coverings as well--everything in fact except the engine and propeller.
On the basis named the framework would weigh 686 pounds, and the power plant 344. These figures are liberal, and the results desired may be obtained well within them as the novice will learn as he makes progress in the work.
Figuring on Surface Area.
It was Prof. Langley who first brought into prominence in connection with flying machine construction the mathematical principle that the larger the object the smaller may be the relative area of support. As explained in Chapter XIII, there are mechanical limits as to size which it is not practical to exceed, but the main principle remains in effect.
Take two aeroplanes of marked difference in area of surface. The larger will, as a rule, sustain a greater weight in relative proportion to its area than the smaller one, and do the work with less relative horsepower. As a general thing well-constructed machines will average a supporting capacity of one pound for every one-half square foot of surface area. Accepting this as a working rule we find that to sustain a weight of 1,200 pounds--machine and two pa.s.sengers--we should have 600 square feet of surface.
Distributing the Surface Area.
The largest surfaces now in use are those of the Wright, Voisin and Antoinette machines--538 square feet in each. The actual sustaining power of these machines, so far as known, has never been tested to the limit; it is probable that the maximum is considerably in excess of what they have been called upon to show. In actual practice the average is a little over one pound for each one-half square foot of surface area.
Allowing that 600 square feet of surface will be used, the next question is how to distribute it to the best advantage. This is another important matter in which individual preference must rule. We have seen how the professionals disagree on this point, some using auxiliary planes of large size, and others depending upon smaller auxiliaries with an increase in number so as to secure on a different plan virtually the same amount of surface.
In deciding upon this feature the best thing to do is to follow the plans of some successful aviator, increasing the area of the auxiliaries in proportion to the increase in the area of the main planes. Thus, if you use 600 square feet of surface where the man whose plans you are following uses 500, it is simply a matter of making your planes one-fifth larger all around.
The Cost of Production.
Cost of production will be of interest to the amateur who essays to construct a flying machine. a.s.suming that the size decided upon is double that of the glider the material for the framework, timber, cloth, wire, etc., will cost a little more than double. This is because it must be heavier in proportion to the increased size of the framework, and heavy material brings a larger price than the lighter goods. If we allow $20 as the cost of the glider material it will be safe to put down the cost of that required for a real flying machine framework at $60, provided the owner builds it himself.
As regards the cost of motor and similar equipment it can only be said that this depends upon the selection made. There are some reliable aviation motors which may be had as low as $500, and there are others which cost as much as $2,000.