The Theory and Practice of Model Aeroplaning Part 11

-- 7. =Silk.=--This again is a _sine qua non_. Silk is the strongest of all organic substances for certain parts of aeroplane construction. It has, in its best form, a specific gravity of 13, and is three times as strong as linen, and twice as strong in the thread as hemp. Its finest fibres have a section of from 00010 to 00015 in diameter. It will sustain about 35,000 lb. per sq. in. of its cross section; and its suspended fibre should carry about 150,000 ft. of its own material. This is six times the same figure for aluminium, and equals about 75,000 lb. steel tenacity, and 50 more than is obtained with steel in the form of watch springs or wire. For aerofoil surface no substance can compare with it. But it must be used in the form of an "oiled" or specially treated silk. Several such are on the market.

Hart's "fabric" and "radium" silk are perhaps the best known. Silk weighs 62 lb. per cub. ft., steel has, we have seen, 490 lb., thus paying due regard to this and to its very high tensile strength it is superior to even steel wire stays.

-- 8. =Aluminium and Magnalium.=--Two substances about which a great deal has been heard in connection with model aeroplaning; but the writer does not recommend their use save in the case of fittings for scale models, not actual flyers, unless especially light ones meant to fly with the wind. Neither can compare with steel. Steel, it is true, is three times as heavy as aluminium, but it has four or five times its strength; and whereas aluminium and magnalium may with safety be given a permissible breaking strength of 60 per cent. and 80 per cent. respectively, steel can easily be given 80 per cent. Being also less in section, resistance to air travel is again less as in the case of wood. In fact, steel scores all round. Weight of magnalium : weight of aluminium :: 8:9.

-- 9. =Alloys.=--During recent years scores, hundreds, possibly thousands of different alloys have been tried and experimented on, but steel still easily holds its own. It is no use a substance being lighter than another volume for volume, it must be _lighter and stronger weight for weight_, to be superior for aeronautical purpose, and if the difference be but slight, question of _bulk_ may decide it as offering _less resistance_.

-- 10. =Sheet Ebonite.=--This substance is sometimes useful for experiments with small propellers, for it can be bent and moulded in hot water, and when cold sets and keeps its shape. _Vulcanized fibre_ can be used for same purpose. _Sheet celluloid_ can be used in the same way, but in time it becomes brittle and shrinks. _Mica_ should be avoided. _Jointless cane_ in various sizes is a very useful material--the main aerofoil can be built of it, and it is useful for skids, and might be made more use of than it is.[38] _Three ply wood_, from 1/50 in. in thickness, is now on the market. Four or five ply wood can also be obtained. To those desiring to build models having wooden aerofoils such woods offer the advantage of great strength and extreme lightness.

Referring to Table V. (Timber) at the end of the book, apparently the most suitable wood is Lombardy poplar; but its light weight means increased bulk, i.e. additional air resistance. Honduras mahogany is really a better all-round wood, and beech is not far behind.

Resilience is an important factor. Ash heads the list; but mahogany's factor is also good, and in other respects superior.

Lombardy poplar ought to be a very good wood for propellers, owing to its lightness and the ease with which it can be worked.

_Hollow reeds_, and even _porcupine quills_, have been pressed into the service of the model maker, and owing to their great strength and extreme lightness, more especially the latter, are not without their uses.

FOOTNOTES:

[38] The chief advantage of cane--its want of stiffness, or facility in bending--is for some parts of the machine its chief disadvantage, where stiffness with resilience is most required.

CHAPTER VIII.

HINTS ON THE BUILDING OF MODEL AEROPLANES.

-- 1. The chief difficulty in the designing and building of model aeroplanes is to successfully combat the conflicting interests contained therein. Weight gives stability, but requires extra supporting surface or a higher speed, i.e. more power, i.e. more weight. Inefficiency in one part has a terrible manner of repeating itself; for instance, suppose the aerofoil surface inefficient--badly designed--this means more resistance; more resistance means more power, i.e. weight, i.e. more surface, and so on _ad infinitum_.

It is because of circ.u.mstances like the above that it is so difficult to _design_ really good and efficient flying models; the actual building of them is not so difficult, but few tools are required, none that are expensive or difficult to use.

In the making of any particular model there are special points that require special attention; but there are certain general rules and features which if not adhered to and carefully carried out, or as carefully avoided, will cause endless trouble and failure.

-- 2. In constructing a model aeroplane, or, indeed, any piece of aerial apparatus, it is very important not to interrupt the continuity of any rib, tube, spar, etc., by drilling holes or making too thinned down holding places; if such be done, additional strength by binding (with thread, not wire), or by slipping a small piece of slightly larger tube over the other, must be imparted to the apparatus.

-- 3. Begin by making a simple monoplane, and afterwards as you gain skill and experience proceed to construct more elaborate and scientific models.

-- 4. Learn to solder--if you do not know how to--it is absolutely essential.

-- 5. Do not construct models (intended for actual flight) with a tractor screw-main plane in front and tail (behind). Avoid them as you would the plague. Allusion has already been made in the Introduction to the difficulty of getting the centre of gravity sufficiently forward in the case of Bleriot models; again with the main aerofoil in front, it is this aerofoil and not the balancing elevator, or tail, that _first_ encounters the upsetting gust, and the effect of such a gust acting first on the larger surface is often more than the balancer can rectify in time to avert disaster. The proper place for the propeller is behind, in the wake of the machine. If the screw be in front the backwash from it strikes the machine and has a decidedly r.e.t.a.r.ding action. It is often contended that it drives the air at an increased velocity under (and over) the main aerofoil, and so gives a greater lifting effect. But for proper lifting effect which it can turn without effort into air columns of proper stream line form what the aerofoil requires is undisturbed air--not propeller backwash.

The rear of the model is the proper place for the propeller, in the centre of greatest air disturbance; in such a position it will recover a portion of the energy lost in imparting a forward movement to the air, caused by the resistance, the model generally running in such air--the slip of the screw is reduced to a corresponding degree--may even vanish altogether, and what is known as negative slip occur.

-- 6. Wooden or metal aerofoils are more efficient than fabric covered ones. But they are only satisfactory in the smaller sizes, owing, for one thing, to the smash with which they come to the ground. This being due to the high speed necessary to sustain their weight. For larger-sized models fabric covered aerofoils should be used.

-- 7. As to the shape of such, only three need be considered--the (_a_) rectangular, (_b_) the elongated ellipse, (_c_) the chamfered rear edge.

[Ill.u.s.tration: FIG. 48.--(_a_), (_b_), (_c_).]

-- 8. The stretching of the fabric on the aerofoil framework requires considerable care, especially when using silk. It is quite possible, even in models of 3 ft. to 4 ft. spread, to do without "ribs," and still obtain a fairly correct aerocurve, if the material be stretched on in a certain way. It consists in getting a correct longitudinal and transverse tension. We will ill.u.s.trate it by a simple case. Take a piece of thickish steel pianoforte wire, say, 18 in. long, bend it round into a circle, allowing in. to 1 in. to overlap, tin and solder, bind this with soft very thin iron wire, and again solder (always use as little solder as possible). Now st.i.tch on to this a piece of nainsook or silk, deforming the circle as you do so until it has the accompanying elliptical shape. The result is one of double curvature; the transverse curve (dihedral angle) can be regulated by cross threads or wires going from A to B and C to D.

[Ill.u.s.tration: FIG. 49.]

[Ill.u.s.tration: FIG. 49A.--MR. T.W.K. CLARKE'S 1 OZ. MODEL.]

The longitudinal curve on the camber can be regulated by the original tension given to it, and by the manner of its fixing to the main framework. Suitable wire projections or loops should be bound to it by wire, and these fastened to the main framework by binding with _thin_ rubber cord, a very useful method of fastening, since it acts as an excellent shock absorber, and "gives" when required, and yet possesses quite sufficient practical rigidity.

-- 9. Flexible joints are an advantage in a biplane; these can be made by fixing wire hooks and eyes to the ends of the "struts," and holding them in position by binding with silk or thread. Rigidity is obtained by use of steel wire stays or thin silk cord.

[Ill.u.s.tration: FIG. 49B.--MR. T.W.K. CLARKE'S 1 OZ. MODEL.

Showing the position of C. of G., or point of support.]

-- 10. Owing to the extra weight and difficulties of construction on so small a scale it is not desirable to use "double surface" aerofoils except on large size power-driven models.

-- 11. It is a good plan not to have the rod or tube carrying the rubber motor connected with the outrigger carrying the elevator, because the torque of the rubber tends to twist the carrying framework, and interferes with the proper and correct action of the elevator. If it be so connected the rod must be stayed with piano wire, both longitudinally (to overcome the pull which we know is very great), and also laterally, to overcome the torque.

[Ill.u.s.tration: FIG. 49C.--A LARGE MODEL AEROPLANE.

Shown without rubber or propellers. Designed and constructed by the writer. As a test it was fitted with two 14 in. propellers revolving in the _same_ direction, and made some excellent flights under these conditions, rolling slightly across the wind, but otherwise keeping quite steady. Total weight, 1 lb.; length, 6 ft.; span of main aerofoil, 5 ft. Constructed of bamboo, cane, and steel wire. Front skids steel wire. Back skids cane. Aerofoil covering nainsook.]

-- 12. Some builders place the rubber motor above the rod, or bow frame carrying the aerofoils, etc., the idea being that the pull of the rubber distorts the frame in such a manner as to "lift" the elevator, and so cause the machine to rise rapidly in the air. This it does; but the model naturally drops badly at the finish and spoils the effect.

It is not a principle that should be copied.

[Ill.u.s.tration: FIG. 49D.--A VERY LIGHT WEIGHT MODEL.

Constructed by the author. Provided with twin propellers of a modified Fleming-Williams type. This machine flew well when provided with an abnormal amount of rubber, owing to the poor dynamic thrust given by the propellers.]

-- 13. In the Clarke models with the small front plane, the centre of pressure is slightly in front of the main plane.

The balancing point of most models is generally slightly in front, or just within the front edge of the main aerofoil. The best plan is to adjust the rod carrying the rubber motor and propeller until the best balance is obtained, then hang up the machine to ascertain the centre of gravity, and you will have (approximately) the centre of pressure.

[Ill.u.s.tration: FIG. 49E.--USEFUL FITTINGS FOR MODELS.

1. Rubber tyred wheels. 2. Ball-bearing steel axle shafts. 3. Bra.s.s wire strainers with steel screws; breaking strain 200 lb. 4. Magnalium tubing. 5. Steel eyebolt. 6. Aluminium "T" joint. 7. Aluminium "L"

piece. 8. Bra.s.s brazed fittings. 9. Ball-bearing thrust. 10. Flat aluminium "L" piece. (_The above ill.u.s.trations taken (by permission) from Messrs. Gamage's catalogue on Model Aviation._)]

-- 14. The elevator (or tail) should be of the non-lifting type--in other words, the entire weight should be carried by the main aerofoil or aerofoils; the elevator being used simply as a balancer.[39] If the machine be so constructed that part of the weight be carried by the elevator, then either it must be large (in proportion) or set up at a large angle to carry it. Both mean considerably more resistance--which is to be avoided. In practice this means the propeller being some little distance in rear of the main supporting surface.

[Ill.u.s.tration: FIG. 49F.--USEFUL FITTINGS FOR MODELS.

11. Aluminium ball thrust and racket. 12. Ball-bearing propeller, thrust, and stay.

(_The above ill.u.s.trations taken (by permission) from Messrs. Gamage's catalogue on Model Aviation._)]

-- 15. In actual flying models "skids" should be used and not "wheels"; the latter to be of any real use must be of large diameter, and the weight is prohibitive. Skids can be constructed of cane, imitation whalebone, steel watch or clock-spring, steel pianoforte wire. Steel mainsprings are better than imitation whalebone, but steel pianoforte wire best of all. For larger sized models bamboo is also suitable, as also ash or strong cane.