The Theory and Practice of Model Aeroplaning Part 6

The number of petrol motor-driven model aeroplanes that have actually flown is very small. Personally I only know of one, viz., Mr. D.

Stanger's, exhibited at the aero exhibition at the Agricultural Hall in 1908.

[Ill.u.s.tration: FIG. 20.--MR. STANGER'S MODEL PETROL ENGINE.]

[Ill.u.s.tration: FIG. 21.--MR. STANGER'S MODEL PETROL ENGINE.]

In Fig. 21 the motor is in position on the aeroplane. Note small carburettor. In Fig. 20 an idea of the size of engine may be gathered by comparing it with the ordinary sparking-plug seen by the side, whilst to the left of this is one of the special plugs used on this motor. (_Ill.u.s.trations by permission from electros supplied by the "Aero."_)

-- 31. The following are the chief particulars of this interesting machine:--The engine is a four-cylinder one, and weighs (complete with double carburetter and petrol tank) 5 lb., and develops 1 H.P.

at 1300 revolutions per minute.

[Ill.u.s.tration: FIG. 22.--ONE-CYLINDER PETROL MOTOR.

(_Electro from Messrs. A.W. Gamage's Aviation Catalogue._)]

The propeller, 29 in. in diam. and 36 in. in pitch, gives a static thrust of about 7 lb. The machine has a spread of 8 ft. 2 in., and is 6 ft. 10 in. in length. Total weight 21 lb. Rises from the ground when a speed of about 16 miles an hour is attained. A clockwork arrangement automatically stops the engine. The engine air-cooled. The cylinder of steel, cast-iron heads, aluminium crank-case, double float feed carburetter, ignition by single coil and distributor. The aeroplane being 7 ft. 6 in. long, and having a span 8 ft.

-- 32. =One-cylinder Petrol Motors.=--So far as the writer is aware no success has as yet attended the use of a single-cylinder petrol motor on a model aeroplane. Undoubtedly the vibration is excessive; but this should not be an insuperable difficulty. It is true it is heavier in proportion than a two-cylinder one, and not so efficient; and so far has not proved successful. The question of vibration on a model aeroplane is one of considerable importance. A badly balanced propeller even will seriously interfere with and often greatly curtail the length of flight.

-- 33. =Electric Motors.=--No attempt should on any account be made to use electric motors for model aeroplanes. They are altogether too heavy, apart even from the acc.u.mulator or source of electric energy, for the power derivable from them. To take an extreme case, and supposing we use a 2-oz. electric motor capable of driving a propeller giving a static thrust of 3 oz.,[23] on weighing one of the smallest size acc.u.mulators without case, etc., I find its weight is 4 oz.

One would, of course, be of no use; at least three would be required, and they would require practically short circuiting to give sufficient amperage (running them down, that is, in some 10 to 15 seconds). Total weight, 1 lb. nearly. Now from a _pound_ weight of rubber one could obtain a thrust of _pounds_, not ounces. For scale models not intended for actual flight, of course, electric motors have their uses.

FOOTNOTES:

[12] Also there is no necessity for gearing.

[13] In his latest models the writer uses strands even three times and not twice as long, viz. fourteen strands 43 in. long.

[14] This refers to 1/16 in. square sectioned rubber.

[15] Of uniform breadth and thickness.

[16] In practice I find not quite so high a proportion as this is always necessary.

[17] Steel pinion wire is very suitable.

[18] See Appendix.

[19] As high a pressure as 250 atmospheres has been used.

[20] There was a special pump keeping the water circulating rapidly through the boiler, the intense heat converting some of it into steam as it flowed. The making of this boiler alone consumed months of work; the entire machine taking a year to construct, with the best mechanical help available.

[21] Model Steam Turbines. "Model Engineer" Series, No. 13, price 6_d._

[22] See Introduction, note to -- 1.

[23] The voltage, etc., is not stated.

CHAPTER V.

PROPELLERS OR SCREWS.

-- 1. The design and construction of propellers, more especially the former, is without doubt one of the most difficult parts of model aeroplaning.

With elastic or spring driven models the problem is more complicated than for models driven by petrol or some vaporized form of liquid fuel; and less reliable information is to hand. The problem of _weight_, unfortunately, is of primary importance.

We will deal with these points in due course; to begin with let us take:--

THE POSITION OF THE PROPELLER.

In model aeroplanes the propeller is usually situated either in front or in the rear of the model; in the former case it is called a TRACTOR SCREW, i.e., it pulls instead of pushes.

As to the merits of the two systems with respect to the tractor, there is, we know, in the case of models moving through water a distinct advantage in placing the propeller behind, and using a pushing or propulsive action, on account of the frictional "wake" created behind the boat, and which causes the water to flow after the vessel, but at a lesser velocity.

In placing the propeller behind, we place it in such a position as to act upon and make use of this phenomenon, the effect of the propeller being to bring this following wake to rest. Theoretically a boat, model or otherwise, can be propelled with less horse-power than it can be towed. But with respect to aeroplanes, apart altogether from the difference of medium, there is _at present_ a very considerable difference of _form_, an aeroplane, model or otherwise, bearing at present but little resemblance to the hull of a boat.

Undoubtedly there is a frictional wake in the case of aeroplanes, possibly quite as much in proportion as in the case of a boat, allowing for difference of medium. Admitting, then, that this wake does exist, it follows that a propulsive screw is better than a tractor. In a matter of this kind constructional considerations, or "ease of launching," and "ability to land without damage," must be given due weight.

In the case of model aeroplanes constructional details incline the balance neither one way nor the other; but "ease in launching" and "ability to land without damage" weigh the balance down most decidedly in favour of a driving or propulsive screw.

In the case of full-sized monoplanes constructional details had most to do with the use of tractors; but monoplanes are now being built with propulsive screws.[24]

In the case of models, not models of full-sized machines, but actual model flyers, the writer considers propulsive screws much the best.[25]

In no case should the propeller be placed in the centre of the model, or in such a position as to _shorten the strands of the elastic motor_, if good flights are desired.

In the case of petrol or similar driven models the position of the propeller can be safely copied from actual well-recognised and successful full-sized machines.

-- 2. =The Number of Blades.=--Theoretically the number of blades does not enter into consideration. The ma.s.s of air dealt with by the propeller is represented by a cylinder of indefinite length, whose diameter is the same as that of the screw, and the rate at which this cylinder is projected to the rear depends theoretically upon the pitch and revolutions (per minute, say) of the propeller and not the number of blades. Theoretically one blade (helix incomplete) would be sufficient, but such a screw would not "balance," and balance is of primary importance; the minimum number of blades which can be used is therefore _two_.

In marine models three blades are considered best, as giving a better balance.

In the case of their aerial prototypes the question of _weight_ has again to be considered, and two blades is practically the invariable custom.[26] Here, again, constructional considerations again come to the fore, and in the case of wooden propellers one of two blades is of far more easy construction than one of three.

By increasing the number of blades the "thrust" is, of course, more evenly distributed over a larger area, but the weight is considerably increased, and in models a greater advantage is gained by keeping down the weight than might follow from the use of more blades.

-- 3. =Fan versus Propeller.=--It must always be most carefully borne in mind that a fan (ventilating) and a propeller are not the same thing. Because many blades are found in practice to be efficient in the case of the former, it is quite wrong to a.s.sume that the same conclusion holds in the case of the latter.

By increasing the number of blades the skin friction due to the resistance that has to be overcome in rotating the propeller through the air is added to.

Moreover a fan is stationary, whilst a propeller is constantly _advancing_ as well as _rotating_ through the air.

The action of a fan blower is to move a small quant.i.ty of air at a high velocity; whereas the action of a propeller is, or should be, to move _a large quant.i.ty of air at a small velocity_, for the function of a screw is to create thrust. Operating on a yielding fluid medium this thrust will evidently be in proportion to the ma.s.s of fluid moved, and also to the velocity at which it is put in motion.