The Theory and Practice of Model Aeroplaning Part 1

The Theory and Practice of Model Aeroplaning.

by V. E. Johnson.

PREFACE

The object of this little book is not to describe how to construct some particular kind of aeroplane; this has been done elsewhere: but to narrate in plain language the general practice and principles of model aeroplaning.

There is a _science_ of model aeroplaning--just as there is a science of model yachting and model steam and electric traction, and an endeavour is made in the following pages to do in some measure for model aeroplanes what has already been done for model yachts and locomotives. To achieve the best results, theory and practice must go hand in hand.

From a series of carefully conducted experiments empirical formulae can be obtained which, combined later with mathematical induction and deduction, may lead, not only to a more accurate and generalized law than that contained in the empirical formula, but to valuable deductions of a totally new type, embodying some general law hitherto quite unknown by experimentalists, which in its turn may serve as a foundation or stepping stone for suggesting other experiments and empirical formulae which may be of especial importance, to be treated in _their_ turn like their predecessor. By "especial importance," I mean not only to "model," but "Aeroplaning" generally.

As to the value of experiments on or with models with respect to full-sized machines, fifteen years ago I held the opinion that they were a very doubtful factor. I have since considerably modified that view, and now consider that experiments with models--if properly carried out, and given due, not _undue_, weight--both can and will be of as much use to the science of Aeronautics as they have already proved themselves to be in that of marine engineering.

The subject of model propellers and motors has been somewhat fully dealt with, as but little has been published (in book form, at any rate) on these all-important departments. On similar grounds the reasons why and how a model aeroplane flies have been practically omitted, because these have been dealt with more or less in every book on heavier-than-air machines.

Great care has been exercised in the selection of matter, and in the various facts stated herein; in most cases I have personally verified them; great pains have also been exercised to exclude not only misleading, but also doubtful matter. I have no personal axe to grind whatever, nor am I connected either directly or indirectly with any firm of aeroplane builders, model or otherwise.

The statements contained in these pages are absolutely free from bias of any kind, and for them I am prepared to accept full responsibility.

I have to thank Messrs. A.W. GAMAGE (Holborn) for the use of various model parts for testing purposes, and also for the use of various electros from their modern Aviation Catalogue; also Messrs. T.W.K.

CLARKE & CO., of Kingston-on-Thames. For the further use of electros, and for permission to reproduce ill.u.s.trations which have previously appeared in their papers, I must express my acknowledgment and thanks to the publishers of the "Model Engineer," "Flight," and the "Aero."

Corrections and suggestions of any kind will be gratefully received, and duly acknowledged.

V.E. JOHNSON.

INTRODUCTION.

-- 1. Model Aeroplanes are primarily divided into two cla.s.ses: first, models intended before all else to be ones that shall _fly_; secondly, _models_, using the word in its proper sense of full-sized machines.

Herein model aeroplanes differ from model yachts and model locomotives. An extremely small model locomotive _built to scale_ will still _work_, just as a very small yacht built to scale will _sail_; but when you try to build a scale model of an "Antoinette" monoplane, _including engine_, it cannot be made to fly unless the scale be a very large one. If, for instance, you endeavoured to make a 1/10 scale model, your model petrol motor would be compelled to have eight cylinders, each 052 bore, and your magneto of such size as easily to pa.s.s through a ring half an inch in diameter. Such a model could not possibly work.[1]

_Note._--Readers will find in the "Model Engineer" of June 16, 1910, some really very fine working drawings of a prize-winning Antoinette monoplane model.

-- 2. Again, although the motor const.i.tutes the _chief_, it is by no means the sole difficulty in _scale_ model aeroplane building. To reproduce to scale at _scale weight_, or indeed anything approaching it, _all_ the _necessary_--in the case of a full-sized machine--framework is not possible in a less than 1/5 scale.

-- 3. Special difficulties occur in the case of any prototype taken.

For instance, in the case of model Bleriots it is extremely difficult to get the centre of gravity sufficiently forward.

-- 4. Scale models of actual flying machines _that will fly_ mean models _at least_ 10 or 12 feet across, and every other dimension in like proportion; and it must always be carefully borne in mind that the smaller the scale the greater the difficulties, but not in the same proportion--it would not be _twice_ as difficult to build a -in. scale model as a -in., but _four_, _five_ or _six_ times as difficult.

-- 5. Now, the _first_ requirement of a model aeroplane, or flying machine, is that it shall FLY.

As will be seen later on--unless the machine be of large size, 10 feet and more spread--the only motor at our disposal is the motor of twisted rubber strands, and this to be efficient requires to be long, and is of practically uniform weight throughout; this alone alters the entire _distribution of weight_ on the machine and makes:

-- 6. "=Model Aeroplaning an Art in itself=," and as such we propose to consider it in the following pages.

We have said that the first requisite of a model aeroplane is that it shall fly, but there is no necessity, nor is it indeed always to be desired, that this should be its only one, unless it be built with the express purpose of obtaining a record length of flight. For ordinary flights and scientific study what is required is a machine in which minute detail is of secondary importance, but which does along its main lines "_approximate_ to the real thing."

-- 7. Simplicity should be the first thing aimed at--simplicity means efficiency, it means it in full-sized machines, still more does it mean it in models--and this very question of simplicity brings us to that most important question of all, namely, the question of _weight_.

FOOTNOTE:

[1] The smallest working steam engine that the writer has ever heard of has a net weight of 4 grains. One hundred such engines would be required to weigh one ounce. The bore being 003 in., and stroke 1/32 of an inch, r.p.m. 6000 per min., h.p. developed 1/489000 ("Model Engineer," July 7, 1910). When working it hums like a bee.

CHAPTER I.

THE QUESTION OF WEIGHT.

-- 1. The following is an extract from a letter that appeared in the correspondence columns of "The Aero."[2]

"To give you some idea how slight a thing will make a model behave badly, I fitted a skid to protect the propeller underneath the aeroplane, and the result in r.e.t.a.r.ding flight could be seen very quickly, although the weight of the skid was almost nil.[3] To all model makers who wish to make a success I would say, strip all that useless and heavy cha.s.sis off, cut down the 'good, honest stick' that you have for a backbone to half its thickness, stay it with wire if it bends under the strain of the rubber, put light silk on the planes, and use an aluminium[4] propeller. The result will surpa.s.s all expectations."

-- 2. The above refers, of course, to a rubber-motor driven model. Let us turn to a steam-driven prototype. I take the best known example of all, Professor Langley's famous model. Here is what the professor has to say on the question[5]:--

"Every bit of the machinery had to be constructed with scientific accuracy. It had to be tested again and again. The difficulty of getting the machine light enough was such that every part of it had to be remade several times. It would be in full working order when something would give way, and this part would have to be strengthened.

This caused additional weight, and necessitated cutting off so much weight from some other part of the machinery. At times the difficulty seemed almost heartbreaking; but I went on, piece by piece and atom by atom, until I at last succeeded in getting all the parts of the right strength and proportion."

How to obtain the maximum strength with the minimum of weight is one of the, if not the most, difficult problems which the student has to solve.

-- 3. The theoretical reason why _weight_ is such an all-important item in model aeroplaning, much more so than in the case of full-size machines, is that, generally speaking, such models do not fly fast enough to possess a high weight carrying capacity. If you increase the area of the supporting surface you increase also the resistance, and thereby diminish the speed, and are no better off than before. The only way to increase the weight carrying capacity of a model is to increase its speed. This point will be recurred to later on. One of Mr. T.W.K. Clarke's well-known models, surface area 1 sq. ft., weight 1 lb., is stated to have made a flight of 300 yards carrying 6 oz. of lead. This works out approximately at 21 oz. per sq.

ft.

The velocity (speed) is not stated, but some earlier models by the same designer, weight 1 lb., supporting area 1 sq. ft., i.e., at rate of 16 oz. per sq. ft., travelled at a rate of 37 ft. per second, or 25 miles an hour.

The velocity of the former, therefore, would certainly not be less than 30 miles an hour.

-- 4. Generally speaking, however, models do not travel at anything like this velocity, or carry anything like this weight per sq. ft.

An average a.s.sumption of 13 to 15 miles an hour does nor err on the minimum side. Some very light fabric covered models have a speed of less than even 10 miles an hour. Such, of course, cannot be termed efficient models, and carry only about 3 oz. per sq. ft. Between these two types--these two extremes--somewhere lies the "Ideal Model."

The maximum of strength with the minimum of weight can be obtained only:--

1. By a knowledge of materials.

2. Of how to combine those materials in a most efficient and skilful manner.

3. By a constant use of the balance or a pair of scales, and noting (in writing) the weight and result of every trial and every experiment in the alteration and change of material used. WEIGH EVERYTHING.

-- 5. The reader must not be misled by what has been said, and think that a model must not weigh anything if it is to fly well. A heavy model will fly much better against the wind than a light one, provided that the former _will_ fly. To do this it must fly _fast_. To do this again it must be well powered, and offer the minimum of resistance to the medium through which it moves. This means its aerofoil (supporting) surfaces must be of polished wood or metal. This point brings us to the question of Resistance, which we will now consider.