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 circumstances 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 Blériot 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
retarding 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.
[Illustration: 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 illustrate it by a simple case. Take a
piece of thickish steel pianoforte wire, say, 18 in. long, bend it
round into a circle, allowing ½ 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 stitch 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.
[Illustration: FIG. 49.]
[Illustration: 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.
[Illustration: 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.
[Illustration: 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, 1½ 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.
[Illustration: 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.
[Illustration: FIG. 49E.--USEFUL FITTINGS FOR MODELS.
1. Rubber tyred wheels. 2. Ball-bearing steel axle shafts. 3. Brass wire
strainers with steel screws; breaking strain 200 lb. 4. Magnalium
tubing. 5. Steel eyebolt. 6. Aluminium "T" joint. 7. Aluminium "L"
piece. 8. Brass brazed fittings. 9. Ball-bearing thrust. 10. Flat
aluminium "L" piece. (_The above illustrations 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.
[Illustration: FIG. 49F.--USEFUL FITTINGS FOR MODELS.
11. Aluminium ball thrust and racket. 12. Ball-bearing propeller,
thrust, and stay.
(_The above illustrations 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.
§ 16. Apart from or in conjunction with skids we have what are termed
"shock absorbers" to lessen the shock on landing--the same substances
can be used--steel wire in the form of a loop is very effectual;
whalebone and steel springs have a knack of snapping. These shock
absorbers should be so attached as to "give all ways" for a part side
and part front landing as well as a direct front landing. For this
purpose they should be lashed to the main frame by thin indiarubber
cord.
§ 17. In the case of a biplane model the "gap" must not be less than
the "chord"--preferably greater.
In a double monoplane (of the Langley type) there is considerable
"interference," i.e. the rear plane is moving in air already acted on
by the front one, and therefore moving in a downward direction. This
means decreased efficiency. It can be overcome, more or less, by
varying the dihedral angle at which the two planes are set; but cannot
be got rid of altogether, or by placing them far apart. In biplanes
not possessing a dihedral angle--the propeller can be placed
_slightly_ to one side--in order to neutralise the torque of the
propeller--the best portion should be found by experiment--unless the
pitch be very large; with a well designed propeller this is not by any
means essential. If the propeller revolve clockwise, place it towards
the right hand of the machine, and vice versa.
§ 18. In designing a model to fly the longest possible distance the
monoplane type should be chosen, and when desiring to build one that
shall remain the longest time in the air the biplane or triplane type
should be adopted.[40] For the longest possible flight twin propellers
revolving in opposite directions[41] are essential. To take a concrete
case--one of the writer's models weighed complete with a single
propeller 8½ oz. It was then altered and fitted with two propellers
(same diameter and weight); this complete with double rubber weighed
10¼ oz. The advantage double the power. Weight increased only 20
per cent., resistance about 10 per cent., total 30 per cent. Gain 70
per cent. Or if the method of gearing advocated (see Geared Motors) be
adopted then we shall have four bunches of rubber instead of two, and
can thereby obtain so many more turns.[42] The length of the strands
should be such as to render possible at least a thousand turns.
The propellers should be of large diameter and pitch (not less than
35° at the tips), of curved shape, as advocated in § 22 ch. v.; the
aerofoil surface of as high an aspect ratio as possible, and but
slight camber if any; this is a very difficult question, the question
of camber, and the writer feels bound to admit he has obtained as long
flights with surfaces practically flat, but which do, of course,
camber slightly in a suitable wind, as with stiffer cambered surfaces.
Wind cambered surfaces are, however, totally unsuitable in gusty
weather, when the wind has frequently a downward trend, which has the
effect of cambering the surface the wrong way about, and placing the
machine flat on the ground. Oiled or specially prepared silk of the
lightest kind should be used for surfacing the aerofoils. Some form of
keel, or fin, is essential to assist in keeping the machine in a
straight course, combined with a rudder and universally jointed
elevator.
The manner of winding up the propellers has already been referred to
(_see_ chap. iii., § 9). A winder is essential.
Another form of aerofoil is one of wood (as in Clarke's flyers) or
metal, such a machine relying more on the swiftness of its flight than
on its duration. In this the gearing would possibly not be so
advantageous--but experiment alone could decide.
The weight of the machine would require to be an absolute minimum, and
everything not absolutely essential omitted.
It is quite possible to build a twin-screw model on one central stick
alone; but the isosceles triangular form of framework, with two
propellers at the base corners, and the rubber motors running along
the two sides and terminating at the vertex, is preferred by most
model makers. It entails, of course, extra weight. A light form of
skid, made of steel pianoforte wire, should be used. As to the weight
and size of the model, the now famous "one-ouncers" have made some
long flights of over 300 yards[43]; but the machine claiming the
record, half a mile,[44] weighs about 10 oz. And apart from this
latter consideration altogether the writer is inclined to think that
from 5 oz. to 10 oz. is likely to prove the most suitable. It is not
too large to experiment with without difficulty, nor is it so small as
to require the skill of a jeweller almost to build the necessary
mechanism. The propeller speed has already been discussed (_see_ ch.
v., § 15). The model will, of course, be flown with the wind. The
_total_ length of the model should be at least twice the span of the
main aerofoil.
FOOTNOTES:
[39] This is a good plan--not a rule. Good flying models can, of
course, be made in which this does not hold.
[40] This is in theory only: in practice the monoplane holds both
records.
[41] The best position for the propellers appears to be one in front
and one behind, when extreme lightness is the chief thing desired.
[42] Because the number of strands of rubber in each bunch will be
much less.
[43] Mr. Burge Webb claims a record of 500 yards for one of his.
[44] Flying, of course, with the wind. _Note._--In the "Model
Engineer" of July 7, 1910, will be found an interesting account (with
illustrations) of Mr. W.G. Aston's 1 oz. model, which has remained in
the air for over a minute.