Compressed-air Engine for Model Aeroplane
Signs are not wanting that compressed air as a motive power for model
aeroplanes will become equally as popular as the twisted skein of
rubber, which has practically held the field since it was introduced
about the year 1870 by Alphonse Penaud.
One of the chief disadvantages of the rubber motor is that experiments
of a full-size scale nature cannot be undertaken, owing to the length
of frame required in order that the necessary power and duration of
run may be obtained, and also owing to the disposition of weight,
and consequently of the centre of gravity, not being tantamount to
that obtaining in full-size practice. With a compressed-air plant
these disadvantages are eliminated, since the weight can be kept well
forward, thus making possible the designing of a model which represents
in essential proportions a full-size machine.
Particulars are here given of a highly successful plant, for which the
machine described in the next chapter was especially designed. Several
of the illustrations in this present chapter are exaggerated to render
the construction clear, and it is thought that the details given will
be found comprehensive.
[Illustration: Fig. 136]
[Illustration: Fig. 137]
[Illustration: Fig. 138]
[Illustration: Fig. 139
Figs. 136 to 139.—Arrangement and Details of Engine]
Fig. 136 gives a plan view of the engine, which is rotary with, of
course, a stationary crank-shaft. The five cylinders are soft-soldered
to what may be termed the crank-case, which consists of two circular
brass discs of the gauge indicated. In order that the cylinders may be
accurately located round the plates, a wooden jig should be made with
slots to receive the cylinders, and a recess to take the plate. Five
lightening holes are drilled in the two plates as shown. The front
plate, that is, the one carrying the propeller bolt, is, however, left
off until the pistons, crank-shaft, and sleeve are assembled.
[Illustration: Fig. 140.—Details of Crank]
[Illustration: Fig. 142.—Connecting Rod]
[Illustration: Fig. 141.—Section of Cylinder]
[Illustration: Fig. 143.—Piston Tongues]
Fig. 137 gives a side elevation of the engine, with the crank-shaft and
sleeve shown in section. It will be seen that the sleeve butts to a
shoulder, a slight undercut being given to the shaft when turning this
portion to ensure a good joint. From this figure the inlet and exhaust
principle will be manifest. It will be noticed that as each inlet
pipe coincides with the right angular inlet in the shaft, so does it
receive a charge of compressed air. The pressure on the piston revolves
the engine, thus shutting off inlet to that particular cylinder and
bringing the next cylinder in line with the inlet. As soon as the
first cylinder nears the bottom of its stroke it begins to exhaust
through the diametrically opposed exhaust port. Needless to say, the
crank-shaft and sleeve must be turned a good running fit, otherwise
there will be considerable waste of power. The best method to employ is
to turn the shaft a push fit within the sleeve, and then to grind it in
with rottenstone. When soldering the inlet pipes into the sleeve, care
must be taken to ensure that they do not become “choked” with solder.
The sleeve should afterwards be reamed out to remove all superfluous
solder. When soldering the sleeve into the back plate care must also be
exercised to ensure that it is truly at right angles to the plate.
It must be clearly understood that the engine revolves with the sleeve
as a bearing. The five holes which are drilled round the sleeve to
receive the inlet pipes must be equidistant, so that the periods of
inlet are synchronous.
Fig. 138 gives an enlarged view of the crank-shaft and sleeve, and is
self-explanatory. Observe that the exhaust port is larger in diameter
than the inlet.
Details of the pistons are shown by Fig. 139. The connecting-rods
are soldered to tubular distance pieces, which rock on the ¹/₃₂-in.
silver-steel gudgeon-pins, which pass through the pistons, being cut
shorter than the outside diameter of the piston to avoid possible
scoring of the bore of the cylinders. The gudgeon-pins are soldered
into position, the superfluous solder being scraped from the piston
walls. To ensure airtightness of the pistons and cylinders, cupped
leather washers are fixed to the piston-heads by means of tin tongues
soldered to them, and which are forced through the washer and bent
over. The ordinary cycle-pump washer is admirably suited to the
purpose, but the height of the washer when within the cylinder should
not exceed ⅛ in.
[Illustration: Fig. 144.—Container]
Fig. 140 gives dimensions of the crank and throw, to exaggerated scale,
to avoid crowding the details. The important point to bear in mind when
beginning this portion of the construction is to obtain the correct
stroke, since the cylinders are designed to take a stroke of ½ in.
only. See also that the crank-pin revolves truly, that is, at 180° to
the shaft.
Fig. 141 is a longitudinal section of the cylinder. As there shown, the
cylinder-head is “let in” the head and soldered there. The inlet pipes
should be packed with resin prior to bending, this being afterwards
melted out. The connecting-rods are shown by Fig. 142, the important
dimension, obviously, being the centre distance of the holes for the
crank-pin and gudgeon-pin respectively. These are of No. 20 B.W.G.
brass. The tin clips used to secure the cupped leather washers to the
piston head (four of which are used for each piston, so that twenty in
all will be required) are shown by Fig. 143. They are of No. 30 S.W.G.,
and are bent along the dotted line to a right angle, the ⅛-in. portion
being the end to be soldered to the piston.
The compressed-air container shown by Fig. 144 is made from copper
foil of the thickness shown. This is folded round a wooden former of
circular cross-section, and tied tightly in place while the lapped
joint is being soldered. The two faces of the joint that are in contact
should first be tinned, using Fluxite or resin as a flux; spirits of
salt should on no account be used, as this has a deleterious effect on
metal of so fine a gauge; and a mediumly heated iron should be used to
solder the joint.
Wind the body with the No. 35 S.W.G. piano wire, soldering each spiral
at each revolution so that it maintains its correct pitch. Now attach
one of the half-balls (which for preference should be provided with a
stepped flange as shown) while the body is still on the wooden former,
first tinning the two surfaces in contact, and then “running” the
solder round with a mediumly heated soldering bit, and so sealing the
joint. Prior to attaching the second half-ball to the other end, a
tension wire must be attached to the flange, either of the valve or the
tap (according to which half-ball was attached first), by soldering.
This is then passed through the body of the container (the wooden
former, of course, now having been removed), and threaded through a
hole drilled in the half-ball at a convenient point near the centre.
Tension is now applied to the wire and the second half-ball eased into
position, and while still pulling on the wire it is soldered into the
hole through which it passes, afterwards being cut off sufficiently
long to form a coil on the end.
It will, of course, be clear that the valve (of the Lucas type) and
tap are soldered to the half-ball before the latter are affixed to the
container body.
The container should be inflated and immersed in paraffin to test for
leakages, and when these are stopped up the container and engine may
be connected by a short length of tubing. The engine is then ready for
running. Thin machine oil should be used for lubricating purposes, and
where necessary the connecting-rods must be staggered for clearance.
In conclusion, it should be pointed out that the plant should not weigh
more than 10 oz. complete, and is capable of flying a machine weighing
2 lb., provided that it is efficiently constructed. The container
should be inflated to a pressure of not less than 100 lb. Fig. 145
shows a similar compressed-air model aeroplane engine complete.
[Illustration: Fig. 145A.—Compressed-air Plant for Model Aeroplane]
The accompanying photographic reproduction shows a model compressed-air
plant for model aeroplanes which is similar in general design to the
one illustrated. The difference is that inlet takes place through
hollow connecting-rods, which are ball-ended and fit into ball
seatings. The cylinders oscillate, and the connecting-rods, being
rigidly attached to the pistons, by their angularity during revolution
form the inlet and exhaust mechanism. The propeller is geared up in the
ratio 2: 1.
[Illustration: Fig. 145.—Three-cylinder Engine]
The design is analogous to a very early French engine much in evidence
in the early days in model experiments.
It is thought that the photograph will give the reader an idea of the
general arrangement of the plant previously described.
=Driving Small Biplane=.—Fig. 145 is a front elevation, with the front
plate removed, of a 3-cylinder engine on similar lines, which would
be sufficiently powerful for models up to 12 oz. weight. As will be
obvious, the three cylinders are fixed (by solder) to two circular
discs of No. 20 gauge brass forming the crank chamber. The cylinders
and various component parts should be assembled _before_ the front
plate is fixed. A length of brass tube is soldered into the back plate,
and equidistant round its periphery three ⅛-in. holes must be drilled
to receive feed pipes which pass to the cylinder heads. A piece of
brass rod to form the crank-shaft must be turned to make a good running
fit within the tube and inlet, and exhaust holes drilled as indicated
by the dotted lines. The pistons should be made an easy fit. Pieces of
by-pass tubing are soldered into the small ends of the connecting-rods.
Through these tubes pass the gudgeon-pins which are anchored to the
piston walls. The position of the connecting-rods in relation to the
piston is thus maintained. The container (into which air is compressed
with a foot pump to from 100 lb. to 120 lb. per square inch) is
constructed from copper foil of three-thousandths (·003) of an inch
thickness, and is of the same dimensions as the five-cylinder one.