MOTIVE POWER FOR AERIAL CAMERAS
As long as circumstances permit, hand operation still remains the most
reliable and satisfactory method of driving a camera. It is always
available, can be applied to just the amount desired, and at the time
and place needed. For instance, in a magazine of the Gaumont type (Fig.
40), what is needed is the periodic application of a very considerable
force rather quickly, and while this can be done quite simply by hand,
no mechanism has even been attempted to go through this same operation
automatically. Instead, the fundamental design of automatic magazines
has been made along other lines calculated to utilize smaller forces
more steadily applied.
It must be granted, however, that for war planes, and particularly for
single seaters, cameras should be available which are capable of
operating semi-automatically or automatically. This necessarily means
the employment of artificial power, whose generation, transmission to
the camera and control as to speed present a mechanical problem of no
small difficulty.
=Available Sources of Power.=—The sources from which power may be drawn
on the plane are four, although the various combinations of these
present a large number of alternative approaches to the problem. These
sources are:
1. The engine of the plane.
2. Wind motors.
3. Spring motors.
4. Electric motors.
These may first be considered largely from the descriptive standpoint,
leaving questions of performance and efficiency for separate treatment.
Power may be derived directly from the engine through a flexible shaft,
similar to that used for the revolution counter. This source of power is
inherently the most direct and efficient, since the engine is the seat
of all the lifting and driving energy of the plane. There is no loss
through transformation into other forms of energy, such as electrical;
or by the use of more or less inefficient intermediary apparatus, such
as wind propellers. Against the direct drive of the camera from the
engine may, however, be urged that the usual distance between engine and
camera is too great for reliable mechanical connection, as by flexible
shafting. Objections also arise from the standpoint of speed. This
cannot be controlled by the camera operator; and varies over too wide a
range, as the engine changes from idling to full speed, to fit it for
automatic camera operation. The first objection may be met by that
combination of methods of power drive which consists in transmitting the
power electrically; that is, by letting the engine operate a generator
from which cables run to a motor close to the camera. This method, of
course, sacrifices efficiency, and it breaks down when the engine speed
drops below the speed necessary to generate the requisite voltage. This
defect may in turn be met by floating in storage batteries, which brings
up the whole question of electrical drive, to be treated presently.
While use of the engine for direct drive or for generating electric
current has not been adopted in the American service, it is known that
some German planes were supplied with electric current in this way.
Coming next to the wind motors, these possess one very great merit: they
utilize a motive power that is always present as long as the plane is in
motion through the air. On the other hand, the process of using the main
propeller of the plane to pull another smaller propeller through the air
appears a roundabout way to utilize the driving power of the airplane
engine. Yet on the whole it is probable that some form of propeller or
wind turbine is the simplest and most convenient device we have for the
operation of airplane auxiliaries. As long as the amount of power
required is small, such inefficiency as is inherent in its use is offset
by its convenience and reliability. An advantage of the propeller is
that its speed is almost directly proportional to that of the plane
through the air, a desirable feature in automatic cameras provided the
proportionality is under control. Yet it is just in this matter of
varying the speed at will that the propeller presents difficulties, to
be met only by additional mechanisms for gearing down or governing.
Propellers have the practical disadvantage that they present an easily
bent or broken projection to the body of the plane (Figs. 83 and 84).
The strength of small propellers for operating auxiliaries is never so
much in question with reference to their resistance to whirling and
thrust of air as it is to their ability to withstand the inevitable
knocks and careless handling that will fall to their lot. The propeller
bracket is just what the pilot is looking for to scrape the mud off his
boots before climbing in.
The wind turbine has the advantage over the propeller that its speed can
be varied rather simply by exposing more or less of its face to the
wind. A turbine fitted with an adjustable aperture for admitting the
wind is shown in Fig. 64, in connection with the type K automatic film
camera. The turbine has the advantage of being compact and lying close
against the body of the plane. In the form figured, altogether too much
head resistance is offered—just as much for low as for high speeds—but
with proper design this need not be the case. It is, moreover, quite too
small to give the needed speed regulation, as it only begins to operate
near its full opening.
[Illustration:
FIG. 64.—U. S. Type “K” (Folmer) automatic film camera, with wind
turbine and Venturi tube.]
[Illustration:
FIG. 65.—Type “K” camera, open, showing suction plate.]
Spring motors have the very real advantage that by their use the camera
can be made entirely self contained. The simplest application of the
spring motor would be to the semi-automatic camera, where no close
regulation of speed is required. In such a camera the operation of
exposing the shutter would release the spring, which would then change
the plate or film and re-set the shutter, repeating this operation as
long as the spring retained sufficient tension. Small film hand-cameras
of this type, using self-setting between-the-lens shutters, have been
designed, though not for aerial work. The possibilities of using springs
as motive power in semi-automatic cameras have not apparently been
seriously considered.
When a spring motor is used for automatic camera operation it at once
becomes necessary to add to the motor an elaborate clock mechanism for
controlling and regulating its speed of action. Springs are much better
fitted for giving power by quick release of their tension than by slow
release, and the necessary clock mechanisms for their regulation become
very heavy, as well as complicated and delicate, when they are made
large enough to do any real work. For their repair they require the
services of clock makers rather than the usual more available kind of
mechanic.
Coming next to electric motors, we meet with a source of power of very
great flexibility both in its derivation and in its application. If a
source of electric current is already provided for heating and lighting
as it is on the fully equipped military plane, and if it has sufficient
capacity to handle the camera, its use is rather clearly indicated,
irrespective of how efficiently or by what method it is produced.
Especially is this the case, from the standpoint of economy and
simplicity, if a propeller-driven generator is the source of current,
and the alternative power drive is an additional propeller for the
camera. If, on the other hand, the camera must have its own source of
electric power, the advantages and disadvantages must be closely
scrutinized. In this case either a generator must be provided, or resort
be made to storage batteries, or a combination of the two.
Ruling out a special propeller-driven generator, we are left with either
the generator driven from the engine or the storage battery. Inasmuch as
storage batteries are practically indispensable with generators, in
order to maintain the voltage constant at all speeds, it is on the whole
advisable to rely upon batteries alone. An advantage of their use is
that the power plant is entirely within the plane: All projections such
as propellers are avoided. Another merit is that the power is drawn upon
only as needed. Against storage batteries is their weight, the need for
frequent charging, and their loss of efficiency at low temperatures—a
loss so serious with those of the Edison form as to preclude their use.
When once the source of electrical energy is decided upon, its method of
application needs to be considered. Here we meet at once the peculiar
merit of electrical energy, namely, the ease and convenience with which
it may be transmitted. All we need is a pair of wires, led to any part
of the plane by any convenient route and connected by simple binding
posts. It may with equal ease be turned on or off by merely making or
breaking a contact with a switch. For operating semi-automatic cameras
this feature may be utilized in the interest of economy, if the power is
automatically turned off as soon as the plate-changing operation is
finished. Exceptionally reliable make and break contacts are necessary
to insure the success of this latter scheme.
Two methods of transforming the supply of electrical energy into
mechanical motion are available. The first is by the use of a solenoid
and plunger. This is a device practically restricted to semi-automatic
cameras, in which the operation consists of a straight to-and-fro
motion, initiated at the will of the operator. It has been used little
if at all. The second motion is the continuous rotary one secured by the
use of an electric motor. This motion is the most practical one for the
continuous operation of any mechanism, but on the other hand requires
that the imposed load be reasonably uniform at all times through the
cycle of operations. Assuming that the camera mechanism is of this
character, the motor may be attached directly to the camera, or if it
must be so large as to cause danger by vibration, it may be connected
through a flexible shaft. This use of an electric motor is very
practical for semi-automatic cameras such as the “L” or the American
deRam, in planes supplied with a suitable source of current.
When it comes to entirely automatic cameras, where uniform and
regulatable speed is required, as in making overlapping pictures for
mapping, the electrical drive is not so convenient. The shunt-wound
motor runs at nearly constant speed, while the series-wound motor in
which the speed can be regulated by the interposition of resistance, has
nothing like a sufficient range of variation for the purpose (at least
five to one is imperative) before it fails to carry the load. Hence we
must either incorporate in the camera some mechanism for varying the
interval between exposures while the speed of the motor remains
constant, or introduce an auxiliary device to effect the required
transformation in speed. If we do use an auxiliary device the train of
apparatus, consisting of battery (or generator), motor, speed control
and camera, is altogether too long; it is apt to cause annoying delays
in connecting up in an emergency, and it offers an excessive number of
chances for break-down.
=Performance and Efficiency Data.=—The first step in deciding upon
methods of power drive, and indeed in deciding whether power drive is
feasible at all, is to assemble definite data as to the power required
to drive representative cameras. Approximate figures for some of the
cameras described in previous chapters are:
L camera, 26 watts,
deRam, 60 watts,
“K” film, 30 watts.
These requirements—not exceeding ⅒ horse power—are insignificant in
comparison with the total of 100 to 400 horse power available for all
purposes from the plane's engine.
_Propeller characteristics._ Data on the performance of small propellers
are somewhat meagre. However, the results of the rather extensive
researches on large ones, suitable for driving planes, may be applied,
with proper reservations, to give a fair guide to the study of the
application of small propellers for driving plane auxiliaries.
The first factor to be considered is the thrust or _head resistance_
offered by a propeller to motion through the air. This varies as the
_square of the velocity_, as the _density of the medium_, and as the
_area of the body_ projected normally to the wind, the formula being
_T_ = _cdaV_^2
where _T_ = thrust, _d_ = density, _a_ = area, _V_ = velocity. Data on
the L camera propeller are shown in Fig. 66, where its thrust both when
free and when loaded with the camera is given, as well as that of a
solid disc of the same diameter as the propeller. For this propeller,
which is double-bladed, and six inches in diameter, _cda_ = .000275 with
the load on. The total thrust amounts to only about three pounds when
the plane velocity is 100 miles per hour. The head resistance of the
whole plane is a matter of hundreds of pounds, so that the propeller
resistance is quite negligible.
[Illustration:
FIG. 66.—Wind propeller data.]
The next factor is the speed of revolution of the propeller, expressed
in revolutions per minute. This varies with the design—the number of
blades, their area, and pitch. For a given design the speed of
revolution is _directly proportional to the speed of motion through the
air_, and to _the density of the air_. Representative data for the L
camera propeller are shown in Fig. 67. It will be noted that the speed
goes up to 8000 for 120 miles per hour air speed. This illustrates the
necessity for great strength to withstand centrifugal force. Propellers
should be constructed of tough material, and subjected to whirling tests
up to speeds considerably in excess of any the plane will attain in any
maneuver. At low speeds the linear relationship fails, as a critical
velocity is reached—about 3500 r. p. m. for this propeller—where it
refuses to turn.
[Illustration:
FIG. 67.—Relation between air speed and propeller revolutions.]
The fact that the speed of the propeller depends on the density of the
air has an interesting corollary, which is that a propeller adequate at
low altitudes will fail at high ones. The density of the air varies with
altitude according to the following figures:
At 3000 meters, 72 per cent. of sea level
5000 meters, 59 per cent. of sea level
6000 meters, 52 per cent. of sea level
If we take the r. p. m. at 90 miles per hour at sea level as 6000, then
at the above altitudes the speeds will be 4300, 3500, and 3000,
respectively. The last figure is below that for which this size of
propeller stalls with its normal load, as noted in the last paragraph.
Consequently, if flying is to be done at these altitudes a larger
propeller must be carried, which will still deliver enough power at the
lower density.
The next factor to be considered is the _power furnished by the
propeller_. As a representative figure may be quoted the performance of
the L propeller. This gives 27 watts at 3600 revolutions per minute (56
miles per hour). From this figure the performance of other propellers
may be deduced from the basic laws, which are: that the _power varies as
the density of the medium_ and as the _cube of the velocity_ (assuming
constant efficiency). Since the power delivered by the six inch diameter
L propeller is already adequate at 60 miles per hour, the necessary
dimension to function satisfactorily at 100 miles per hour would need to
be only a little more than three inches, except for the desirability of
a safety factor for high altitudes and low air densities.
The _efficiency_ of the propeller is defined by the relation—
power delivered by the propeller
Efficiency = ————————————————————————————————
power supplied to the propeller
The denominator of this fraction is the thrust times the velocity, for
which the curves of Fig. 66 supply us data for the L propeller. Using
the figures 3600 r. p. m., 56 miles per hour, and 27 watts, we find the
efficiency to be about 50 per cent. This increases with the velocity,
with a possible upper limit of 70 to 80 per cent. Since the main
propeller of the plane is not over 80 per cent. efficient we have at
most an efficiency of 64 per cent. in using a propeller drive, as
compared with taking the power directly off the engine.
In considering the use of _spring and clock-work motors_ we meet at once
with the problem of comparing the effect on the performance of a plane
of a carried weight, as against a head resistance. The efficiency of a
spring motor is measured in terms of its weight, that of a propeller in
terms of its head resistance. The general answer to this question is
given by the relation that _a pound of dead weight is equivalent to ⅕
pound head resistance_.
In order to apply this relation to the study of spring motors for
driving cameras, data are necessary on the power delivery per pound
weight of such mechanisms. Such data are not easily accessible, largely
because clock-work has not generally been seriously considered as a
motive power for large apparatus. To arrive at an approximate figure we
may take the fact that in an 8 × 10 inch film camera designed by one of
the manufacturers who have utilized clock-work, the motor weighed 30
pounds. This is equivalent to six pounds head resistance. Now the type
K, 18 × 24 centimeter film camera is operated, even with the addition of
a friction drive speed control, by means of the L camera propeller. As
shown in Fig. 66, at 100 miles per hour the head resistance of this
propeller is still less than three pounds. Consequently, it appears that
from the efficiency standpoint the clock mechanism is quite outclassed
by the wind propeller.
Coming next to the _electric motors_, the L camera and the K are both
operated satisfactorily with a 1/20 horse power motor, weighing 6
pounds. For the deRam a ⅒ horse power motor has been adopted.
Taking up efficiency considerations, we have, if the current is supplied
by a generator from the engine, a transformation factor of 70 to 80 per
cent. from mechanical to electrical energy and a similar factor in using
a motor for the camera. When batteries are employed the matter of weight
_versus_ head resistance again arises. The batteries found most
satisfactory for operating the K and deRam cameras are of the six-cell
12 volt lead type. Their capacity is 40 ampère hours at three ampères or
36 at five ampères—more than is necessary for a single reconnaissance,
but a practical figure when economy of charging and replacement are
considered. The weight of this unit is 27 pounds. To this must be added
the weight of the motor—6 lbs.—making a total of 33 pounds, equivalent
to a head resistance of nearly 7 pounds. This is more than twice the
propeller head resistance invoked to do the same work.
These considerations of efficiency have been gone into because they are
usual in studying any engineering problem and because of the insistent
demand from the plane designer that every ounce of weight and head
resistance be saved. Actually, as already stated, the load imposed by
any method of power drive is trivial in comparison with the whole load
of the plane. There is, however, an important reservation to be made,
which applies against clock-work and batteries: This is, that while the
equivalent head resistance of any camera motive power carried as dead
weight is small, its effect on _balance_ may not be so. While the use of
a propeller need not disturb the plane's balance, the weight of the
camera alone, without any driving apparatus, is already seriously
objected to on this score. The merely mechanical superiority of the
propeller as a source of motive power is on the whole rather marked.
=Control of Camera Speed.=—In the semi-automatic camera the only control
required on the speed of the operating motor is at the upper and lower
limits. It must not go so fast as to anticipate the completion of any
steps in the cycle of camera operation, such as the fall of plates or
pawls into position, which would jam the camera. On the other hand, it
must not be so slow that pictures cannot be obtained with the requisite
overlap for maps or stereoscopic views. In the American deRam camera the
cycle of operations cannot safely be put through in less than four
seconds, a short enough interval for most purposes. It is also highly
desirable in the semi-automatic camera to have the motive power capable
of stopping completely. This saves wear and tear on both motor and
camera mechanism.
In the automatic camera an extreme range of speed is called for by the
several problems of mapping, oblique photography, and the making of
stereoscopic views. For mapping alone, the shortest likely interval may
be taken as that required for work at approximately 1000 meters
altitude, for a plane speed of 150 kilometers per hour, which demands an
interval of six seconds with a ten inch lens on a 4 × 5 inch plate. For
vertical stereos at the same altitude and speed this interval is divided
by three, and low oblique stereos need even quicker operation. Hence a
range of from 1 to 30 pictures per minute should be provided for. This
requirement is difficult to meet with any simple mechanism.
From the standpoint of simplicity in speed regulation the wind turbine
of adequate vane surface has much to recommend it. It is only necessary
to present more or less of its vane area to the wind in order to secure
a considerable range of speed. The method of doing this by a shutter
interposed in front is uneconomical, but it is probable that the design
can be so altered that more or less of the turbine is exposed beyond the
side of the plane, possibly by varying the angle, to secure the same
result without introducing useless head resistance. A serious practical
objection to the turbine lies in the large vane surface necessary to
give adequate power combined with proper speed variation. In the
automatic film camera (Type K) this area should be as much as 40 to 50
square inches.
The wind propeller does not lend itself at all well to speed variation.
It cannot be partially covered from the air stream, as can the turbine,
because of the resulting strain on its mount. A possible form of
variable speed propeller, one which, however, has not yet been
practically developed, is a propeller with controllable variable pitch.
If this could be made mechanically sound it would be well-suited for
camera operation. That such a propeller could be worked out is indicated
by the good performance of a constant speed propeller developed for
radio generators and used on the French deRam camera (Fig. 54).
Parenthetically, it maybe questioned whether a constant speed propeller
is really desirable with an airplane camera. What is required is not
exposures at a definite time interval—although most of the data are in
that form—but exposures at definite intervals with respect to the motion
of the plane, which practically means with reference to its air speed.
Rather than build a camera calculated to give exposures at intervals of
so many seconds when it is attached to a constant speed propeller, we
would do better to use a propeller which responds to the speed of the
plane, in conjunction with some form of tachometer to show the rate at
which exposures are being made. This in turn should be coördinated with
the indications of a proper camera-field indicating sight.
One solution of the problem of speed control with a propeller of
practically fixed speed, is to use a governor and slip clutch as in the
English Type F film camera (Fig. 57). Here the propeller shaft and the
camera driving axle are connected by two friction discs. That on the
camera mechanism is forced against the other by a spiral spring, whose
tension is controlled by a ball governor. If the camera speed becomes
too high the governor reduces the tension on the spiral spring and the
discs slip over each other. The point where this slipping occurs is
determined by the position of the governor as a whole, and this is
controlled by a lever on top of the camera.
Another speed control device, perhaps more positive but certainly more
complicated and wasteful of power, consists of a large flat disc, driven
by the propeller or electric motor, and from which the camera is driven
by a shaft from a smaller friction disc which may be pressed against any
point from the center to the periphery of the larger disc. The speed
range attainable in this way is limited only by the size of the large
disc. An application of this idea is shown in the speed control (Fig.
68), designed for the American Type K camera when operated on an
electric motor or on a simple propeller. The same idea is utilized in
the Duchatellier film camera, in connection with the constant speed
propeller already described.
[Illustration:
FIG. 68.—Friction disc speed control.]
On the whole it is eminently desirable from the standpoint of power
operation that the automatic camera should embody its own means for
altering the interval between exposures, so that all the external
attachment needed is a single connection to a source of power either of
constant speed, as an electric motor, or of speed proportional to that
of the plane, as with a simple wind propeller. This makes the camera
largely independent of the nature of the power supply, whereas a camera
designed for a special variable speed device is of little use on a plane
where this is not available.
=Transmission of Power to the Camera=—It has already been pointed out
that the ease of transmission of electrical energy makes it particularly
convenient for use in a plane. All other sources of power, except
clock-work incorporated in the camera, require flexible shafting, so
that the question of bearings and connections becomes a serious one,
especially when the shaft runs continuously for long periods at very
high speeds.
The shafting found most suitable is the spirally wound form commonly
known as dental shafting. This must be encased in a smoothly fitting
sheath, flexible enough to permit of easy bends. The ends of the shaft
should be equipped with square or rectangular pins to fit into
corresponding slots in the motor and camera shafts. The ends of the
shaft casing may be fitted either to attach by bayonet joints or by
smoothly fitting screw collars. At the point of attachment to the camera
it is desirable to have some form of junction adjustable as to the
direction from which the shaft may be connected, so that it need be bent
as little as possible. A right angle bevel gear offers one means of
doing this. Bearings, such as those of the propeller, should be of the
ball variety, while heavy lubrication, such as vaseline, should be
freely used, both in the bearings and in the shaft casing.
An important feature of any power drive system should be a safety
device, so that the power will race in case of any jam or stoppage in
the camera. This will often prevent serious damage through the breakage
of some relatively weak part of the camera mechanism on which the whole
force of the driving apparatus is suddenly thrown. The “L” camera
propeller is fitted with a spring friction clutch with the idea that if
the camera refuses to operate the propeller will slip instead of
wrenching the shaft to pieces.