United States. National Advisory Committee for Aer.

Annual report of the National Advisory Committee for Aeronautics online

. (page 27 of 77)
Online LibraryUnited States. National Advisory Committee for AerAnnual report of the National Advisory Committee for Aeronautics → online text (page 27 of 77)
Font size
QR-code for this ebook

so the stem does not emerge from the guide. Otherwise the residt
is qtdte the same as if the shoulder were not present, except that a
larger diameter of valve is possible than without such extension of
the bore. This valve trouble is supposed to be quite prevented by
side-pocket location of valves, but is not, because should the valve
drop into the pocket there is every chance of it diding over on the

Eiston under the influence of a suction stroke, especiculy if the flat
ridge inclines downhill, as it usually does in single cam shaft V
engines, for example, though placing the valve on the opposite or
downhill side would prevent it, but would require two cam shirfts.

Prevention of undue wear on shaft and pin bearing surfaces is
entirely a question of bearing pressures and lubrication. These
bearing pressures are all subject to pretty accurate determination by
computation, so the design of an engine with excessively high bear-
ing pressures, judged by general machinery bearing experience, is a
pure technical mistake, not to be excused by the admtion of elaborate
forced systems of pump oil supply. Bearings shoidd be lai^e enough
to not need elaborate special oils or oil-apphcation systems, out these
should be added to make assurance doubly sure, in short; as safety
attachments, not as essential elements. Weight reduction secured
by cutting down main and pin bearings is too dearly bought to be
worth the price. Cylinder and piston oearing wear, while mvolvinff
the same elements as main bearings, have to endure the additional
difficidty of high temperature, but this is not serious if due attention
is paid to the principles of heat abstraction. Violation of these
principles, coupled with a rise of side thrusts, a^avated by side cock-
ing that follows undue shortening of pistons, is another case of pure
n^ect. Pistons should be as long and as thick at the top as is con-
sistent with weight-speed limits, and where observance of these
limits fails to reduce the pressures and temperatures to values known

Digitized by



to run properly in other engines, then definite special remedies can
be suggested, only one of which is excessive use of lubricating oil and
the last to be adopted instead of the first.

Seizing of runmng parts at bearing surfaces is entireljr a question
of relative size or of clearances, and its prevention a question of main-
tenance of the cold clearances after the parts become heated, which,
of course, is least necessarv, the better tne provision for abstracting
and dissipating the heat derived from combustion or developed by
friction. Next to cooling, which in main and crank -pia bearings is not
attempted, though it might well be, and which in cylinders and
pistons is their Big problem, material selection is most important,
some materials have low relative frictional coefficients for their
lubricated surface and are properly related as to thermal expansion.
Nothing better than the soft-metal lined or bronze can be found for
steel shafts and pins, especially as these expand more per degree rise
than the steel, so heating tends to loosen and oppose seizing by assist-
ing lubrication, which by lowered oil viscosity tends to Mcome less
effective. The boxes must, however, be stiff enough to really dis-
tribute stress. Piston and cylinder bearing surfaces are somewhat
more difficult, as the outer part, the cylinder, is normally much cooler
than the inner part, the piston. The temperature difference is greater
the thinner the pistons, and the difference is much ^ater than in the
case of tne standard box on the pin or shaft. It is, therefore, more
neceesaryHo care for these clearances. This is done when the ma-
terials are the same, cast iron on cast iron, by making the initial
clearance high^ far higher than would be feasible on shafts. This
tends to promote side knocks and leaks at part load. For equally
good cooling the steel cylinder with cast-iron piston gives about the
same expansion relations as do the bronze box and steel shaft, but
not such good antifriction qualities. Steel selection and heat treat-
ment will undoubtedly lead to improved antifrictional results, perhaps
even equal or superior to cast iron, after proper research. This seems
to be a rational and promising line of development, especially if the
cylinders are kept symmetrica, as they can be with head valves.

Reliability so far as carburation, ignition, mixture distribution,
and cylinder treatment processes are concerned, has already been
discussed. Any derangement' whatever here leads to impaired
power output or increased and perhaps very much increased fuel
consumption. Serious derangement of these processes means stop-
page even though the whote engine structure be perfect. Most
operating troubles are directly traceable to these process derange-
ments, 'miich if sufficient in degree, mean stoppage, and even if slight,
constant tinkering and anxiety.

Adaptability of an internal-combustion en^e to aeronautic service
is promoted by certain features of the engme that play no part in
metal reduction, in mean effective pressure and efficiency increases
or in its reUability, though of course low weight of engine and of fuel
per horsepower are themselves adaptability factors, as is also any
element oi reliability.

General external shape and position of points of attachment are
subject to a far wider range in aero than m auto engines. In one
respect aero adaptability imposes a direct requirement, that of end
shape for least head resistance. Engines directly exposed to the air
or their casings when covered have a relative movement always

Digitized by



approaching, and sometimes exceeding, 100 miles per hour. This
must always impose a resistance which is larger, the larger the end
area facing the direction of flight and the less smooth the exposed
surfaces are. In this respect the rotating-ejrlinder engines are hj
far the worst and the sinde line of cylinders oi the auto type of multi-
CTBJjJs. engine is best, neany twice as good as the V engine for example.
Air-cooled engines if similarly arranged to water cooled offer more
head resistance except for the radiator of the latter which may be
very highly resistant but is not necessarily so. But apparently the
reqiiirements of low head resistance is losmg in importance, at least
for war machines, since in these the fuselage is roomy enough gen-
erally to accommodate any type of engine.

Ease of starting and a control of speed are also required of aero as
of automobile ana boat engines, but with some elements of difference.
Electric self-starters with generator-motor and storage batteries are
prohibited by weight limits, for even if the craft could carry them
their weight would be much better disposed in the engine by adding
either more horsepower of the same unit weight, more fuel for the
same engine to make longer flights, or for 6qual flights and engine
power by using a heavier ouilt and therefore less sensitive engine of
longer life. Wnen starting from the ground a starting crank on the
shait end often would be inaccessible and even if it were within reach,
engines of largepower could not be hand rotated against their normal
compression. It has been a general practice to start these engines
by hand turning of the propeller blades, a practice that is most
dangerous, does the blades no good and certamly requires an extra
man because at the moment of starting the operator must be in his
seat. All hand-starting difficulties are removed if the compression
is relieved and the accessibility of a starting crank can be met with
equal ease by a chain and sprocket having a self-releasing ratchet
aud hand crank on a short auxiliary shaft, near the operator's seat.
It may therefore be regarded as necessary that aero engines, cer-
tainljr the larger ones, and this means most of all if not all of those to
be built in the near future, be provided with compression-release cams,
equivalent to those so long used on hand-started stationary ensines
and lever operated from the seat. This same compression release
gear will serve as a speed control, should speed vanation be neces-
sary, by permitting escape of part of the charge thou^, of course,
witn waste of gasoline. It serves as a supplement to the throttle
valve of the can>ureta, and which is not so wasteful of fuel. Speed
reduction by spark retardation should not be practiced on aero
engiaes, though a starting retard is necessary, automatic or manual,
because of the serious overheating effects that follow, and aero
engines at best are hard enough to keep cool at their hi^n speeds.

Muffling may be regarded as a necessity, however much free
exhausts nave. been used in the past, and whatever unfavorable
weight and power effects are imposed must be regarded as warranted.
Noise from the exhausts of so many cylinders operating at higli
speeds becomes a loud roar. There are at 1,200 revolutions per
minute from the 20 cyUnders of the Le Rhone engine, for example,
600 X 20 « 12.000 air impacts per minute, and at 2,400 revolutions
per minute tne eight cyhnders of the Sunbeam engiae ^ve 1,200 X 8 »
9,600 impacts. With such a disturbance cloFe to him no operator
can be expected to keep his head as clear for the serious busmess of

Digitized by



flying as if the noise were absent. To detect engine defects by the
noise changes in the machine before they become serious is absomtdy
impossible, though this is the main reliance in operating any other
kind of machine. Free exhausts must be classed, ther^ore, not as
annoyances but as preventers of engine-trouble detection, no matter
what the type of machine, and for military machines they are the
finest kind oi approach signal to the enemy, being audible long before
the machine is visible.

Mufflers can be made, due to automobile development, that are
quite effective with no more than 2 pounds per square inch back
pressiu*e. and possibly less. Hiis will reduce engine output 2 per
cent if tne mean effective pressure is 100 pounds per square inch, as
it is in aero engines, less than 2 per cent for higher, and more for
lower mean pressures. The weight increase is almost n^ligible,
being between one-tenth and two-tenths of a. pound per horsepower.

Just as soon, however, as mufflers are demanded as a necessity the
rotating cylinder engine must be changed or abandoned, as normaJly
the exhaust valve is placed in the center of the head, usually held
in place by an open cage screwed to the cylinder, discharging directly
into the air. To attach a muffler will require a change oi the cage
to a closed form with pipe attachment and additional cooling to keep
the now inclosed valve as cool as the open one. The muffler would
have to be disposed symmetrically about the shaft and inwardly
radial pipes held against centrifugal force at the muffler, fitted to the
exhaust cage by shp ioints. These pipes must, moreover, be circum-
ferentially sunportea to prevent distortion by variaole angular
velocities, and: they will impose additional windage resistance. The
net effect will be a greater reduction of power and a greater increase
in weight than muffler attachment imposes on fixed cylinder engines.

It goes without saying that no aero engine with tanks and connec-
tions complete is adapted to its purpose if tilting even to very con-
siderable degrees interferes with its operation, and if it stops on
tilting to any an^le that is remotely possible in real flying it certainly
must be rejected as failing in adaptability. There is considerable
uncertainty as to the angle and direction of tilt that aero-enjgine
adaptabiUty requires, but the 15° required in the German and British.
contests seems to be a very modest requirement. No one will deny
that the greater the angle of tUt and the more independent of direc-
tion, the better the adaptability factor. The conditions when tilting
in flight may be quite different from those existing in a tilted engine
at rest, especially when the motion is in curves developing centrifugal
forces in all masses as well as in the lubricating oil and fuel feed
system. Therefore, in considering engine independence of tiltinfi^,
rapid change of motion as to speed and direction, but especialfy
direction, must be included.

Any changes of direction of motion that the planes could withstand
can have no appreciable effect on the motion or friction of the moving
masses, but the effects on lubricating oil in the crank case or separate
tank or pipes and on the gasoline m the carburetor float chamber,
tank, and pipes may easily be as great as in extreme tilting. It is
quite possiole to imagine a resistance to flow, for example, purely
gravitational or purely centrifugal, or both, j^reat enougn to cause
engine trouble, in the one case from failure of the carburetor and in
the otiier from overheating of bearings robbed of oil, or from flooding

Digitized by



of oombustion chamberB whose pistons get an excess. It is likewise
possible that the water-drculation system be similarly deranged by
opposition to circulation, causing steam to generate in a jacket,
expelling all water, and causing an overheating, with a possibility of
a crack, or by a drainage of water from the radiator vent. If an
enmne could so be designed that it could work on end, lyinfi^ on its
siae, or even upside down for a short time, but preferably indefinitely,
this would be the ideal. No such possibility is in sight, though
engines are now operating in machines moving in curves and circks
in norizontal planes, turning the engine on its side, but centrifugal
force replaces gravity and no flows are disturbed. Similarly, looping
or circle flying in a vertical plane turns the engine so that it operates
first on end and then upside down, but, as oefore, the centrifugal
force replaces gravitjr. Such is not the case, however, in a steep
climb or descent nor in the uptilting of one end of the plane due to
wind gusts. Here gravity flows are disturbed by the amount of side
and end angle. Crank shafts and pin bearings must receive new
and end thnists which are not difficult to handle if they all are
properlv joumaled.
Crank-shaft torque that is most uniform is best adapted to pro-

Seller drives, as these propellers being made of wood for Ughtness may
e broken by sudden torque changes. Such changes ako reduce the
avers^e propeller efficiency and produce reverse rocking forces in the
machme frame. Any engme with insufficiently steady torque for pro-
peller safety and for maintenance of nigh average efficiency may be
adapted by addition of sufficient fly-wheel effect oetween engine and
propeller. The same fly-wheel effect increases the crank-shaft tor-
sional distortion and crank deflection and adds to engine weight.
Engines that can give sufficiently uniform torque for the purpose
without fly-wheels must displace others, and while the four cylinders
in line engine seems to serve, it is true that the effort falls to zero
on dead center. Anythiug less than four cylinders is out of the ques-
tion, because the gas-pressure effort is entirely absent for a part or a
whole stroke or more. Increase of number of cylinders over four
makes the actual effort or residtant tangential force due to com-
bined gas pressure and reciprocating inertia forces depart less and
less from the constant mean effort and minimizes the angular
velocity variations of the propeller without any other fly wheel
than itself. From this standpoint the more cylinders the better,
though from others discussed this is not the case.

Arrangement of a given number of cylinders radially about one
crank produces the same torque curve as the same cylinders in line,
provided their cranks in the latter case are separated by the same
angles as their cylinder axes in the former. When, however, these
cyfinders in line nave cranks parallel in pairs, as in the four and six
crank arrangements, the torque will not be as uniform as when these
are radially disposea about one crank. It appears, however, that the
6 cylinder in Ime, 6-crank arrangement, in which the torque never
drops to zero, is quite uniform enough tor practical work, and the 8
and 12 cylinder V arrangements are progressively better. There is
no reason for adopting the radial arrangement if, as is the case,
other objectionable elements are introduced, because the above is
good enough and anything better not worth another disadvantage.
Comparison of turning efforts for any arrangement of cylinders and

Digitized by



cranks is easy if they be plotted to a crank angle or crank path
base by the usual standard methods. Many of these curves nave
already been worked out and may be found in the literature, including
the inertia as well as gas-pressure force effects, and for such reference
is made to the bibhograpny in the appendix. In no case may a fly
wheel be introduced in aero engines to dampen torque vanations
because of weight liniitations.

Balance of reciprocating parts in view of .the light and flexible char-
acter of the engme supports which are part of the flying-machine
structure, is probably the most important of the adaptability fac-
tors, because lack of balance means free shaking forces or moments
on the whole system, and these being regular and periodic may

Eeriodically synchronize with the naturd periods of wires, struts, and
cams, and so cause displacements of such increasing amplitude as
may be responsible for rupture. In no other engine, including the
automobile, motor boat, and even the light shell of tne racing boat,
which comes nearest, is the support so frail and of such smeSi mass
capacity for absorbing vibration forces. Therefore, all unbalanced
forces or couples and the full displacements or vibration of the
engine as a unit are communicated directly to the flying-machine
structure practically without any modification. Moreover, aesro
engine weight being so small m comparison with other engines, its
own mass resists cUsplacement by its free unbalanced forces and
couples less than any other. For these reasons good balance is
essential to aero engines, but absolutely perfect bakoice is not.

Shaking forces and moments in en^es are due to both recipro-
cating and rotating masses, and vibration or rocking is the result of a
fiulure to balance these forces and moments. Shaking forces due
to rotating masses can be balanced perfecthr by other rotating
masses disposed on opposite sides of the shaft center with proper
numerical relation between centers of gravity, radii, and weights.
If the plane of rotation of the ori^al rotating mass is not the same
as that of its balance weight or weights, then mere will be an unbal-
anced couple even if the centrifugal forces are in balance, unless
balancing masses be disposed properly in separate planes, themselves
properly related. Due observation of these simple and well-known
relations make it a perfectly easy and simple matter to balance
rotating parts of an engine by adduig suitably disposed extra rotating
balance masses. Such dead oalance weights are, nowever, prohibited
by the service requirements pi least weight per horsepower, so the
actual rotating working parts must themselves be so oisposed as to
balance each other. These parts include the cranks, crank pins, and
rod ends principally, but also such small parts as the cams. If
cranks, pins, and rod ends are balanced, other minor rotating parts
may be neglected, though they set up inevitably some small ^akin^
forces, especially as the speeds are so nigh, and these forces vary with
the square of the speed.

Accordingly, to balance centrifugal forces and couples, due to
cranks and their attached rotating masses, of fixed cylmder engines
similar cranks must be suitably £sposed with r^erence to the first.
To avoid unbalanced couples with balanced forces more than two
such cranks are necessary and in different planes. Two similar
cranks at 180°, three at 120°, or aoy number equallv spaced will
result in force balance, because each introduces an equal force vector.

Digitized by



and, the sum of the rector angles beuu; 300^, these vectors will form
a closed equilateral force polygon, which means, of course, a zero
resultant. Each set of such ecjuaily spaced cranks is characterized
by a free couple, to balance which a similar and opposite couple must
be introduced by adding a similar set of cranlos with equal but
reversed angular spacing.

Applying this reasonmg to fixed cylinder engines, it appears that
the least number of cranes that can ^ve couple ana force balance is
four, set at 0^, 180^, 180^, and 0^, and the next smallest number, six,
set at 0°. 120°, 240*", 240**, 120'', and O"*. Of course any multiples
of these lour and six crank arrangements will also vield such balance.
This indicates a condition of inferiority of the meed cylinders star
engine with many cvhnders circumferentially disposed about each
crank, compared to tne single-row and double-row V engines of equal
number of cylinders. These star arrangements must have as many
multicylinder stars, each working on its own crank, as the single and
double rows of parallel arrangement has cylinders, in order to secure
equally good rotating mass oalance. This would impose on such
fixed star cyUnder engines an excessive number of cyhnders, unless
crank counterbalance weights were introduced, with consequent loss
of the weight advantage otherwise due to the star arrangement.

Rotating cylinder star engines are peculiar, because with fixed
cranks all parts of the ^igine are rotatm^ — cylinders and frames in
purely circular paths, pistons and wrist-pm ends of rods in a sort of
oval path, while crank-pin ends of rods are fixed. According to
this tne cyhnder and frame are in force balance when axis angles are
equal, and all being in one plane there is no unbalanced moment.
Tne centrifugal force due to tne rotation of the piston is a maximum
and radially outward when the piston is at outcenter, and a minimum
at the incenter position with regular symmetrical gradations between.
The net effect is a resultant force constant in amount and direction
acting radially outward along the crank and exerting a lifting action
if the crank points up, but not producing any vibration so long as
the speed is constant. From the balance standpoint, therefore, the
rotatmg star is superior to the fixed star arrangements, but is no
better man the four and six cranks and their multiples with parallel
rows of cylinders.

Reciprocating masses of fixed cylinder engines, such as pistons,
wrist pms, ancTan appropriate part of the connecting rod, develop
inertia forces for uniform rotary motion ot the crank that can be
expressed by an equation of the form of Fourier's infinite series, each
successive term being proportional to a trigonometric function of a
multiple of the angle of rotation from inner dead center and to in-
creasing powers of the ratio of crank to connecting rod length. The
reciprocating inertia force of one set of reciprocating parts is there-
fore the sum of an infinite number of forces of di£^rent periods or
frequency, the first being largest and its period that of an ^wne
speed, each successive one being smaller ana of longer period. These
reciprocating forces and the couples due to them must be balanced
partectly if possible; and if not, as well as possible. The forces due
to valve and valve^ear reciprocation with accelerations determined
by cam form niay be neglected, though of course if these could be
balanced in a simple way it would be desirable.

Digitized by



Balance of main reciprocating forces is possible only by opposing
equal and opposite masses of e(]^ual simultaneous acceleration, or by
arran^g reciprocating masses m groups, so that the vector sums of
their mertia forces become zero, liiere is, however, a partial balance
possible by the use of crank coimterweights or otherwise disposed
rotating masses frequently used on stationary and locomotive engines,
but normally prohibited on aero engines, on the principle of exclusion
of all dead weights, even for balance purposes. A rotating crank
counterweight exerts a radial centrifugal force which may be resolved
into an axial and a right-angle component. This axial component
may be made equal to the nrst-penod inertia force, and, being, of
course, opposite, it serves to balance this force. The right-angle
component is, however, left and of equal intensity, and so, of course,

Online LibraryUnited States. National Advisory Committee for AerAnnual report of the National Advisory Committee for Aeronautics → online text (page 27 of 77)