*Principles and Use of Propellers*. A propeller converts the energy of
the engine into the thrust required to overcome the resistance of the
aeroplane. To maintain flight the thrust, or force exerted by the
propeller, must always equal the total resistance of the aeroplane. A
total resistance of 400 pounds requires a propeller thrust of 400, and
as the resistance varies with the speed, the engine revolutions must be
altered correspondingly. The propeller is the most complicated and least
understood element of the aeroplane, and we can but touch only on the
most elementary features. The inclined blades of the propeller throw
back an airstream, the reaction of which produces the thrust. The blades
can also be considered as aerofoils moving in a circular path, the lift
of the aerofoils corresponding to the thrust of the propeller. The
reactions in any case are quite complicated and require the use of
higher mathematics for a full understanding.
*Pitch and Velocity*. When in action the propeller rotates, and at the
same time advances along a straight line parallel to its axis. As a
result, the tips of the propeller blades describe a curve known as
"Helix" or screw-thread curve. The action is very similar to that of a
screw being turned in a nut. For clearness in explanation we will call
the velocity in the aeroplane path the "Translational velocity," and the
speed of the tips in their circular path as the "Rotational velocity."
When a screw works in a rigid nut it advances a distance equal to the
"Pitch" in each revolution, the pitch of a single threaded screw being
equal to the distance between the threads. Since the propeller or "Air
screw" works in a fluid, there is some slip and the actual advance does
not correspond to the "Pitch" of the propeller blades. The effective
pitch is the distance traveled by the propeller in one revolution. The
actual pitch or the angle of the blades must be greater than the angle
of the effective helix by the amount of slip.
If N = Revolutions per minute, P = effective pitch in feet and V =
translational velocity in miles per hour, then V= NP/88. With an
effective pitch of 5 feet, and 1200 revolutions per minute, the
translational velocity of the aeroplane will be: V = 1200 x 5/88=68.2
miles per hour.
[Illustration: Excelsior Propeller, an Example of American Propeller
Construction. This Propeller Is Built Up of Laminations of Ash.]
The actual pitch of the blades would be from 15 to 25 per cent greater
than the effective pitch because of the slip. To have thrust we must
have slip. With the translational velocity equal to the blade-pitch
velocity, there is no airstream accelerated by the blades, and
consequently there is no thrust due to reaction. The air thrown to the
rear of a propeller moves at a greater speed than the translation when
thrust is developed, and this stream is known as the "slipstream." The
difference between the translational and slipstream velocity is the
slip.
The angle of the blade face determines the pitch. The greater the angle
of the blade with the plane of propeller rotation, the greater is the
pitch. This angle is measured from the chord of the working face of the
table, or from that side faced to the rear of the blade. In the majority
of cases the working face is flat. The front face is always heavily
cambered like a wing section, with the greatest thickness about
one-third the chord from the entering edge. As in the case of the wing,
the camber is of the greatest importance.
A uniform pitch propeller has a varying blade angle, smallest at the tip
and increasing toward the hub. With a uniform pitch propeller, every
part of the blade travels through the same forward distance in one
revolution, hence it is necessary to increase the angle toward the hub
as the innermost portions travel a smaller distance around the circle of
rotation. Theoretically, the angle at the exact center would be 90
degrees. The blade angles at the different points in the length of a
uniform pitch propeller are obtained as follows: Draw a right angle
triangle in which the altitude is made equal to the pitch, and the base
is equal to 3.1416 times the propeller diameter. The angle made by the
hypotenuse with the base is the blade angle at the tip. Divide the base
into any number of equal spaces and connect the division points with the
upper angle. The angles made by these lines with the base are the angles
of the different blade sections.
*Blade Form*. The blade may be either straight-sided or curved. In the
latter case the most deeply curved edge is generally the entering edge,
and the maximum width is about one-third from the tip. Much care is
exercised in arranging the outline so that the center of pressure will
not be located in an eccentric position and thus harmfully distort the
blade when loaded. If this is not attended to, the pitch will vary
according to the load. In one make of propeller the blade is purposely
made flexible so that the pitch will accommodate itself correctly to
different flight speeds and conditions. This, however, is carefully laid
out so that the flexure is proportional throughout the blade to the
changes in the load.
[Illustration: The Lang Propeller, Having Straight Edges, Slightly
Tapering Toward the Tips. The Tips Are Sheathed With Thin Copper for
Protection Against Spray. This Outline Is Often Known as the "Normale."
Type From the French Propeller First Using This Outline.]
[Illustration: A "Paragon" Propeller With a Curved Leading Edge. The
Maximum Width Is About One-Third the Blade Length from the Tip and §o.
Toward the Tip So That It Is Very Narrow at the Outer End. The Steel
Propeller Flange Is Shown in Place on the Hub.]
*Propeller Diameter*. The largest propellers are the most efficient. The
propeller should be as large as can be safely swung on the aeroplane.
Large, slow revolution propellers are far superior to the small high
speed type. It is more economical to accelerate a large mass of air
slowly with a large diameter than to speed up a small mass to a high
velocity. The diameter used on any aeroplane depends upon the power
plant, propeller clearance, height of chassis and many other
considerations. Approximately the diameter varies from about 1/3 the
span on small speed scouts, to 1/5 or 1/6 of the span on the larger
machines.
*Air Flow*. The greater part of the air is taken in through the tips,
and is then expelled to the rear. This condition prevails until the
blade angle is above 45 degrees, and from this point the flow is
outward. Owing to the great angles at the hub, there is little thrust
given by the inner third of the blade, the air in this region being
simply churned up in a directionless mass of eddies. At the tips the
angle is small and the velocity high, which results in about 80 per cent
of the useful work being performed by the outer third of the blade. In
some aeroplanes a spinner cap is placed around the hub to reduce the
churning loss and to streamline the hub. The blade section is very thick
at the hub for structural reasons.
The "Disc area" of a propeller is the area of the circle swept out by
the blades. It is the pressure over this area that gives the thrust, and
in some methods of calculation the thrust is based on the mean pressure
per square foot of disc area. The pressure is not uniformly distributed
over the disc, being many times greater at the outer circumference than
at the hub. The average pressure per square foot depends upon the blade
section and angle. Because of the great intensity of pressure at the
circumference, the effective stream is in the form of a hollow tube.
*Number of Blades*. For training, and ordinary work, two-bladed
propellers are preferable, but for large motors where the swing is
limited, three or four blades are often used. A multiple-bladed
propeller absorbs more horsepower with a given diameter than the
two-blade type. In general, a four-bladed propeller revolving slowly may
be considered more efficient than the two-blade revolving rapidly. Where
the swing and clearance are small, a small four-blade may give better
results than a larger and faster two-blade. A three-blade often shows
marked superiority over a two-blade even when of smaller diameter, and
the hub of the three-blade is much stronger than the four-blade,
although neither the three or four is as strong as the two-blade type.
*Effects of Altitude*. At high altitudes the density is less, and
consequently the thrust is less with a given number of revolutions per
minute. The thrust can be maintained either by increasing the speed, or
by increasing the pitch. For correct service at high altitudes the
propeller should undoubtedly be of the variable pitch type, in which the
pitch can be controlled manually, or by some automatic means such as
proportional blade flexure.
*Effects of Pitch*. Driven at a constant speed, both the thrust and
horsepower increase with the pitch up to a certain limiting angle.
For a given horsepower the static thrust depends both on the diameter
and the pitch. If the pitch is increased the diameter must be decreased
in proportion to maintain a constant speed. As the pitch is regulated by
the translational speed and revolutions, the static thrust of a high
speed machine is very small. As the translational speed increases, the
pitch relative to the wind is less, and consequently the thrust will
pick up until a certain limiting speed is reached.
*Thrust and Horsepower*. The calculation for thrust and power are very
complicated, but the primary conditions can be given by the following:
Let V = the pitch velocity in feet per minute, T = thrust in pounds, and
H = horsepower, then H = TV/33000E from which T = 33000HE/V, the
efficiency being designated by E. Since the pitch velocity is NP, where
N = revs. per minute and P = pitch in feet, then T = 33000HE/PN.
Assuming a 5-foot pitch, 1200 revs., the efficiency = 0.75, and the
horsepower 100, the thrust will be:
T = 33000 x 100 x 0.75/5 x 1200 = 412.5 pounds. The pitch in this case
is the blade pitch, and the great uncertainty lies in selecting a proper
value for E. This may vary from 0.70 to 0.85. The diameter is also an
unknown factor in this primitive equation.
*Materials and Construction*. The woods used for propeller construction
are spruce, ash, mahogany, birch, white oak, walnut, and maple. Up to 50
H. P. spruce is suitable, as it is light, and strong enough for this
power. In Europe walnut and mahogany are the most commonly used,
although they are very expensive. Birch is very strong and comparatively
light for its strength, and can be used successfully up to 125
horsepower. Ash is strong, light and fibrous, but has the objectionable
feature of warping and cannot withstand moisture. Maple is too heavy for
its strength. White oak, quarter-sawed, is the best of propeller woods
and is used with the very largest engines. It is strong for its weight
and is hard, but is very difficult to work and glue. For tropical
climates, Southern poplar is frequently used as it has the property of
resisting heat and humidity.
One-inch boards are rough dressed to 7/8 inch and then finished down to
13/16 or 3/4 inch. After a thorough tooth planing to roughen the surface
for the glue, they are thoroughly coated with hot hide glue, piled
together in blocks of from 5 to 10 laminations, and then thoroughly
squeezed for 18 hours in a press or by clamps until the glue has
thoroughly set. Only the best of hide glue is used, applied at a
temperature of 140°F. and at a room temperature of 100°. The glue must
never be hotter, nor the boards cooler than the temperatures stated. The
propeller after being roughed out is left to dry for ten days so that
all of the glue stresses are adjusted. If less time is taken, the
propeller will warp out of shape. The propeller is worked down within a
small fraction of the finished size and is again allowed to rest. After
a few days it is finished down to size by hand, is scraped, and tested
for pitch, tracking and hub dimensions.
The finish is glossy, and may be accomplished by several coats of spar
varnish or by repeated applications of hot boiled linseed oil well
rubbed in, finishing with three or four coats of wax polish. There
should be at least 5 applications of linseed oil, the third coat being
sandpapered with No. 0 paper. The wood should be scraped to dimension
and must not be touched with sandpaper until at least two coats of
varnish or oil have been applied.
The wood must be absolutely clear and straight grained, and without
discolorations. The boards must be piled so that the edge of the grain
is on the face of the blades, and the direction of the annular rings
must be alternated in the adjacent boards.
[Illustration: Plan and Side Elevation of the S.P.A.D. Speed Scout.
Courtesy "_Aerial Age_".]