THE PROPELLER, OR "AIR-SCREW"
The sole object of the propeller is to translate the power of the engine
into thrust.
The propeller screws through the air, and its blades, being set at an
angle inclined to the direction of motion, secure a reaction, as in the
case of the aeroplane's lifting surface.
This reaction may be conveniently divided into two component parts or
values, namely, Thrust and Drift (see illustration overleaf).
The Thrust is opposed to the Drift of the aeroplane, and must be equal
and opposite to it at flying speed. If it falls off in power, then the
flying speed must decrease to a velocity, at which the aeroplane drift
equals the decreased thrust. The Drift of the propeller may be
conveniently divided into the following component values:
_Active Drift_, produced by the useful thrusting part of the propeller.
_Passive Drift_, produced by all the rest of the propeller, _i.e._, by
its detrimental surface.
_Skin-Friction_, produced by the friction of the air with roughness of
surface.
_Eddies_ attending the movement of the air caused by the action of the
propeller.
_Cavitation_ (very marked at excessive speed of revolution). A tendency
of the propeller to produce a cavity or semi-vacuum in which it
revolves, the thrust decreasing with increase of speed and cavitation.
THRUST-DRIFT RATIO.--The proportion of thrust to drift is of paramount
importance, for it expresses the efficiency of the propeller. It is
affected by the following factors:
_Speed of Revolution._--The greater the speed, the greater the
proportion of drift to thrust. This is due to the increase with speed of
the passive drift, which carries with it no increase in thrust. For this
reason propellers are often geared down to revolve at a lower speed than
that of the engine.
_Angle of Incidence._--The same reasons as in the case of the aeroplane
surface.
_Aspect Ratio._--Ditto.
_Camber._--Ditto.
[Illustration:
M, Direction of motion of propeller (rotary).
R, Direction of reaction.
T, Direction of thrust.
AD, Direction of the resistance of the air to the passage of the
aeroplane, _i.e._, aeroplane drift.
D, Direction of propeller drift (rotary).
P, Engine power, opposed to propeller drift and transmitted to
the propeller through the propeller shaft.]
In addition to the above factors there are, when it comes to actually
designing a propeller, mechanical difficulties to consider. For
instance, the blades must be of a certain strength and consequent
thickness. That, in itself, limits the aspect ratio, for it will
necessitate a chord long enough in proportion to the thickness to make
a good camber possible. Again, the diameter of the propeller must be
limited, having regard to the fact that greater diameters than those
used to-day would not only result in excessive weight of construction,
but would also necessitate a very high undercarriage to keep the
propeller off the ground, and such undercarriage would not only produce
excessive drift, but would also tend to make the aeroplane stand on
its nose when alighting. The latter difficulty cannot be overcome by
mounting the propeller higher, as the centre of its thrust must be
approximately coincident with the centre of aeroplane drift.
MAINTENANCE OF EFFICIENCY.
The following conditions must be observed:
1. PITCH ANGLE.--The angle, at any given point on the propeller, at
which the blade is set is known as the pitch angle, and it must be
correct to half a degree if reasonable efficiency is to be maintained.
This angle secures the "pitch," which is the distance the propeller
advances during one revolution, supposing the air to be solid. The air,
as a matter of fact, gives back to the thrust of the blades just as
the pebbles slip back as one ascends a shingle beach. Such "give-back"
is known as _Slip_. If a propeller has a pitch of, say, 10 feet, but
actually advances, say, only 8 feet owing to slip, then it will be said
to possess 20 per cent. slip.
Thus, the pitch must equal the flying speed of the aeroplane plus
the slip of the propeller. For example, let us find the pitch of
a propeller, given the following conditions:
Flying speed ... 70 miles per hour.
Propeller revolutions ... 1,200 per minute.
Slip ... 15 per cent.
First find the distance in feet the aeroplane will travel forward in one
minute. That is--
369,600 feet (70 miles)
----------------------- = 6,160 feet per minute.
60 " (minutes)
Now divide the feet per minute by the propeller revolutions per minute,
add 15 per cent. for the slip, and the result will be the propeller
pitch:
6,160
----- + 15 per cent. = 5.903 feet.
1,200
In order to secure a constant pitch from root to tip of blade, the pitch
angle decreases towards the tip. This is necessary, since the end of the
blade travels faster than its root, and yet must advance forward at the
same speed as the rest of the propeller. For example, two men ascending
a hill. One prefers to walk fast and the other slowly, but they wish to
arrive at the top of the hill simultaneously. Then the fast walker must
travel a farther distance than the slow one, and his angle of path
(pitch angle) must then be smaller than the angle of path taken by the
slow walker. Their pitch angles are different, but their pitch (in this
case altitude reached in a given time) is the same.
[Illustration]
In order to test the pitch angle, the propeller must be mounted upon
a shaft at right angles to a beam the face of which must be perfectly
level, thus:
[Illustration]
First select a point on the blade at some distance (say about 2 feet)
from the centre of the propeller. At that point find, by means of a
protractor, the angle a projection of the chord makes with the face of
the beam. That angle is the pitch angle of the blade at that point.
Now lay out the angle on paper, thus:
[Illustration]
The line above and parallel to the circumference line must be placed
in a position making the distance between the two lines equal to the
specified pitch, which is, or should be, marked upon the boss of the
propeller.
Now find the circumference of the propeller where the pitch angle is
being tested. For example, if that place is 2 feet radius from the
centre, then the circumference will be 2 feet x 2 = 4 feet diameter,
which, if multiplied by 3.1416 = 15.56 feet circumference.
Now mark off the circumference distance, which is represented above by
A--B, and reduce it in scale for convenience.
The distance a vertical line makes between B and the chord line is
the pitch at the point where the angle is being tested, and it should
coincide with the specified pitch.
You will note, from the above illustration, that the actual pitch line
should meet the junction of the chord line and top line.
The propeller should be tested at several points, about a foot apart, on
each blade; and the diagram, provided the propeller is not faulty, will
then look like this:
[Illustration:
A, B, C, and D, Actual pitch at points tested.
I, Pitch angle at point tested nearest to centre of propeller.
E, Circumference at I.
J, Pitch angle at point tested nearest to I.
F, Circumference at J.
K, Pitch angle at next point tested.
G, Circumference at K.
L, Pitch angle tested at point nearest tip of blade.
H, Circumference at L.]
At each point tested the actual pitch coincides with the specified
pitch: a satisfactory condition.
A faulty propeller will produce a diagram something like this:
[Illustration]
At every point tested the pitch angle is wrong, for nowhere does the
actual pitch coincide with the specified pitch. Angles A, C, and D,
are too large, and B is too small. The angle should be correct to half
a degree if reasonable efficiency is to be maintained.
A fault in the pitch angle may be due to (1) faulty manufacture, (2)
distortion, or (3) the shaft hole through the boss being out of
position.
2. STRAIGHTNESS.--To test for straightness the propeller must be mounted
upon a shaft. Now bring the tip of one blade round to graze some fixed
object. Mark the point it grazes. Now bring the other tip round, and it
should come within 1/8 inch of the mark. If it does not do so, it is due
to (1) faulty manufacture, (2) distortion, or (3) to the hole through
the boss being out of position.
3. LENGTH.--The blades should be of equal length to 1/16 inch.
4. BALANCE.--The usual method of testing a propeller for balance is as
follows: Mount it upon a shaft, which must be on ball-bearings. Place
the propeller in a horizontal position, and it should remain in that
position. If a weight of a trifle over an ounce placed in a bolt-hole on
one side of the boss fails to disturb the balance, then the propeller is
usually regarded as unfit for use.
[Illustration]
The above method is rather futile, as it does not test for the balance
of centrifugal force, which comes into play as soon as the propeller
revolves. It can be tested as follows:
[Illustration]
The propeller must be in a horizontal position, and then weighed at
fixed points, such as A, B, C, D, E, and F, and the weights noted. The
points A, B, and C must, of course, be at the same fixed distances from
the centre of the propeller as the points D, E, and F. Now reverse the
propeller and weigh at each point again. Note the results. The first
series of weights should correspond to the second series, thus:
Weight A should equal weight F.
Weight B should equal weight E.
Weight C should equal weight D.
There is no standard practice as to the degree of error permissible, but
if there are any appreciable differences the propeller is unfit for use.
5. SURFACE AREA.--The surface area of the blades should be equal. Test
with calipers thus:
[Illustration]
The distance A--B should equal K--L.
The distance C--D should equal I--J.
The distance E--F should equal G--H.
The points between which the distances are taken must, of course, be at
the same distance from the centre in the case of each blade.
There is no standard practice as to the degree of error permissible. If,
however, there is an error of over 1/8 inch, the propeller is really
unfit for use.
6. CAMBER.--The camber (curvature) of the blades should be (1) equal,
(2) decrease evenly towards the tips of the blades, and (3) the greatest
depth of the curve should, at any point of the blade, be approximately
at the same percentage of the chord from the leading edge as at other
points.
It is difficult to test the top camber without a set of templates,[18]
but a fairly accurate idea of the concave camber can be secured by
slowly passing a straight-edge along the blade, thus:
[Illustration]
The camber can now be easily seen, and as the straight-edge is passed
along the blade, the observer should look for any irregularities of the
curvature, which should gradually and evenly decrease towards the tip of
the blade.
7. THE JOINTS.--The usual method for testing the glued joints is by
revolving the propeller at greater speed than it will be called upon to
make during flight, and then carefully examining the joints to see if
they have opened. It is not likely, however, that the reader will have
the opportunity of making this test. He should, however, examine all the
joints very carefully, trying by hand to see if they are quite sound.
Suspect a propeller of which the joints appear to hold any thickness of
glue. Sometimes the joints in the boss open a little, but this is not
dangerous unless they extend to the blades, as the bolts will hold the
laminations together.
8. CONDITION OF SURFACE.--The surface should be very smooth, especially
towards the tips of the blades. Some propeller tips have a speed of over
30,000 feet a minute, and any roughness will produce a bad drift or
resistance and lower the efficiency.
9. MOUNTING.--Great care should be taken to see that the propeller
is mounted quite straight on its shaft. Test in the same way as for
straightness. If it is not straight, it is possibly due to some of the
propeller bolts being too slack or to others having been pulled up too
tightly.
FLUTTER.--Propeller "flutter," or vibration, may be due to faulty pitch
angle, balance, camber, surface area, or to bad mounting. It causes a
condition sometimes mistaken for engine trouble, and one which may
easily lead to the collapse of the propeller.
CARE OF PROPELLERS.--The care of propellers is of the greatest
importance, as they become distorted very easily.
1. Do not store them in a very damp or a very dry place.
2. Do not store them where the sun will shine upon them.
3. Never leave them long in a horizontal position or leaning up
against a wall.
4. They should be hung on horizontal pegs, and the position of
the propellers should be vertical.
If the points I have impressed upon you in these notes are not attended
to, you may be sure of the following results:
1. Lack of efficiency, resulting in less aeroplane speed and climb
than would otherwise be the case.
2. Propeller "flutter" and possible collapse.
3. A bad stress upon the propeller shaft and its bearings.
TRACTOR.--A propeller mounted in front of the main surface.
PUSHER.--A propeller mounted behind the main surface.
FOUR-BLADED PROPELLERS.--Four-bladed propellers are suitable only when
the pitch is comparatively large. For a given pitch, and having regard
to "interference," they are not so efficient as two-bladed propellers.
[Illustration:
SPIRAL COURSES OF TWO-BLADE TIPS.
SPIRAL COURSES OF FOUR-BLADE TIPS.
Pitch the same in each case.]
The smaller the pitch, the less the "gap," _i.e._, the distance,
measured in the direction of the thrust, between the spiral courses of
the blades (see illustration on preceding page).
If the gap is too small, then the following blade will engage air which
the preceding blade has put into motion, with the result that the
following blade will not secure as good a reaction as would otherwise be
the case. It is very much the same as in the case of the aeroplane gap.
For a given pitch, the gap of a four-bladed propeller is only half
that of a two-bladed one. Therefore the four-bladed propeller is only
suitable for large pitch, as such pitch produces spirals with a large
gap, thus offsetting the decrease in gap caused by the numerous blades.
The greater the speed of rotation, the less the pitch for a given
aeroplane speed. Then, in order to secure a large pitch and consequently
a good gap, the four-bladed propeller is usually geared to rotate at a
lower speed than would be the case if directly attached to the engine
crank-shaft.
[Footnote 18: I have heard of temporary ones being made quickly by
bending strips of lead over the convex side of the blade, but I should
think it very difficult to secure a sufficient degree of accuracy in
that way.]