*General Notes*. Up to the present we have considered only two wing
outlines, the rectangular and the wing with raked tips. In addition, we
have considered only the effect taking place on monoplane surfaces. For
the purpose of obtaining longitudinal stability, or for distributing the
lift upon two or more following surfaces, the plan view in some
aeroplanes has been somewhat modified as in the Dunne, Langley and Ago.
Undoubtedly the simple rectangular wing, or the wing with raked tips,
have proved the most efficient from an aerodynamic standpoint, but as
the layman is usually interested in distorted wing shapes, or
odd-looking outlines, I will describe the effect of such stabilizing
forms upon the lift and drag.
Figs. 1-9 show the usual range of wing forms, at least those that have
been used on well known machines, and all of them have flown with
varying results. In any case, the variations in the values of the lift
and lift-drag are not excessive, the extreme cases varying possibly not
more than 20 per cent on either side of the values for a plain
rectangular wing. While almost perfect longitudinal stability can be
obtained in other ways, by less loss than by changing the plan form,
certain manufacturers still adhere to one or more deviations from the
more usual rectangular form.
Fig. 1 shows a plan view of a machine with a rectangular wing, and No. 2
shows the machine provided with raked tips. Fig. 3 is a wing with an
inclined entering edge, as used on the English Mann biplane. Fig. 4 is
the German Ago with a diamond form surface, the evident purpose of which
is to simplify the wing spar bracing as shown by the dotted lines. By
bringing the wing spars ’together at the tips, the spars themselves form
a triangle to resist the drag stresses. Fig. 5 is the common form of
"sweep back" or "retreated wing" as used in the Standard H-3 training
biplane, and several other modern biplanes. While this arrangement
undoubtedly assists longitudinal stability, it causes certain losses
that will be described later. The inherently stable Dunne is shown by
Fig. 6 in which the sweep back is increased to almost 90 degrees. In
fact the retreat is so great that no tail or stabilizing surfaces are
used at all, the elevator functions being performed by the ailerons. It
should be noted that the swept back tips really act as stabilizers since
they trail back far behind the center of gravity and center of pressure.
The ailerons (a) are not needed for lateral balance, hence ascent and
descent—and also turning in a horizontal plane—are performed by the
ailerons in setting them in different relative positions. Being far to
the rear they are very effective elevators, although their action and
the extreme retreat of the wings, causes a considerable drag. This is
somewhat offset by the absence of tail resistance. An Austrian or German
"Taube" is shown by Fig. 7 with the negative trailing wing tips (a),
that greatly assist longitudinal stability, but which are decidedly
inefficient. This is evidenced by the fact that neither the Germans nor
Austrians build this machine at present—at least for active service. The
tips (a) are bent up at the rear and thus form a "negative" angle of
incidence with the main lifting surface. This was the original invention
of Igo Etrich, an Austrian, and as with everything else, the idea was
promptly grabbed by the Germans at the beginning of the war and claimed
as their own idea. Etrich was one of the pioneers in aviation, a science
that did not prosper in Germany until the impracticability of the
Zeppelin for universal service was an accepted fact.
[Illustration: Figs. 1-9. Plan Views of Different Wing Arrangements and
Wing Outlines.]
Fig. 8 is the wing outline of the Bleriot monoplane, a representative
wing used on the earlier monoplanes. This is really reversed rake, and
hence does not stand for efficiency in lift. A tandem aeroplane is shown
by Fig. 9 in which the leading surface is (m) and the trailing wing is
(n). The tail (t) may, or may not be included. This is a type that has
been neglected in its practical development although it has been
repeatedly proposed. The Langley machine, the Montgomery glider, and the
Richardson are of this type.
In all the figures the wing ribs are indicated by the thin full lines
passing across the width of the wing, with the tail at (t). The arrow
represents the line of flight. It will be noted that with any but
rectangular wing outlines the rib lengths are different, throughout the
wing. This makes this wing a bad manufacturing proposition, and a
difficult and expensive wing to repair. To provide against emergencies
the aviator must keep a complete extra wing in reserve for repair parts,
while the manufacturer is put to the expense of a great number of rib
molds, and must also keep a large number of ribs in stock. There is a
constant difficulty due to the fact that mistakes are often made in
ordering the ribs for repair, and altogether, anything but a rectangular
wing is a decided nuisance.
*Sweep Back or Retreat*. With swept back wings, as shown in Fig. 5, the
center of pressure movement is peculiar. The C. P. moves forward when
reducing the incidence at small angles, and thus tends to reduce head
diving, but at very large angles the C. P. again moves forward, tending
to increase the angles further and thus stall the machine. This reversal
of C. P. movement takes place at about 10° to 12°, and the movement is
sharper and further with each increase in the sweep back. At ordinary
angles of flight, say at from 0° to 6° the forward C. P. movement is
satisfactory, but at low speeds and high angles stability is only
partially secured, and hence for the total performance sweep back is not
to be desired. The wing section used in the above investigation was an
R.A.F.-6 with an aspect ratio of 6.
In regard to the lift coefficient Ky, it was noted that this factor was
decreased with every degree in the angle of sweep back. At the incidence
angle of 4° for the maximum value of L/D, the value of Ky decreased from
0.00143 with a sweep back of 0° (Straight wings), to 0.00120 with a
sweep back of 30°. At the same incidence, but with a sweep back of 10°,
the lift became: Ky = 0.00130. With a retreat of 20°, Ky- 0.00129. The
lift-drag also suffered with an increase in retreat, this being 17.00
with straight wings, at 16.5 at 10°, 16.2 at 20°, and 12.8 at 30°. Up
to, and including a retreat of 20°, the loss in lift-drag is not so bad,
but in the change from 20° to 30° there is a very great loss.
[Illustration: Fig. 10. (Upper) Various Angles Made by Wings During
Experiments. (Below) The Center of Pressure Movement with Varying
Angles.]
The "angle of retreat" herein specified is such that each wing is moved
back through an angle of one-half the total given retreat (r) angle in
Fig. 5. That is, with a retreat of 30°, each wing section makes an angle
of 15° with the entering edge of a pair of straight wings. It is more
usual to specify the included angle between the leading edges as
indicated by (S) in Fig. 5. In the upper portion of Fig. 10 is shown the
various angles of the wings during the experiments, while below is the
C. P. movement according to the different angles of retreat. The above
is based on experiments made by H. E. Rossell and C. L. Brand, assistant
Naval Constructors, U. S. Navy, and published in "Aerial Age."
The center of pressure referred to is that at the forward point of the
middle longitudinal section of each wing, and with a given incidence the
C. P. is thrown to the rear by about 0.2 of the chord by a sweep back of
10 degrees, and 0.4 of the chord for a sweep back of 20°. The center of
gravity of the machine will thus have to be moved to the rear if sweep
back is employed.
In making a turn a machine with sweep back has a natural tendency to
bank up in the correct attitude, and even a retreat of 10° will add
nearly 100 per cent to the banking tendency when compared to a pair of
straight wings. It will be seen that with swept back wings, the leading
edge is radial, consequently meets the air stream at more nearly a right
angle. This gives more lift to the outer wing than would be the case
with straight entering edges, while the inner end losses increase
correspondingly in lift and hence tends further to depress the inner
tip. The greatest value of sweep back is found in its resistance to side
slip. If the machine should be "over banked," and tend to slip down
toward the inner side of the turn, the backward angle of the inner wings
will cause the entering edge to meet the side stream at more nearly
right angles, and thus tend to reduce the bank and the inner slide slip.
In this respect the retreat is an aid to lateral stability. On the other
hand, the sweep back tends to keep the machine rolling in rough weather
for side gusts then meet the inclined wing edges at an effective angle.
This is particularly noticeable in landing.
*Raked Tips*. This subject was discussed under "Standard Wing Section,"
but it may be repeated here, that Eiffel in an experiment with the
Coanda Wing (Eiffel 38), found that the L/D of the raked wing was 20 per
cent higher than with the same wing in rectangular form. This value
would not be safe to assume with all sections.
[Illustration: Fig. 11. Influence of "Wash Down" on the Rear Wings of a
Tandem Pair.]
[Illustration: Fig. 12. Tandem Arrangements Used in Eiffel Experiments.
(1) Chords in Straight Line, (2) Rear Wing at 2.5°, (3) Rear Wing at
5°.]
*Tandem Arrangement*. In tandem wings as shown by Fig. 9, the downward
wash of the front wing (m) will affect the rear wing (n) by causing a
change in the relative direction of the flow. If the front wings are at
an incidence of (i) degrees (Fig. 11), the deviation or washdown of the
air stream to the rear will be expressed by: d = (0.5i + 1). If i =
incidence of rear wing measured from the chord of the front wing, then
the angle of incidence made by the rear chord to the horizontal will be:
I = (i-i’) - (0.5i + 1), where I is the incidence of the rear plane with
the horizontal.
Experiments by Eiffel on tandem planes with circular cambered aerofoils
gave exceedingly good results for certain combinations (Fig. 12). These
arrangements were used, (1) Chords in a straight line (2), Rear aerofoil
tilted down at a negative angle of 2.5°, (3), Rear Plane tilted down at
a negative angle of 5°. In all cases the camber was 1/13.5 of the chord,
and the front and rear wings were spaced two chord widths apart. While
the drag did not change much for any of the arrangements, the lifts
varied widely, and arrangement (2) is by far the more efficient in
lifting capacity. No. 2 is 50 per cent greater than (1), and has twice
the lifting ability of (3). For the same angle of incidence, the front
wing does the same amount of lifting in all cases, the difference being
entirely due to the changes in the rear surface. In (2) the lift of the
rear aerofoil is actually 13 per cent greater than the front plane. The
following tables give the results:
Demo Table TABLE OF LIFTS IN GRAMS FOR THREE ARRANGEMENTS.
──────────────────────────────────────────────────────────────────
ANGLE OF ARRANGEMENT ARRANGEMENT ARRANGEMENT
INCIDENCE (1) (2) (3)
──────────────────────────────────────────────────────────────────
3 665 1094 334
──────────────────────────────────────────────────────────────────
6 987 1568 703
──────────────────────────────────────────────────────────────────
9 1315 2068 965
──────────────────────────────────────────────────────────────────
12 1540 2326 1347
──────────────────────────────────────────────────────────────────
Average at all 1127 1764 837
angles
──────────────────────────────────────────────────────────────────
Percentage 0.64 1.00 0.47
──────────────────────────────────────────────────────────────────
[Illustration: Fig. 13. Drzewiecki Tandem Arrangement for Longitudinal
Stability.]
The lifts in the above table are for the two planes working together,
and the angle of incidence is the angle of the front aerofoil, or rather
the angle of the combination. The wings were 15 x 90 centimeters, aspect
ratio = 6. Fig. 12 shows the construction clearly. This is only true for
circular arched surfaces of the camber given.
[Illustration: Fig. 14. Drzwiecki Tandem Wing Arrangement for
Stability.]
M. Drzewiecki working with Eiffel’s results on the above combinations,
produced an inherently stable tandem monoplane, in which the front and
rear wings were of different cambers and were set at different
incidences. The front wing is Eiffel No. 8 set normally at 8° incidence,
and the rear wing is Eiffel No. 13-bis (Bleriot 11-bis), set normally at
5°. The center of gravity is approximately half-way between the two
wings, and the front is smaller than the trailing surface. Because of
the difference in area, the lift of the front wing varies less rapidly
than the rear when the angle of the machine changes because of disturbed
air. Should the machine "head up," the rear wing increases faster in
lift than the front, and hence restores the machine to a horizontal
position. Should the front surface drop, the incidence is reduced, but
as incidence of the rear wing is less than the front (8°), the rear wing
is reduced to nearly a zero angle of incidence—(With little lift). The
front wing is still inclined at a considerable incidence: (3°) when the
rear is at zero. This drops the rear, and raises the front wing so that
the normal attitude is restored. Lateral stability is obtained by moving
the two halves of the front wing in relation to one another, the
relative movement being similar to that of ailerons.
[Illustration: Typical Wing Assembling Shop]