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Flying Wings
By Henry Cole
The rubber version of this design produced flights of over a minute and a half. It
proved slightly tricky as a gas job; the wings were found too weak.
No. 3 design is recommended by the designer as adaptable for towline or hy-start
gliders. Slotted aileron was found to greatly increase the model's stability.
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The absence of fuselage and tail surfaces
makes the flying wing aerodynamically and
structurally superior to conventional types of aircraft.
Nevertheless, despite these advantages, there have
been few successful tailless designs -- and yet birds
prove that high performance is attainable.
The most well-known among man-made
tailless airplanes are
Waterman's Arrowbile and, more recently, the
Northrop Flying Wing. The development of these
planes and the remarkable flights of the birds indicate
that further experiment will pay great dividends.
Presented in this article are three basic designs
which were developed through glider experiments
and bird observation. The many problems which
come up in building a tailless model will be discussed
so that the reader will have an insight into tailless
design and avoid many of the pitfalls of experimen-
pressure movement in any one section. The slight
washout is built in by making the dihedral break at a
five-degree angle.
Two models of this design were built and
flown, one a rubber job, and the other an Elf-powered
gas model. For the length of motor, the rubber model
turned in remarkable performance, averaging over a
minute and thirty seconds. Approximately thirty
flights were made with the gas model, and although it
was much slower than Design No. 1, it was still too
fast. The model made a number of good flights, but
was far from consistent, a defect which may be
attributed to structural weakness of the high -aspect
ratio wing. The wings could actually be seen to flutter
in flight and on one occasion broke in midair.
However, tests were encouraging on the whole, and
the model showed tendencies toward a fast climb and
exceptional glide. A larger model with a low wing
loading should turn the trick. The one weakness of the
model was its slow stalling angle. A slot like that used
on No. 1 would eliminate this undesirable
characteristic and add much to the stability of the
Design No. 1 was selected from a number of
balsa test gliders because it showed the greatest
stability and could be made to circle very tightly
without spiraling in. This enables the model to stay in
the slightest updraft and ride the wind like the birds, a
great advantage over normal craft. After thorough
glider tests, the design was scaled up to Class A size
and powered with all Elf Single. The symmetrical
Davis section and slotted wing tips were the key
points of the design.
Test flights verified the stability of the design,
but two defects were brought out: (1) the streamlined
airfoil induced excessive speeds; (2) the tractor
arrangement increased the prop shortage. The model
as it stands would make an excellent speed job, but
for endurance purposes it is out of the question. The
model could be slowed down by building it larger and
decreasing the wing loading, or by using a high-lift
wing with washout.
Design No. 2 shows the changes which were
made to produce a slower model. The use of a high-
lift section insured a high positive pressure, but also
induced a diving moment. Rather than turn up the
ailerons to excessive angles for control, the diving
moment was compensated for by varying the airfoil
section and by incorporating a slight washout. Note
that the forward section is the very-high-lift Davis
No. 5 which gradually changes to a Clark Y near the
tips. This produces a moment which counteracts the
airfoil diving tendencies. (The principle is analogous
to the lifting stabilizer.) With this arrangement the
center of gravity must be moved back, and a high-
aspect ratio must be used to minimize the center
Design No. 3 came directly from the birds.
Through careful study of the seagull and the albatross,
several new principles were discovered: (I) the
flexible slotted-tip aileron, (2) the dihedral-chord re-
lationship. The flexible tip aileron when used in
connection with gull dihedral increases lateral
stability by decreasing the pressure at the tips in the
side slip. The spring adjustment is quite sensitive and
is not advised for everyday flying. The ailerons
should, instead, be locked at the proper setting. The
dihedral-chord relationship on the model increases
lateral and longitudinal stability. In simple terms it is
the ratio of the chord to the height above the center
line. Note how the chord is largest at the high point
and decreases progressively in the lower sections. The
albatross section used was developed through
observation of the albatross as applied to the Davis
airfoil formulas. Tests indicate that it is a stable
section and possesses a
higher lift than other stable sections of the same
thickness. It has many of the characteristics of some
of the N. A. C. A.'s famous five-numbered series.
At present the model has been tested only with
the high start. With two strands of 3/16" rubber
twenty-five feet long and seventy-five feet of towline,
the model shoots skyward at a fast rate, releases and
sets into a slow, steady glide. The whole flight is
exceptionally smooth and the model soars with all the
grace of a bird. The consistency of the flights
indicates that the design will make a good gas model.
(The plans show the top view drawn flat for
The elements of tailless design are based
primarily on three factors.
Longitudinal stability is mainly dependent upon the
type of airfoil used. With stable sections a very
consistent model can be produced with only small
amounts of sweepback and washout. Note how little
sweepback was necessary on No. 3. Some good stable
sections are N. A. C. A. M-6, U. S. A. 27, and the
albatross section presented first in this article. With
high-lift sections more sweepback and washout must
be used in connection with high-aspect ratios. Good
sections are Clark Y. Eiffel 400 and Davis No. 5.
Variation of the airfoil as used on No. 2 is best when
using high-lift sections.
On all tailless models a small amount of
washout is necessary. Adjustable tip ailerons are the
best way to get this effect, for the framework often
twists with the tightening of the covering and any
built-in washout is lost. The most effective way to get
longitudinal stability is with a large slotted aileron as
used on Design No. 3. Small deflections give the
desired effect and have the advantage of low drag.
The exact position of the center of gravity is
best determined by experiment. Many glide tests
should be made, first with a low wing loading and
later with a high wing loading. Any radical changes
with the C. G. should be noted and their cause
Lateral stability is dependent mainly on the
dihedral and the height of the center of lift above the
C. G. The position of the rudders also has a
pronounced effect on the lateral stability of a tailless
design. Note that on all three designs rudders at the
tips have been avoided because it was found that they
have a detrimental effect upon spiral and lateral
stability. In deciding upon the dihedral, the
importance of keeping the center of lateral area low
should be considered. for it determines the spiral
characteristics of the airplane. In addition, excessive
dihedral induces the plane to rock. causing great loss
in efficiency.
Design No. 3 is a perfect example of keeping
the C. L. A. low and yet incorporating sufficient
lateral stability. The dihedral-chord relationship
builds up a high pressure at the peak and the gull tips
keep the C. L. A. low. The result is a stable model
with smooth flying characteristics.
Directional control is one of the greatest
problems of tailless models. Since the rudders must
be placed close to the C. G., the directional moments
are small unless billboard-sized rudders are used. The
best solution is to place the rudders where they will
be most effective without changing the lateral
stability. In the case of a tractor (Design No. 1), the
rudder should be placed directly in the slipstream
with most of the area below the wing. In the case of
pushers, the rudders must be placed outboard on the
wing. It was found that the area above the wing has
practically no effect, so the rudders should be placed
entirely underneath the wing. The most effective
place is at the points of high pressure. On No. 2 the
rudder is placed on the flat section of the dihedral.
The ideal setup is on No. 3, where the fin is at the
high-pressure point at the peak.
Possibly sufficient directional control can be
obtained by using extrenme sweepback and depressed
tips as on the Northrop, but the loss of lift due to
sweepback is not worth the little extra drag of
auxiliary rudders. The position of the C. G. is
important for directional control; the most trouble will
be experienced with tailless models using high-lift
airfoils which require that the C. G. be moved back.
To present a complete and absolutely accurate
report on a highly experimental type like the tailless
design would not be possible at this time. The three
designs and the discussion on stability should serve as
a guide to the inexperienced designer. The increase in
performance from Design No. 1 to Design No. 3
indicates that the basic ideas are sound and will
eventually lead to models as efficient as the birds.
Model builders should have no illusions about
developing a super contest model of this type under
the present AMA rules. Conventional models are
allowed a large stabilizer upon which no loading
penalty is placed. Consequently, the surface loading
required is twenty-five percent less on conventional
models. Therefore, it is suggested that for comparison
tailless models should be built with a six-ounce-per-
square-foot wing loading. All of the designs presented
should be scaled up for contest flying.
At present the tailless design does have two
fields of possibility in competition, as a towline glider
and in control-line speed contests. It is hard to
understand why it has not been developed in these
fields before, for the tailless design presents the ideal
setup for soaring, the ideal setup for speed. The
following recommendations can be made: No. 1 for
speed; No. 2 for powered endurance models; No. 3
for towline and high-start gliders. Remember that No.
2 should be scaled up and the wing loading kept to 6
oz./sq. ft.
A complete explanation about adjustments
would be too lengthy; it is advised that the builder
experiment with small gliders before building the
larger models. In short, the procedure is to turn the
ailerons slightly up and add weight to the nose until a
smooth glide is obtained. The ship is then power-
flown and stalls, dives, or turns are ironed out with
thrust adjustments. For towline gliders, the rudders
are set for the desired turn and the towline hook is set
to one side so that the model tows straight. The high-
start glider should be tried by all model builders. With
a well-stretched line, the model starts out at
tremendous speed and climbs high overhead before
releasing -- and all of this without running or cranking
the motor.
The next time you see a bird soaring high
overhead, watch its slow, majestic flight and see how
truly remarkable it is. Essentially a tailless model, the
bird is the perfect flying machine, representing the
goal which we seek.
Scanned From April 1943
Air Trails
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