tailless.pdf

Flying Wings

By Henry Cole

FLYING WINGS REPRESENT THE THEORETICAL ULTIMATE IN AIRCRAFT DESIGN.

USE THESE IDEAS, AVAILABLE AFTER A YEAR, OF RESEARCH,

TO DEVELOP PRACTICAL MODELS.

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.

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-

tation.

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.

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

construction.)

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

determined.

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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