Development of the Anderson Greenwood AG-14

INTRODUCTION

Once the philosophy had been completed, many preliminary designs were sketched before that which became the AG-14 was selected. The one chosen seemed to offer the best compromise between the requirements of the design philosophy and the practical problems that arise in the development of any airplane.

DESIGN PHILOSOPHY

In order to be useful as a medium of air transportation, an airplane must have a reasonably high cruising speed and in addition carry a reasonable load. Designers have always been speed conscious and because of this and the fact that speed is proportional to the cube root of the equivalent drag area, there has been little difference in the speeds of similarly powered competitive airplanes. Designers often give credence to the fact that one airplane is 3 or 4% faster than another airplane but this means little as far as utility is concerned. As a sales point it may have some value and for this reason it was decided that a sincere effort should be made to keep the design of the 14 as aerodynamically clean as possible.

The ability of an airplane to carry load depends on several factors, principally, the power, wing area and weight empty. Once the wing area and power have been selected, the battle against excessive weight begins. Designers are also weight conscious, and it was doubtful whether any substantial improvement in the weight empty could be expected over competitive aircraft, particularly if an unorthodox design were attempted.

To sum up thus far, it was doubted that any substantial improvements could be made in speed or in load carrying ability. Every effort, however, was to be made to do a good job in these respects in order to maintain a good competitive position.

As an observation platform definite improvements could be made and it was in this field and in the field of increasing the pleasure of personal flying that our attention was particularly attracted.

Considerable thought was given to the problem of increasing the pleasure of flying and one of the questions which immediately arose was what constitutes pleasure. The sensation of flying involves many seemingly associated factors and it was hard to separate the good ones from the bad. This was particularly noticeable when trying to get the opinion of other people.

In attempting to apply logic to the problem we decided to list all the various factors which make up the sensation of flight and examine each on the basis of its effect on man's basic senses; sight, smell, touch, hearing, and taste. The various factors which were able to isolate are listed below opposite the basic sense which they affected.

SENSE	SENSATION
touch	vibrations accelerations temperature
smell	exhaust odors fuel vapors oil vapors
hearing	power plant noises air noises
sight	vision of clouds and objects on the earth
taste	not affected

It would be a hard job to try to sell a customer on any of these sensations as being desirable except those associated with sight. Maintaining correct temperature is certainly desirable but adds nothing that can not be staying at home in a well ventilated and temperature regulated dwelling. What then is the factor which makes flying so enjoyable? By the process of elimination we arrived at the answer, VISION.

Although vision has recently been given some attention particularly in post war designs, it is doubtful if its full importance has yet been recognized. A completely new world is opened to the view of the aeronaut whenever he leaves the ground and his ability to enjoy it is directly proportional to the degree of vision which his aircraft affords him.

Flight without vision is certainly not enjoyable.

Vision downward and over the nose is particularly desirable; downward because objects are then at their nearest distance to the aircraft and over the nose because this is the direction of flight and it is possible to approach a given site and at the same time keep it in full view. Panorama vision should also be considered because at reasonable altitudes, scenery changes slowly and if observation is limited to a small area it soon loses its interest and flying tends to become dull.

If flying is to be made as enjoyable as possible consideration would also have to be given to reducing the physical and mental strain associated with it by designed an airplane that is easy and safe to fly and one in which there is adequate vision to avoid maid-air collisions.

Thus the basic philosophy was first of all to design a good airplane; one that would be competitive both in performance and in load carrying ability and would be easy to fly. In addition it was to be designed so as to have the best vision which could reasonably be provided. And a serious attempt was also to be made to reduce the annoyances due to vibration, odors, noises, etc.

The results of this design philosophy is the AG-14, a two place, all-metal, personal airplane with tricycle landing gear and pusher powered with a Continental 90 horsepower engine. (See February issue of the Aeronautical Engineering Review for a more complete description.)

Flight tests on the prototype have proved very gratifying. The cruising speed is definitely competitive and we are hopeful that the cruising speed of the production airplanes will be even better since they will have many improvements and refinements not found on the experimental mode. The weight empty of the 14 is about the same as other similarly powered competitive airplanes and we feel that this is somewhat of a moral victory because of the 14's unorthodox design.

As an observation platform the 14 is in a class by itself. (Excluding rotary wing aircraft.) Visibility is excellent in all directions except straight aft. This makes the 14 ideally suited to various types of patrol and survey duties and in addition will be exceedingly valuable if it ever decided to adapt it to dusting or spraying duties. In addition to excellent vision, the 14 had many other features which are intended to make flying more enjoyable such as low noise level, easy entrance, large roomy cabin, etc.

UNCONVENTIONAL FEATURES

A complete structural description of the 14 was given in the February issue of the Aeronautical Engineering Review so it will not be repeated here.

CORRUGATED SHEAR WEBS

Their use is of course not new and for this reason it was surprising that so little data existed for structural design. Republic1 had published some data on this but it covered only that range in which the sheet did not buckle between corrugations or beads. For the very light skin used on the 14 this would have necessitated a very large number of corrugations. If the sheet were allowed to buckle first between the corrugations, then the Republic data is unconservative. It was therefore decided to investigate the shear allowable for use in the range where the sheet buckled between corrugations before the overall sheet buckled as a whole.

Sheet thicknesses from .016 to .051 were tested with web depths of from 6 to 12 inches. Both corrugations and beads were tested. Two panels extended into the Republic range and verified the use of an equivalent skin thickness based on moment of inertial for determining the shear allowables. Corrugations were 60 degree with depths of from one quarter to one half inch. Spacing was 2 and 4 inches.

One result which is interesting was that as long as the sheet buckled between corrugations first, the allowable was independent of the size or spacing of the corrugations. In fact the only parameter which gave consistent results was the ration of the height of web to the skin thickness.

At first glance this result might be taken to mean that the number of corrugations can be materially reduced but their elimination extends the panel into the Republic range and buckling of the sheet as a whole will occur first.

Fig. 1 shows the relation of the Anderson Greenwood data in which buckling occurs first between corrugations, to the Republic data in which the sheet buckles as a whole. On this plot the Republic data is a family of curves, only one of which is shown.

Fig. 2 gives empirical data for use in the design of such corrugated shear webs. This data should be used with some precautions until a much larger number of specimens are tested but it is believed that the data is sufficiently accurate within the range of the tests as outlined above.

The procedure for the design of corrugated shear webs are as follows;

1. Determine the equivalent skin thickness of the panel based on equivalent moments of inertia.

2. Determine the critical buckling stress for the overall panel

f_s2 = K E (T/B)²

where
K = K of ANC-5
T = equivalent skin thickness
B = panel depth

3. Determine the critical buckling stress of the material between corrugations in the same manner

f_s3 = K E (t/d)²

Where
t = skin thickness
d = distance between corrugations
K = based on ratio of B to d

4. If the inter-corrugation buckling stress is higher than f_s2 then the value of f_s2 should be used and

q = f_s2 t

5. If f_s3 is lower, then the empirical data from the curve of fig. 2 should be used as the allowable (except that the value of the allowable determined in 2. can never be exceeded. K of this empirical curve is based on the ratio of B to the length of the panel and is the same as 1/5 the K of ANC-5.

BEADED STABILIZER SKIN

However a male die was developed which stretched the skin as the beads were being formed. Results were satisfactory. A cross section of this die is shown is fig. 3. The amount of stretch was determined experimentally. The main portion of the die is an aluminum casting with polished surface. Beads are machined separately and pinned to the casting.

These skins as well as nearly all hydropressed parts of the 14 are formed in the ST condition.

WING LOCATION

Power tends to decrease the unfavorable pressure gradient of the wing-body intersection and to prevent separation, so that this is not a problem at the lower cruising lift coefficients. The favorable effort of power is absent, however, in landing when the largest possible lift coefficient is needed.

The wing-body arrangement in the 14 avoids the difficulties of a converging body in the vicinity of upper wing surface. The only part of the body projecting above the wing is the part that houses the flat engine, and convergence is kept to a minimum. Below the wing, the body is highly tapered and separation might again become a problem on the lower surface of the wing at low life coefficients (corresponding to cruise and high speed flight) were it not for the favorable effect of power.

At 100 mph with power off, there is a small region of separation at the body-lower wing junction near the trailing edge. The application of power cleans up the flow in this region completely. At the higher lift coefficients the flow is smooth with or without power.

SPIRAL STABILITY

In flight, the spiral characteristics were such that continuous attention to the controls was required to maintain a heading, and it left to itself the speed and acceleration would build up to rather high values. This characteristic was particularly objectionable during flight tests when instruments demanded most attention.

A reinvestigation of the spiral question turned up two factors which had not previously been considered. The importance of aspect ration (9.6) had not been given sufficient attention and the effect of the fuel shifting in the long spanwise center section tank had not been considered.

The effect of fuel was eliminated by making all flights during this phase with full tanks. In addition the effective dihedral was increased by the use of wing tips slanted upward several inches above the wing at approximately 45 degrees.

With this configuration, spiral characteristics were considered satisfactory. The airplane changed heading very very slowly and could be left to itself indefinitely. The effect of the tips is equivalent to increasing the dihedral of the outer panels 2 degrees, and the production airplanes are being built with this increase. A central baffle has been added in the fuel tank which will reduce the rolling moment due to shifting fuel to approximately one fourth of its previous value.

CONCLUDING REMARKS

REFERENCES

Illustrations:

Fig. 1
Shear Allowable
Corrugated Webs

Fig. 2
Shear Allowable
60 degree Corrugated Web
24ST Alclad

Profile View

Fig. 3
Stabilizer
Beaded Skin Die