Anderson Greenwood AG-14

What constitutes the ideal private lightplane based on the present state of the art and market availability? Many details tempered by sobering reality would be the most logical answer. And that was the goal two young men set out to achieve during World War II. When Ben M. Anderson and Marvin H. Greenwood first got together, their creative minds immediately jelled into one driving desire, to design a personal airplane that would have pleasure and ease of flying as the uppermost consideration. Idealism therefore dominated the initial design philosophy, and the reality of engineering and construction was dealt with secondly.

Writing for the Institute of the Aeronautical Sciences, Document 227, Marvin Greenwood related in great detail how development of the Anderson-Greenwood Model 14 airplane began - as a pure design philosophy. In the initial study neither man had any particular configuration for the airplane in mind and both tried to keep this problem from affecting the achievement of their ideals; the main emphasis being placed on increasing the utility and personal pleasure of air transportation. A small, low cost airplane was selected for their initial efforts. Whether fact or fancy, each placed his ideals in the hat, but until the study was completed, preliminary design of the actual airplane was not considered.

Shorty after summing up their design philosophies, the first reality hit them. If their ideals were to be realized, it became apparent that no matter which way the ultimate design or construction went, no substantial improvements could be made in speed or load carrying ability and all indicators pointed to the need for an unorthodox design. This was not unexpected. Still, both men felt that an airplane involving such a departure from the norm could be produced and would maintain a good competitive position in the flying boom expected after the war.

The sensation of flying involves many associated factors. This particularly effects any effort to get the opinion of various individuals. In attempting to apply logic to this elusive problem, a list was made of the effects flight has on man's five basic senses. Ben and Marvin were then able to isolate, in a generalized form, the attendant effects of flight on these senses.

      

SENSE		SENSATION
Touch		Vibration Acceleration Temperature
Smell		Exhaust odors Fuel odors Oil odors
Hearing		Powerplant noises Air noises
Sight		Vision of clouds and objects on the earth
Taste		Not affected

With the exception of sight none of these sensations would be considered as desirable by a customer. Flight without sight is not enjoyable. By the process of elimination then the primary factor which makes flying most enjoyable was clearly - vision. Although vision had long been recognized as one of the most important factors in any aircraft design, Anderson and Greenwood set out to put this design factor foremost in their work. It was this sort of analysis that produced the AG-14 airplane.

Anderson, Greenwood and Company (AGCO) was formed in Houston, Texas in 1940 by Ben M. Anderson, Marvin H. Greenwood and Lomis Slaughter, Jr. Greenwood was the principal technical man, being an ME graduate of Rice Institute of Houston and with a master's degree from California Institute of Technology (Cal. Tech.) of Pasadena, California. Slaughter was an ME graduate of the University of Texas while Anderson was primarily a business man.

Greenwood and Slaughter were working on the BM-10 military biplane trainer for Southern Aircraft Company in Houston. When they joined with Ben in privately developing a conventional, two place, tandem seat, low wing monoplane with a retractable landing gear, AGCO was formed. Unfortunately, this original aircraft design died on the drawing board as the AG-11 when, in early 1941 it became apparent that materials would be so difficult to obtain, due to military priorities, that the aircraft could never be completed. With the need for qualified aeronautical engineers and offers of high pay by major aircraft manufacturers with military contracts, all three principals of the six month old Anderson Greenwood & Co. joined the Boeing Airplane Co. in Seattle, Wash. early in 1941.

The war years kept the trio busy on preliminary design and project work but when the war ended in 1945, Ben immediately returned to Houston, reestablished AGCO and resumed work on the low wing AG-12. Shortly thereafter Marvin also returned to Houston suggesting they devote all their efforts to the pusher model that they had been nurturing during the war years, and work on the AG-14 resumed with greater enthusiasm than ever. A few months later Lomis joined them at the old, Sam Houston Airport, which no longer exists. The new plane emerged quietly, without fanfare, the result of the personal resources of these three men.

Having ascertained that vision was a primary factor, the pusher configuration was a natural choice. But, while such a design has many advantages - excellent visibility, quietness, propeller safety, etc. - it also possesses a number of its own particular problems. A few of the more important ones are: wing-body interference, balance, engine cooling, soundproofing and an unconventional structure. Overcoming these problems was a more difficult job than it appeared when AG-14 engineering got underway. Relatively few pushers had been built, so that applicable research data was very limited. Engineering libraries were practically dry on the subject, leaving Greenwood and Slaughter pretty much to their own resources in solving the complex problems involved in the new design.

Only two of the problems connected with the development of the AG-14 were unconventional; the structure and the installation of the powerplant. Most of the other components, such as the tricycle landing gear, twin tail booms, pusher propeller and steerable nose wheel, had been developed and used by other manufacturers and only detail design was necessary to adapt these features to the -14 configuration.

Conventional tubular construction with fabric or partial metal covering was not a desired feature for a modern aircraft and the result of such construction methods could prove, in the case of the -14, to be too expensive to manufacture on a production basis. An all-metal structure was chosen as most desirable but new construction methods had to be devised if the AG-14 was to remain competitive in the post war market. It became necessary to make extensive use of metal pressings, formings and corrugated shear webs in the structural design. This was not an entirely new form of construction, yet surprisingly little data existed on its use in 1946.

It was hoped that some structural data would be available on the Republic RC-3 Seabee, but the data which could be obtained was limited to giving the range in which the sheet metal did not buckle on the wings between corrugations. Nothing was reported on the buckling effects where this did occur. An independent study for the AG-14 at first showed that the number of external rib corrugations used in the Seabee wing could be materially reduced, but it was then learned that if this was done, buckling of the entire sheet panel would occur. As a result the AG- 14 wing, designed with probably the highest aspect ratio (9.6) ever used on a personal plane up to that time, used internal webbing, serving in the same capacity as external rib corrugations, but relied upon greater airfoil thickness to acquire the strength needed. Under static structural tests, the Seabee wing began to buckle along the entire panel while the AG-14 wing at the same stress load point began to buckle between ribs only and required considerably more torsional stress to buckle the entire wing. The relatively thick airfoil employed NACA 4418. allowed fewer ribs to be used, a lighter gauge sheet metal surface and a savings in weight. It also improved stalling characteristics but at some expense to maximum speed.

Corrugated sheet skinning was used on the ailerons, flaps, elevator and stabilizer, with corrugated shear webbing used in the outer wing main spars. Each component had to be engineered anew and entirely new formulas arrived at. Much of this required special tools, dies and fixtures but would, in production, produce a sturdy, simple and low cost structure allowing the little plane to be made available at an economical price.

Many factors entered into the choice of the wing position. Unless a high wing configuration is used, wing-body interference becomes a major problem in the pusher design. The stumbling block is in attaining a reasonably high, power off, maximum lift coefficient. Airflow over this wing-body juncture portion is high while power is full or nearly so. but as the power is decreased, such as in landing, this air burbles or separates and prevents sufficient lift, just when it is needed most. There is no propeller blast to help out. Many low wing pusher designs have had disastrous results because of early separation at this critical wing body intersection. During flight tests of the prototype AG-14, some separation in a small region near the wing trailing edge was experienced during a 100 mph power off glide, but the application of a little power cleaned up the airflow completely. The -14 did not suffer from this problem in its final refined configuration.

Wing placement in a shoulder high rather than a high wing position, was considered most advantageous. The wing was basically a monospar design with an auxiliary spar well aft to carry all moveable surfaces, which distributed and subsequently dissipated these aerodynamic loads to the ribs and then to the main spar. In the prototype plane, double opening, split landing flaps were installed to minimize change in trim during flap operations but this proved unnecessary and the more conventional plain single surface negative type flaps were fitted.

Flight characteristics of a pusher type aircraft can be quite different from the more conventional full bodied tractor type. Spins could be of some concern, but engineering on the AG-14 had made it virtually spin-proof. Only two items created some unsatisfactory characteristics in the prototype; the high aspect ratio of the wing and fuel shifting in the long spanwise centersection.

In original planning a 5 degrees dihedral was considered a reasonable value for stability, but the wing area of only 120 sq. ft. proved marginal in spin attitudes. Wing tips were reworked, using fibreglass sections, and slanted upwards several inches above the mean wing chord line, while dihedral in the outer wing panels was increased 2 degrees, from 5 to 7 degrees.

This corrected the tendency to accelerate and attain undesirable headings during a spiral situation. The fuel problem was eliminated by adding a baffle to the tank which stopped the rolling movement of the gasoline.

In its final form (production model) the AG-14 would spin one turn, in only one direction with full power only and with full up elevator - if the pilot knew how to force it into such a situation. If it did spin, it would automatically fly out after one turn, coming under control at the same time, even with full up elevator and full rudder still applied. This characteristic, when coupled with the use of only one small rudder on the left vertical tail surface and a restricted elevator travel, made the AG-14 virtually spinproof. Its simplified control system is not to be misconstrued as making it a two-control aircraft but comes as close to it as possible while still retaining many of the advantages of the standard three control system.

A Continental C-84-12F engine and a wooden propeller was used on the prototype aircraft with about a 4" plug placed on the shaft between the two. This metal collar allowed the engine to be located further forward, nearer the c.g., where it gave better longitudinal balance.

Powerplant cooling becomes a critical problem in a pusher design. In a tractor type airplane, the propeller forces air through the entire powerplant system, where as the pusher does not have this benefit. To retain the AG-14's high wing design, thus obtaining satisfactory airflow at the wing-body juncture, its engine had to be in a buried position. No problem was encountered during actual flight operations of the AG-14, but on the ground some sort of induced cooling had to be incorporated. Employment of a fan was ruled out due to the increased expense and complications.

Satisfactory cooling was achieved by tucking twin rectangular cooling air intakes under the wing, one on each side of the fuselage. Air entered at the wing leading edge and went into the lower engine compartment, up through baffles, over the engine itself, and aft out a fairly large cowl opening around the propeller shaft area. The exit opening was placed as close as possible to the propeller so that even at this low pressure area, the propeller was able to induce sufficient airflow to cool the engine satisfactorily during ground operations. During early tests, the oil temperature and the temperature of one cylinder head ran over limits on several occasions. This was corrected by installing a blast tube and deflector inside the left hand air duct. No oil cooler was necessary after this modification and no further engine cooling problems were encountered.

Early one morning in September 1947, Ted Yarborough of Fort Worth was hired to make the first flight in the AG-14 from Sam Houston airport. After the flight he found the only serious deficiency in the new plane was that the elevator was too small. A larger one was made and installed that same day. Ted flew it again in the evening, finding everything fine and ultimately putting in five hours flying time, checking out Marvin Greenwood in the process. The small outboard ailerons with large travel subsequently proved ineffective and the plane was returned to the shops where longer ones were made to occupy the space between the originals and outboard end flaps.

Without notoriety, Marvin Greenwood assumed test pilot duties throughout 1948-49 and proceeded to put the AG-14 through a series of rigorous tests. During this period various modifications were made in addition to those aforementioned. The horizontal tail was moved from its original position at the bottom of the vertical tails to the extreme top of the verticals. Extensive engine cooling tests led to better baffling, and venting and a molded Plexiglas windshield was formed to replace the original automotive type safety glass. In addition, the shape of the aft fuselage was changed to decrease drag, and the rectangular engine cooling air ducts were modified to the then latest NACA triangular, turbo-jet inlet shape. In the process a lot of excess weight was eliminated. The two years were devoted to redesign, cleaning up and the achievement of greater fineness of details.

By the summer of 1949 the decision was made to finalize the design and build five units based on the data obtained from nearly two years of operating the prototype - NX80828. A new powerplant was chosen for the pre-production models, the Continental C-90-12FP, and a new ground adjustable Hartzell propeller was fitted. The Continental -12FP was specially modified by Continental for pusher installation, the suffix P indicating pusher. The metal extension plug or spool was again fitted between the propeller and the engine. In addition to offering the ability to adjust the blades for maximum performance under various conditions, the new Hartzell propeller also had metal leading edges which better resisted the severe abrasion experienced on the original propeller when operating from the oyster shell runways at the Sam Houston airport. The thin, more efficient, blades of the Hartzell, and power increase to 90 hp, provided the responsiveness the AG-14 needed to make it not only competitive but also a sales threat in the personal airplane market.

First of the new airplanes, S/N 1, N3901K, was first flown by Marvin in June 1950. (No serial number had been assigned to the prototype-test machine, NX80828.) During the summer, all certification flying was carried out and the final check-out was made by CAA engineering test pilot John Paul Jones of the Fort Worth Regional Office. In September, Type Certificate 4A1 was issued. The little pusher performed flawlessly, displaying safety and comfort rivaled only by a few aircraft which cost twice as much.

She was conventional and yet unconventional in handling characteristics. A cruising speed of 110 mph, climb of over 700 ft. per min., exceptional visibility and simplicity of operation elicited nothing but praise from all who flew her. Through extensive use of sound proofing the cabin was very quiet despite the engine location just aft of the occupants' heads. Appointments were comfortable and airy, with plenty of spreadout room in the 44 inch wide cabin area. To reduce complexity and expense in the production models, the left hand door was eliminated. The AG-14 was set to retail for $4,200 to $4,500. For 1950, the Anderson-Greenwood pusher was definitely a new airplane.

It was during the late summer of 1950, however, that the Korean War started and once again materials for building personal planes began to dry up fast; so fast, in fact that AGCO found it impossible to continue and only the four remaining aircraft, S/N 2 thru 5, N 3901K thru N 3904K, were completed. Sufficient material had already been procured for this initial batch. In the meantime the experimental airplane NX 80828 was scrapped together with the static test airframe. Anderson-Greenwood & Co. were forced to seek sub-contract engineering work in order to stay in business. This they were able to do. Today AGCO manufactures valves - primarily used in pipelines, petrochemical plants, gas storage facilities for industry, and some airborne and ground support products for the military.

All five of the AG-14 pushers were eventually sold to private owners. FAA records for as late as the end of 1972 showed three of the -14s were still registered and in flying condition. Only the prototype N3900K, S/N 1, registered to the Yahwehs Kingdom Society of New Jersey, had its certificate revoked as of 17 Nov. '71, and N 3901K, owned by Mississippi State University and used in a special test program, was out of flying status. Over the years owners of AG-14s were reported from California and Texas to Minnesota, New Jersey, Georgia and Mississippi.

Company interest in the AG-14 never waned but conditions kept preventing further development. In 1962 Anderson Greenwood & Co. Iicensed three individuals to produce the AG-14, C. G. Taylor (of Taylorcraft fame), LeRoy W. "Roy" Hubert and Edward Phinney. Together they owned N 3902K and Hubert had it based at Flabob airport near Riverside, Calif. The plane had tentatively acquired the nickname of "Space Coupe" and plans were afoot to build the little pusher on a limited production basis at Oxnard, Calif. Some parts were produced but several problems intervened. The manufacturing facilities were subsequently relocated several times but the financial ability of the trio was not strong enough to put the business on a sound basis. In due course the contract with AGCO expired with no airplane having been built.

In 1968 another contract was signed with Strato Engineering Co., Inc. of Burbank, Calif. Roy Hubert was associated with Strato at the time, but again was unable to get the "Space Coupe" into orbit. This license also expired by its own terms. At present AG-14 manufacturing is in limbo. One AG-14 remains registered and in use by Mississippi State, and two others are believed still in storage but out of flying condition.

It might be well to mention at this point that the AG-14 and the men behind it were in no way related to the Stearman-Hammond, Kaiser-Hammond or the Wheelair IIIA pusher type aircraft of the same era. All these aircraft were entirely separate designs.

What would AGCO do if they could again produce the -14, some 23 years later? Mr. Ben Anderson replies, "Not much. If we were to redesign for today's market, I imagine we would add a few feet of wing area for improved takeoff characteristics, add more horsepower, say to 125 or so, perhaps go to a controllable pitch propeller if people really wanted a short field takeoff, change to rudder pedal nose wheel steering (the AG-14 had the nosewheel linked to the control wheel), provide adjustable seats if the airframe structure in this area adapted itself and maybe a larger nose wheel for better handling on unimproved fields." Thus it is believed that even today only minor updating would be required to make this 1946 design competitive with any 1973 standard model in its category.

Captions:

Photo 1
Anderson-Greenwood AG-14, s/n 1, N3900K, the first production model, Houston, Texas, 1950. [AGCO]

Photo 2
Prototype - experimental AG-14, NX 80828 of 1947 (left) shows rectangular engine air intakes at wing-fuselage juncture, automotive type windshield, and horizontal tail in original position. [AGCO]

Photo 3
Below is s/n 1, first production model with curved windshield, triangular air intakes and repositioned horizontal tail, (1950). N3900K was an entirely new airframe, not the experimental machine reworked. [AGCO]

Photo 4
Marvin H. Greenwood, left, and Ben M. Anderson inspecting the chest high engine location. Position made engine easy to work on, safe and quiet in operation. The 4" extension plug on the propeller shaft is evident. [AGCO]

Photo 5
Visibility from the AG-14 was exceptional. Here we are on final to Sam Houston Airport in 1948 tests. This downwind landing was made for photo purposes only. Normally aircraft would be higher and descending at a steeper angle for an into-the-wind landing. [AGCO]

Photo 6
AG-14, N 3902K, s/n 3, owned for a brief time in the mid-'50s by Air Associates of Burbank, Calif. Aircraft was listed as experimental at the time. [John Underwood]

Photo 7
Perky pusher topped out at 120 mph, cruised at 110 and landed at 49 mph. Power was 90 hp Continental C-90-12FP. Virtually spin-proof, AG-14 had a small rudder in the left vertical only. [AGCO]

Photo 8
The AG-14 obtained much of its high performance and stability through the high-aspect ratio wing and great, 7-degree, dihedral. Finish of the five AG-14s built was natural aluminum with red trim. [John Underwood]

Photo 9, Photo 10
Close-up views show roominess, clean lines and simple design and construction of cabin pod... the mark of a quality lightplane. [AGCO]

Sketches of the more prominent engineering features of the AG-14.
Drawing 1
Wing center section is composed of a monospar and auxiliary rear spar. The spar in the center portion of the wing is a 24ST extrusion with caps and web extruded as one piece. This has the advantages of simplicity and low cost. Simplicity and elimination of rivets are important in this case, since this spar web also forms the aft side of the structurally integral fuel tank.

Drawing 2
Structural detail of outer wing showing monospar and auxiliary spar. In the outer wing, auxiliary spar picks up movable surfaces and also carries local air loads to four main ribs which in turn carry loads to main spar.

Drawing 3
A keel box beam is employed as the load-carrying member in the Model 14 body as shown in this structural diagram. Completely below the floor level, it permits a full-sized low level door in body side and large front and side windows with narrow, nonstructural corner posts, allowing convenient entrance and egress, as well as a large field of vision.

Drawing 4
Boom structure consists of a top pan, sides and bottom made from one bent-up sheet, "W" sections added at bottom corners to increase compression strength of boom, and an intercostal midway between top pan and bottom to increase buckling strength of sides.