The accelerated development of aeronautical knowledge and design from 1903 to the years just after World War I was a heady mix of discovery, science, and engineering, with the occasional entrepreneurial influence.
But if everybody had a new and better idea, the postwar U.S. Army Air Service determined that all the newfound ingenuity had to be quantifiable if the service was to make sound aircraft procurement decisions.
The Air Service had performance and design standards, and industry was given a handbook to guide their work intended for the military.
Readers of this column may remember its creator, the late Peter M. Bowers, as a one-of-a-kind wizard of historical aeronautical knowledge. Pete was a purposeful packrat who had at his fingertips a technical library of amazing depth and breadth. He lent me a copy of the Air Service’s Third Edition of the “Handbook of Instructions for Airplane Designers” from June 1922, which I quickly copied and filed for reference. Let this article be another ongoing tip of the hat to Pete Bowers.
The foreword to the handbook is inscribed by Maj. Thurman H. Bane, who strove to make military aviation developments methodical and precise at McCook Field in Dayton, Ohio, after World War I. The handbook instructed aircraft manufacturers to adhere to the standards it contained when responding to an Air Service request for bids on a design type.
The specifications called for accurately scaled and dimensioned three-view drawings of the aircraft being proposed: “This drawing shall be in sufficient detail to indicate accurately all the physical characteristics of the airplane. All distances that are important for the stress analysis of the wings shall be dimensioned on the drawing.”
Detailed weight estimates and design characteristics were spelled out on forms provided by the Air Service. It was incumbent upon designers to follow these forms, the service said.
“Having the data in standard form will enable the Engineering Division to make a direct comparison with other designs submitted or, when available, with actual data similarly tabulated for complete airplanes.”
In the 19 years since the Wrights first flew, the design of aircraft was being codified and quantified with precision.
The Air Service’s requirements for submission of an aircraft design included the construction of a small scale model of the proposed aircraft. The handbook elaborated: “This model is not for wind tunnel tests and is requested in addition to the three-view drawings to facilitate comparison with other designs, particularly for such features as vision and field of fire. It will enable the designer to present the features of his design in a much clearer manner. The scale of the model will be 1/20 full size for less than 40 foot span and 1/40 full size for over 40 foot span, unless otherwise specified.”
The 1922 manual specified the need for a mockup once a bid was accepted by the Air Service. The mockup was to depict a portion of the fuselage with all cockpits, engine and fuel and cooling systems, center section, and a portion of the wing(s), armament, equipment, instruments, accessories, and controls.
“All pieces of cowling must be cut to shape and proper form, and all structural members must be accurate as to size and location.”
The handbook did not require the mockup to be built entirely of actual hardware. “The quality and kind of materials used in the mockup are not important. If actual engine, armament, equipment, and accessories are not available, dummies correct in dimension shall be used.”
The Air Service said the government would construct any needed wind tunnel models based on the design dimensions provided by the submitting companies “in order to secure uniform accuracy and permit a proper comparison of the results with existing data.”
It behooved the designer to make sure his dimensioned drawings contained the information the Air Service wanted, including airfoil number, or a detailed airfoil cross-section drawing if it was nonstandard for the day.
The 1922 Air Service standard called for pursuit aircraft to be designed with load factors as high as 8.5, trainers 8.0, and long-distance night bombers 4.0. The Air Service spelled out requirements for landing gear loads, since not every landing is a pilot’s best.
Many aircraft of the day relied on spruce for wing spars, and the handbook provided stress information for designers to use in computing the right size and load expectations on spruce structures.
Center of gravity (CG) was life and death for good designs, and the Air Service instructed manufacturers that their designs: “In order to obtain proper balance, the center of gravity of the airplane will in general be between 25 and 33 percent of the mean aerodynamic cord for conventional airplanes.”
The 1922 manual was a one-volume aeronautical engineering education in rationale and methodologies for computing vital aircraft measurements and values. Bright engineers in the Air Service wanted to ensure that the design submissions they were expected to evaluate were derived with sound methods in the still-new field of aeronautical engineering.
In the era before nosewheel designs, the Air Service handbook said: “The propeller should clear the ground by at least 9 inches with the airplane in either flying or landing position.”
All multi-engine designs were to incorporate starters, as were single-engine designs of greater than 450 horsepower. Looking ahead, the Air Service said all aircraft should provide space for the possible future installation of starters.
The Air Service’s advice to designers on pursuit pilot visibility was, not surprisingly, to minimize blind spots: “A good view forward is required for landing, and a good view to the rear for defense.”
It was suggested that good pursuit designs would eliminate blind spots through the use of small movements through any axis. Interestingly, in 1922 the Air Service suggested a parasol design might afford the best vision for the pilot, but the handbook also singled out two fighters from the recently ended World War — the sesquiplane French Nieuport 27 and biplane German Fokker D.VII — as having excellent visibility for the pilot.
The handbook reminded designers of the practice of placing bolts and other fasteners “head uppermost on assembly of the airplane, and the fittings shall be designed accordingly. This procedure will eliminate the possibility of the pin or bolt falling out in the event of failure to properly safety it with wire or cotter pins.”
Control sticks were to have sufficient strength to withstand a pull of 200 pounds, which the Air Service said is the maximum that could normally be applied by a pilot in flight. To ensure this with a margin of safety, sticks were to be calculated to withstand an even greater force of 300 pounds. Fore and aft stick motion of about 18 inches was desired.
Airfoil rib stitching within the diameter of the propeller arc was to be have spacing no wider than 2-1/4 inches. Over the rest of the airfoils, stitch spacing could be no wider than 4 inches. Aircraft grades of either cotton or linen were allowable for fabric covering.
Students of World War II Japanese aircraft have noted the deep red-brown color of Japanese propellers. Possibly this dates back to an Air Service observation on propellers in the 1922 handbook: “The finished propeller is deep maroon in color, as it has been found from tests that this reflects the least amount of light when an airplane is landing at night by the aid of lights and flares.”
For the convenience of designers, the handbook lists the weights of several common types of propellers of the day. A propeller for a Liberty 12 engine, made of lightweight poplar wood, weighed 35.8 pounds, 63.2 pounds if hewn from laminated oak, and 42.2 if made of walnut.
This handbook, produced less than four years after the end of World War I, is an elegant antiquity. Sophisticated in its era, it came on the cusp of rapidly changing design tenets that would soon see steel tube fuselages replace wooden structure. Designs, and their designers, became ever more capable rapidly.
(This handbook is cited only for historical purposes and not for actual use.)