|This article originally appeared in issue #54 of Skyways magazine: April 2000
In a few years, two P-26 Peashooter reproduction aircraft under construction at Golden Age Aeroplane Works in Seymour, Indiana will take to the air. Not since the early 1940ís has America seen two Peashooters flying in formation.
The Boeing P-26A Peashooter began service as a front line fighter with the U.S. Army Air Corps in the early 1930ís. It was a revolutionary new monoplane design that made biplane pursuit aircraft obsolete. Of the 148 production aircraft, only 2 original P-26 aircraft are known to exist today - one in the Smithsonian Air and Space Museum and other at the Planes of Fame Museum in Chino, California. A non-airworthy reproduction is also on display in the U.S. Air Force Museum at Dayton, Ohio.
The little fighter experienced a long career for aircraft of the era and was still in the U.S. Army Air Corps inventory when Pearl Harbor was attacked (a number of operational P-26ís were destroyed on the ground during the attack). A few even soldiered on with the AAF 6th Air Force until 1943. Several found their way to Guatemala and were still active in 1955! The P-26 was also used as a front line fighter for several other countries during the mid 1930ís.
A total of 11 Model 281ís (P-26Cís) were delivered to the Chinese Government for use as a front line fighter against the invading Japanese forces. The P-26 proved itís abilities in combat by racking up a number of kills, but, was eventually taken out of service due to a lack of spare parts. A twelfth P-26 was sold to Spain without machine gun equipment installed. This aircraft was subsequently modified by the Spanish Air Force in the field to carry a couple of .50 caliber machine guns under the wings but was shot down before scoring any kills. Another service that successfully utilized the P-26 as a front line fighter was the Philippine Air Force which scored several aerial victories against the invading Japanese forces before being overtaken.
The first process in building reproductions of the P-26 began in 1991, and involved drafting full scale drawings (from original prints). The second step saw the construction of full scale wooden mock-ups for the fuselage and landing gear wheel pants. This process was used to insure the contour of the fuselage was correct and verified the wheel and tire clearance. Fuselage profile was critical since Army Air Corp test pilots complained of wind buffeting and noise in the cockpit during test flights of the prototype (XP-936). In order to solve the problem, Boeing temporarily widened the fuselage by installing specially shaped skin patches in the area immediately above and behind the cockpit on both sides of the fuselage. Finding success, the production fuselage was redesigned to accommodate a wider shape/contour on all bulkheads aft of the firewall.
On the P-26 there are 8 primary bulkheads of varying complexity and 13 secondary circumferential bulkheads. The secondary structure consists of numerous short stiffener sections of hat shaped aluminum riveted together by .032 aluminum ďtieĒ plates. The firewall (Station 2) was the first bulkhead to be fabricated. It consists of an aluminum web box spar with heat-treated 4130 rectangular tube spar caps of differing shapes and various formed parts.
Data for the aluminum hat sections used on this aircraft was found to be non-existent. Nine months of intensive blueprint research, along with many hours of reverse engineering finally unlocked the secrets relating to the cross section and thickness of the 30 hat sections required. Unfortunately, once this was accomplished, it was discovered that none of the hat sections could be readily purchased and would have to be fabricated. Blueprints were then drafted up for each hat section and then forwarded to several aviation sheetmetal shops for bidding. A majority of the shops balked at the tight tolerance between flange heights and wanted the allowance increased. After several months of searching, a shop was finally located that could meet the requested specifications of + .005. The only problem, however, was that the shop didnít have a die that could form certain hat sections due to the depth required (Station 2 hat section required a 1 1/2 inch in depth with a 5/8 width). A large machining shop in the Seattle shipyards was contracted to machine a10 foot long die with specific tip hardening requirements. The die was subsequently delivered and the hat sections fabricated to perfection in a timely manner.
Some fabrication problems are caused by the construction material used years ago. One concern was the strength values and properties of modern metals (aluminum grades 2024, 6061 and 7075) as compared to the metals used on the original aircraft (17ST, 17SOHT, 17SO and 24S) . Data from a 1930ís Boeing engineering manual shows the metals for the period are not the same as today (17ST is approximately 12% weaker than 2024-T3). In addition, the older metals utilized half the bend radius allowance as compared to those in use today. Therefore, manufacturing parts become difficult due to considerations for fit and size.
Some construction problems are not obvious until assembly is required. For instance, how did Boeing rivet the aluminum and steel tubes to large sections of sheetmetal plate when the tubes do not have holes on the opposite side for bucking bar access? This was going to be a major problem if a solution couldnít be found since several areas of the structure such as the firewall, pilots bulkhead (station 3), and the floor use the tube and sheetmetal combinations. The solution was found while talking with a retired Pan Am Clipper sheetmetal mechanic. Amazingly, it required very little practice and worked very well. All the bulkheads for both aircraft have been assembled and installed. In order for the hat section stringers and longerons to be installed they had to first be formed to the desired curvature by a roller die process. It should be noted that each circumferential stiffener hat section (on one side) had a different curvature and had to be individually rolled to the proper shape before preliminary fitting. Due to the severe forming required many of the sections had to be rolled in the soft condition and trimmed, joggled, and deburred before being sent out for heat-treatment. Each piece was then Rockwell and conductivity tested to insure proper hardness before fitting, painting and installation took place.
Due to compound curves on skins around the cockpit area a full-scale wooden mock-up (form) had to be made. Another difficulty involved fuselage access panels that are recessed into the compound curved skins through a stamping process. This is different than modern structural techniques that normally use a reinforcement piece of sheetmetal flush riveted to the inside of the skin. The stamped access panel feature is typical of the P-26 design, which was the first flush riveted aircraft ever produced. The fuselage has 31 different skins Ė many with compound curves. Each skin was installed and removed between 6 to 11 times before final riveting. This was necessary since each skin was sequenced for assembly and had to be custom fit, countersunk/ dimpled, trimmed, edges filed, edges rolled, burnished and primed before riveting. Castings have been made and fitted to the airframe while the endless machined parts continue to be manufactured. Rudder pedal assemblies which have hand formed aluminum and steel along with a multitude of machined parts have been fabricated and installed. Instrument panels have also been machined, fitted and installed.
The P-26 used narrow wheel assemblies with streamline contoured tires. Since such tires havenít been manufactured in over 40 years and new old stock have serviceability and safety concerns, a slight contour change of the landing gear fork was made in order to allow for the use of modern wheels and tires. Fortunately, this will not affect the profile or width of the sheetmetal landing gear covers (wheel pants/spats). The wheel pants have already been assembled and are awaiting installation.
While researching the P-26, misinformation in the form of myths was shared on occasion. For example, phrases such as: "just change it.... back then they didn't really know what they were doing" were heard. While some small aircraft manufacturers in the 1920's and 30's might not have had the engineering expertise needed for a good and/or safe design, large manufacturers such as Boeing were very professional with large staffs of engineers and aerodynamic testing personnel. According to retired Boeing Archivist Marilyn Phipps, Boeing laboratory tested structural subassemblies (to destruction) on every production aircraft type since the second Boeing design in 1916. This enabled Boeing to verify engineering estimations ("g" loading) and provided a means for learning how to optimized structural design for strength and weight savings. With regards to the P-26, Boeing not only tested prototype subassemblies to destruction, but, spent a fair sum of money doing wind tunnel testing to verify proof of concept and airfoil design.
Another area of misinformation involves the structural changes made from the prototype to the production aircraft. In some published articles, statements have been made that the changes were minimal. Not so! The production aircraft had significantly beefed up stabilizers with almost twice the number of nose ribs and the addition of multi-layered spar cap strips on the upper and lower areas of both spars. The floor was also redesigned with brackets added from the floor tubing to stub wing in order to make the structure stiffer. The headrest also went through a transformation during production. The original structure was comprised of formed aluminum pieces riveted to the skin immediately above the pilotís bulkhead. After a pilot was killed in a roll over incident, Boeing completely redesigned the headrest assembly. The new headrest was comprised of 4 parts including left and right rectangular steel tubes welded to formed hat section assemblies which were bolted and riveted to the forward face of the pilotís bulkhead. In addition, behind the steel structure was a rear wedge assembly comprised of 10 "V" shaped hat sections riveted to aluminum side panels and topped of with a heavy aluminum cap. To hide the forward part of the structure, Boeing used a formed aluminum (heat-treated) fairing and added a cushion in the center to protect the pilotís head.
Other areas of redesign involved the shape of the entire fuselage (wider and more pear shaped) so that cockpit wind buffeting was eliminated. In addition, the wing shape went from round to elliptical, leading edge rivet spacing was reduced, and the rudder box had stiffeners added. In-service enhancements included the addition of flaps, a completely new tail wheel and steering assembly, reinforcement panels bolted to the forward stabilizer bulkhead, several circumferential stiffener additions to the mid fuselage area, and the addition of stiffeners to top of the fuselage between the headrest and stabilizer.
This project has required several people with great talent. Special thanks are to be given to Dan Laing for his Engineering support, Gayla and Nicholas OíConnor, which have assisted in every difficult assembly job encountered, Kevin Huber and Wayne Bolton for their building assistance. Rex OíConnor needs to be mentioned also, for without his encouragement this project would have never been started. Further information on this project can be found on our web site at: www.peashooter.net
About the Author
The author has been involved with aviation since he was a young man flying with his father (a retired Air Force fighter pilot). Stinson Gullwings and the P-26 have been Timís favorite airplane since childhood. In 1978, Tim and his father purchased a basket case Stinson SR-9E. It was a dream come true when after 10 years of restoration work on the Stinson SR-9, it garnered a ďGrand ChampionĒ award at the Antique Aircraft Fly-in at Denton, Texas.
Tim has been in commercial aviation for 25 years working in various positions with 3 different airlines and Boeing. Tim is celebrating his twelfth year at UPS Airlines and is currently working as a Technical Support Supervisor for Aircraft Line Maintenance. Tim also teaches AMT college courses (part time) for Embry-Riddle University (Louisville branch). He holds 2 degrees from Embry-Riddle University, an A&P and a pilotís license.