The convertible aircraft, a vehicle which can function either as a direct lift, rotary aircraft or as fixed wing airplane, with conversion from one to the other type as required, has long been a dream of aeronautical inventors. Combining the vertical flight capacity of the helicopter with the greater efficiency in forward flight of the airplane, the convertiplane seemingly had the potential to revolutionize both the military and commercial aviation sectors; in practice, it has proven quite difficult and expensive to develop a vehicle that is both useful and safe. NASA (and NACA, its predecessor) has long had an interest in this type of aircraft, as shown in the following four reports dating from the early postwar period. These documents offer an excellent summary of the convertible aircraft field in the late 1940s and feature numerous rare images of unconventional projects and prototypes, some published here for the first time.
Symposium on Convertible Aircraft—March 2, 1945
The first report summarizes a symposium on convertible aircraft that took place on March 2, 1945 at the Engineers’ Club of New York City. Notable attendees included Charles H. Zimmerman of Chance Vought Aircraft, designer of the V-173 and XF5U-1 flying pancakes; S.D. Robins, who designed a bizarre combination airplane helicopter which sat on the ground nose down and took off in a helicopter manner, tail first; Gerard P. Herrick, a pioneer in the field who had built and flown the only successful convertible aircraft up to that point, the HV-2A Vertaplane; Lloyd H. Leonard, a former NACA engineer who patented a series of tailsitter aircraft that turned 90° in the air after take-off with large counter-rotating rotors as propellers to drive the machine forward; Arthur M. Young, distinguished inventor and designer of the Bell helicopter; and E. Burke Wilford, a gyroplane pioneer whose WRK Gyro of 1931 was the first rotorcraft with cyclic pitch control and rigid rotor system. Wilford presented a variety of interesting projects, including an incredible convertible transoceanic flying wing with an auxiliary single-bladed rotor for take-off and landing. Images from this report are shown above left, with high resolution versions available at the end of the article; the main body of the report is shown immediately below.
The Franklin Institute’s Proposed Program for the Evaluation of Convertible Aircraft—April 28, 1947
The next report found in NACA files was produced by the Franklin Institute of Philadelphia, Pennsylvania and dates from April 28, 1947. This organization proposed investigating all known and projected methods of combining fixed wings and rotating wings, with the object of determining which basic types and variations were feasible. The analysis and evaluation would be based on existing knowledge of fixed and rotary wing design and on proven processes of manufacture. The end product would be a recommendation of the most promising combinations of fixed and rotating wings. The final section of the report features illustrations (shown above left) of some of the convertible aircraft types the Franklin Institute proposed investigating; recognizable are configurations similar to those developed by Herrick, Wilford, and Leonard, as well as a tiltrotor of the Platt-LePage type. The main body of the report is shown below.
Grover Loening’s Investigation of Convertible Aircraft Status—July 22, 1947
This report was followed by one authored by the great Grover Loening titled “Investigation of Convertible Aircraft Status.” Dated July 22, 1947, this study reported on the interest in the convertible aircraft (helicopter to airplane) aircraft, and was accompanied by preliminary estimates on their possibilities. Also included were several suggestions for research, including recommendations from some of the more prominent engineers of the industry that were interested in this subject. Visits were made to investigate this matter through the month of June to various helicopter manufacturers and, in particular, to the aforementioned Franklin Institute, where conferences were held with staff, and who had already suggested to NACA that a study of convertible aircraft be made. Conferences were held with Zimmerman, Leonard, Wilford, and his associate Louis De Monge of France. Loening also reviewed the earlier work done by Herrick. Conferences were held frequently with members of the Langley Laboratory staff; in particular, Messrs. Wilson and Dingeldein cooperated in investigating one of the principal problems involved, and their study was included as part of the report, to illustrate the problem in transforming a helicopter rotor into an airplane propeller status. The Loening report features numerous illustrations and photos of a variety of interesting convertiplane types, including a particularly odd machine designed by De Monge.
NACA Langley’s Convertible Airplanes and Personal Aircraft Research—November 5, 1948
Finally, we have a memorandum from H.J.E. Reid, Director of NACA Langley dated November 5, 1948 concerning the application of convertible airplane technology to the personal aircraft class. The memo summarized NACA research into this area and included some generic illustrations of the major configurations considered applicable to the field; these included both the parallel and tandem tiltrotor, the vertically-rising airplane, the combination rotor and propeller aircraft (both rotor-streamlined and rotor as wing), the direct take-off airplane, and the Custer channel-wing. The drawings are shown at left, while the memo is shown below.
Before closing, we must mention a November 18, 1949 letter from E. Burke Wilford to Joseph Ames, Chairman of NACA, concerning the Convertible Aircraft Congress, which was held in Philadelphia on December 9 of that year. The author recalls seeing an interesting booklet that resulted from this meeting, but unfortunately did not have the opportunity to scan it. If any reader knows of where one might find a digital copy of this document online, feel free to comment below or email the editor.
In reading these reports, one is struck by the hope and enthusiasm of the figures involved in the convertible aircraft field; these engineers truly believed that they would revolutionize flight and merge the best characteristics of the airplane and helicopter into one superior vehicle. Looking from the vantage point of the early 21st century, this optimism appears to have been premature, as only one type of true convertiplane, the V-22 Osprey, has reached production, and not without considerable expense and controversy. Prior to the Osprey, there were many experimental convertible aircraft built in several countries, but few were judged useful enough to warrant production. However, the type seems to have a much brighter future in the unmanned aircraft category, with many new UAVs emerging which combine the characteristics of both helicopters and aircraft, such as the Aerovel Flexrotor. It has taken many decades and technological advancements, but the convertiplane may finally live up to its potential and fulfill the expectations of those pioneers who first championed the concept so long ago.
All images from NARA Archives II, College Park, MD, RG 255
Readers may purchase a high resolution PDF download of the 27 images previewed above for just $1.49 through our shop on Gumroad.com; an example high resolution image is shown at left.