Preliminary Wind Tunnel Testing of WWI Fighters

On June 4 and 5, 2005, a quarter scale Fokker D.VII and a quarter scale Fokker Eindecker E.III were tested at the University of Washington’s Kirsten wind tunnel. The short tests were performed to determine basic performance characteristics of the two WWI fighters and to serve as a preliminary study in preparation for more complete stability and control testing performed in November and December of 2005. The data obtained, although preliminary, confirms pilot reports and data collected shortly after the war. The initial goal of reaching one-quarter flight Reynolds number was not achieved, due to model structural constraints, and the initial limits on angle of attack prevented data collection past stall during these initial tests. In this paper, a discussion of the test, the motivation for the test and preliminary results for lift and moment characteristics are discussed. A brief preliminary description of the November and December tests will also be discussed. 1 Associate Professor, currently Configuration Engineer, Boeing Commercial Aircraft, Associate Fellow, AIAA Copyright © 2006 by D. Scott Eberhardt. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission 44th AIAA Aerospace Sciences Meeting and Exhibit 9 12 January 2006, Reno, Nevada AIAA 2006-333 Copyright © 2006 by D. Scott Eberhardt. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. 2 American Institute of Aeronautics and Astronautics Introduction Pilots of the First World War pioneered fighter tactics still in use today. They developed techniques in primitive aircraft just seven years after the Wright Brothers demonstrated their airplane to European audiences in 1908. Two airplanes best characterize the development of fighter aircraft if the First World War. They are the Fokker Eindecker E.III, the first successful fighter, and the Fokker D.VII, the best all-around fighter of the war. On June 4 and 5, these two WWI fighters were tested in the University of Washington’s Kirsten wind tunnel. The tests were performed for an upcoming production on the history of military aircraft for Granada Media, of Manchester, England, scheduled to air on the Public Broadcasting System in early 2006. The goal of the wind tunnel test was to gain reliable data to determine performance characteristics of the two airplanes. The test also served as a shake down test for more extensive stability and control testing performed in November-December, 2005. These later tests will be reported in the AIAA Region 6 student conference to be held at UC Irvine in April 2006. No such data has been found for testing of the E.III. Wind tunnels were rare in 1915 and the scale small. The available Reynolds number was low, which resulted in misleading results. Low Reynolds number results were the primary reason airplanes of WWI used thin airfoils. Several wind tunnel tests were performed on the D.VII in Germany, England and the United States on the D.VII. None came close to the scale tested in this study. The test in England, performed in 1921, was a 1/10-scale model tested at 60 ft/sec, for a Reynolds number of 2x10 based on the chord. It is thought that the test was mostly laminar. One goal of this test was to increase the Reynolds number by at least one order-ofmagnitude. The Fokker Eindecker E.III was a marginal airplane at best when it reached the front in September 1915. The E.III came about as a challenge to counter a menace on the western front. The famous French pilot, Roland Garros, had been successful hunting and shooting down German aircraft. Somehow, Garros was able to shoot directly through his propeller, making his airplane an accurate gun platform. On April 16, 1915, Garros was forced down inside German held territory, revealing his secret. The trick Garros used was to mount steel defector plates to the propeller, which would deflect the copper-jacketed bullets used by the French. The Germans used steel-jacketed bullets, so this technique would not work for them. Dutch airplane designer, Anthony Fokker, turned to a 1913 patent for an interrupter gear, held by Franz Schneider of the LVG Aircraft Works, and developed the Eindecker E.I. The interrupter gear is a system that prevents the machine gun from firing while the propeller is in the path of the bullets. Several months and modifications later, the airplane evolved into the E.III model. The fixed, forward-firing machine gun gave the Fokker Eindecker E.III an enormous advantage in combat. It quickly became popular as a German fighter as Germany gained air superiority from September 1915 to about April 1916, in a period known as the “Fokker Scourge”. It was not only the interrupter gear that gave the Germans the advantage, it was the tactics they were starting to put into action. Two well-known German pilots made their fame in E.III’s. The first was Oswald Boelcke. Boelcke was Germany’s Roland Garros. He gained experience in his E.III and wrote down a series of seven “dicta” for fighter combat. The “Dicta Boelcke” were the first written rules for fighter tactics and engagement. Many are still practiced today. Max Immelmann, a “moody, vegetarian teetotaller, known as the “Eagle of Lille” developed the maneuver named after him. The Immelmann is a one-half loop followed by a one-half roll. The result is a quick change in direction and increase in altitude. Aerobatic and fighter pilots today are familiar with the “Immelmann” maneuver. The tactics and maneuvers developed in the E.III are especially remarkable given the challenges of flying this airplane. An E.III captured by the British was tested and the following comments made: the plane was “unstable laterally, longitudinally and directionally.” It was “tiring to fly in all but still air.” It was also found to fly “persistently with the right wing down.” It should be noted that the Eindecker’s used wing warping for roll control. Both Boelcke and Immelmann were killed in E.III’s. Boelcke suffered a mid-air collision with a student