Komet 163 • Chief test pilot Rudy Opitz tells it like it was • Walking on the Edge—the challenges of data acquisition • Popular Wisdom vs. a Test Pilot’s Experiences (interview with Rudy Opitz) • A Fighter Ahead of Its Time (includes aircraft specs)

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Messerschmitt Me 163B
Wingspan: 30 ft., 7 in. Length: 19 ft., 2 in. Max. speed: 596mph between 10,000 and 30,000 ft. powered time remaining after climb at 495mph: 2 1/2 minutes. Max. takeoff weight: 8,707 lb. Inital climb rate: 16,000 ft. per min. Armament: Two 20mm or two 30mm cannons.

Adding a landing flap and/or aerodynamic drag brake to the airplane steepens the glide, making it easier for the pilot to hit his landing spot precisely. Putting flaps on the Komet was not simple. The small moment arm of the elevons made the use of conventional flaps impossible. The elevons were not powerful enough to trim out the nose-down pitching moment generated by flaps.

The solution was to place landing flaps under the inboard half of the wings, with the hinge line well forward of the trailing edge. These could be deflected without affecting pitching moment. The craft also carried inboard, trailing-edge trim flaps.

While several of the faster piston-engine fighters of the time had encountered difficulties due to compressibility effects, the Komet was the first fighter to charge headlong into the world of transonic flight. As air flows over an object, it changes speed. In near the nose, the air is slowed. Further aft, where the air is accelerated, the local airspeed is actually higher than the speed of the airplane. As the speed of the airplane approaches the speed of sound, the local flow will go supersonic on some parts of the airplane. The “critical Mach number” of the airplane is the flight Mach number at which the local flow first goes supersonic on the wing.

When flying above the critical Mach number but below the speed of sound, the airplane is in the “transonic” flight regime. The airspeed is less than Mach 1, but there are local bubbles of supersonic flow embedded in the overall airflow. The critical Mach number depends on the configuration of the airplane. The thicker the wing, the more the air accelerates when passing over it. Some of the early WW II fighters, notably the P-38 Lightning, began to run into some transonic aerodynamic effects at Mach numbers as low as 0.68 or 68 percent of the speed of sound. The Komet had a critical Mach number of about 0.84.

When supersonic flow begins to appear on a wing or tail surface, the aerodynamic center moves aft, causing a nose-down pitching moment. As the Mach number increases, a shock wave forms at the aft boundary of the supersonic-flow bubble. When the shock gets strong enough it will cause the airflow to separate aft of the shock, leading to a loss of lift. This condition is called “shock stall.”

On the Me 163, the combination of the aft shift in aerodynamic center and shock stall led to a dangerous condition known as “Mach tuck.” If the Mach number exceeded approximately 0.85, the airplane would begin to nose down on its own. The pilot would naturally react by pulling on the stick and deflecting the elevons upward. This would cause a shock wave to form on the underside of the wing at the elevon hinge line. The elevons would shock stall and be unable to bring the nose up, causing the airplane to pitch over into an ever-steepening dive. The only hope for recovery was to wait until the airplane had dived to a lower altitude where the speed of sound is higher, thus reducing Mach number, and the elevons would regain effectiveness.

After the War, both the British and the Americans rediscovered this delightful characteristic of thick-winged tailless airplanes at high speeds. The British de Havilland D.H. 108 Swallow research aircraft had the same Mach- tuck characteristic along with a nasty tendency to porpoise violently at high speed. It eventually killed three pilots, including Geoffrey de Havilland.

Relative size Me 163B Komet vs. P-51D Mustang 3-View by Lloyd S. Jones

While it was not quite as nasty as the D.H. 108, the American Northrop X-4 ran into similar difficulties during flight testing. Later tailless combat aircraft benefited from what had been learned with the Komet, the Swallow and the X-4. They had thinner wings and elevons shaped to prevent the loss of control power these pioneers encountered.

Alexander Lippisch was aware of the causes of Mach tuck and tried to design the operational Me 163B to have as high a critical Mach number as possible. This dictated that the wings have very little washout. Most wings are twisted so the tip is at a lower angle of attack than the root; this twist is called washout. It is used to make the wing stall at the root first and keep the airplane from dropping a wing or spinning when stalled. Washout is particularly important for swept wings, which more readily tip-stall (causing a wing to drop).

The problem Lippisch faced with the Komet was that washout caused the lower surface of the wingtips to shock stall at high speeds, bringing on Mach tuck. Since washout had to be kept to a minimum, he used fixed leading-edge slots (called C-slots) to delay the stall of the wingtips.

The Komet was a daring attempt to push fighter performance to a new level. Ironically, the Komet, along with several other advanced German weapons, was buried under the onslaught of overwhelming Allied forces equipped with more archaic equipment.

—Barnaby Wainfan

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