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BREAKING THE SOUND BARRIER

The first pilot, who was in contact with the sound barrier was Messerschmitt Test Pilot Heini Dittmar. He intended to cross the then magical 1,000km/h line with his rocket fighter Me 163 on 2 October 1941. He described the flight as follows: "When I approached the measured distance, the speedometer indicated 910, then 950, 970 and finally 980km/h. This time the engines did not cut out on me. When I looked at the instruments again, I had gone over the 1,000km/h mark, but the airspeed indicator was unstable, the elevator started to vibrate, and at the same time the aircraft plummeted out of the sky, gathering speed. I could not do anything. I immediately turned off the engine and was certain that the end was near, when I suddenly felt the steering responding and managed to get the Me out of her nose-dive relatively easily. Nothing else happened. The so-called Mach phenomena, which I was the first pilot ever to experience, were nothing else than the knocking at the sound barrier with an aircraft, which had not been designed to break through this barrier". (from: Mano Ziegler, Rocket Fighter Me 163, Motorbuch-Verlag). When the measurements were evaluated, it became apparent that the aircraft had been travelling at a speed of 1,004km/h. It was a lot faster than any previous speed record.

What Dittmar calls the Mach phenomenon, i.e. the effects occurring in the boundary area, the so-called trans-sonic area, are caused because the air is compressed. Compressibility means that a substance reacts to a change in pressure with a change in density. In liquids this change is so minimal that as a rule, it is negligible. However, gases like air react differently. Air pressure, which is working on an aircraft during flight is the result of static and dynamic pressure. While static pressure changes with altitude, the dynamic pressure is dependent on speed: If the aircraft's speed increases, the dynamic pressure also grows and thus the density of the air in front of the aircraft. One can easily imagine this phenomenon as "piled up air" which causes the air to compress. The character of the air stream changes fundamentally at high speeds.

The influence of compressibility is measured in the dimensionless Mach (M) number. It describes the relation of speed in a medium to the sonic speed of that medium. At M 0.3 the relative change in density of the air is around five per cent. This quantity is generally seen as the limit at which the compressibility of the air has to be taken into account mathematically, i.e. always when it comes to designing aircraft.

At first glance the interdependence of sonic speed, compressibility, pressure and changes in density is not all that obvious. It does become clearer, when one realises that sound is nothing more than a change in pressure in the surrounding medium (air, water). The speed of sound indicates how fast a change in pressure can spread. If an aircraft approaches the speed of sound the changes in pressure cannot escape, but concentrate in front of the aircraft in a wave front. If you are "hit" by the concentrated wave of the Mach cone, the change in pressure is experienced as supersonic bang.

However, it is not always that easy to distinguish between supersonic and subsonic. At the speed of around M 1.0, i.e. in the transonic regime, the aircraft experiences both subsonic and supersonic airflows. If one looks at a wing profile at a moderate speed of M 0.5, a subsonic airstream acts on the entire profile. On the top of the profile the air stream's speed is increased, the pressure is lowered. On the bottom of the profile the reverse is the case; there is less of an acceleration and a slight increase in pressure. The difference in pressure across the profile corresponds with the lift created by the wing.

When looking closely at transonic speed, it is interesting that the speed of the airflow is higher on top of the profile than the flight velocity. From a certain velocity value, the so-called critical Mach number, supersonic areas appear for the first time on the upper side of the wing. This happens, depending on the used wing profile, at approximately M 0.8.

Heini Dittmar hit the speed regeim, which is colloquially known as the sound barrier, i.e. the transition from subsonic to supersonic, when he was flying his Me 163. This explains why he was unable to control the aircraft. Two significant effects contributed to the fact that this phenomenon was called sound barrier, because transgressing the transsonic seemed initially impossible: On the one hand drag increased drastically, on the other hand a drastic change in the sudden alteration of momentum was experienced. Both are caused by shock waves in varying intensities.

The increase in drag has two causes: With the wave drag a new drag factor comes into action, which only exists at supersonic speeds. On the other hand the shock waves at the rear end of the supersonic area on the wing cause an airflow separation in the boundary layer. The biggest drag occurs at M 1.1 to M 1.2.

The separation of the boundary layer is a non-stationary process, which means that it happens irregularly and is subject to constant change. As a result the aircraft shakes violently, the so-called buffeting. The changes in momentum are being caused by shock waves. The pressure distribution at the wing is changed, when compared with a pure subsonic current. The scale of the described effects depends on the Mach number, the attitude and the wing profile. Thickness and the degree of wing sweep are especially important.

During transsonic flight the drag is unavoidable, just like the buffeting and the change in momentum. It can however, be minimised by optimizing the aircraft design.

For this speed region, the so-called super critical profiles were developed in the 60s. They are adjusted to the air stream properties of around M 1.0. A special profile geometry is designed to reduce the intensity of shock wave impact on the wings. The lift vector is moved further to the back. It has to be said that these profiles are better suited for transonic and not for supersonic flight.

During supersonic flight there are no shock waves on the upper and lower wing surface. These only occur at the leading and trailing edge. This means that drag decreases, buffeting and swaying vanish at the existing momentum. At high Mach numbers other aspects are becoming increasingly important: At high speeds, built up pressure and temperatures are increased considerably. This fact must be addressed when designing the aircraft. Furthermore new propulsion concepts like ramjet engines are necessary.

This poses the question of how an aircraft should be designed to make it cope with both transonic and supersonic speeds. Obviously the Me 163 was not able to withstand these speeds. Although it was fitted with swept wings, the wing profile was rather thick. By tapering the front of a wing the effective air stream velocity is reduced, and thus the critical Mach number rises. However, at low supersonic speeds with their relevance to drag and momentum these features are less effective. They become increasingly significant as the speed increases. On the other hand the intensity of buffeting and of wave resistance are lower at supersonic speeds.

That is why swept wings are advantageous for supersonic and transonic flight. Aircraft operating mainly in high supsonic speed regions, such as commercial aircraft travelling between M 0.8 and 0.9, cannot do without swept wings. However, for slow flights straight wings are better. This consideration led to the development of variable-sweep wings.

The profile thickness of the Me 163, i.e. the ratio of thickness to profile depth, was 0.14 at the root of the wing and 0.08 at the wingtip. In modern supersonic fighters this number is 0.05. If one examines the Me 163's fuselage the situation is similar. It was not slim enough. The thickness of the profile has a direct bearing on the strength of buffeting and the size of wave drag: The thicker the profile, the higher the wave drag, which is the dominating drag component during supersonic flight.

The shape of the profile nose is also important in connection with the profile thickness. While a round shape will prevent airflow separation during subsonic flight, during supersonic flight it will lead to shockwave separation. The rounder the nose, the higher the vertical component. A wedge shaped nose cannot be realised practically, since a thin tip cannot withstand the increase in temperature. As far as drag is concerned an increase in the angle of attack corresponds with a thicker profile.

There will never be an aircraft, which will perform perfectly at all speeds. The conditions at subsonic, transsonic and supersonic flight are simply too different. Every design must be a compromise, which will ensure that the aircraft will perform at its best at the speed it will mainly be flying at.

From page 46 of FLUG REVUE 11/99