FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1942
1942 - 0721.PDF
APRIL 2ND, 1942 FLIGHT 3T5 TOPICAL AIRCRAFT PROBLEMS—II Pressure Cabins : Super-sonic Speed Limitations : Airscrew and Control Difficulties at High Speeds : Flutter Bv PROF. DR.-ING. GONTHER BOCK (Continued from page 294) IN the first part we examined the design solutions which on the part of the power plant were needed to enable it to fulfil the requirements at great altitudes. On the part of the airframe, too, special precautions are necessary. In the human organism disturbances arise at altitudes of more than 12,000 metres (40,000ft.), even when breathing pure oxygen. It is therefore necessary to raise the pressure of the air used by the crew for breathing to something above the external -pressure and for that reason to place the crew in a pressure cabin. The arrangements necessary in the pressure cabin for retaining the air pressure for breathing are shown diagram- ihatically in Fig. 16. The air for breathing can either be tapped off from the engine supercharger or it can be drawn in from the free air. After compression it is reduced to the desired temperature by a cooler and flows then through a filter in which any oil that might be present is drawn off, and finally into the pressure cabin. A constant pressure is maintained in the cabin by pressure-retaining valves controlled by barometer capsules. In order to make quite sure of preventing an excess of pressure an additional high- pressure release valve may be provided. In order to RADIATOR FILTER Fig. 16. Diagrammatic representation of the equipment pressure cabin. of a prevent the pressure in the cabin from falling below the pressure of the outer air, after a high-altitude flight, and to prevent the possibility of the cabin walls being forced inwards by the external pressure, it is advisable to provide also a low-pressure valve. Heating Arrangements As the outer air at great heights has a temperature of from -50 deg. to -60 deg. C, it is necessary to provide heating of the cabin. The simplest way of doing this is to heat* the air. As a rule the temperature rise caused by the compression is sufficient. Sometimes an additional heat regulator is desirable. To keep the heat losses of the cabin ddwn to a minimum, the walls should be provided with a heat insulator, which might be aluminium foil, and the windows should be double. As the heated air enters it first strikes the windows and thus keeps them free from misting and icing. The fundamental features of such a pressure cabin were incorporated with success in the Junkers alti tude aircraft, the Ju.49, built as long ago as 1929. Approaching the Speed oi Sound From what has been said it emerges that speeds of 800 km/hr. (500 m.p.h.) and over are attainable within a foreseeable period. By further increase in speedr however, another limiting factor makes its appearance, which arises from an approach of the air speed to the speed of sound. When this limit is approached, the air flow round the wings is fundamentally changed. This can be most easily 1.0 _P PSes 0.8 1 TOP SURFACE M = 0.6 0.4f SPEED OF SOUND NACA 0015-6+ Fig. 17. Pressure distribution on N.A.C.A. 0015-64 wing section at high speeds. . appreciated by a comparison of the pressure distributions at different speeds. The wing section shown in Fig. 17 has the pressure dis tribution shown in dotted lines at a Mach figure, M, of 0.57. By the Mach figure is meant the ratio of the tunnel test speed to the speed of sound. If the tunnel speed is raised, the pressure distribution becomes that shown in full lines in Fig. 17. The depression which corresponds to the attainment of the speed of sound is particularly marked. This is followed, on the top surface, by consider ably exceeding the speed of sound, until a sudden rise in pressure follows ; this is called the compressibility shock. After the compressibility shock the air flows on at sub sonic speed. The pressure distribution at local super-sonic speeds is thus exactly similar to that found at sub-sonic speeds. Consequently the magnitude and location of the air resultant on the wing section must change considerably. For constant angle of incidence the force of the air on the wing section increases approximately as the square of the speed. Its location remains the same. If, for instance, the air speed of the wing section shown in Fig. 18 is in creased to 700 or 800 km/hr. (435-500 m.p.h.), then the force of the air grows correspondingly without changing its location. If one exceeds 900 km/hr. (560 m.p.h.) the resultant of the ah force moves forward and becomes smaller instead of, as one would expect, greater. Even
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
