And Hypersonic Flight

in Germany at that time. It is not a point, as Millikan says,of nationality.Let's look at some of the important developments of thepostwar period. First, there is the so-called sound barrier(Figure 3). I object to the word "barrier" because it impliesprecisely a kind of collisional approach. It has nothing todo with barrier; there is no barrier, there is nothing there.There is just air, like anywhere else. The point is, that if youget near the speed of sound to about 0.7 Mach, then underthose circumstances the drag coefficient on the airfoil increasesvery steeply, exponentially, until you in fact reachthe speed of sound.The reason for this is that through the development ofshock waves, which affect the airflow over the airfoil, acertain amount of the lift energy is converted into shockformation. That energy is taken away from the lift capabilityof the plane, and under those circumstances you experiencevarious kinds of instabilities and difficulties with theplane itself, which have to be countered simply from thestandpoint of understanding the problem. You have to makethe kind of geometrical adjustments in wing design, or anythingelse, that are necessary to do that.One of the principal adjustments in wing design that canbe made was invented by Busemann, the so-called sweptwingdesign, the arrow design. You can see (Figure 4) howthe critical zone for the development of shock waves thatinfluence flight and lift negatively is at the 0° angle; that is,if the wing is at right angles with the fuselage, you get theonset of the critical area at 0.7 Mach and then the dragcoefficient declines afterwards.If you have a 60° angle of the wings, then not even halfthe drag coefficient develops and you get it also at a muchlater point; namely, beyond Mach 1. And if you have a 70°inclination with the fuselage of the wings, then you get to apoint where you get a very low, very late onset of the criticalphase. Also, the amount of reduction in lift or the amountof increase in the drag coefficient is not very substantial. Itis there, it will always be there, because Shockwaves form.Shock waves are real, as was certainly determined bythese methods of research in aerodynamics carried out inthe 1930s and 1940s in Germany primarily under Busemann'sdirection in Braunschweig. They are not what Rayleighhad said, when he criticized Riemann's 1859 paper.He said, shock waves do not exist. What exist, are singularitiesin the mathematical formulation of the wave equations,but we cannot assign any reality to such singularities.All it means, is, that we have failed to come up with a solution.As Riemann said, these things are real, and he said it 50years before Rayleigh made that idiotic criticism. It wasprecisely because of the realization that the shock wavesare real that they were taken into account when supersonicFigure 3THE SO-CALLED SOUND BARRIERThe sound barrier has nothing to dowith a barrier. When a plane gets nearthe speed of sound to about 0.7 Mach,the drag coefficient on the airfoil increasesvery steeply because shockwaves develop that affect airflow overthe airfoil.Figure 4SHOCK WAVES THAT INFLUENCEFLIGHTThe critical zone for the developmentof shock waves that negatively influenceflight and lift is at the 0° angle. Ifthe wing of the plane is at right angleswith the fuselage, you get the onset ofthe critical area at 0.7 Mach. But if thewings are at a 60° angle, then not evenhalf the drag coefficient develops. Witha 70° inclination, there is a very low,very late onset of the critical phase.Figure 5THE D-558 DESIGN BEFOREAND AFTER BUSEMANNThe Douglas D-558, which wasdeveloped simultaneously withthe Bell X-7 as a supersonic designin 1945, had a conventionalstraight wing (a). After von Karmanand others visited Germanyand interviewed Adolf Busemann,their design was modifiedto his swept-wing design (b).48 January-February 1986 FUSION