(pleasant, lively music) (jet engine roaring) – [Announcer] The Secret of Flight, a new series of programs on aerodynamics For our first program, we present a preview of things to come Your host is Dr. Alexander Lippisch (machine whirring) – We called this series of programs “The Secret of Flight.” And we could have probably called it also the mystery story of the streamline, to be up-to-date And you might wonder what’s all this secret about? Can’t you buy yourself an airliner ticket, drive out to the airport, board the plane, take your seat, fasten your seat belt, and, with roaring engines, up you go After the plane has climbed to the proper attitude, there comes the voice of the captain through the loudspeakers telling you that you are flying at 20,000 feet altitude, 320 miles per hour, and that the weather is fair ahead, and you will have a pleasant flight So, you make yourself comfortable in your seat, read your favorite magazine, slide the seat guard, and probably look out through the window to the wide plains, to the little farms and towns And you might also have a glimpse through your window of the wing and a view of the engine, against some beautiful cloud formations, which you see at the far horizon Well, there’s certainly no secret anymore That this slender wing, and these four powerful engines support an aircraft of more than 100,000 pounds, which connects towns, countries, and even continents It’s the fastest means of transportation What was once a dream of the pioneers of aviation 50 years ago became a reality through a scientific and technical development of unforeseen rapidity But while you are looking out there, something is still mysterious to you Where is a road on which you travel? Where is this element which supports your flight with such invisible power? Where you feel the forces of the air and the aircraft, and the pilot can tell you how these forces react if he changes his controls And you probably could even ask a scientist, aerodynamicist, and he will give you long mathematical explanations of the laws of fluid motion, and how you can figure out these forces, at least, approximately Well, this would not satisfy your search for better understanding If you only could see what happens, if you could make these forces visible, you would at once conceive, basically, what’s going on And it was one of my pet projects to make an instrument where we could visualize basic flow pulses You know, a picture is better than 10,000 words, and if we can make nature to show this flow process visually, then we would have a mean of simple conception, and we would not need all this

complicated mathematical explanation And therefore, I built this model And there is the view field of our test section Now, the white lines which you see there are really streamlines You might think that we have put some threads, white threads in there, but this is not so I can prove it to you When I shut the tunnel off, and the motion slows down, then, at once, you see that this is clearly a smoke pattern And it really gets a beautiful pattern, as you see it now If I start it again, there are our straight streamlines Now, if we have something in the tunnel, some model, we would see the flow pattern which is formed around the model What’s this? That is the streamlines now, don’t, as you see, bounce just against the plate, oh no They go smoothly over it But, at the corner of the plate, they start away, and don’t come back to the original position in this tunnel Oh no, there is behind this plate a large area of dead air And when you would think you would stand there you might feel only the local turbulence and a little flow from the backside But it is especially interesting to see this tangent, this transition while this plate flips up And therefore, we have made a slow motion picture of this action First, we see the normal view field, as you saw it out there And now, the plate comes up And watch this, it forms first the small circular vortex behind the plate, which floats away, and then, the whole large area of dead air in the lee side of the plate begins to form You actually see a little oscillation of the plate itself, because it’s spun up by a spring, which makes some (mumbles) oscillations until it gets into this steady position You see on the border of the dead air and the flow itself, you see what we call a discontinuity line, where the mixture happens between the outer air and the flow in the lee zone of the obstacle You know, to see what’s really on such a flow picture, you have to get acquainted with it, and it might be quite new to you Therefore, we want to show you this flip-up plate once more, in normal speed, as it is here Now, watch it again There we go There you see again this nice pattern of flow lines which go over this obstacle And observe the reverse flow which you have behind the plate It’s really something which you might often experience in nature itself, where the wind suddenly comes from the other side, due to such a reverse flow in a dead air zone Now, we want to go again and show it in slow motion So, straight parallel flow lines, and pretty soon you will see it here how this plate comes up, and how this circular vortex is formed first,

and then floats away, and the large dead air zone behind this obstruction is formed In the boundary between the lee dead air, and the outer flow you have small vortices, which you can see here Which always protrude into the dead air zone and form also such a wavy front between the flow, outer flow and the calm zone between the, behind the obstruction On the other hand, one thing is interesting here, that the flow lines of the undisturbed flow go much higher than the height of the obstruction is So, if you stand behind such a thing, you easy can look over it without having any flow in your face That is the secret of a windshield While we are preparing the tunnel for another test, let me just briefly explain to you how this instrument works Now, you might first ask what is the wind tunnel Well, a wind tunnel is an instrument to measure the forces of air on models, airplane paths And we might even look into the Webster to find the right definition of a wind tunnel Let me just read it to you I think it’s there There it is A wind tunnel: a tunnel-like passage through which air is blown at a known velocity to determine the action of wind pressure on structures, airplanes, et cetera, that are often in the form of models of reduced dimensions Well, Webster certainly knows it Now, this wind tunnel is a special one, and you can see here the top view of this whole installation It consists mainly of three major sections: the intake section, the test section, and the blower section At the intake section, we have a large nozzle, and go into a silencer chamber, where we have fine mesh wire screens in it to break up the large disturbances, which might come from the outer, and also, to damp out any noises which would come from the outer and disturb our flow After we have made through these fine screens, out of the air which comes in, the micro-turbulent air, plug it up and all little fine bubbles We contracted through nozzle, and due to this contraction, we get all the turbulence out and get a complete smooth, steady flow into the test field Then, we blow the air from the test field into the blower section, where we have a little settling chamber, take it out with the blower, and through a smokestack, blow it out into our system We control the models from a control panel And our smoke is introduced at the nozzle, where we have a smoke strut, streamline-shaped strut where very fine tubes come out, and through each tube, we blow the smoke into this accelerated flow You might be interested to see such a smoke generator, which we have here put up for you, so that I can demonstrate it to you This instrument actually was designed by British scientists, and it works in this way We have here an oil container, and through a communicating system,

the oil goes into this tube where we have a heating wire on it Heat it up, evaporates the oil, which then goes into this chamber through a very fine nozzle This jet stream of this oil vapor, which comes out there, is then cooled down by air, fresh air, which is flown through a pressure system into it Due to the sudden cooling and expansion, the oil vapor forms a fine mist, which comes then out here as this smoke, a very fine smoke, which does not plug up your lines Now, let’s have a look around the tunnel Here, you look into the intake, the large nozzle, which goes into the silencer chamber The structure itself is mainly wooden structure Here we have the silencer chamber, where we have the screens in They are put in here and can be exchanged so that we can clean them, because to have clean screens is very important, otherwise our flow would be disturbed From there, the flow, as I told you, goes into this nozzle, which is large converging nozzle of a convergence of one to 20 In front here, we have the control panel Now, these three wheels are the model controls, where we can turn with them the models to give them certain motions of the angle itself, of flaps, and things like this This is a mechanical connection through the back of the tunnel to three concentric tubes, where you attach the model in it at the rear glass wall Here we have the usual switches, the main power, and here are the lights, (clicking) and from here, you control the blower, the speed of the tunnel, and the blower itself And with this little instrument, we can measure how much speed we have through the test section This tunnel we have built at the Collins Aeronautical Research Laboratory, with the help of the Office of Naval Research in Washington Now, we get here to the test section, and let me show how we put a model into it We open up the front window Supported because it’s quite heavy And here we have the model You see, that is a kind of a bayonet attachment, where we put it in (machine whirring) So, and then, close the tunnel again And through these controls, I can now move the model from here up and down And now, let’s turn the smoke on There we have it And there is our flow field, with the plate parallel to the direction of the flow So you’ll see practically no disturbance, except this small turbulent layer, which is caused by the friction of the air on the surface of the plate But now watch when I turn the plate across the flow First of all, it begins to start to break away a little bit And then, when I turn it on more and more, and finally have it at 90 degrees towards the action of the stream, we get that violent oscillating motion, a so-called Karman street Since Professor Theodore von Karman was the first who investigated this phenomenon You might see it even better if I turn the blower down, so that the whole field slows down and we see it in a slower motion

See how these vortices are generated from the tips of the plate, always intermittent See now, the new field fills up with smoke And when we start again, (machine whirring) then we have the tangent picture of this flow, which we actually can observe much better if we have a slow motion picture First, we see a straight wake with straight air, extending from the plate backwards Both sides are the same, but we see at once beginning an oscillation, back there at the end of this wake, like on a flag And this is transmitted slowly against the flow direction towards the plate And finally, this results in an pulsating intermittent formation of wake vortices from the corners of the plate This we call a Karman street, since well-known dynamicist Professor Karman investigated this for the first time and found that only such an intermittent motion and position of these vortices could exist due to stability conditions We will come to this later in one of our other programs Observe here mainly the magnificent pattern of the flow, and the speed, which you have locally in these vortices, which is quite high because you must think that this low-speed picture makes about a second to a minute You might think that this generation of these intermittent vortices is mainly caused by the sharp corners on the flat plate, but unfortunately, that is not so Even a circular cylinder, a rod, would cause such oscillation, and we have put one cylinder into the tunnel, and when we turn the smoke on, then we see that we have the same type of a vortex street behind the circular cylinder So actually, we have to have another form of body to diminish this disturbance behind such a body Now, my assistants have put in a streamlined body in the wind tunnel, and to explain the shape, the evolution of the shape to you, we have probably to go back to this phenomenon of the vortex street behind the body Actually, you can observe this quite often in your normal life For instance, when the wind blows against a wire, this fluctuation excites the wire and makes it to vibrate So that’s what you hear These wire whistling is nothing than such a, as we call it, Karman street behind the wire Or you want to move an oar through the water, or round rod, and you will find that you can’t move it straight It always oscillates due to these vortices which you generate with the oar or with the rod Now, this is certainly a phenomenon which we don’t want to have in airplane design, because it would cause large oscillations, and also lots of drag To get rid of it, what you would do? First of all, you must see that the front part,

on the front part of the body, the flow was nice and smooth, and your trouble started at the rear Therefore, you have to do something at rear, not make the front pointed Oh no, let it be rounded, but fill out this rear part with a body which is slightly convergent, so that this vortex generation can never start Here you see a typical streamline shape And you see no vortex generation anymore A small disturbance back there, this little large part where they air gets turbulent Because we have friction on the walls of the body, which necessarily retards the airflow and makes small disturbances which cause here our smoke lines to blow out Now, if we turn the body, we can also see it when we have not just a straight flow against it, but the flow comes now at a certain oblique angle You see that then, the disturbance gets somewhat larger All these things I will explain to you in many of the later lectures, and I wanted to show you today mainly that such a device where we can make the flow visual, where we can see what happens is a very helpful tool in our aeronomic research As we see it all, when you just want to make such flow phenomena understanding to other people In the next lecture, we will show you some of these interesting pictures in our smoke tunnel And we will explain to you how to analyze from what you see, what kind of forces and pressures are excited on these different models I hope that you will be interested to see our programs again (calming orchestral music) – [Announcer] You have just seen Preview, the first in a series of programs explaining the secret of flight Your host has been Dr. Alexander Lippisch, Director of the Aeronautical Research Laboratories of the Collins Radio Company This program was produced for the educational television and radio center by the State University of Iowa (jet engine roaring)

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