The Secret of Flight 2: Laws of Fluid Motion

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(instrumental music) – [Narrator] The Secret Of Flight. A series of programs on aerodynamics. Program 2: The Laws Of Fluid Motion. Your host is Dr. Alexander Lippisch. – In our last program, we have seen several pro pictures in the smoke tunnel and I have explained to you how this tunnel works. Today, I want to show you how we can determine the forces which act on a model in a tunnel at different velocities which are there. Well, the forces which are produced by a flow over a body apparently are forces of the air pressure against such a body. For instance, when an aircraft flies, the lift is produced by pressure forces on the surface of the wings. And this keeps the airplane flying. Have you ever seen an airplane take off? Well, to show you this, time when the lift begins to go, we have made a picture of a light plane taking off on the runway of an airport. And there it is. Let’s look at it. First, we see the plane rolling on the ground. Then it gains speed and then the pressure forces are so large that they lift the plane from the ground and it flies, supported by the air pressures in the same way as, for instance, a weight would be supported which is here lying on the table by the pressure from the material of the table to the weight. It’s exactly the same thing. Now, how can we do and see it in this smoke tunnel? We see there only a section of the floor. We see only a cross section. And to explain it to
you more specifically, I have here a model of a light plane as we just saw him in our movie picture. And when I now take this wing off and show you the side view of it, then you might be able to compare it with what we have here as a model in the wind tunnel. There we have part of the wing and we have outlined the fuselage with a thin piece of plexiglass on the rear wall of the tunnel. If I now turn the flow on, (faint buzzing) and we have our smoke generator going, then we can see the flow over this model. Especially over its wing. First as it took off it was flat and then the pilot turned it over and we got small lift and it lifted from the ground. Now, apparently we cannot see any pressures. So we can only see the scales, for instance, on a pressure measuring instrument. And in flow where we have quite low
pressure differentials, we use what we call a monometer. A fine pressure gauge
which works this way. We have a huge tube, and this huge tube is
filled with colored water to a certain level. If we now blow into
one tube, for instance, through this leather hose, like this, then you see that the pressure which I exerted here through this hose on the water level is to be read by the difference between these two water levels. Here for instance, let’s see, two and a half inches of water. Now, (laughs) what is two and a half inches of water? Well, we have a wonderful, simple, English system of units and then when we figure this all out, which takes us quite a while, then we find out that one inch of water equals 5.2 pounds per square foot. Now, what is pounds per square foot? You probably are not familiar with this type of unit. You usually measure pressures in PSI as you measure that means in pounds per square inch as you measure it, for instance, on the tires of your automobile where you have about 20 PSI hindrance. And you measure the same PSI’s on a pressure tank which you probably have in your basement for anything. So what is a PSI when we think of inches of water? Then we find out that one PSI equals twenty-seven point seven inches of water. So that means when we have here one inch of water, or sometimes even less in such pressures which we measure in a flow, then this is extremely small. Let me show how we, for instance, can measure how much flow we have in this blow of a vacuum cleaner. If I put, for instance, this hose direct against it, you see then that this blast gives us about, oh, not quite an inch of water. And with some tricky formulas we can figure out how much velocities this is. I don’t want to bore you with this. But you must think that this small pressures don’t amount to anything when we think of the pressure which we have, for instance, in our drawer tires, where we have compression of the air. So if we have a few inches of water, there is practically no compression or expansion of the air and therefor the
scientist treats such flow as we have it in normal wind tunnels or as we have it up to 300 mph flight speed as the flow of an incompressible fluid. Now I want to explain to you the correlation between the velocity and the pressure on the surface where we blow the flow over it. There’s one important thing to understand. That is that when the velocity at a certain point of our model is larger than the average velocity, the pressure on the surface of this body is lower than the average pressure and if the velocity is smaller than the average velocity, our pressure is higher
than the average pressure in the whole fluid. That means we have to know all the time that if velocity increases, pressure decreases and if velocity decreases, pressure increases. Very important to know. And to show you this, to explain it as good as we can do it in this tunnel, we have put here a so-called multi-monometer that means a multi pressure gauge on a certain model in our smoke tunnel. Now, let’s have light so you can see the model and if I now blow through the tunnel, then you see that this monometer reads certain pressures along the surface of this body. How we did it? Now, we have fine holes on the surface of the body. These fine holes are connected through rubber hoses through the model here and out there back there on the tubes
of this multi-monometer. And the other ones are all connected with the average pressure which we read in front of this test field where our flow goes in with its mean velocity. We see now, since this is converted, the velocity must increase and since the velocity increases, our pressure must decrease that means we must have kind of suction on the surface. The air must suck away some fluid out of this line and this makes the
water column go up here. So if the water column goes up, we have suction and where we have the highest velocity we must have the highest suction or the lowest pressure that would be on such point like here. Now let’s see. This point corresponds to this line and you see at once this is the highest water column which we read here. We have the highest velocity through this cross section. Now if we turn the smoke on and I will let our smoke generator go, then we can see this flow field and we see that when
the velocity increases, our distance between the flow lines gets smaller and smaller. So, if flow line are very narrow together, we have a high velocity and we have a low pressure. How can we see this velocity business? Now, we make a trick and that is we pulsate the flow. That means we let pulses into the smoke system and so our smoke comes out in front little puffs. But we can see it even better if we observe it through an optical system that means we take our high speed movie camera and make a slow motion picture of this pulsating flow process and then we see at once that our velocity in this region is much higher than here. Let’s just look at this film. There we are. You see a very interesting thing here. That the velocity increases first in the center of the smoke front and then goes with even velocity through the narrowest part of this channel. We call such a line which goes through there a timeline because actually this is indicates the particles of the air which come all at the same time into the flow and travels through there and the different shape of this line shows us the time history of the flow. Now, to see another pressure distribution as we call it, that means the distribution
of the pressure over the surface, we have taken out of the tunnel one side of this nozzle and then we have the flow over a hillside or over a hump and we can see then how the pressure records now. There we are. I turned the flow on and you see, when we takes this out and we have more space of velocity doesn’t go as high as it went when we went through this narrow nozzle. Therefor the pressure doesn’t read as high as it did read before. And we see one other thing. We see that the highest point is now this one. That means here. It reads here the highest velocity and the velocity here as it was before the highest is now smaller and declines since the whole flow begins to slow down to the original cross section which we have at the rear of the tunnel. Now, if we turn the smoke on then we can see the flow field itself. Here it is. And we see at once that in this region, we have the highest velocity, because there our flow lines are the most pushed together. Here down there we have even a little bit pressure. We see that because this one records a little lower than the average pressure. Now, if we do the same thing again with pulsating smoke, then we can see also the velocity field. You see, when we have normal velocity, that goes too fast, so I have to cut the speed out and slow the tunnel down then we can see it by eye. But again, better we see it in a slow motion picture which we have here. There it is. There we see that the velocity at the lower part of the tunnel decreases considerably before it hits the bump and then it suddenly accelerated, but the whole front bends over. Now, we have another little trick. Because we want to demonstrate to you the flow and the pressure distribution due to this flow over a wing section or type of a wing section now we add this hump when you think the upper surface of this hump had almost the same shape as the upper surface of a wing. Slightly rounded. So we take our hump and lift it up into the center of the view field. And then we have a large wing section like model in the tunnel and we can see now the pressure recordings
on the upper surface of this wing section. When I turn the blower on, then you see here in this multi-monometer or pressure gauge, you see that we have again, suction. Lower pressure on the upper surface due to the higher velocity which we have up here. Interesting to see first of all the first one, which is this one record a little lower than the average. That means here we have actually some pressure on the surface. From there on, the speed increases and the pressure goes down and we have suction and this suction on the surface sucks our waterlines up and this shows the pressure distribution over this surface of a wing section. If we now have smoke lines, then we can see the flow picture. And you see very clear that the velocity here is much higher than there. See how narrow the lines are here and how wide they are in this range? On the other hand you see that here, the flow somewhere goes to a point on the surface and this we call the stagnation point where the air flow stagnates. It has stopped completely and would record if you would have hear pressure the stagnation pressure, which is a higher pressure than the average pressure of the flow. Now if we take again our method of pulsating the smoke, then we can see this velocity distribution and to make it visible to you in this flow field, I stop the blower so that the tunnel slows down and we see a little bit at least this smoke fronts going over the wing section. Better we see it in the slow motion picture which we have here. And there we see very clearly that the velocity on the upper surface of the wing section is higher than below because we have a very distinct shift of the two parts of the smoke front which goes over the wing section. This shift of the timeline of the smoke front indicates lift on the wing section and by some tricky
mathematical calculation I didn’t want to explain to you here because you would sleep with it, you can calculate the lift you have on such a section from the shift which you see here. Now, you have seen this space shift of the big wing section in our tunnel. And some of you might say all right, that’s due to the fact that this thing is so large and the tunnel is quite small and therefor we have blocked some air here above and the velocity must be higher and you have cheated me. And I will tell you I
did not cheat you at all. And I can prove it. Because we put now a small model in this tunnel. Like this here. And when we do the same thing, we pulsate the smoke again you will see that again, the upper flow is much faster and behind the wing section, still far away behind it you can see very clearly, that you have this phase
shift in the timelines. Now let’s first look at this thing with the normal smoke on. There is the flow and now we get the smoke generator going and there is our flow picture. You see this as an angle of attack almost zero. We measure it from the lower surface and what you see here also very interesting is a little stagnation point this is a stagnation point of this section in this angle of attack range. And if we pulsate the smoke now then you see this velocity field. Well, you can’t see much here. But if we look at the high speed movie or the slow motion picture, are, as the smoke runs here, then we might detect this shift. Let’s look at this here. There we have the same condition as we have put it here in the slow motion picture and you see again very clearly this shift in the timelines, the same way as we saw it on the bend, big wing section. Now, if we see this with a higher angle of attack, then the lift is larger and therefor our shift in the timelines must also be larger. Now let’s go back to our smoke lines in the smoke tunnel. We can see other things too, especially if I change the angle of attack, see, when I change the angle of attack then you see that the lines on, down here, get wider and wider as we get more pressure underneath and the lines up here get narrower together so we have more speed especially in this region, we increase the lift. Another thing we see is that behind this section we get a larger area of turbulence of air which doesn’t have any more the main velocity of the flow because it’s blocked off and also slowed down by the flow on the surface. If we go much higher then this blocking is so strong that the whole thing stalls away as you see it here. And then we have no lift. A very difficult condition and very dangerous for somebody who wants to fly with an airplane. You have to learn that if you have too much angle of attack you’re flow stalls as we say it and breaks away from the upper surface where we have most lift and your airplane falls down and you have severe accidents if you stall an aircraft. Now I think we have understood how we can analyze such a flow picture and we got a little idea why an airplane flies. In our next program we want to give you a little story of the past. You see, this flow and basic law of fluid motion was well known for quite a while before any activity where went on to try solve the problem of the flying machine. And the first, I think, whoever at least made a model of a flying machine was a Frenchman named Alphonse Penaud He made a small model of an airplane with a propeller on it and had a rubber motor on it and flew it in 1870 before some of the members of the Academy of Science of France. And as we saw in an institute in Washington, there is a replica of this model and I have here other one which I can fly. Unfortunately, this area which we have here is a little bit too small to do this and so we made a little film in the outside where I can show you how this Penaud model, which has originated in 1870, that means it is now 76 years old, how it flies. From there we will go through the up and downs of the first development
of the flying machine and we will see how the invention finally was achieved
by the brother’s Wright of this country. (instrumental music)

 

One Response

  1. Mani shankar Yadav

    August 25, 2019 3:05 pm

    Please add mathematical treatment to the above phenomena.
    Please upload necessary videos with crystal clear explanation of the governing equations namely mass, momentum & energy equations.

    Thank you very much for preserving such videos on aerodynamics. I really enjoyed it.

    Reply

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