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AIRPLANE FLIGHT ANALOGY
1997 William Beaty

Two rows of stacked disks approach from the left, then at one spot they suddenly move downards together while counterrotating

The controversy about wings and the lifting force has a definite origin. It arises because two-dimensional airfoils are used to teach us about pressure and flow patterns... and then we apply those concepts to 3D wings.

This is a major mistake. The behavior of 3D wings is fundamentally different than the behavior of 2D airfoils. Going from 2D to 3D is not a trivial change.

In our 3D world, an airplane produces a downstream wake with net downwash. It injects momentum into the nearby air, and this air begins moving downward. But in the 2D world of a wind-tunnel there is no such downward-moving wake behind the wing. Instead, the 2D upwash must always be equal to the 2D downwash, and the wing does not push upon the air. Instead, all the momentum is injected directly into the distant floor/ceiling. Even more important, 3D wings have finite dimensions, while 2D wings act as if they have infinite span. An infinite wingspan gives some strange results; results never produced by finite 3D wings. For example, if a 2D infinite wing should ever deflect even a tiny portion of the oncoming air downwards (deflecting a streamline,) it would deflect an infinite amount of air and produce an infinite lifting force. As a result, a 2D infinite airfoil does an odd thing: it applies a finite force to an infinite mass of air. In response a net amount of air does NOT move downwards (since it has infinite mass.) The incoming and outgoing streamlines remain horizontal. The wing acts like a reaction engine, but an engine where the "exhaust gasses" have zero velocity and infinite mass. This strange effect only applies to 2D airfoils and infinitely long wings, and is never seen with 3D airplanes flying through 3D air.

The controversy about "Bernoulli versus Newton" is probably a controversy about two-D wings versus three-D. It's a fight between the vortex- shedding flight of real aircraft, versus the instant-forces of ground-effect flight seen in airfoil diagrams and in 2D wind tunnels. It's a controversy over the physics of short 3D wings in a 3D world, versus flight in strange two-dimensional "flatland" worlds.

I could attempt to explain the problem in words, but words are easily misunderstood (especially on hot-button issues where emotions run high.) A visual analogy works much better. Below is my explanation for how a three-dimensional airplane flies through 3D air. It is very different than the typical 2D explanations found in most textboooks. My "circulation" is flipped ninety degrees!

Imagine a huge, disk-shaped helium balloon floating in the air. The disk stands on edge. It is weighted for neutral buoyancy so it neither rises nor sinks. A small platform sticks out of its rim. (If you feel the need, you can imagine a counterweight on the opposite rim to the platform, so the balloon hovers without rotating.) See fig. 1 below

            _____
         _--     --_
        /           \    fig. 1  DISK-BALLOON WITH
     __|      .      |           A SMALL PLATFORM
       |             |
        \_         _/
          --_____--

Now suppose I were to leap from the top of a ladder and onto the balloon's small platform. The balloon would move downwards. It would also rotate rapidly counterclockwise, and I would be dumped off.

Next, suppose we have TWO giant disk-shaped balloons stacked adjacent to each other like pancakes standing on edge.

           ____
        _-- _____
      /  _--     --_
   __|  /           \    fig. 2 TWO DISK-BALLOONS,
     __|      .      |          STACKED ADJACENTLY
       |             |
        \_         _/
          --_____--

They do not touch each other. Both have platforms. If I jump onto the first platform, but then I immediately leap onto the next platform, I can stay up there for a tiny bit longer.

Next, suppose we have a row of these disk-balloons one KM long. It looks like fig. 2 above, but with hundreds of hovering balloons. Now I can run from platform to platform, and I will stay aloft until I run out of balloons. Behind me I leave a trail of rotating, downward-moving balloons. I can remain suspended against gravity because I am flinging mass downwards. The mass takes the form of helium mass trapped inside the balloons. I am also doing much more work than necessary, since the energy I expend in rotating the balloons does not contribute to my fight against gravity. (In truth, all my work is really not necessary, I could simply walk along the Earth's surface with no need to move any massive gasbags!)

To make the situation more symmetrical, let me add a second row of platform-bearing balloons in parallel to the first row:

            _____                 _____
         _--     --_           _--     --_     fig. 3
        /           \         /           \    END VIEW OF TWO LONG
       |      .      |__   __|      .      |   ROWS OF DISK-BALLOONS
       |             |       |             |
        \_         _/         \_         _/
          --_____--             --_____--


There's one platform for each of my feet. I can run forwards, leaving a trail of "wake turbulence" behind me. The "wake" is composed of rotating, descending balloons. Fig. 4 below shows an animated GIF of this process.

[GIF animation: descending balloons]
Fig. 4 Forcing the balloons downwards

Also see: Smoke Ring animation

"DISK BALLOONS" BEHIND AIRPLANES

A 3D aircraft does much the same thing as me and my balloons: it remains aloft by shedding vortices: by throwing down a spinning region of mass. This mass consists of two long, thin, vortex-threads and the tubular regions of air which are constrained to circulate around them. The balloons crudely represent the separatrix of a vortex-pair: the cylindrical parcels of air which must move with closed streamlines.

So, how do airplanes fly? Real aircraft shed vortices. They inject momentum into air which as a result moves downwards. They employ "invisible disk-balloons" to stay aloft. The two rows of invisible balloons together form a single, very long, downwards-moving cylinder of air. This single cylinder has significant mass and carries a large momentum downwards. Airplane flight is vortex-shedding flight.


         _____                 _____
      _--     --_           _--     --_     fig. 5
     /           \    |    /           \    FRONT VIEW OF AIRCRAFT, W/
    |      ___    |   |   |    ___      |   AIR MASSES ROTATED BY THE
    |         ---____/ \____---         |   WINGS' PRESSURE DIFFERENCE
     \_         _/   \_/   \_         _/ 
       --_____--             --_____--



               \      |      /
                 \    |    /
      ______      |   |   |     ______      fig. 6
     /  ___  \    |   |   |    /  ___  \    THE CROSS-SECTION OF AN
   /   /   \   \   |  |  |   /   /   \   \  ACTUAL WAKE HAS STREAMLINES
   |  |  o  |  |   |  |  |   |  |  o  |  |  WHICH DO NOT PERFECTLY
   \   \___/   /  |   |   |  \   \___/   /  RESEMBLE A PAIR OF ROTATING
     \_______/    |   |   |    \_______/    BALLOONS.  THE BALLOONS ARE
                 /    |    \                A CRUDE ANALOGY.
               /      |      \


view behind Learjet flying just above fogbank: miles long downwash vortex-pair punches a clear slot
The downward-moving "reaction exhaust" below a wing made visible
(See Hyperphysics and other photos.)


Scanned laser smoke downwash

FLY FASTER FOR LESS DRAG

My forward speed makes a difference in how much work I perform. If I walk slowly along my rows of balloons, each platform sinks downwards significantly. I must always leap upwards to the next platform, and each balloon is thrown violently downward as I leap. I tire quickly. On the other hand, if I run very fast, my feet touch each platform briefly, the balloons barely move, and the situation resembles my running along the solid ground.

Similarly, if a real aircraft flies slowly, it must fling the vortex-pairs violently downward. It performs extra work and experiences a very large "induced drag." If it flies fast, it spreads out the necessary momentum-changes, and therefore it needs only to barely touch each parcel of mass (each "balloon.") Hence, faster flight is desirable because it requires far less work to be performed in moving the air downwards. And if a slow-flying, heavily-loaded aircraft should fly very low over you, its powerful wake vortices will blow you over and put dust in your eyes.

All of my reasoning implies that modern aircraft actually remain aloft by vortex-shedding; by launching "smoke rings" downwards. Imagine one of the flying cars in the old 'Jetsons' cartoon, the ones with those little white rings shooting down out of the underside. But rather than launching a great number of individual rings, modern aircraft throw just one very long ring downwards, and they are lifted by the upward reaction force.


A CRUDE PREDICTION

How well does the "disk balloons" model correspond to the real world? Well, we can pull an equation out of the motions of the balloons, and use it to predict both aircraft energy-use and induced drag. If the equation is at all similar to the actual aerodynamics of a real-world airplane, then the "disk balloons" are a useful model. If the equation is faulty, then the model only has weak ties with reality.

Suppose the "disk-balloons" contain air which rotates as a solid object, (or imagine radial membranes in the balloons.) If I add together the work done in creating the circulatory flow, plus the work done in projecting the constrained air downwards, I arrive at a predicted aircraft power expenditure of:

Power = 8 * (M * g)^2 / [ pi * span^2 * V * density ]
M * g being aircraft weight, V is velocity of horizontal flight, and "density" is the density of air. Induced drag should then be power/V:
Induced Drag = 8 * (M * g)^2 / [ pi * span^2 * V^2 * density ]
What happens if I assume that the air within the disk-balloons is not "solid", but instead it is made to whirl faster near the center of the balloon, such that the tangential velocity of the air is constant regardless of its distance from the center of the balloon? (Imagine a wing which produces a downward velocity of net downwash which is constant at each point along the whole span of the wing.) If the "downwash" is constant across the wingspan, then the modified "balloon equation" predicts a power expenditure of 2x that above.

How does this match reality? I'm looking for information on this at the moment. I'm told that these two equations are identical to the equations of real aircraft, except that the number "8" is replaced by a factor which is dependent upon the particular geometry of the wing. Pretty good for an "amateur aerodynamicist", eh?

One final note. The downwash vortices of real airplanes contains rapidly rotating air. This represents wasted energy, since only the "shell" of each "balloon" needs to rotate as the region of air moves downwards. Is there a wing which can produce a downwash vortex-pair without any spinning cores? Maybe it would use less fuel than modern wings.


(See also J. Denker's critique and my response, 8/99)


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