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I am curious about a method to enhance the efficiency of passenger aircraft during cruise speeds by integrating treadmill-like structures on the fuselage and beneath the wings without changing the original shape of the aircraft and the airfoil.

These structures, designed to move with air friction without altering the traditional airfoil shape, aim to decrease drag and increase lift. The idea is that as the structures move with air friction, the relative speed between the surfaces and the air decreases, leading to reduced drag induced by the friction.

Placing these structures under the wings could potentially augment lift by increasing pressure due to the decreased relative speed.

Additionally, even these structures do not create any lift or reduce the drag, we can produce kinetic energy with these moving structures, which otherwise wasted in the form of heat energy due to friction between the air stream and the wing surfaces.

I seek insights from the Engineering Stack Exchange community on the feasibility, potential benefits, challenges, mistakes associated with this concept and the reasons why this method will not work, emphasizing its potential to improve fuel efficiency without requiring significant alterations to current aircraft design.

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    $\begingroup$ that's a really interesting idea $\endgroup$
    – jsotola
    Nov 14, 2023 at 18:38
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    $\begingroup$ The boundary layer already doesn't move. If you stick anything unnecessary beyond that so that it does move with the air...well...more drag. $\endgroup$
    – DKNguyen
    Nov 14, 2023 at 19:47
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    $\begingroup$ Its a thing. Its called Moving Surface Boundary Layer Control. Drag reduction is a usually a secondary concern, it is mostly used to fiddle with other lifting body performance issues. Several hundred vehicles have been built experimenting with the idea, going back well before airplanes. There are a handful of commercial applications. $\endgroup$
    – Phil Sweet
    Nov 15, 2023 at 1:04
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    $\begingroup$ "Placing these structures under the wings could potentially augment lift by increasing pressure due to the decreased relative speed." – Why would decreasing the relative speed result in increased pressure? (Note that Bernoulli's principle does not lead to that conclusion in any obvious way.) $\endgroup$ Nov 15, 2023 at 4:52
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    $\begingroup$ Your treadmills also have friction...plus if you want to extract energy from a treadmill the treadmill friction will be higher...you basically want to run a dynamo by blowing at the wheel $\endgroup$ Nov 15, 2023 at 11:01

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You can't reduce friction with machines.

The nature of machines is that everything you do adds inefficiency. A moveable wing surface adds weight, complexity, and additional losses. Even if you did it for zero extra weight, you can't do it with no losses. You're using air to power a non-functional moving surface. You also cannot make energy out of taking energy from a vehicle's movement, because the energy you are trying to remove had to come from the engines in the first place, and each conversion has losses.

Any mechanical engineer will look at this as a cavalcade of losses and non-reversible processes.

In response to comments, let's be clear - there are machines that are used to reduce friction, like bearings, as several commenters have noted. But we aren't going to be able to add machines onto something working to try to scavenge oddball savings. This is the classic "can I add a windmill to my car to capture wind energy." No, you can't. For an airplane, its flight is based on the friction and pressures and momentum changes of the air it passes through. Trying to capture energy from the air will reduce the energy you can put into the air to keep you aloft. That doesn't mean that it wouldn't be possible to do something like use a Peltier device to grab heat from the skin of the aircraft. The problem is that such devices will have a high bar to clear in any vehicle in terms of weight and cost, and that bar is very high (no pun intended) for aircraft, for which weight is a primary design consideration.

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    $\begingroup$ I understand your point which I believed to be true in the case of efficiency, but I think the energy the machine will convert actually will come from the heat loss because of the friction. In other words, this machine will prevent the energy loss as heat due to friction between the wing and the air and convert this part of fuel energy to kinetic energy. But this is also an idea which may be insufficient, I just wanted to explain what is in my mind, I look forward to your reply. $\endgroup$
    – Siames
    Nov 14, 2023 at 21:18
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    $\begingroup$ I don't think this is true in general. Assume a jet-powered vehicle scraping along the ground -- adding wheels would certainly improve the efficiency. $\endgroup$
    – Toffomat
    Nov 15, 2023 at 8:18
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    $\begingroup$ While I agree with the body of your answer, that title seems very misleading. I mean, a bearing is a simple machine that's primary purpose is reducing friction. $\endgroup$
    – DBS
    Nov 15, 2023 at 11:21
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    $\begingroup$ A thermoelectric generator can turn heat generated by friction into usable electrical energy. If you already have frictional losses, you can recover some of that energy to reduce further losses. I'm imagining an apparatus that drags along the ground but uses a TEG to recover some of the energy lost, which is used to compress a spring and leap forward periodically. This machine can move further for some fixed energy cost; it loses less total energy to friction. I agree you need to make the process very efficient, but it is not theoretically impossible as this answer implies. $\endgroup$ Nov 15, 2023 at 14:09
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    $\begingroup$ This is just nonsense. Of course you can reduce friction with machines, actively (power-driven) and passively (bearings etc.). It is done all the time and has nothing to do with the "windmill on a car" fallacy (but everything with ball bearings on the wheels of a car). Magnetic levitation is one spectacular way to actively reduce friction; in principle you don't need much energy for it (no mechanical work is done through the levitation, and you could use super conductors to reduce energy losses). $\endgroup$ Nov 21, 2023 at 6:59
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Can you reduce friction by making a surface frictionless? Yes.

Will that reduce drag? Most of the drag comes from lift. Without lift, the airplane falls out of the sky. Most of the rest of the drag comes from turbulence. Only a tiny amount of drag comes from surface friction.

But can we reduce that small amount of friction drag? We need to include a small amount of friction drag to get the air to flow cleanly and smoothly. We don't want less friction drag than we have now.

My car doesn't need lift. Can I put treadmills on it? If you could find something more slippery than air, but just as dense, you could blow a slippery boundary layer along the surface of your car. Fire crews put soap in their water to make it more slippery. Treadmills are more slippery than running tracks-- you can run in place. Unfortunately, treadmills are not more slippery than air.

But could I produce energy from treadmills? Airplanes have frequently had windmills running generators for one reason or another, including sometimes for use as an independent power source. But it doesn't "produce" energy. It's just another way of using up energy and diverting some of it to another use.

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    $\begingroup$ The contribution of induced drag decreases as you increase speed, so jetliners and Cessnas will have dramatically different breakdowns for skin friction/form/induced drag. $\endgroup$
    – costrom
    Nov 16, 2023 at 14:40
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    $\begingroup$ Treadmills are more slippery than running tracks-- you can run in place - on my treadmill, I can run in place because it has a 2kw motor to overcome the friction $\endgroup$
    – Haukinger
    Nov 17, 2023 at 12:49
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    $\begingroup$ @Haukinger Or the new curved unmotorized treadmills where gravity drags you down and the belt down. $\endgroup$
    – DKNguyen
    Nov 17, 2023 at 16:48
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Let's be honest here...

From an engineering point of view, this is totally impractical and could never work with anything like current technology.

Consider that the surface of an aircraft is not a perfect rectangle, so you would have to cover a significant portion of your aeroplane's surface with dozens or hundreds of tiny treadmills to fit its shape. The belts have to be flexible, so they have no structural function, but strong enough to not flap about, so they would be heavy. They would have to be supported by rollers which would need to be strong enough to withstand the considerable tension needed. All this adds a lot of weight. There would be gaps between adjacent belts that would add air resistance. The ends of the belts would have to somehow be made flush with the other parts of the surface. There would be a gap - more air resistance. The belt material would probably have higher friction than a painted or bare metal surface. If you are going to generate power, you would need generators which add weight.

The treadmills would be mounted on top of the structural surface, so the total surface area of the aircraft would be much greater, increasing friction, not reducing it.

The belt material would have to withstand mechanical wear, extreme weather and UV light as well as an alloy or painted surface. The whole setup would need to be maintained, worn belts replaced, etc.

There is not a hope this would work.

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Air on a wing or aileron or any airfoil does not slide on the surface of these, a very thin layer of air (boundary layer's contact surface) sticks to the surface and moves at the same speed as the wing or airfoil. Checking a cross-section of the wing one will see the relative speed of air to the wing surface is zero next to the wing.

That's why you can not dry clean your car by driving it fast down a freeway. Because the wind doesn't slide over the body of the car. It slides over a very thin layer of air moving with the car.

In the diagram, we see the transition of speed from zero to the wind speed. The boundary layer behaves as a viscous media and the layer outside behaves inviscid. source Aviation stack

boundary layer

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    $\begingroup$ but friction is still transferred to the skin via friction between air molecules $\endgroup$
    – Tiger Guy
    Nov 14, 2023 at 20:17
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    $\begingroup$ If this skin itself was a treadmill / belt, the layer of air "stuck" to it would also be moving, so the total speed difference would be smaller across the gradient between skin layer and free stream. $\endgroup$ Nov 15, 2023 at 21:40
  • $\begingroup$ Hm-hm. Of course you can dry-clean (parts of) your car by driving. Old-school photographers used to wave their wet copies around to air-dry them. Blowing on a wet hand or moving it through the air (it's the relative motion that counts) clearly cools it, indicating an increased rate of evaporation. $\endgroup$ Nov 21, 2023 at 6:47
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Unless the treadmills are independently powered, which they aren't according to your description, then they will only move if there is friction with the airflow to make them move. If we pretend there is no boundary layer here, the air flows over the treadmills, and speeds the treadmills up, but in doing so the air is slowed down. So is the relative airspeed between the aerofoil and the airflow actually less?

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  • $\begingroup$ "So is the relative airspeed between the aerofoil and the airflow actually less?" Yes. $\endgroup$ Nov 21, 2023 at 6:49
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Treadmill-like belts (with or without little "shovels") are problematic as a design. Part of it is weight, as rghome explained. Another problem is that the belt must return somehow. For a wing, this return path introduces a slit or two (for the belt on the underside) on the leading edge, which must be aerodynamically bad.

I wondered whether one could have a smart stationary surface which reduces drag. For example, golf balls have indentations which make them fly farther. They produce a less laminar flow which, paradoxically, reduces overall drag. But this is because the ball is a sphere, not a wing. The laminar flow detaches from a smooth sphere very early, leading to a large turbulent wake which causes a lot of pressure drag. The indentations cause more frictional drag but also let the flow curve around the ball farther, thus reducing pressure drag. This effect overcompensates the additional friction.

Wings have comparatively little pressure drag to begin with; their shape is optimized to let the laminar flow "stick" to them all the way. Therefore, a "golf ball surface" on a wing would mostly just increase frictional drag.

Another idea I had was covering the surface with small balls which are 90% or so embedded in the wing. Their upper parts would be exposed to the passing air so that the balls would start to spin, turning the wing surface into a kind of ball bearing. That might reduce the speed gradient between the surface (which now moves with the passing air) and the air, hence reducing friction.

The main disadvantages are surely the constructive problems and the increased weight; I'm also afraid that the net effect might be negative, because the bumpy surface increases turbulence. If we want to make the surface less bumpy, we have to embed the balls further, but then most of the wing is not balls, which reduces the "ball-bearing" effect.

But I still have a feeling that there may lurk an ingenious idea there somewhere regarding "smart" surfaces (e.g., google "compliant walls" and dolphins), and I didn't like the dismissive attitude of some answers here.

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