Back-spinning conveyors instead of wings?

[WCOLtd]

I had an idea I thought I would try out - the idea is based on the concept of backspin like on a tennis ball applied to create lift on an aircraft.

Instead of having wings, have a pair of back-spinning high-speed conveyor belts (the bottom of the conveyor belt goes in the direction the plane is traveling) - going against the flow air and creating high pressure beneath the craft. and the upper part goes with the flow of air creating low pressure above.

I realize drag might be a problem depending on the viscosity between the treadmill and the air, but I want to know; would such a machine lift off the ground?

 

[Cyrus]

Not only would this not work, it doesn't make any sense. Having a treadmill doesn't create a positive pressure on the lower surface of your imaginary wing. All it does is induce shear flow.

 

[WCOLtd]

I figured as much. a flawed idea :(

 

[Cyrus]

There's no harm in trying to think outside the box, just make sure you validate your reasoning with the fundamental principles of fluid dynamics.

 

[russ_watters]

It certainly wouldn't be viable, but if a rotating baseball or cylinder creates lift, I don't see why a treadmill wouldn't create lift in exactly the same way.

 

[minger]

Right, I would agree with Russ. It is fairly well known that a counter-rotating cylinder will generate lift according to:
<br /> F = \rho U \Gamma<br />
Where Gamma is the strength of the vortex circulation, and U is freestream velocity. While it is entirely not plausible or efficient by any means, the principles do allow it to happen.

 

[Cyrus]

Right, I would agree with Russ. It is fairly well known that a counter-rotating cylinder will generate lift according to:
<br /> F = \rho U \Gamma<br />
Where Gamma is the strength of the vortex circulation, and U is freestream velocity. While it is entirely not plausible or efficient by any means, the principles do allow it to happen.

You're absolutely right. I made the mistake of convoluting the requirement that an initial velocity would have to be given to the wing, like in the case of a ball. I forgot that there is this thing called an 'engine' which provides that velocity .

From a practical standpoint though, a moving conveyor belt means a moving surface that will have some RMS value of wing contour (no belt perfectly holds a shape) and I can see that easily creating a large amount of drag inside turbulent boundary layer. But, I am curious to see what would happen if the bottom belt moves exactly opposite to the airplanes speed. The no slip condition would mean the boundary layer gradients should be pretty small (which would be a good thing for overall drag).

 

[RonL]

I figured as much. a flawed idea :(

Forget the belt, but focus your attention on the tube that would have driven the belt, if it forms the leading edge of the wing it will entrain a larger amount of air to flow up and over the top surface.

Just something to look at.

 

[Cyrus]

Forget the belt, but focus your attention on the tube that would have driven the belt, if it forms the leading edge of the wing it will entrain a larger amount of air to flow up and over the top surface.

Just something to look at.

Why would you worry about entraining air at the leading edge of an airfoil? The problems of separation are due to a lack of energy in the boundary layer to keep it attach near the trailing edge.

 

[WCOLtd]

Forget the belt, but focus your attention on the tube that would have driven the belt, if it forms the leading edge of the wing it will entrain a larger amount of air to flow up and over the top surface.

Just something to look at.

I am no aerospace engineer, but

I thought about that - have a giant breadroller thing for a wing instead of a belt. I thought I had an even better idea though,
Position engines both infront and behind the conveyor belt - in order to create a more laminar flow. If the belt will be made of very low viscosity material and air from the props is vented in order to distribute flow more evenly across the entire surface of the belt I think it is more likely to produce greater lift. The design model calls for higher windspeeds and belt speeds to compensate for lower lift created by the lower viscosity of the belt.

 

[Cyrus]

Position engines both infront and behind the conveyor belt - in order to create a more laminar flow.

Come again? Now your just making stuff up as you post.

 

[WCOLtd]

I am going to build this thing in order to test the concept out.

I am going to use wrapping paper as the belt, and the rollers will be made out of wooden dowels, the dowels will have gears attached to the ends and will have an electric rotor from a remote control car power them. The inside of the belt will be made of sheets of copper or aluminum - or something else which has a low coefficient of friction with the wrapping paper. The length and width of the conveyor belt will be easily adjustable. Allowing me to alter the design in between tests in the mock windtunnel I will make with just a pair of fans with a tubular encasing.

Wish me luck.

 

[WCOLtd]

Come again? Now your just making stuff up as you post.

That wouldn't work? to have a more tubular air flow?

If I have just one engine, the air will tend to want to disperse and slow down the further down the air is from the engine,

If I have an engine on the other end of the conveyor belt, it will tend to want to pull air back in and speed it back up - that's the idea anyway. To create a more steady, tubular kind of airflow.

The idea is that the speed of the air along the top will be faster than the speed of air along the bottom. The effect of the conveyor belt on the bottom surface is to slow down the passing air, and the top is to speed it up or keep it going at nearly the same speed. Hopefully if a laminar flow is maintained, the belt will produce greater lift.

The idea is to have the air flow distributed as evenly as possible throughout the conveyor belt. Smoothly slow down the air below the wing, and have air on the top glide over it with very little resistance.

Your thinking of the design aerodynamically - to improve the efficiency of aircraft,

The way I am thinking is to maximize lift (even if that does mean lots of drag along the bottom of the wing)

There are many ways a backspinning conveyor belt could be installed for use on an aircraft, it could have the bottom recessed into the body or wing of an aircraft, with the top going about the same speed as the air as you said, in order to reduce viscous forces across the top. But the application I am talking about is for increasing the lift, and I think if it is very finely tuned it is possible to create lift with a conveyor belt.

 

[Cyrus]

The flow from the exhaust of an engine will not promote laminar airflow. Having a propeller in the wake of another propeller is a bad idea from a thrust and vibration level, and is why you don't see this done on real airplanes.

Honestly, I wouldn't waste time building this idea.

 

[WCOLtd]

The flow from the exhaust of an engine will not promote laminar airflow. Having a propeller in the wake of another propeller is a bad idea from a thrust and vibration level, and is why you don't see this done on real airplanes.

Honestly, I wouldn't waste time building this idea.

Too late, you already have. :P

 

[cepheus]

Isn't this principle normally applied to rotating cylinders and called the Magnus effect? Some ships and wind turbines have used it. I recall it was seriously considered as a supplement for conventional ship propulsion a few decades ago. The ship can evidently travel at a greater angle into the wind than with conventional sail designs.

http://www.grc.nasa.gov/WWW/K-12/airplane/cyl.html

http://www.mecaro.jp/eng/products.html

However adapting it to a conveyor belt should produce greater surface area at the cost of additional complexity of design, so I don't see it as fundamentally bad idea.

I had a similar idea for a 'kite' many decades ago back at Uni, and everyone said I was mad!

 

[cepheus]

The flow from the exhaust of an engine will not promote laminar airflow. Having a propeller in the wake of another propeller is a bad idea from a thrust and vibration level, and is why you don't see this done on real airplanes.

Honestly, I wouldn't waste time building this idea.

Isn't this a good idea when contra-rotating since it strightens out the airflow? I think aircraft manufacturers are considering this for future turboprop designs

http://en.wikipedia.org/wiki/Contra-rotating_propellers

 

[Cyrus]

Isn't this a good idea when contra-rotating since it strightens out the airflow? I think aircraft manufacturers are considering this for future turboprop designs

http://en.wikipedia.org/wiki/Contra-rotating_propellers

It's like a coaxial helicopter. Its only really used when you absolutely have no other choice. It is better to simply have a larger actuator disk. Its a bad idea because the rotor that lies in the wake of the first is in a far from optimal flow field.

Here is a nice little quiz question for you, what would you expect the performance of a coaxial duct to be as compared to a single disk duct?

 

[cepheus]

Well the link doesn't work but the wiki artcle suggests that Contra-rotating propellers have been found to be between 6% and 16% more efficient than normal propellers[1].

 

[Cyrus]

Well the link doesn't work but the wiki artcle suggests that Contra-rotating propellers have been found to be between 6% and 16% more efficient than normal propellers[1].

I know a professor who did this test in a wind tunnel and its actually worse. Also, if you have a coaxial helicopter the ideal spacing between the two rotor blades huge (1-Rotor Diameter). The point is, coaxial rotors are finicky and need to be highly optimized for little (or no) gain. The rotor downstream of the main rotor has highly unsteady air-loads and induced velocities, which is not good.

 

[RonL]

Why would you worry about entraining air at the leading edge of an airfoil? The problems of separation are due to a lack of energy in the boundary layer to keep it attach near the trailing edge.

Maybe I used the wrong word, but control of air movement over and under the wing seems to be the entire function of the leading edge of an air foil.
To have any control of air flow, other than angle of attack, seems to offer some way to influence what happens as the air moves past a wing.

Does the problem of separation apply to any wing, regardless of speed, size and weight?

The idea might be of no use and would be a reason that I have never seen it in any aviation book, or the mechanical difficulties be too many to justify the gains, but new and different ideas are getting harder to find.

 

[WCOLtd]

thanks for the links they're great.

 

[Cyrus]

Maybe I used the wrong word, but control of air movement over and under the wing seems to be the entire function of the leading edge of an air foil.

An interesting approach to looking at an airfoil you have presented here . I would add two things: first, the leading 1/4 - 1/3 of an airfoil is the most critical lift generating surface. So you want to very nice, clean leading edges with good geometric accuracy in your manufacturing. Having a wobbling belt here does not seem like a good idea. The second point is that geometry of the airfoil (everywhere) matters, not just the leading edge. Otherwise, why would camber matter? We put control surfaces (mostly) at the trailing edge of a foil, with of course the normal exception of leading edge slats.

To have any control of air flow, other than angle of attack, seems to offer some way to influence what happens as the air moves past a wing.

The current state of the art is active flow control, so there is a distribution of little taps along a body. The taps are connected to something (maybe a piezzo actuator, synthetic jet actuator, or spark-jet actuator) that injects air into the boundary layer. So effectively you are changing the boundary layer itself without changing the airfoil physically.

Does the problem of separation apply to any wing, regardless of speed, size and weight?

For an airfoil in steady conditions, separation will be a function of AoA, Reynolds Number (Speed & Size), and Mach number (high speed). Weight has no bearing on aerodynamic performance, it has an indirect effect on the value of AoA though. A given weight requires a given lift, and therefore specifies an AoA: but weight does not change the flow pattern around an airfoil.

The idea might be of no use and would be a reason that I have never seen it in any aviation book, or the mechanical difficulties be too many to justify the gains, but new and different ideas are getting harder to find.

The 60s were really the wild west of aviation where everything under the sun was tried. I wouldn't be surprised if someone has tried this before.

 

[RonL]

An interesting approach to looking at an airfoil you have presented here . I would add two things: first, the leading 1/4 - 1/3 of an airfoil is the most critical lift generating surface. So you want to very nice, clean leading edges with good geometric accuracy in your manufacturing. Having a wobbling belt here does not seem like a good idea. The second point is that geometry of the airfoil (everywhere) matters, not just the leading edge. Otherwise, why would camber matter? We put control surfaces (mostly) at the trailing edge of a foil, with of course the normal exception of leading edge slats.



The current state of the art is active flow control, so there is a distribution of little taps along a body. The taps are connected to something (maybe a piezzo actuator, synthetic jet actuator, or spark-jet actuator) that injects air into the boundary layer. So effectively you are changing the boundary layer itself without changing the airfoil physically.



For an airfoil in steady conditions, separation will be a function of AoA, Reynolds Number (Speed & Size), and Mach number (high speed). Weight has no bearing on aerodynamic performance, it has an indirect effect on the value of AoA though. A given weight requires a given lift, and therefore specifies an AoA: but weight does not change the flow pattern around an airfoil.



The 60s were really the wild west of aviation where everything under the sun was tried. I wouldn't be surprised if someone has tried this before.

Thanks for the smilie

The rotating tube as a leading edge item, could be done, but I see too many reasons why the belt cannot work at all.
If for a moment all logic is put aside and the belt is considered a working option, there might be three things that might be positive point questions. (on a slow speed craft)

1. The underwing belt area is less, if this is moving forward at the same speed as flight, will the friction increase be more costly in energy required, than the gain from the complete removal of friction on the greater upper area of the wing?
2. Would the removal of friction drag on the upper surface have a positive effect to air flow or would the dynamics of lift be compromised in a negative way?
3. Could an iceing condition be eliminated by a belt that carries any buildup to the rear and cracks it off as it makes the very small 180 degree turn?

Let me state again this will never fly, but then look at history and how many things have been labled impossible, but were later successful.

Ron

 

[WCOLtd]

If I recess the conveyor so that the conveyor is exposed to only the upper leading section of the wing, I don't see why that wouldn't help increase the pressure difference between the top and bottom of the wing.

Sure there might be difficulties in manufacturing the thing, but I am sure those could be overcome if the benefit is significant enough to warrant production. Which it is highly unlikely to be.

Another problem with the idea that I haven't seen addressed is that the conveyor would have to be going at pretty high speeds in order to produce the desired effect (of having low viscous forces) but the higher the speed the greater the gyroscopic force is over the front of the wing, and the harder it would be to maneuver such an aircraft to the left or right - or barrel roll maneuvers would be very difficult. So it wouldn't be very useful for military fighter aircraft who need that maneuverability.

Also I'd imagine that it would help reduce parasitic drag over the wing if the conveyor was extended over the entire upper surface of the wing.

The faster the plane, the more likely this design is to be useful and the more challenging it will be to make it work.

Again I am no expert, that is just how I am thinking of it.

I agree that having the under part of the wing as exposed conveyor probably woudn't be terribly beneficial for producing lift, and might even cause a large amount of drag - so the best way is to probably cover up the lower section of the conveyor, but I could be wrong about that.

 

[WCOLtd]

I talked with a local hobby shop owner, and I told him about my idea. He was really helpful in discussing the various types of airfoil for different types of planes, and we settled on one that was a design for a lifting RC aircraft. He told me that the leading edge roller will have to be really small, because if I make it larger than the leading edge of the existing airfoil, it will likely increase drag. I was worried that this would dramatically increase the requirements for rotational speed of the motor - and that I would likely be very difficult for me to find such a motor. (the motor would have to be able to create super sonic prop speeds for prop blades as small as 3 inches from the center of rotation!)

He came up with a simple idea to have an unpowered roller along the leading edge of the wing, and the powered roller inside the thicker area of the airfoil - significantly reducing the RPM requirements of the motor.

now the issue is finding a high rpm bearing capable of 50,000 rpms with - ideally an outer diameter of only 5/16 of an inch (it could also be a bearing that has an inner diameter of 5/16 of an inch - but that would be less ideal - because at the edges of the wings, there would be a bearing sticking out, and the outer part of the wing would have to be cut to accommodate for the larger bearing. - if the bearing had an outer diameter of 5/16 of an inch, it could fit into the grove carved out for the roller - it would be less obstructive)

The airfoil design of the conveyor will be made to match the contour of an existing custom r/c airplane. The wing material is made out of light wood, it's easy to build and is relatively inexpensive. Due to the 30 lb lifting capacity of the r/c plane, the weight of the conveyor will likely have minimal effect on the handling characteristics of the aircraft - of course, depending on the weight of the belt.

Another advantage to this airfoil design, is the maximum thickness of the wing - compared to other r/c aircraft. This makes it easier for the roller to be recessed into the airfoil, it will either be powered by a pair of propeller engines outward from the fusalage and have a bearing on the other end, or, ideally, will have an axle powered by a rc car motor across the fuselage of the the plane in between the area of maximum thickness of the airfoil. (this will ensure that both rollers are going at the same speed)

the thickness of the airfoil will determine the limitations of how thick the roller can be. The thicker the roller, the lower the rpm requirements will be for the engine (but the higher the torque).

due to the high speed of the belt, the inside area of the belt should not make contact with the wing, additional rollers would be needed to match the contour of the wing or if the roller does contact the wing surface, the surface must have very low friction with the belt.

In order to first test this out, I will use a wind tunnel made from a leaf blower, with wind directed outwards across the surface of the wing. (I will probably need more than one and I will need a way of judging airspeed coming from the leaf blower)

Other Considerations

There are certain design limitations in the airfoil, for instance, the curvature of the airfoil must always be positive, because conveyor belts can not match negative (concave) curvature. Also, the belt must have high tensile strength - because to maintain constant contact with the rollers, the belt must be taught. Also the belt must be elastic enough to bend around the 5/16th of an inch diameter roller, and be inelastic enough to prevent low pressure over the top of the wing from deforming the shape of the belt.

 

[RonL]

You seem to be thinking through things, if you live in a place that has a motion control dealer, they will have bearing catalogs, as well as belt suppliers and their catalogs, these are generally free and have all engineering data needed to find what the limits are.

I think you will find that weights and mechanical limits will never come close to reaching your goal.

Above all be safe to a fault, and no matter what the results, you will learn things that you will always remember. The price of what you do, might prevent you from doing something else, so try to buy things that can be used in other applications.

Ron

 

[Brian_C]

I think I'll stick with wings.

 

[RonL]

I think I'll stick with wings.

Good Plan

 

[minger]

I applaud you WCOL (oddly very similar call letters our CBS affiliate here). It's one thing to try and do something that cannot work. It's another thing to try and do something which probably won't work.

The one thing here that's important is that the OP realizes that the design is inefficient, and probably won't work. But, as engineers, how many times have we all done something (most likely dumb) just to prove a point to someone, or to even prove it to ourselves that it can be done.

As far as bearings, high speed bearings can be extremely pricey. Have you tried calling around to see what you can get off the shelf?