# Back-spinning conveyors instead of wings?

1. ### WCOLtd

87
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?

2. ### 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.

3. ### WCOLtd

87
I figured as much. a flawed idea :(

4. ### 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.

Last edited: Feb 9, 2010

### Staff: Mentor

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.

6. ### minger

1,498
Right, I would agree with Russ. It is fairly well known that a counter-rotating cylinder will generate lift according to:
$$F = \rho U \Gamma$$
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.

7. ### Cyrus

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, Im 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).

8. ### RonL

907
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.

9. ### Cyrus

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.

10. ### WCOLtd

87
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.

Last edited: Feb 9, 2010
11. ### Cyrus

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

12. ### WCOLtd

87
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.

13. ### WCOLtd

87
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.

Last edited: Feb 9, 2010
14. ### 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.

15. ### WCOLtd

87
Too late, you already have. :P

16. ### cepheus

15
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!

17. ### cepheus

15
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

18. ### Cyrus

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?

19. ### cepheus

15
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].

20. ### Cyrus

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.

Last edited: Feb 9, 2010