Comment by carapace | Hacker News Reader

Comment by carapace | Hacker News Reader

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The interesting thing about the Magnus effect is that the lift goes up with the second power of the radius of the rotor, so twice as large gets you four times as much lift. The largest Magnus effect vehicle I know of was a huge sphere with the crew cabin hanging below it from the axle. The whole sphere was the lifting body.

The old OMNI magazine had an article wherein they calculated that you could put Magnus effect rotors on a bus and it would fly at (speeds as low as) 35mph.

I've had plans for years now to build large flying buildings (if I can get my act together I might eventually do it) using geodesic cellular kites and the Magnus effect. (Look up Alexander Bell's kites, and Bucky Fuller's "Cloud Nine" aerial city concept.)

- - - -

http://www.rexresearch.com/flettner/flettner.htm

http://www.rexresearch.com/skybow/mueller.htm

http://www.rexresearch.com/aero/1aero.htm#thompson

http://www.rexresearch.com/aero/1aero.htm#flettner

And I found some images but not the name of the upside-down-hair-band-sphere design on this blog post: https://steemit.com/airship/@everittdmickey/magnus-effect-ai...

The interesting thing about the Magnus effect is that the lift goes up with the second power of the radius of the rotor

Due to E = mv^2, doesn't the energy required to spin the rotor also go up with the square of the radius? Thus, the amount of energy is directly proportional to the amount of lift, so you don't get any magical quadratic gains for free like your post implies.

Think of the cross-section of one of these rotors, it's a circle, yeah? We're not interested in the interior of the circle, just the perimeter, which increases linearly with the radius. (We can make the interior out of wire spokes like a bicycle wheel.)

Unless you use carbon fiber or other super light materials. In other words, weight is more or less disconnected from size.

The kinetic energy required to spin up a rotor mounted on low-friction bearings says very little about its efficiency.

Don't forget that skin friction against the air also resists the rotation force, and does so to the tune of the square of velocity.

Ah yes, but you have the option to use more power. The way I describe it is: The Magnus effect rotor is to the wing as the wheel is to the sled runner.

But the whole game of airfoil and wing design is to optimize the lift to drag ratio, i.e. how much lift is produced per propulsive power needed. Sure, given enough power, you can just remove wings altogether and simply fly an engine.

Would it increase the drag if you spun the rotor faster? My intuition says not, or not by much, but I don't know.

That's what I'm getting at, not increasing propulsive power, rather adding power to the rotation to get more lift.

It probably (?) wouldn’t increase the drag of the aircraft significantly, but you’re still spinning a cylinder in a viscous fluid and that takes energy. So over all, you increase the power you need to put into the system to create a certain amount of lift. Whether the energy expense comes as an additional propulsion requirement to overcome drag or as an additional motor to spin the cylinder does not really matter if you’re interested in the lowest power expense for a desired lift.

Hmm, that makes sense. From your username I suspect that you may know (much?) more about this than me. :)

But it's misspelled! ;-)

But increasing the radius would absolutely increase the drag.

Can you get your act together and build this? I'd like to see the end result, and the process.

You and the other folks responding to this so positively have got me doing some soul-searching. One of my personal limiting factors on this is that there hasn't been a lot of interest so far. It's like, why perform if there's no audience?

("But, if we don't perform, in what sense do we have an act?" ~B.B. Rodríguez)

So, um, wanna join my mailing list? mailto:~sforman/heliotrope.pajamas+subscribe@lists.sr.ht (Web interface: https://lists.sr.ht/~sforman/heliotrope.pajamas )

I'm joking, but I'm also serious: if you really want to see this happen the most useful thing you could do would be to pay attention to it. If I know folks are interested that's hella motivating, y'know?

Cheers!

I for one find this fascinating, please work on this!

Cheers! I think I will.

I just posted some more info here: https://lists.sr.ht/~sforman/heliotrope.pajamas/%3C202012290...

Based on the responses so far, consider “How much money would I need to actually commit enough time to this?”, and then share that number.

If it ‘doesn’t stick’, you’ll still be able to keep it as plan refi ing for now, but if it does, investors might surprise you.

I’m pretty sure a lot of folks from HN will help you fund such research and prototyping if needed, me included :)

Well aren't you sweet! Cheers!

I'm not sure it would be wise to take people's money. I've burned a lot of investor money in my career as a computer programmer and I've gotten to the point where I'm reluctant to do it again. Not to mention that there's no good reason to suspect I've got what it takes as a founder. (And I've got no team and few connections, so...)

In any event, FWIW, I have a plan for bootstrapping: starting with toys and kits, then moving into certain niche applications (such as drone swarms to collect and deliver water to contain and extinguish wildfires) and finally winding up with large structures for mass shipping, human migration, launch platforms for space rockets, etc.

Like I said to dragosmocrii, if you really want to encourage me just join my mailing list. :)

mailto:~sforman/heliotrope.pajamas+subscribe@lists.sr.ht

Potential flow theory tells us that the lift L is L = rho U Gamma, where rho is the air density, U the flight velocity, and Gamma the circulation, i.e. the tangential velocity of the rotating surface times the circumference.

L is then only proportional to the radius, not to the square of the radius.

But it sure does produce enough lift to let things fly! https://www.youtube.com/watch?v=K6geOms33Dk&ab_channel=Peter...

Dunno what to tell ya, I'm not an expert, Wikipedia says: Gamma = 2πr²ω where "ω is the angular velocity of spin of the cylinder (in radians / second) and r is the radius of the cylinder (in metres)." https://en.wikipedia.org/wiki/Magnus_effect

Ok, omega * R is what I referred to as tangential velocity, you’re right.

I don’t know if that’s a useful way to look at it, though. It’s like saying “if I just made my car tires a mile in diameter, geez, I could be so much faster!”

Oh interesting, I’ll need to look into where that discrepancy comes from!

Regarding flying spheres, there's also electro(static) lift caused by Earth's electric field pointing upwards. The drag is microscopic, but some tiny insects have figured how to use it for flying. I wonder if a light sphere could use an electromagnetic motor to create a strong electric potential and use that to move vertically? In an ideal world, this electrostatic drag would also grow quadratically with the sphere size so it would be possible to lift a hundred kg with a reasonable sized sphere.

There are a lot of interesting applications of electric fields with this, have you heard of Lord Kelvin's Thunderstorm?

On the other hand, I'm not sure flying around in a giant capacitor is wise? :)

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