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I'm currently reading Arthur Clarke's Rendezvous with Rama. Rama is a 50 km long, 16 km diameter cylindrical alien artifact that is rotating fast enough to provide a 0.6G artificial gravity on its long inner surface. The characters in the book enter Rama through an air lock at the hub at one end of the cylinder's central axis, where they are weightless; they then descend a series of ladders and stairs, getting heavier as they go, until they reach full weight at the surface.

One character has a flying contraption with him. He sets off from the hub, weightless, with the intent to ride along the axis the entire length of Rama. He is warned not to drop down towards the surface, as lowering at all will increase the weight (or centrifugal force) he feels. But I'm not sure if this is true.

Centrifugal force is not like gravity, which reaches out to pull you down. If there is nothing pushing him along the rotation, he shouldn't feel any centrifugal force at all, right? Of course, the air itself is going to be moving along with Rama's rotation, but would the air near the hub be moving fast enough to exert a serious sideways force on him, causing him to take part in Rama's rotation? Theoretically, if there was no air, he could lower himself almost all the way to the surface (which from his vantage point would be moving by very fast), and still remain weightless the entire time.

So, am I thinking about this entirely wrong? Or is Clarke?

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    $\begingroup$ The air inside is rotating along with the cylinder. The "flying contraption" is embedded in the air. You asked, "but would the air near the hub be moving fast enough to exert a serious sideways force...?" No. It wouldn't. That is the reason the character was told to stay near the center line. If he flew too close to the "ground", then the air in that region would be moving fast enough... $\endgroup$ – Solomon Slow May 25 '16 at 17:23
  • $\begingroup$ FWIW, to achieve 0.6g acceleration, the cylinder needs to do one revolution in approx 231 seconds, so the speed at the surface is approx 217 m/s = 781 km/h. $\endgroup$ – PM 2Ring Dec 3 '18 at 2:12
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It's been a while, since I read the book, but I believe that his flying contraption runs at low air speed. And of course the atmosphere rotates with the can (give or take a little).

That being the case as he moves out he picks up roughly the same speed that he would have standing on a surface at the same "height" and consequently experiences roughly the same pseudo-gravity. He can alter that a little be flying spinward for extra pseudo-gravity or anti-spinward for less. But low air-speed makes this a relatively small effect.

Next you should ask yourself how a device designed to fly in a gravity field will maneuver in the absence of one. Or more importantly how a pilot used to flying with gravity would handle the situation. I'm betting that he'd intentionally move down to where he had enough pseudo-gravity to orient the craft. That shouldn't take a lot and he can adjust his control surfaces for less lift but at least the craft will fly like he expects it to.

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I just found your question because I was thinking the same, as I read the passage about 'Dragonfly.' As Dmckee said, there must be aerodynamic effects from the rotation (the air-bike wouldn't move otherwise), but it doesn't make sense that he'd feel .3g or whatever, since he's not in a gravity well. It would be more like scuba diving in a pool with a current, or a slow-moving river. You only feel "centrifugal forces" when something is changing the direction of your linear acceleration, which, in this case, nothing is.

When the crew is ascending the staircases, they WOULD indeed feel a pseudo-gravitational force vector from that effect, that would lessen as they approached the axis of rotation.

They could also float all the way back down to the external surface from the 'pole,' but they'd be in for some road-rash when they made contact.

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With no air, you are correct.

The air makes this entirely different. I don't recall mention of there being heavy winds near the surface, meaning that the air is moving with the cylinder. The air is accelerating outward ("downward"), and the dragonfly would be accelerated with it. Along the hub, the air would not be moving much, and there would be a gradient of steadily increasing wind-speed as you move away from the hub.

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  • $\begingroup$ Why do you say that the air is accelerating outward? $\endgroup$ – PM 2Ring Dec 3 '18 at 2:27
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In addition,

If on the ground, running one direction would be very hard, and the other direction might cause you to come off the floor.

So,

Flying with rotation, very slowly, would completely zero out the friction of air rotation flow, if that happens at all. Literally at all? Air flow from wind would like be farcin excess of rotational flow.

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    $\begingroup$ Note that there is an "edit" button so that you don't have to post a second answer to the same question. $\endgroup$ – Kyle Kanos Dec 2 '18 at 18:23
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Yes. You’re right. This is totally wrong. It must stand as one of very few mistakes Clarke ever made.

Also in the chapter he mentions suit thrusters. They would have been more than sufficient to fly the axis.

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    $\begingroup$ As it stands now your answer is really just a comment since it does not contain any materially new information. $\endgroup$ – ZeroTheHero Dec 2 '18 at 19:25

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