3 ms·

I think you're getting at it, but at the limit, you throw out the air, which Derek did in the follow up. Once you have two surfaces moving relative to each othe

I think you're getting at it, but at the limit, you throw out the air, which Derek did in the follow up. Once you have two surfaces moving relative to each other, energy can be extracted. Doesn't matter how fast you're going relative to the two surfaces.

gibba999 5y ago
I actually find the limit to be somewhat less insightful. It sorta shows that it's possible, but it doesn't really show why it works in this case. 1. Air isn't solid. 2. There aren't two rollers. The analogy is a little bit distant. I just need to find time to make mine more eloquent.
- ummonk 5y ago
  A propeller in air is fundamentally just like a roller on the ground, but with some efficiency loss.
  - gibba999 5y ago
    I understand that. However, that's not something I find easy to explain to a someone who doesn't understand that. The analogy is a little distant. For example, "some efficiency loss" is complex and a little bit inaccurate. It's not just efficiency loss: - In the mechanical analogy, I can extract infinite power from a board moving at a given speed. There is no upper bound on the force the board can apply to me. - In the case of a fan, there is an upper bound on the power which can be extracted given a specific air speed, even given perfect efficiency. Now, it's possible to correctly argue that as the fan size increases, I can make a closer and closer to this analogue. However: - Once you start making arguments like that, cognitive load increases, and beginners start to drop off. You need to keep more and more steps in your head. - And more often than not, an imprecise argument simply reinforces or introduces some misconception, rather than giving an honest understanding. I think a force*distance argument (which is probably a clearer way of stating the argument I was making above) has the virtues of: 1. Being 100% precise 2. Being compact enough someone who understands force, power, and energy can keep the whole argument in their head

gibba999 5y ago

I actually find the limit to be somewhat less insightful. It sorta shows that it's possible, but it doesn't really show why it works in this case. 1. Air isn't solid. 2. There aren't two rollers. The analogy is a little bit distant. I just need to find time to make mine more eloquent.

ummonk 5y ago
A propeller in air is fundamentally just like a roller on the ground, but with some efficiency loss.
- gibba999 5y ago
  I understand that. However, that's not something I find easy to explain to a someone who doesn't understand that. The analogy is a little distant. For example, "some efficiency loss" is complex and a little bit inaccurate. It's not just efficiency loss: - In the mechanical analogy, I can extract infinite power from a board moving at a given speed. There is no upper bound on the force the board can apply to me. - In the case of a fan, there is an upper bound on the power which can be extracted given a specific air speed, even given perfect efficiency. Now, it's possible to correctly argue that as the fan size increases, I can make a closer and closer to this analogue. However: - Once you start making arguments like that, cognitive load increases, and beginners start to drop off. You need to keep more and more steps in your head. - And more often than not, an imprecise argument simply reinforces or introduces some misconception, rather than giving an honest understanding. I think a force*distance argument (which is probably a clearer way of stating the argument I was making above) has the virtues of: 1. Being 100% precise 2. Being compact enough someone who understands force, power, and energy can keep the whole argument in their head