4 ms·

But is there any situation where the combination is more effective (produces a higher net thrust) than the fan alone? I imagine that the conservation of momentu

But is there any situation where the combination is more effective (produces a higher net thrust) than the fan alone? I imagine that the conservation of momentum rules it out.

dahart 8y ago
I’m not certain this counts, but I think jet engines do what you’re asking about. They have second stage fans that are propelled by the air being pushed out of the first stage. It seems magical to me, it seems like putting something in the way of the thrust would just eat any possible savings, but it works and all modern turbofans do it. It feel similar to me to blowing air into your sails.
- mannykannot 8y ago
  I think you are referring to the stationary blades that take the swirling air coming off the fan and direct it straight backwards. In doing so, additional thrust is generated.
  - dahart 8y ago
    No, that's interesting, but that's not it. I was thinking of how turbofans have at least 3 sets of moving blades: the fan, the compressor, and the turbine. The turbine blades are behind the combustion chambers and they are pushed by the thrusting exhaust in order to power the fan at the front of the engine. (https://en.wikipedia.org/wiki/Turbofan https://en.wikipedia.org/wiki/Turbofan) The general theory behind this is called the Brayton Cycle (https://en.wikipedia.org/wiki/Brayton_cycle https://en.wikipedia.org/wiki/Brayton_cycle), and you can see if you read about it that they are all some form of blowing on your own sails, it's a basic design principle that increases the engine efficiency by capturing some of the output. And it still seems to defy intuition to me, I find it hard to understand how reducing the output and putting it back through something that is less than 100% efficient can end up saving energy or increasing output... it's counter-intuitive. Anyway, I wonder if installing a fan on your sailboat technically makes it a Brayton Cycle engine... mmm maybe not since the sail doesn't power the fan...

But is there any situation where the combination is more effective (produces a higher net thrust) than the fan alone? I imagine that the conservation of momentum rules it out.

dahart 8y ago

I’m not certain this counts, but I think jet engines do what you’re asking about. They have second stage fans that are propelled by the air being pushed out of the first stage. It seems magical to me, it seems like putting something in the way of the thrust would just eat any possible savings, but it works and all modern turbofans do it. It feel similar to me to blowing air into your sails.

mannykannot 8y ago
I think you are referring to the stationary blades that take the swirling air coming off the fan and direct it straight backwards. In doing so, additional thrust is generated.
- dahart 8y ago
  No, that's interesting, but that's not it. I was thinking of how turbofans have at least 3 sets of moving blades: the fan, the compressor, and the turbine. The turbine blades are behind the combustion chambers and they are pushed by the thrusting exhaust in order to power the fan at the front of the engine. (https://en.wikipedia.org/wiki/Turbofan https://en.wikipedia.org/wiki/Turbofan) The general theory behind this is called the Brayton Cycle (https://en.wikipedia.org/wiki/Brayton_cycle https://en.wikipedia.org/wiki/Brayton_cycle), and you can see if you read about it that they are all some form of blowing on your own sails, it's a basic design principle that increases the engine efficiency by capturing some of the output. And it still seems to defy intuition to me, I find it hard to understand how reducing the output and putting it back through something that is less than 100% efficient can end up saving energy or increasing output... it's counter-intuitive. Anyway, I wonder if installing a fan on your sailboat technically makes it a Brayton Cycle engine... mmm maybe not since the sail doesn't power the fan...