Fanjet efficiency questions.
I have a series of fanjet questions that I haven't been able to get answers to. Here we go:
1. It's a known fact that the bigger you make a propeller/impeller/fan, the less horsepower it needs to move a set amount of air. This is why we see high-bypass fanjets getting bigger and bigger fans in the front. At some point, this fan is going to be too big to shroud in a housing and would have to be un-shrouded. Why, in the name of fuel efficiency hasn't this happened yet?
2. Any prop or fan will lose efficiency when the blade tip get close to transsonic speeds. In a big fanjet, the fan is normally driven by a second turbine stage at the end of the core turbojet engine. And since the blades are fixed pitch - how does the fan rpm stay below transsonic when it's direct driven by the turbine at the outlet? And if it doesn't (which I suspect), then it must lose a lot of efficiency. Which leads me to the my third question...
3....Why don't fanjets have variable pitch blades like the turboprop has?
4. Many blades in a fan or prop adds solidity and will require less power to shift air. The reasons turboprops rarely have more than 5-6 blades is that they have variable pitch hubs. A fanjet doesn't have VP, so they can have more blades. But there's a disparity here - if more blades are more efficient (as they keep telling us), then how come fixed pitch aircraft don't have more blades? Or reversed - why don't fanjets have fewer but variable pitch blades? You can't have your cake and eat it too - something has to be better than the other.
5. If a bigger fan increases fuel efficiency is taken to its logical conclusion, soon the fans will be so big as they will be hard to hang under wings without scraping the ground. Would it therefore be beneficial and fuel efficient to add more engines with higher bypass ratios so as to shift more air? Example: instead of having 2x engines rated at 10kN (in the core turbine, I'm not talking total thurst from the fan part) each and with a 100sq.ft fan area - would it be more beneficial to have 4x 5kN engines with 200 sq.ft fan in instead?
Last edited by AdamFrisch; 28th November 2008 at 00:44.