Watson wrote:Talking strictly about aerodynamics, when both systems are in use, there will always be a more efficient ducted fan design than a propeller design for a given condition, but not the other way around. This is for the same reason that a glider in ground effect will glide more than the same glider not in ground effect. Basically, using tip vortex to your advantage, outweighs the duct drag.
You are correct that a properly designed duct will increase the efficiency of the blade. You are not correct in stating that a ducted fan therefore is more efficient than an open prop. The efficiency is lost because a ducted fan must accelerate the air to many times the flying speed. That is wasteful. On a slow speed aircraft you want to accelerate the air as little as possible. A large diameter propeller is the most efficient way to do this. It is the same reason you see long slender wings on a glider. They sweep a large amount of air, accelerating it a little.
However, as the aircraft speed increases the large diameter propeller gets impractical - tip speed become surpersonic and they become inefficient. The blades then need to be enclosed.
Also, as further evidence of the "move a lot of air a little bit" method, just look at the turbofans on todays modern airliners compared to the turbojets of yesteryear. The difference? Big, huge fans. They put those huge fans on them because it is more efficient. The amount of acceleration is less, but is imparted to a larger volume of air. That is what makes them more efficient. The same reason a large open prop will outperform a small ducted fan in slow speed applications.
Watson wrote:However, when we talk about the propulsion itself, in order to turn these aerodynamic surfaces, there are more things that come to play. Ducted fans usually need to spin faster, while propellers need to spin slower. So, when only piston engines were available (lower RPM), propellers were the only plausible answer. But, with the invention of gas turbines (high RPM), ducted fans grew in popularity, however their cost is still too high for some applications.
Many piston engines have reduction unit to further slow the prop speed - so they can swing the largest possible prop! Many thousands of gas turbines also have reduction units - so they can swing the largest possible propeller!
Watson wrote:Now, with electric engines, there has been a change in paradigm. The newest electric engines (with rare earth magnets) have a higher efficiency at high RPMs and are dirt cheap! You can still put a gear box to change from high to low RPM (or the other way around), but every time you go through one of those you necessarily loose efficiency through the system.
True, but no need to gear them down, just use the appropriate motor for the desired rpm (it's in the windings and diameter of motor bell).
Watson wrote:So, this is the reason why I believe we are going to see more and more ducted fans paired with electric engines. They really have unprecedented efficiency numbers, that even people in the area are not used to it.
Readily available efficient brushless motors will make EDFs gain in popularity. Still, if efficiency is your priority, you will choose a large open prop.
Please remember that an efficient motor does not mean an efficient thrust producer. It only means that the motor is very good at converting electrical energy into rotary motion. It does not mean that it is producing more pounds of thrust per watt than an open propeller can.