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Old 12-25-2010, 09:30 AM   #9
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Originally Posted by skyman View Post
I don't see why it wouldn't work. So then what is the point of ducted fans in the first place if they are so inefficient? Do they just produce more thrust for high speeds than a prop would or what? If it's such a big deal why don't they just use little propellors in place of fans? Seems to me that there has got to be a better way here... perhaps I could make some money on a new electric jet system.
I never implied that it "wouldn't work".. just that it will be inefficient.
As insomniac say: The main reason people use ducted fans is because they want the plane to look like a jet and that means not have an external prop, so the fan/prop has to fit inside the fuselage. In order to fit inside the fuselage fan or prop diameter must be small. A ducted fan is the best way of getting a lot of power into a small diameter, it might not be efficient but if you are stuck with a small diameter then it's the best you can do other then by having a 'proper' turbojet.
Ducted fans do start to work better and their efficiency increases at higher speed, but they will never match a prop for efficiency unless their diameter is similar to the prop.

So 'yes' there is a better way to do it if you want efficiency; just use a large diameter prop.. but that spoils the look of your jet model. For model airplanes efficiency is not the only consideration, In fact for scale/fun models it's hardly any consideration at all.

The ducted prop on the real Optica is a bit different.. It is not 'hidden' inside the fuselage and it's probably close to the diameter of a normal prop, so it's efficiency might be quite good. They probably went with a ducted system to prevent the prop striking the ground and because the duct can form part of the plane's structure.

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