RC Ducted Fan JetsRC ducted fan jets are the most common type of RC jet. There are also pusher RC jets which simply have a pusher propeller at the rear of the aircraft. Gas turbine RC jets have real turbine engines just like full scale jets. This page describes the different types of RC ducted fans. The tube prevents losses in thrust from the tips of the fan blades. This greatly increases the efficiency of RC ducted fans compared with conventional propellers. RC ducted fans have shorter and more blades than conventional propellers. This allows them to run at much higher speeds. Electric RC Ducted Fan Jets (EDF) To understand the role of each component of an electric RC ducted fan jet, lets take a look at the above image. This is an image of GreatPlane's Hyper Flow RC Ducted Fan System. Rotor Cone & Front Housing Flange: Fan Rotor: Housing: Motor: Aft Cone: Stator: RC Ducted Fan Jets for Beginners
The "tube" of most RC ducted fans is the aircraft itself. As a matter of fact, all GDF's that I am aware of have internal ducted fans. Some EDF's have ducted fan units mounted under the wings or near the rear of the fuselage. Take a look at the
A7 Tornado
ducted fan jet shown to the right, which has an internal ducted fan. The fan pulls air into the front of the fuselage and pushes it out of the rear, much like a full scale just.
 Hobbico's Diabolo (EDF) RC Ducted Fan Jet Trainer is another excellent aircraft for learning to fly RC Jets. The Diabolo is significantly larger and flies faster than the A7 Tornado. A Hyper Flow RC Ducted Fan System like the one at the top of this page is installed in Hobbico's Diabolo. Both of these electric RC ducted fan jets are designed specifically for beginner jet pilots. Keep in mind that these jets, especially the Diabolo, are not suitable for pilots without prior RC flying experience. You need to have mastered a basic trainer airplane or beginner park flyer before attempting to fly an RC jet. Glow Powered RC Ducted Fan Jets (GDF)
A tuned pipe is needed when using a glow engine with a ducted fan. The tuned pipe is that long tube you see extending from the engine. A tuned pipe basically pushes unused fuel from the exhaust back into the engine to increase the power. The tuned pipe must be "tuned" to the maximum RPM by adjusting its length. O.S has a great page explaining what tuned pipes are and how do they work.
Most Common GDF ManufacturersDynamax Fan -can be found at Jet Model ProductsTurbax fan - can be found at Jet Hangar Hobbies. Jethangar Hobbies has a wealth of information pertaining to RC ducted fan jets. This pages is loaded with information on glow ducted fans, electric ducted fans, and turbines. It's an excellent site to explore for information on RC model jet airplanes. Examples of GDF Installations The image to the left is a Balsa USA Enforcer (GDF) RC ducted fan jet with an O.S. 91 DF large head glow engine used in conjunction with a Byro-jet fan package. Notice how the engine faces the rear of the jet. This is a called a pusher fan set-up. Most ducted fans are tractor fans, meaning the engine faces the front of the plane and pulls the are over the engine. Tractor fans are more efficient because the suction end of the fan is not restricted by the engine. Tractor fans also do a better job of cooling the engine since all the air is pulled across the engine. But as you can see, pusher fans are also very common. Don't confuse an RC ducted fan jet with a pusher fan with an RC pusher jet. An RC pusher type jet has a convententional pusher propeller with no tube.
To the right is a close up of this ducted fan installation. This fan is mounted on top of the aircraft with its own tube as opposed to being housed inside the fuselage.
Special thanks to KC Moore for contributing these images. Back to RC Jets
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Glow engines designed for ducted fans are made a little differently. Take a look at the OS .46 VX-DF engine to the left. Notice how the carburetor is in the rear of the engine instead of being in the front. Notice how the exhaust is also in the rear. The carburetor is moved to make room for the front of the engine to fit into the fan duct. The exhaust port is in the back so the tuned pipe can run straight back so that the entire GDF unit will fit nicely inside the fuselage. 
