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 A standard ducted fan setup OS91 VRDF
Ducted fans are basically large air pumps, which draw surrounding air in and accelerate it past the fan to produce thrust. Full size examples of ducted fans technology can be seen on commercial airliners with a large fan at the front and a turbine engine to drive the fan. In model jets, the turbine is replaced with a glow engine or an electric motor, which spins at very high speed, a multi blade propeller commonly known as an impellor. This in turn accelerates the air passing through the blades onto the stators, which help stabilise the air or trust, e.g.(as the air is accelerated by the impellor, it introduces a large amount of swirl into the airflow, therefore the stators are just special vanes that counteract the swirl so the airflow exits more efficiently hence giving more thrust). Many impellors these days are injected moulded, either of glass-fibre or carbon-fibre reinforced nylon. Some impellors such as the RAMTEC fan are made up of individual segments (modular) securely clamped together by a metal hub.One piece impellors are inexpensive to manufacture however they are not as rigid as modular ones and often require some balancing. Both fan units for electric and ic powered units, must be balanced for best performance and safe operation A reasonable balance can be obtained by using the Top Flite magnetic balance. Set the fan up in the balancer as instructed. If the fan rotates to the same position every time, then one side is heavier than the other. To over come this problem on a solid one piece impellor,add weight to the lighter side by using epoxy or CA glue. Another alternative is to use silicon glue normally available from your local hardware.  92mm EDF unit from flyfly Basically the Shroud is a cylindrical tube in which the impellor spins inside and ensures that all ingested air passes through and is accelerated pass the stators.The shroud normally has tangs moulded or bolted to it and serve as a form of mounting to the airframe. The stators are located at the rear of the fan unit, then help straighten the “swirling” effect that the impellor causes as it accelerates the air. Installation
Most ducted fan units are installed into the airframe by use of fasteners with self locking nuts. Shock mounts are normally used between the airframe and the ducted fan unit. This has two benefits. First it reduces excessive vibration imparted on the airframe. Secondly it reduces the noise that can be generated by a high energy fan/engine combinations. It is also important to reduce any gaps between the fan and airframe during installation. Similarly steps should be avoided between the intake duct and the ducted fan shroud. It is important that the airflow to the ducted fan unit to be as smooth as possible. This will ensure that no turbulence is created that could otherwise reduce the effective thrust of the system.  A brushless motor for a large fan unit
The most popular engine to date is the OS91 VRDF engine. This engine is still available today. This engine has become the industry standard for its power output and reliability and is highly recommended as a first engine. Many spares are available either as new or second hand on eBay.There are many other makes available. K & B manufacture engines from 45, 82 & 100. Rossi also have engines available from 65, 80, 90 & 105.
Electric brushless motors have become very popular due to their high power output.Some of the smaller motors for the small light jet, are available at a reasonable cost, however the bigger models require the more powerful range of brushless motors, and these can be very expensive. Many ducted flyers are converting their older 90 gas powered ducted fans into electric with amazing results. Even the gas turbine flyers are using electric power for its simplicity and lower price tag.
In order to drive electric brushless motors, light high power batteries are required. The battery of choice today is the LiPo battery pack. Most top end electric ducted fan jets use 2 x 5 cell 5000mAh Lipo packs. High quality packs are not cheap to purchase e.g.(a 3700mAh Lipo cost approx £100) but when well maintained can give over 300 flights, which can be cheaper to run than gas engines in the long run. When using Lipo batteries, also use a balancer/charger unit to charge your packs, not only will this give you max power and longer life of the battery, but will also reduce the risk of fire, lipo's are dangerous when mishandled, always follow the manufacture's recommendations on using lipo packs.
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