This section is intended to provide the beginner some valuable information about ducted fan units. In short ducted fans are just air pumps. They draw surrounding air in and accelerate it past the fan to produce thrust. The acceleration of this air is based on a change in moment principle the same way in which a jet engine or rocket motor works.
We see ducted fans in common use today with full size aircraft. The most common is the high bypass turbofan engines used on most commercial airliners. The large fan at the front of the engine is powered by a small gas turbine engine. In our models the gas turbine is replaced by a piston engine or electric motor.
The motor spins an impellor (a multi blade propeller) at high rpm to accelerate the surrounding air. Stators are positioned behind the impellor to essentially straighten the airflow. What do we mean by straighten the airflow? Well as the air is accelerated by the impellor, it introduces a large amount of swirl into the airflow. The stators are just special vanes that counteract the swirl so the airflow exits more efficiently hence giving more thrust. Let’s now look at each component in detail.
Impellor
Many impellors these days are injected moulded, either of glass-fibre or carbon-fibre reinforced nylon. They typically have an odd number of blades generally between 5 to 11. Some impellors such as the RAMTEC fan are made up of individual segments (modular) securely clamped together by a metal hub. Some others are moulded as one piece such as the VIOFAN.
Generally one piece impellors are inexpensive to manufacture however they are not as rigid as modular ones and often require some balancing. The best way to balance an impellor is to use a dynamic balancing machine, however this is out of reach for most modellers. A basic balance can also be obtained using Top Flite magnetic balancer. Simply set the fan up in the balancer as shown in the instructions. If the fan rotates to one position every time then once side is heavier than the other.
For one piece moulded impellors, 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. You may need to make several iterations to get it right. For modular impellors, disassemble the impellor and shift the location of the blades around until a suitable balance can be obtained. Keep in mind that a perfect balance is difficult to achieve so be patient and do not strive for perfection.
When the impellor is installed into its shroud, it is important that the gap between the impellor and shroud should be kept as small as possible. The minimum clearance improves the efficiency of the fan. Typical minimum clearance is in the order of 1mm but some other units on the market have been known to have larger clearances. If the impellor is grossly out of balance then it is possible for the impellor to scrape the shroud due to excessive vibration.
Shroud
This is a cylindrical tube in which the impellor spins inside and ensures that all ingested air passes through and is accelerated. Most are manufactured from injected moulded nylon, with either glass fibre or a small percentage of carbon-fibre added to enhance the stiffness of the material. The shroud normally has tangs moulded or bolted to it and serve as a form of mounting to the airframe.
Stators
Inside the rear of the shroud are a number of shaped blades which are fixed at an opposing angle to the impellor blades. As mentioned previously the stators straighten the “swirling” effect that the impellor causes as it accelerates the air. The number of stators used varies from design to design, but generally the number of stators used is the same number as the impellor blades. Stators are typically manufactured integrally into the shroud or they can be mechanically fastened. They also serve as a way to mount the motor inside the shroud.
Assembly
Most fan units come as kits to be assembled. All generally have assembly instructions or internet user forums. Pay particular attention regarding the use of thread locking compound. Use the correct one the manufacturer specifies as all ducted fan units driven by piston engines have some degree of vibration.
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.
Another important factor to keep in mind is that the leading edge of the intake duct should have a rounded lip. A sharp lip may cause turbulent flow before entering the fan. In addition a rounded lip is the most efficient design used to draw in surrounding air at low speeds. This will ensure maximum static thrust when accelerating during the ground run.
Power plant
In order to produce reasonable thrust, the ducted fan needs to spin at high RPM. By high RPM we mean in the order of 21000 to 23000 RPM for a 5” diameter fan. Typically sport model engines that we see at our local club will not work as they spin in the order of 9000 to 12000 RPM. In addition, sport engines aren’t able to produce the required horse power to spin the fan at the required speed. So what do we mean by horse power? Well the fan blades have a certain pitch in order to accelerate the airflow. However with increased pitch you get increased drag on each blade. So the higher the RPM the higher the drag which translates into torque back on the engine. Engines with higher power ratings can handle these loads more efficiently. This leads us to an important point for ducted fans; each engine must be matched to the fan it is installed in. For example, the OS91 VRDF engine develops 4.8 bhp at 21700 RPM so for best efficiency it requires an impellor that allows the engine to run at this speed. The best match for the OS91 VRDF engine currently is the Ramtec fan.
Many of the engines available for ducted fan were derived from the pylon racing family. These engines along with ducted fans have tunned pipes which act as superchargers for the engine. Each engine must have a pipe of the correct length in order to produce the power required to drive the impellor. Most pipes available have a fixed length for the engine of choice where other pipes available require some adjustment in length to obtain the power output.
The most popular engine to date is the OS91 VRDF engine. This engine is still available today (see www.towerhobbies.com). 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. BVM also have an engine in the 96 size which is very powerful.
On the other side of the coin are electric brushless motors. These motors have become very popular during recent years for their high power output. Motors manufactured specifically for electric ducted fans are not cheap ($300USD) but are extremely powerful and rival the gas ducted fan setup. 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 6 cell 5000mAh Lipo packs. High quality packs are not cheap to purchase but when well maintained can give many flights of use, which can be cheaper to run than gas engines in the long run. One of the drawbacks of LiPo cells are safety issues during the charge or discharge cycle. For example, if overcharged or misused, LiPo cells can burst into flames. Many safety procedures have been devised to mitigate these risks. On of the techniques is to balance the voltage of each individual cell in the pack. This ensures that each cell charges in a quasi balanced state. Most cell manufactures provide guidelines on how to best maintain these type of packs.
DUCTED FAN BASICS
Enjoy our website....