THRUST TUBE PRINCIPLES
In an electric ducted fan jet, the air flow creates the thrust for forward movement of the plane. This airflow must travel through ducting work in the airplane. Once the air reaches the fan unit, the motor accelerates the air and pushes it out the rear of the fan. A thrust tube is placed at the rear of the fan and extends to the very end at the exhaust side of the jet. A thrust tube can provide many benefits but does share tradeoffs.
Benefits of a thrust tube
A thrust tube is used to control the surface area of the air escaping out the back of the ducted fan jet. By changing the diameter of the exit on a ducted fan jet, a couple parameters are changed. By decreasing the exhaust exit diameter the airspeed rushing out will increase. This may also increase the maximum speed of your model. By increasing the exhaust exit diameter, additional thrust may be gained. It is a balance between thrust and speed that must be achieved according to your goals. One who desires a faster top speed may require a smaller exhasut diameter. One who desires more vertical performance requires a larger exit diameter. It is important to understand that these to variables are trade offs. Increasing exhaust exit speeds will decrease thrust and vice versa.
How to calculate the best exit diameter?
The exhaust exit diameter is best achieved by use of a calculation. Below you will find the link to the online calculator that will simplify this step. First we will cover the details and terminology.
There are two critical dimensions needed in order to perform this calculation. The first dimension is the inner case diameter. This is the diameter of the case on the inside where the fan blades pass by. The second critical dimension is the outer hub diameter. On an EDF unit, the outer hub is the cylinder that houses the brushless motor.
These dimensions outline the Fan Swept Area known as FSA. This is the total area of the largest measured diameter minus the total area of the smallest measured diameter. The difference in area will provide the area for 100% FSA. 100% FSA will provide the maximum amount of thrust. Generally, the smallest percentage one would choose for maximum exhaust exit velocity is 80% FSA. The balance between these two points would then be 90% FSA. It is best to try each interval out to get desired performance.