Why do EDFs need thrust tubes? This article will inform you!
Electric Ducted Fans - What You Need To Know
Before going over why exactly you need to be optimizing your EDF units with thrust tubes, let’s talk a little about their characteristics.
EDFs use higher KV motors than most independent brushless out runners. This is so they can rotate a relatively small fan fast enough to produce sufficient thrust to propel an RC airplane. A side effect of this is that they take a long time to spool up and therefore have worse acceleration than a conventional motor-prop combo. To get the most out of an EDF, you’re going to need an intake ring and a thrust tube to take in and accelerate the air optimally.
Hey! If you want to know more about ducted fans, read this article about building, tuning and flying EDFs to expand your knowledge.
Are EDFs Less Efficient?
Yes, when compared to a conventional brushless motor prop combo, electric ducted fans are generally faster, draw more amps and are less efficient. They draw a lot of power to spin those fans and only have a very narrow speed range where they’re at their prime. At a fast cruise, an EDF powered plane is in its element.
Why do I need a Thrust tube:
Controlling the airflow into and out of an EDF unit is important for getting the blades working at their most efficient speed range and maximizing the performance of your RC airplane. What a thrust tube mainly does is help the air to accelerate. It does this by making the exit diameter of the tube smaller than the size of the EDF unit.
So how does narrowing the exit tube work? Well, by narrowing the exit diameter, the air is compressed more by the fan and is forced out of the exhaust faster. This means that your aircraft can reach higher top speeds. It also means you can fly at a cruise using less throttle.
The thrust tube also can improve the airflow of air exiting the EDF unit. If you think about an EDF unit, with the motor, motor mount, spars, and veins, you’ll probably realize it has a pretty turbulent effect on the air as it passes through. A thrust tube can help to minimize this effect, much thanks to the smooth surface as it compresses the air after it exits the main fan unit.
Determining the Tube Size:
There are two key factors that need to be considered in order to determine the overall tube size.
1. Exit diameter
2. Tube Length
Here is a thrust tube calculator for working out the size of your EDF tube. It's super helpful!
As a rule of thumb, the exit diameter should be about 90% of the fan diameter. For the length, a thrust tube should be around 4x longer than the fan diameter.
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Build an EDF plane:
Article by James Whomsley
Editor of FliteTest.com
YouTube Channel: www.youtube.com/projectairaviation