Electric Ducted Fans - Duct Design

My research at EJF.com into the area of ducting and fan design has been one of the most consuming parts of the development process and at each area of a design question the R.W. Kress articles and the book "Ducted Fans for Model Jets" by David James makes worthwhile reading

efflux
Meaning "outward flow" and it's complement is influx which means "inward flow." Both of these are critical to a successful and usable efflux velocity.

Our goal in ducted fan design at EJF.com is to take an incoming air influx and accelerate it to produce a substantial increased air efflux during dynamic air intake to gain maximum flight performance. The static or zero influx of air measurements as I've discussed before are starting points for the design process and specifically the Mini Fan and HET 6904 EDF units that I've done substantial testing with. For those who are familiar with the wet jets another hidden fundamental difference between gas and electric DF design is the amount of torque produced by a nitro burning power plant under dynamic loads that turn the impeller. As an example one of the leading DF units from Bob Violett produces about 13lbs of thrust at 24,000 RPM with a .91 engine. Now I'm sure you're thinking "just attach an electric motor and off you go." Once doing that you very quickly come full circle back to the electric motor torque issue. In the near future we will see a better matching of this torque as newer electric motors and power packs are released.

It does happen quite often that experimenters can produce a high static efflux velocity and then have an undesirable decreased dynamic efflux velocity that causes either poor performance or a flight failure. For this reason it's important to start with proven EDF units and motors for your initial developments. There are a variety of emerging electric motor designs that will prove important however incremental testing should be done after your first successful EDF design.



Inlet Lip
The inlet lip has been discussed very frequently and many have different approaches to the design. In full scale aircraft you will notice a variety of methods to the inlet lip and air intake design and you can be assured that a serious amount of aerodynamic testing has been applied to their design. The most suggested in print is an inside tapered airfoil that varies with the scale of the inlet. A couple of important inlet lip recommendations that I have is to never have a blunt edge. Make the lip at least a nice rounded edge and construct it to be solid. If the intake lip distorts under dynamic loads this disrupts the air influx and causes turbulence and will cause your aircraft to operate as though it loses power when accelerating or maneuvering. The impeller and motor will then have a great amount of stress during these conditions and the impeller will actually stall for lack of air. If you have a fuse installed that would be the only long term protection from system damage.

Intake tube
The intake tube must be air tight and no less than 85% of the impeller diameter and attached to the EDF unit, it's designed to deliver the air as straight as possible with a constant influx of air to the fan and as turbulent free as possible. Every bump and curve along the way will add to the loss of influx efficiency. When the air is turbulent it works against the movement of the impeller and causes an increased loading on the motor. This means that a smooth air tight and aligned intake tube will deliver the highest efficiency. In addition to that it must be very strong and not collapse under static and dynamic pressure, there is a tremendous vacuum generated as the collected air is increased. As the distance increases it also becomes even more critical. If you can deform it at any point along the way at the very minimum it will cause turbulence, and if it's too flexible it can collapse altogether and cause the impeller stall scenario. When using flexible materials also use some epoxy soaked carbon fiber strips around the intake tube at reasonable intervals to eliminate any flexing. Use the epoxy sparingly since you don't want to add unnecessary weight.

EDF unit
The placement of the EDF unit seems to be a very mysterious thing, some at the front, middle and end. It's now clear that only when the air reaches the fan does the impeller begin to accelerate it as well as pushes it through the stators which also add in the acceleration and smoothes/straightens the air. Yes, the motor wires, brushless controllers and any other items in the way dirty the air and cause a level of exit turbulence. It is important to minimize all of these as best as possible. I would like to note that our wet cousins seem to move the fan unit back towards the middle and create a thrust tube to cover the entire distance of the tuned pipe length and then use a variety of techniques and fairings to smooth/straighten the air and maintain a high efflux. Remember my torque discussion, the wet jets have a higher engine torque so they can push the air with more force over a longer distance. With my aircraft and ducting experiments, I've found that the EDF unit placement has made the biggest difference in generating a high efflux velocity. It is important to place the EDF unit such as the Mini Fan toward the rear of the duct.

Thrust tube
The thrust tube is what directs the generated efflux out the tailpipe. This tube must be air tight and extend from the edge of the fan shroud with a taper not less than 90% of the impeller diameter. I recommend that the thrust tube length be kept between three and seven inches for optimal use. Like the intake tube, it should be deform free, straight and as obstruction free as possible. The expanding air will cause exit turbulence and efflux loss. In some full scale jets the exhaust tailpipe can have a variable geometry and optimize static and dynamic operation while in motion. It's important to have some amount of taper to add in the dynamic efflux velocity. Having it the same size only benefits the static efflux and reduces the dynamic efflux potential. If it's too small back pressure will build up and load the impeller down.

Summary
It seems like a lot of people make the duct design harder than it needs to be. Lets just summarize the above into a few solid principles. The important thing is not to get hung up on trying to design the perfect duct. These principles will get you in the ballpark, which is good enough for most of us.

1. The inlet area should be between 85% and 100% of the impeller diameter
2. The tailpipe should be about 90% of the impeller diameter.
3. The HET 6904 or Mini Fan fan unit should be located toward the rear of the duct, within 3 to 7 inches of the duct's tail.
4. The duct should be as clean and smooth as possible. Turbulence in the duct is a very bad thing.
5. The inlet should be rounded. A sharp inlet will cause the duct to stall which will destroy the duct's efficiency. A model with a blunt inlet will lose power during manoeuvres.
6. Make certain the duct and intake are rigid and will not flex under the pressure created by the fan unit in flight.