More on Ductwork at EJF.com

For an in depth reading of airflow physics goto http://en.wikipedia.org/wiki/Fluid_dynamics

For the benefit of my EJF customers I will stay focused on the real meat and potatoes of EDF Jets with examples. I have spent over 10 years convert many Turbine and fuelled EDFs to Electric including 1.8 scale machine with 12ils thrust for a 12lb craft. I'm not trying to taunt the Physics Police and get pulled over, just present some working solutions. I do enjoy the e-mail so keep it going and be specific. Review the past document lists in previous EDF columns for references.

The 10 Cent Tailpipe

Let's start with the tail and work forward, The tailpipe can be derived from some very common items. I'll be using the Mini Fan and HET 6904 series of EDF units however this technique will work for others. Notice in Figure 1. that these look like ordinary plastic cups. Well yes they are, the 16 oz. and 32 oz. Solo cup once cut to size make a perfect tailpipe that can then be painted and detailed.


The Mini Fan has a 72 mm outside diameter so to better adjust both the coupling of the tailpipe to the EDF unit and adjust the exhaust diameter of the tailpipe a lengthwise cut is required for proper adjustment. Once the position is in place C.A. can be applied for a permanent seam. Figure 3. shows the tailpipe connected to the Mini Fan and Figure 3a. shows how you can use the 32 oz. cup to make a complete nacelle. When the tailpipe is ready for connection it's advisable to tack it into place with C.A. and then use a thickened bead of epoxy around the circumference. Don't try to do both sides at once, unless you're in micro gravity that is.

The HET 6904 series of EDF units  and Pro both have a 70 mm outside diameter and I wasn't required to make a lengthwise cut as you will see. Notice in Figure 4. the cut locations and the resulting Figure 4a. Once each is cut the tailpipe now has a flange that makes a very nice coupled joint at the EDF unit and a clean exhaust path for the air. Figure 5. shows the attached tailpipe ready for action.



The Mighty Ducts
Don't worry these are not shots of my shop vac attachment just so we get that out of the way, This is one side of my twin F-15 intake duct form, Also known as a cheek inlet. Figure 7a. shows what the full scale F-15 intake looks like. Notice the side view in Figure 6. I have included a similar angle as part of the ducting. Also notice the bottom view in Figure 7. do you notice that there is a shape change? By now you may realize that this duct is made from the standard intake duct size as it couples to the EDF unit. The intake lips are actually part of the fuselage not shown.

Now this takes us into the next important issue of duct design, maintaining the volume of air that the EDF unit receives. Just as we see different shapes and sizes of drinking cups that hold the same amount of liquid, We also have different shapes in ducting that are required to keep the same volume of air despite their shape. I've chosen to use a standard duct size and alter the intake area by forming the intake to the desired shape.

Figure 8. shows the intake duct for a F-16 chin inlet and Figure 9. shows that the shape has been alliterated however the volume has been maintained. Again here is the full scale F-16's intake Figure 9a. notice the shape. Also note that the intake lip is part of the fuselage not shown.

What is it?
Animal, Vegetable or Mineral? Don't worry it's not organic, This is a duct system for a wet jet F-117 that uses a single 5" fan unit. Many of the scale wet jets use a single ducted fan unit for both reliability and cost savings. Over the years many manufactures have become very creative in making bifurcated inlets and outlets and this is a whopper. As we have discussed maintaining the air volume is alive and well in this design. When using a bifurcated system the total area of each side must create the required air volume as well as the tailpipe totals.

This is a duct system for a wet jet F/A-18 that uses a single 5" fan unit. I coupled the two ends together just for effect. Where the two pieces are joined is where the fan unit would set.

Here is a sneak peek at one of my EDF test stands where I've spent over a month with cell counts and ducting experiments that has left me speechless with glee!

Summary
I hope by now your feeling of EDF design is less intimidating than when we first started, There are many complex topics involved and I hope I haven't watered them down to much so useful work can be done. After all we want to fly Jet like model aircraft, that's why your reading this.

• 1. Keep the volume of air through the intake matched to the EDF unit despite the shape.
• 2. Keep the inside exhaust diameter of the tailpipe to around 90% of the impeller diameter.
• 3. Keep the inlet and outlet connections to the EDF unit as air tight as possible.