EDF efflux tube?

[SIMON CRAGG]

I am in the process of modifying a Flite test foamboard model to fit a 70mm EDF.

 

I have several models of this type, all without an efflux tube.

 

I am sure it could be a very complicated subject!.

 

However, reading several posts, I have found that for a 70mm EDF, the yardstick is a length of four times the diameter = 280mm and a reduction of circa 20%.

 

Has anybody tried this or similar?

[Lipo Man]

I’ve built a few own design EDFs from insulation foam, and they all perform well - this is what I did. 
 

The most common recommendation I could find was that the outlet of the tube should be about 85% of the SWEPT AREA of the fan - that’s the area of the fan diameter minus the area blocked by the motor. Assuming your 70mm fan has a motor of about 28mm diameter (that seems pretty standard?) than 85% of the FSA would give you a diameter of 59mm the exit of your tube. I suspect the length is not as crucial - mine are about three times the fan diameter, but I think a bit longer or shorter won’t be make or break. 
 

Going a bit larger or smaller diameter will increase or decrease the static thrust and exit velocity of the tube. I tested adding an additional cone to reduce the area of my tube outlet - the intention was to sacrifice a bit of static thrust for some additional speed. It was an epic failure and I didn’t try it again…

 

Good luck - here’s a short video of my latest design as proof of flight! This one is on a 64mm fan. 
 

Good luck!
 

 

[Engine Doctor]

Not sure about the formula but a small plastic coffee cup fitted pushed over the back of a 70mm fan and the bottom cut off about 1 inch up works very well.  Tried this at our field when a member turned up with a EDF zagi . Running on 4s it didnt have any power to maintain flight . He had left off the inlet flare , or whatever its called off and had no eflux tube. We fitted the inlet flare and thrust improved slightly then tried the coffee cup as a experiment as it was all we had in the club shed . It was pushed on and held in place with some insulating tape . The trust was so improved that flight was maintained at less than half throttle . The coffee cup idea has now been used in a few models .

Edited November 12, 2023 by Engine Doctor

[john davidson 1]

I have used the foam coffee cup method too, one point is that it can make the motor work harder , faster flight but reducing duration. All down to experimentation.

[Engine Doctor]

Hi john we used the plastic cup . The foam ones seem to have a bit more of a taper from memory .

[Futura57]

Can I ask about the shape of an efflux tube at it's exit. I am looking for more of an elliptical shaped exit to suit a stealthy design aircraft. Perhaps bordering on flat. This would be like pulling the corners of your mouth open to create a wide smile. Provided I followed the area rule of thumb would this complicate matters?

[PeterF]

For a given flow area, the friction will be higher because there is more duct wall so the back pressure on the fan will be higher which will reduce the air flow a little but it will probably not be a problem. Consider the extreme of a slot 300mm wide and 2mm tall. I did obround (oblong with round ends) outlets on a B2 stealth and measured the thrust vs circular and the difference was small.

[Simon Chaddock]

Much of the air flow reduction in an EDF duct are due to the wall area and surface finish if its walls. The wall friction losses are the result of the air's viscosity slowing down the flow next to the wall thus reducing the average total velocity and thus thrust. It follows that a cylindrical duct will have the smallest wall area for a particular duct area. A calculation of the perimeter of your duct and its area compared to the perimeter of a similar area circular duct would give you an idea of the increase in the wall losses.

In other words the narrower the "smile" of your duct the greater will be the wall losses.

You also have to consider how the duct is going to change from the circle at the EDF to the final nozzle shape.  The more gradual the change the lower should be any losses from turbulence created by a sudden change in direction but the longer the duct required to achieve a smooth shape transition the greater the wall friction losses.

 

Having said all that the wall area from the total length of the duct (inlet and exhaust) is likely to make a bigger contribution to duct losses than the nozzle shape itself unless it is very long and narrow.

[Frank Day]

I  buy from a chap who 3d prints tubes to order, apparently for our purpose as long as the opening is within the 80-90% the shape doesnt matter.

 

He has printed round ends, "squashed" ends etc in different lengths 

[SIMON CRAGG]

Problem solved.

 

Found a programme that works out the FSA and produces the relevant design criteria,

[Engine Doctor]

16 hours ago, Futura57 said:

Can I ask about the shape of an efflux tube at it's exit. I am looking for more of an elliptical shaped exit to suit a stealthy design aircraft. Perhaps bordering on flat. This would be like pulling the corners of your mouth open to create a wide smile. Provided I followed the area rule of thumb would this complicate matters?

If you make an eflux tube with the ammount of taper you require it doesnt matter if the end is made to fit the shape of the outlet. I did yhis to a Tomcat model wi5h a single 70mm fan that looked like it had two outlets/ engines . It made no difference and had excellent power. I dare say the theorists will disagree  but in practice it works. The model was a " Highend technology F18 Tomcat " from around the late 90's to mid 00's.Eflux tube was made from acetate sheet made into a normal tube with taper then fitted through the elongat3d outlet and fitted to fan in the normal way. The oulet end had to be supported as there was a gap either side . I made an infill to sit insde tge dummy motor outlets to support the tube.

Edited November 13, 2023 by Engine Doctor