EDF "Stators" ? to straighten airflow in thrust tube

[Frank Day]

As title, has anyone attempted to straighten the airflow in a thrust tube

[J D 8]

May well straighten the airflow like in a test wind tunnel but I would think any gain would be negated by the extra drag. Never tried it or seen it done though.

[Paul De Tourtoulon]

I did years ago with methanol engines,,,( 1984 onwards )

 

 I 'might' have some pictures somewhere,,,

 

 I treated my turbines as 'air accelerators',,, know what I mean,,,

Edited May 29, 2022 by Paul De Tourtoulon
accelerators

[john davidson 1]

Better to keep it clean, a badly  made tube can seriously reduce efficiency. The one on the Arrows Hawk is a good model to copy, just a very small narrowing, too much narrowing increases speed but places more load on the motor and decreases flight time

[Paul De Tourtoulon]

I was 'taught' to keep the areas all the same from the front to the turbine to the back of the pipe, any difference in the surface is moving the air about which in course of a is wasted in energy,,,,,

[Frank Day]

Thanks for the replies, I have a guy who 3D prints, he's done a few bits for me before, thrust tubes, inlet and outlet rings from my drawings. He may print for others if asked.

 

He can print a tube to suit, I may have to get one with and without efflux and do some static thrust tests

 

Some pictures would be good to see Paul

[Frank Day]

I have a Gary Hethcoat Horten 229 for two 70mm EDF and dont want to run two toilet roll cardboard tubes out the back. The issue is the way the air expands from the EDF in a spiral and causes issues on the TE/Bat tail. This isnt an issue with the turbine version.

 

I've been on Horten the specific forums and no one has mentioned creating vanes or stators in the trust tube to straighten the airflow. It is a possibility that it will cause hideous amounts of loss but doesnt seem to have been tried. 

[Simon Chaddock]

3D printing can be used to create quite effective EDF ducts.

I used them in my small 40 mm EDF Folland Gnat both the for inlet, exhaust.

The duct is printed in several sections with flush overlapping joints that are glued together.

In fact the duct is so rigid it is used as a 'spine' for the lightweight Depron fuselage to be built around it.

The whole duct is set to 1.2 times the FSA so no cheat holes are required. The nozzle on the end of the EDF reduces the area to 95% of the FSA. 

By setting the EDF right at the back the airflow is straight along 97% of the duct length so reducing duct losses.

I have used 3D printing on several EDF ducts.

 

[Frank Day]

Hi Simon,

 

I dont print myself but have found someone who is able to quickly and easily interpret my drawings etc and suggest the most suitable print material.

Before I start building I'm going to have a look at the inlets on the model as i'm pretty certain they are undersized. Once installed they are a straight run through with the fans set about a third of the way back. Im certain the vanes/stators will straighten the flow,  I'll just have to source the fans producing the most thrust etc to overcome losses.

 

Using the duct as a structural component is a great idea.

 

Im currently tinkering with a Robbe Concorde EDF conversion from twin pusher using two FMS 50mm 4s fans. I did fancy creating outlets of  50mm  to bifurcated twin outlets giving it the four engine look but that would take some work/working out to maintain FSA, haven't thought about it  much but I guess its a bit of A =Pi r2

[Simon Chaddock]

frank

Pi r2 is all you need although technically two ducts of the same area as one bigger one will have slightly higher losses due to the boundary layer being the same thickness regardless of the duct diameter.

Keeping a constant area during the transition from one to two ducts does require rather more complex maths!

My bifurcated inlet and exhaust used on my Hawker P1052.

Inlet area is 1.2 the FSA of the 55 mm EDF. The exhausts taper down to give a combined area that is 85% of the FSA.

 

Remember that extra vanes adds to the duct surface area and thus to the losses in the duct. It then becomes a question as to whether the extra thrust gained from flow straightening more than compensates from the extra losses the vanes create.

Flow straightening vanes are definitely used in high pressure low volume fans but I doubt it applies in a high volume very low pressure EDF duct.     

[Frank Day]

I may just have to get both printed and do some test rig time.

 

I've seen pictures of the P1052 but not seen a real one. It looks like the child of Gloster Meteor + DH Venom

 

Print work is fantastic let alone the maths and programming

[Lima Hotel Foxtrot]

I would have thought that unless the moving air needs to be straightened/untwisted for a specific purpose (such as being further accelerated by a second fan in series) then there is no need for stator vanes. Stator vanes exist in full size gas turbines, but for specific reasons.

 

Co-ax props on some drones don't use stators because the air being accelerated by the first prop meets a second prop of a higher pitch and lower diameter because the accelerated air does not form a cylinder of equal diameter; because of pressure differences. It's adding weight, compexity and drag when you don't need to.

 

I'd say that for model aircraft stator vanes are a flawed exercise in applying full size principles in the wrong area.

[Frank Day]

Hi,

The way the "exhaust" exits from the EDF in an expanding spiral affects the TE of the model as the outlets are "inboard" on the HO229 causing serious issues. It has been found that turbine powered models dont have the same problem, so  the thought was to straighten the flow and try to compensate for loss with better more powerful fans.

 

Owners usually just use a couple of push in clear plastic tubes to move the outlet aft of the TE but it looks pony.

[Paul De Tourtoulon]

I used clear stiff plastic from book shops as the outlet duct, don't forget it won't move as it's under pressure,,,

 

[Frank Day]

Im hoping to delete the tubes by straighten the airflow. I did have a conversation with Gary Hethcoat, he was aware that some where working on this. I've been on a number of the forums but no real updates, they are pretty old now.

 

He is aware that turbine versions dont have the problem, so it seems logical that smoothing the airflow should resolve the issue, hopefully!

 

I intend to build with split flaps and will also add the drag rudders in a simplified form and hook up a gyro to them. I noticed on he B2 that the drag rudders seem to be partially deployed all the timeso that may be something to lookat

[Paul De Tourtoulon]

Not much but I did find ma picture of the 'innards' of my Mirage 2000 which I made, as you see from fg /and carbon fitted with an OPS .40 spp rear fan engine and it's noisy pipe, the tube under the pipe is the fg tank, micro mould fan unit where the rear ring was changed from plastic to a turned aluminium  one and a steel penny washer on the front, probably around 135dB at 22.5000 rpm, up to 50% nitro, yes 50% not a typing mistake and a very expensive 'gold' plug per flight,,,?

 

 If you zoom in you might be able to see the aerofoil stators that hold the lot together,,,

[Simon Chaddock]

This does raise the question "Just how spiral is the airflow?" and depending on the degree of spiral "Is is worth straightening it?"

My guess would be that the airflow would have to spiral quite strongly for the gain in thrust to be worth the extra drag of any straightening  'vanes'.

 

To try to aser this question I added short ribbons to the motor supports of this EDF to see how much "spiral" could be detected. The ribbons were just 25 mm behind the fan so should detect the angle of the airflow as it leaves the fan. 

https://www.youtube.com/watch?v=MR9DjSlToBM

It certainly seems to show the degree of "spiral" but is very small and thus unlikely to be worth doing anything about it.

This ducted 4 blade 3x3.5 prop has a relatively fine pitch compared to a more conventional multiblade fan which might indeed generate a more spiral airflow.

 

My own observations suggest that almost anything in the airflow or even the surface quality of the ducting, has a significant detrimental effect to the thrust obtainable.

[Paul De Tourtoulon]

Looking back it's only an air accelerator, with EDF's it would be really easy to experiment with movable vanes and see weather it does make a difference,,,

 so come on Simon,,,,