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Help! Turbulent Air Inside EDF Flow Chamber.

#1
Every now and then a question or idea hits me. Unless you just hang a EDF (Electric Ducted Fan) unit off the wings of your plane, even making what appears to be a modern commercial Bypass Jet, most EDF's are inside after a long tube mounted at the aft of the plane. In this way your basic EDF air chamber looks like a mini-wind tunnel.
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Other than the EDF air chamber NOT having a TEST SECTION, I've never seen anyone put a HONEYCOMB-SCREEN-AIR STRAIGHTENER inside the PLENUM of an EDF system. Wouldn't using some type of AIR STRAIGHTENER, even scrunching a bunch of plastic straws inside the throat of the AIR CHAMBER just past the INLET allow the EDF unit to operate more efficiently working with NON-TURBULENT AIR? Or would the AIR-STRAIGHTENER cause too much drag?
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#3
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I have many pics of homemade or small wind tunnels thinking one day I might build one to test stuff in.
But as far as EDF's go, am I barking up the wrong tree, or could this... actually work?
 
#4
In a wind tunnel the air straightener smooths out the air flow and distributes it more-or-less evenly across the test chamber so that you can better see what happens when it encounters the test surface. Distributing it evenly: The honeycomb results in a pressure differential - higher pressure on the upstream side, lower pressure in the test chamber. The higher pressure on the upstream side causes the air to distribute itself evenly across the honeycomb, with about the same rate of flow through each segment. That gives not just a straightened air flow, but also evenly distributed airflow, for the test surface. That whole high-pressure, low-pressure thing - that's drag. There might be some benefit to having the air flow straightened and evened out, but I'd guess the drag makes it not worth doing.
 
#5
In a wind tunnel the air straightener smooths out the air flow and distributes it more-or-less evenly across the test chamber so that you can better see what happens when it encounters the test surface. Distributing it evenly: The honeycomb results in a pressure differential - higher pressure on the upstream side, lower pressure in the test chamber. The higher pressure on the upstream side causes the air to distribute itself evenly across the honeycomb, with about the same rate of flow through each segment. That gives not just a straightened air flow, but also evenly distributed airflow, for the test surface. That whole high-pressure, low-pressure thing - that's drag. There might be some benefit to having the air flow straightened and evened out, but I'd guess the drag makes it not worth doing.
That's kinda where I was going. But somehow... I think it may depend upon a few factors that would have to be TESTED! Maybe sometime this coming year I'll try a few things and just see what we get!
 

DamoRC

Well-known member
Mentor
#6
I think this is a great idea, at least as an experiment. Static testing should be pretty easy to accomplish. Dynamic testing would require a basic mule plane that the laminar flow generators could be added to and removed from coupled with some telemetry. This sounds like a lot of fun.
 
#8
I'm looking into a Viggen. I'm going to look over the Speed Build Kit build video to see if there is a way to get into the inlet/s to add a honeycomb air straightener. This could easily be tested by using a $20 wind gauge at the exhaust port. Yes?
 

DamoRC

Well-known member
Mentor
#10
I'm looking into a Viggen. I'm going to look over the Speed Build Kit build video to see if there is a way to get into the inlet/s to add a honeycomb air straightener. This could easily be tested by using a $20 wind gauge at the exhaust port. Yes?
Viggen sounds like a good candidate for this. On the measurement piece it depends on what you expect the laminar flow entering the EDF to achieve. You could get increased static thrust which means you will need a scales to measure the effect or you could get increased exhaust velocity which you can measure with a wind gauge (probably). You might also have other impacts such as reduced noise and possibly lower current draw.

I might have a crack at this as well using the thrust stand and just and EDF in a tube. I can measure thrust and exhaust velocity along with the voltage and amps. I might also be able to get RPM. Pretty sure I have a bag of straws around here somewhere.
 
#11
Viggen sounds like a good candidate for this. On the measurement piece it depends on what you expect the laminar flow entering the EDF to achieve. You could get increased static thrust which means you will need a scales to measure the effect or you could get increased exhaust velocity which you can measure with a wind gauge (probably). You might also have other impacts such as reduced noise and possibly lower current draw.

I might have a crack at this as well using the thrust stand and just and EDF in a tube. I can measure thrust and exhaust velocity along with the voltage and amps. I might also be able to get RPM. Pretty sure I have a bag of straws around here somewhere.
DUDE! I'm all in! I'm not sure why this hit me, but it was one of those... "Okay, a wind tunnel looks like the inside of a simple jet engine... so if a honeycomb or micro-tube air straightener helps air flow in the wind tunnel, why not inside an EDF plane?" Let's see. I have a digital Volt/Ohm/Amperage meter, digital mail scale, can pick up a $20 digital wind speed/temp gauge, a few $ for a battery alarm, hmm.
 
#12
DUDE! I'm all in! I'm not sure why this hit me, but it was one of those... "Okay, a wind tunnel looks like the inside of a simple jet engine... so if a honeycomb or micro-tube air straightener helps air flow in the wind tunnel, why not inside an EDF plane?" Let's see. I have a digital Volt/Ohm/Amperage meter, digital mail scale, can pick up a $20 digital wind speed/temp gauge, a few $ for a battery alarm, hmm.
Okay, I just saw the review of the Hobby King F-18 where Bixler installed mock afterburners with amber LED's and a Brushed Motor ESC, and a little bit of fiberglass batting. Say What? Anyone ever put this into a Viggen? Love to see pics!
 
#14
It sounds a great idea to put airflow straighteners in the tube just like adding a dynamo to an electric bike to charge the batter while you go along.
Thank you. I think I'm gonna snatch up a 69 mm EDF, motor, ESC kit, battery, and hit up DT for some more FB. Order a $20 wind speed/temp gauge, break out my mail scale and just see what happens! Oh, and not to forget... STRAWS!
 

Hai-Lee

Old and Bold RC PILOT
#15
Just a thought but I suspect that IF you gain any thrust increase in a static test the gain will not translate to greater thrust when airborne. In the same way that the centrifugal compressors had inbuilt headwind I believe a collimating array of straws would actually cause an increase in pressure in front of the EDF unit and a reduced flow through it!

I sincerely do hope I am wrong but I remember reading something on Whittle and Axial flow compressors a long while ago where there was a similar though rejected collimator used to straighten airflow.

Have fun!
 
#16
Just a thought but I suspect that IF you gain any thrust increase in a static test the gain will not translate to greater thrust when airborne. In the same way that the centrifugal compressors had inbuilt headwind I believe a collimating array of straws would actually cause an increase in pressure in front of the EDF unit and a reduced flow through it!

I sincerely do hope I am wrong but I remember reading something on Whittle and Axial flow compressors a long while ago where there was a similar though rejected collimator used to straighten airflow.

Have fun!
Okay, now wrong me if I'm off here. On the early Whittle jets, didn't they have a form of air straightener at the AFT/EXHAUST end? Maybe I'm looking at this... backward?
 

Hai-Lee

Old and Bold RC PILOT
#17
Okay, now wrong me if I'm off here. On the early Whittle jets, didn't they have a form of air straightener at the AFT/EXHAUST end? Maybe I'm looking at this... backward?
You are quite correct and the person who was an expert on axial compressors actually confessed that whilst he did not understand the "Vanes" that Whittle had included in the design they definitely helped avoid compressor stalls.

It was a long time ago now and all I can remember was the need for unrestricted inlet at speed and even inlet shaping to force the air in at high speed. (All of the theory was subsonic of course because at supersonic the requirement is to slow the air before it enters the compressors.

Have fun!
 

d8veh

Well-known member
#18
Thank you. I think I'm gonna snatch up a 69 mm EDF, motor, ESC kit, battery, and hit up DT for some more FB. Order a $20 wind speed/temp gauge, break out my mail scale and just see what happens! Oh, and not to forget... STRAWS!
It sounds a great idea to put airflow straighteners in the tube just like adding a dynamo to an electric bike to charge the battery while you go along.
In case you didn’t get it, those ideas sound great, but there's something that prevents either from giving you anything positive. It's called the first law of thermodynamics.
 

Chuppster

Well-known member
#19
I'm guessing that you'll want to maximize the surface area of your "straightener" to help mitigate the drag caused by the straws.

I'm thinking a thin-walled 3d printed structure would be ideal here.
 

DamoRC

Well-known member
Mentor
#20
In case you didn’t get it, those ideas sound great, but there's something that prevents either from giving you anything positive. It's called the first law of thermodynamics.
Not sure I see the relationship between this and what I understood to be the goal (although I did like the perpetual electric bike idea).

I believe it is accepted that an EDF works best if the incoming air is laminar.
A craft moving through still air with the EDF exposed at front is essentially in laminar flow territory.
Once we install the EDF inside the plane it requires inlet ducting to guide the air to the EDF and it is generally accepted that this ducting should preserve laminar flow (no sharp turns, rounded lips etc).
If you want to see a really bad example of an inlet, look at what I did for the Diamond.
I believe the goal here is to install a device in the ducting to improve the laminar flow of the air to the fan.
The experimentation will reveal if the additional construction complexity, weight, and drag in the duct can be offset by greater efficiency of the EDF.
I think that the approach could work to improve performance of poor inlet duct but probably won't help with a duct that is already doing a good job maintaining laminar flow in flight.