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Electric Ducted Fan Jets Discuss electric ducted fan jets here including setup tips, power systems, flying techniques, etc.

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Old 02-04-2011, 01:56 PM   #1
quorneng
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Default A low power Douglas A4D Skyray EDF

The DuctFan demonstrated it was possible to fly a low power and slow EDF, so I wanted to try a more conventional 'jet' configuration.
There are several Douglas Skyray EDF kits but could one be built with a wing loading as low as the 6.8oz/sqft of the DuctFan.
Scaling up by 4 from this rather battered 40 year old 1/90 kit plastic kit gives a wing span of 18".
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It would just possible to fit this 2.25" fan inside.
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To keep a 6.8oz/sqft wing loading would mean a target weight of 7.5oz but nearly 50% of the projected area is 'fuselage' and so of questionable lifting capability. A target of 6oz would be a safer bet.
Before going too far I thought I had better built a 'test' wing to develop a suitable construction method.
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A true symmetrical section and 8% thick. It has a 1mm hard balsa strip to protect the leading edge.
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The LH wing complete with elevon.
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With a 3.7g elevon micro servo it will weigh 0.43oz (14g).
On a purely scale note the elevons did not occupy the complete training edge. The inner section was a landing flap that unusually moved up to minimise the required elevon input to ensure full aileron capability at low speed.
Having got this far I do wonder if it is not too small and whether a 22.5" (x5) or even a 27" (x6) version would be easier to fly.


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Old 02-04-2011, 04:11 PM   #2
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Hi,quorneng , anything is possible but EDF will not be efficient on slow moving plane.
EDF need high speed air blasting into its intake lip to run efficiently.


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Old 02-04-2011, 11:02 PM   #3
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I am not sure a ducted fan is ever particularly efficient.
Only well beyond model flying speeds does prop efficiency actually fall below that of a ducted fan.

I think my DuctFan demonstrated that you can fly a ducted fan at low power and speed reasonably well.

The Skyray will of course have a much longer duct but I anticipate its losses will be less than a conventional EDF due to its much lower duct velocity.
We shall see.
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Old 02-05-2011, 05:07 AM   #4
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Originally Posted by quorneng View Post
I am not sure a ducted fan is ever particularly efficient.
Only well beyond model flying speeds does prop efficiency actually fall below that of a ducted fan.

I think my DuctFan demonstrated that you can fly a ducted fan at low power and speed reasonably well.

The Skyray will of course have a much longer duct but I anticipate its losses will be less than a conventional EDF due to its much lower duct velocity.
We shall see.
EDF is not design for use on slow plane . Sure you can use EDF but its a wrong intended applications.

Moreover,EDF is generally less efficient than propeller setup but you can mitigate the inefficient but putting the EDF into an airframe with low drag design, unobstructed inlet & outlet design and improve the flying speed and not trying to fly slow.
By deploying the EDF on a slow flying plane will excebate the inefficiency further.

If you want to setup the plane for slow flight , it maybe better to use a larger prop running at low rpm. You get longer flight or could run at smaller size battery pack an achieve a lighter weight setup.


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Old 02-05-2011, 08:45 AM   #5
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Regardless of efficiency a prop just doesn't look right on a jet.. How can you call it a jet model if it has a big ugly propeller hanging off it?... So I agree ducted fan it must be, efficient or not!


The scale of your proposed model is very close to a freeflight indoor ducted fan Skyray that UK modeller Steven Glass did back in 2003.

Here is a video:
I believe Steve used a 44mm EDF unit in that one. Steve used to wind his own motors and make his own controllers but these days that's not necessary as micro motors etc are readily available. It might be worth considering a commercial 40-50mm EDF unit as I'm sure these will give much better results than any 'home brew' system and the commercial ones will need less power and will be lighter.

My own 17" span Mig 17 is also along roughly similar lines though the Mig is a lot less 'bulky' at that span than the delta winged Skyray would be and so weighed only about 3oz.. the Mig used a 30mm EDF converted to brushless motor (photo attached)

Looking forward to seeing this one come together.


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Old 02-05-2011, 09:17 AM   #6
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Not wanting to hijack the thread, but my Me 163 Komet and Me 262 with pusher props are blistering fast and eat any EDF I have ever made, plus you really cannot even see the props in flight at reasonable heights. They look very much like a jet/rocket model in the air. Of course on the ground they are much more obvious. Still, if you want the purist scale look in the true sense of the word, EDF is the way to go for jets. You just can't expect the same performance. Best of luck with the Skyray
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Old 02-05-2011, 02:02 PM   #7
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[QUOTE=JetPlaneFlyer;782971]Regardless of efficiency a prop just doesn't look right on a jet.. How can you call it a jet model if it has a big ugly propeller hanging off it?... So I agree ducted fan it must be, efficient or not!

Agree that a pusher propeller on a jet looks weird or unrealistic, on static display . However in flight, you can hardly see the propeller. On the same argument if you have a realistic jet plane like my F18 EDF version, but fly slower than a Cessna how can one call the F18 a jet plane. Itís kind of weird too.
What is more important in flight, jet aircraft must have the speed to be realistic.
I have posted this two video before but for the sake or argument or opinion I am going to re-post these two videos.
F18 with 64mm EDF running on a 3.9KV motor
http://www.youtube.com/user/starscre...23/uEV4W-Gpvas

After transferring the electronics, EDF, motor& ESC using the same setup as the F18 to a EDF converted micro-jet .
http://www.youtube.com/user/starscre...32/TFp7tKCWIgY

Tell me honestly which one is more realistic?
Moreover, the benefit of transferring to a higher flying speed airframe are:-
1) Lower current during speed flying
2) Longer flying duration
3) No overheating to the motor.
4) Battery is less taxed and last longer.

Cheers and all the best
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Old 02-05-2011, 03:10 PM   #8
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babylon5,

If realism is the aim than we should aim to fly at the SCALE speed of the real plane, not 'as fast as possible'.

Taking the Skyray: The real plane had a maximum speed of 722mph. Quorneng is modelling his at 1:22.5 scale, so for perfect realism the scale top speed should be 722/22.5 = 32mph

Landing speed of the real one would be in the ballpark of 150mph which scales to 6.7mph

So for optimum realism a 1:22.5 scale Skyray should have a flight speed range between 6.7mph and 32 mph.

The F18 is a faster plane but if modelled to the same scale then the speed range would be about 8-50mph. Small models tearing around the sky at massively over-scale speeds might be a whole lot of fun but to my eye they don't look anything like 'realistic'

Of course if optimum realism is not the only goal and you just want a fast flying fun model then go for it.
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Old 02-05-2011, 06:56 PM   #9
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Although 'scale' speed was not really the object of this build I do rather agree with JetPlaneFlyer.
Low power tends to lead to light weight planes and low speed by necessity.
The down side is that any wind becomes a no no.

Here the wings are in their correct relative positions and it shows just how wide the centre section/fuselage actually is.
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The fan will be set well aft with a short convergent duct, but if I can achieve it, the centre section will be completely hollow giving an inlet cross section area nearly twice that of the fan. This will reduce the inlet air velocity and the duct losses - I hope.


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Old 02-05-2011, 10:39 PM   #10
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Originally Posted by JetPlaneFlyer View Post
babylon5,

If realism is the aim than we should aim to fly at the SCALE speed of the real plane, not 'as fast as possible'.

Taking the Skyray: The real plane had a maximum speed of 722mph. Quorneng is modelling his at 1:22.5 scale, so for perfect realism the scale top speed should be 722/22.5 = 32mph

Landing speed of the real one would be in the ballpark of 150mph which scales to 6.7mph

So for optimum realism a 1:22.5 scale Skyray should have a flight speed range between 6.7mph and 32 mph.

The F18 is a faster plane but if modelled to the same scale then the speed range would be about 8-50mph. Small models tearing around the sky at massively over-scale speeds might be a whole lot of fun but to my eye they don't look anything like 'realistic'

Of course if optimum realism is not the only goal and you just want a fast flying fun model then go for it.
For the sake of a discussion and speaking for myself that my Ff18 is flying so slow that it is not even close to scale speed.
The F18 video that was posted by me, is one of the slowest ducted fan in the field and a slow cessna easily overtake it. After a while I find the F18 too boring to fly. The F18 is so slow that the EDF unit cannot unload its impeller in flight and the systems are prone to overheating

Please try to understand commercial EDF setup usually require speed to run efficiently and to avoid overheating.

The F18 is now a display unit in my home.


To add further, there is no right and wrong if someone want to use the EDF for slow speed setup.


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Old 02-06-2011, 06:28 PM   #11
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Slowly feeling my way.
An elevon servo mounted in the wing.
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In fact the wing is so thin at this point that the upper surface had to be cut out, thinned down to less than 1mm and glued back.
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As the elevon is top hinged and has limited down travel, the servo horn is set forward 45 degrees to give a significant differential action.
I will be using this neat V tail mixer unit so I can still use my basic 6 channel tx.
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The fuselage is next but to make it completely hollow out of 3mm Depron is going to be quite a challenge.


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Old 02-08-2011, 11:49 PM   #12
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The fuselage is slowly starting to take shape but still mounted on its cardboard tube 'plug'.
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The duct diameter was too small to 'wrap' the depron so it was planked.
The outer skin has gentler radii so it should be possible to cover the underside in a single piece.
This will strengthen the structure sufficiently to allow the duct and first former to be cut away to create the bifurcated duct.
The top skin will be added once the wings are in place and the elevon servo cables run along the top of the fuselage.

Well that's the plan!


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Old 02-11-2011, 06:26 PM   #13
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Progress is rather slow as each step has to be carefully thought through as with the fairly complicated shapes involved you have to make sure any subsequent step is still possible!
The single piece bottom skin.
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This was about a big as it was possible to do in Depron as some double curves were required.
With the plug removed the paper lined duct is visible.
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The fuselage has to be built next as it determines the shape of the bifurcated duct.
The five formers.
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To ensure the fuselage remained straight it was built 'free standing' off the bottom keel strip. Very delicate until sufficient planking is added.
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When the planking is complete the fuselage will be grafted into the main duct and the two inlets built around it - I hope!


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Old 02-13-2011, 06:37 PM   #14
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The cockpit section fully planked.
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Amazingly rigid it only weighs 0.275oz.
The cockpit grafted into the centre section and the LH inlet duct completed.
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My only concession to scale is to enlarge the actual inlet to match the greater than scale fan and duct diameter.
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Nozzle theory says that maximum propulsive efficiency is achieved when the plane speed approaches that of the jet velocity hence modern airlines cruise at about Mach 0.8 from the nearly supersonic high bypass fan exit.

My Skray is intended to fly slowly (30moh?) so the fan exit need be no more than say 40mph. The result may be efficient but such a low exit velocity does limit the maximum thrust from the available fan diameter. It is in effect a high bypass fan running in a turbo jet sized fuselage.

To fly at all like this does place a great emphasis on light weight and low wing loading.

Still some way to go.


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Old 02-17-2011, 11:13 PM   #15
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A bit more progress but now each stage has to be carefully thought through to make sure nothing gets left out that would be difficult to rectify later.
The wing attached. With the elevon servos in place it weighs exactly 2oz.
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The issue now is the fact that the motor, battery and RC gear will weigh nearly 3 times that of the basic airframe so their position is critical yet with limited access once complete any subsequent adjustment will be very difficult.
I need to know the CofG position quite accurately.
To do this I built a simple 'same size' Skyray glider.
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I felt it was important to aerodynamically model the fuselage centre section.
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It uses a completely hollow fuselage and the whole plane only requires a total of 10 pieces.
A few tests in the garden established the correct nose weight. It only weighs 1.36oz but I do now have a good idea where the CofG must be.
It is reasonably stable with no trailing edge reflex but for such a light plane it does seem to fly quite fast and the scale version will be over 4 times heavier.


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Old 02-17-2011, 11:25 PM   #16
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Hi
Shes coming along quite nicely
Very impressive
Cant wait to see more
Take care
Hank

"When wild the head-wind beat,Thy sovereign Will commanding, Bring them who dare to fly, To a safe landing."
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Old 02-18-2011, 07:07 AM   #17
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Yeah, looking really good!

Having flown small scale (freeflight) Skyrays along with many other tailless models I'm sure you will need some reflex. Perhaps if you put some reflex in on the glider and edge the CG forward to suit then the glide speed will come down.. At that weight it should glide quite slowly.

Due to the blended fuselage and wing the Cg on the Skyray needs to be well forward. There is a plan for a small scale version on 'ffscale' website with CG shown.. I've seen this model fly so I know it works. For RC you could probably move the CG a little back and decrease reflex a little (Mike needed plenty of reflex on his model as the still photos shoe): http://www.ffscale.co.uk/plans4.htm
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Old 02-19-2011, 11:43 PM   #18
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The first test of the 9 blade 55mm diam fan mounted on an 11g 3500kV motor running on a 500mAh 3s.
YouTube Video
ERROR: If you can see this, then YouTube is down or you don't have Flash installed.

It produces 2.25oz thrust. It should generate a bit more in the close fitting duct and exhaust nozzle but there will be inlet losses as well.
It should still fly - I hope!
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Old 02-20-2011, 03:11 AM   #19
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Well, after seeing how your last project turned out (even with a homemade fan) I bet this will work. Maybe I will scratch build this, but It seems like someone could make a mig-15 with a larger than scale duct and put two props in it rather than one fan. Two tiny props may equal one big, and we would finally have prop efficiency with jet looks.

"I didn't crash... I just did an inverted nose landing!"
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Old 02-20-2011, 02:26 PM   #20
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quorneng,

I hate to always be the one taking the role of naysayer but i think you may find yourself underpowered with only a little over 2oz of thrust, even if you achieve your 6oz target weight. My experience with similar jet models is they need a thrust/weight ratio of around 1:2 minimum to fly halfway decently. Unless thrust is increased substantially one installed (a possibility) then I think you will be badly short of thrust.

There are many options that you could consider to increase your thrust dramatically. In my spare parts box I've got two that would do it:
  • 40mm AEO off the shelf fan/motor unit. My own measurements: All up weight = 20.3g, thrust = 120g (on 2 cell 450mah LiPo)
  • GWS EDF50mm fitted with 12mm 5800kv in-runner. My own measurements: all up weight = 30.4g, thrust = 185g (on 3 cell LiPo)

The AEO 40mm unit would look like a good lightweight option (lighter than what you have now most likely) and would give you about twice the thrust, plus the smaller diameter is easier to fit in the fuselage.

I appreciate that part of the challenge of the project is the DIY fan but it would be a shame to build such a nice model and then for it to be hopelessly underpowered. Perhaps it would be possible to allow for the fan to be swapped out if necessary?

Steve
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Old 02-21-2011, 08:12 PM   #21
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Almost ready to mount the motor 'pod' in the duct.
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It is hollow to allow air to pass through the motor.
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A combination of wood and Depron the complete unit weighs 0.85oz (24.3g).


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Old 02-25-2011, 07:13 PM   #22
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The motor and fan assembly.
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I installed it into the Skyray and set up for an initial test but a bit of a disaster - it no longer worked - the ESC just beeped!
A bit of testing showed I had lost one 'phase', I suspect one of the lead out wires had fractured inside the motor.
The only course of action was to strip it all out again and install a new motor.
The new motor fixed to the mounting.
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With the fan attached.
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The gap between the mount and the fan will be covered by a ring of Depron.
In the Skyray I will mount the fan right at the back to keep the length of duct with the high velocity air as short as possible.
This a scale(ish) drawing of the installation.
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The rather large diameter of the fan hub means that the actual fan area is only 74% that of the inlet duct. The inner and outer exhaust cones reduce this by a further 10%.


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Old 02-27-2011, 03:05 PM   #23
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I'm having a play at low powered EDF as well, though to be accurate it's EDP, Electric Ducted Prop.


Just waiting for a descent flying day to try her out.


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Ray in Wales
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Old 02-27-2011, 08:11 PM   #24
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eflight-ray
That's big! Is it Depron?
When you say a ducted prop I assume you have a conventional prop(s?) inside the fuselage.
I must admit I was very tempted to go down your route - really big and light.
Do let us know how you get on.
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Old 02-28-2011, 01:41 AM   #25
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Originally Posted by eflight-ray View Post
I'm having a play at low powered EDF as well, though to be accurate it's EDP, Electric Ducted Prop.


Just waiting for a descent flying day to try her out.
Wow, that thing is big! Seems like you would want to have multiple props in there, do you?

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