Ducted fan efficiency if engine is taken out of the equation (pure electric case)

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kenkad

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Why is there so much discussion about static thrust? If you are putting this on a LSA, you better find a way to use your front wheel (or some other drive wheel) to get you to takeoff speed. In addition, timing belt drives do not take up 40% of the ducted fan. This forum is supposed to help people make good design choices, not dwell on known issues that will not work in the real world. This thread seems to have become the same as the roadable aircraft thread. I would strongly recommend that everyone interested in shrouded propellers/ducted fans read the available literature (if not already posted here, then Google it)and then run you own tests.
kenkad
 

orion

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Hey Orion,
Let me see if I have this correctly. The shroud is essentially an airfoil made round. An enclosure so precise that the prop tips are not exposed to free air? Do they run in a slotted track of some sort?
BTW, when's the proto going to get airborne?
As always,
Thanks
Gary
In the simplest of terms yes however, if the design is off it's optimal point, the penalties involved can add up very quickly, the net effect of which would be extremely poor performance. For optimum function the shroud design has to be matched to the airframe, to the prop and to the performance envelope. The prop position has to be pretty accurate as well as the incidence of the section. The prop tips also need to be very close to the surface since any leakage defeats the purpose of the shroud. In our case we will have a channel molded into the skin that will be filled with an artificial grass (Astroturf) within which will ride the prop tips. This has been tested by the Rohr Corporation a few decades back and seemed to work perfectly. The artificial grass provides a flow surface but its flexibility allows to prop to operate beneath the tops of the grass blades, thus effectively sealing the interface.

The airframe is moving along well, albeit slow (we're doing two projects at the same time so our time is a bit split up). We just mated the wings to the body and are now working on some of the detail stuff on the sponsons. We should be able to mate the body and sponsons within the next three months or so, after which point we'll work on the empennage and systems. If all goes well we should be taxiing by end of summer (I hope).
 

kenkad

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Gordon,
I said in my thread that an LSA needs to use the front wheel for propulsion (electric motor) to get up to Vrotate (or some other powered mechanism that is not the shrouded propeller/ducted fan).

Orion,
The prop tip in a slot is discussed in the Periera's Thesis I referenced. I am suprised that AstroTurf has long enough threads to do what you are suggesting. What is the slot width and depth you are using? I am going to have the students try the slot with and without the AstroTurf type of sealing you are suggesting. I wanted to actually see the effect if it is measureable. Can you give me the title of the reference you said had done this before. Are you using counter rotating props or a single prop?
kenkad
 

Lucrum

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My intended method for dealing with blade tip/duct spacing is to incorporate a concentric ring around the blade tips. The ring will be flush with the duct. I hadn't yet devised how to "seal" those gaps though. The Astroturf idea certainly has promise.
 

karoliina.t.salminen

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Very interesting and informative replies so far, thanks. And thanks Gordon for the spreadsheet as well!

In reply to Gordon's replies:
However, as I mentioned, I was thinking about distributing the power to multiple fans.
I was thinking about this kind of specifications (based on your assumption of 100 kW):
Velocity 100 m/s
only 10 kW per fan (an electric motor (brushless DC) on each fan)
10 fans
totalling together 100 kW (100 kW based on Gordon's example).
energy from batteries, no engine.

Therefore the disc loading of each fan will be low because instead of one little fan, there are 10 little fans doing the same as this one little prop.
The difference is that what I have been thinking, construction wise, the fans could be installed to a wing like that, but (pusher) props (behind the wing) would require an extension where they would be installed.

I did not take however in account the problem of fans needing to be angled towards down to not cancel the lift. Indeed wing needs to throw the air down
or lift does not happen. This could hamper this idea somewhat. But doesn't this also mean that the props needs to be angled downwards if they are behind the trailing edge of the wing and yet again the case would not be so much different for the fan versus props, either way there would be problems?

NOTE: Indeed having additional motors in the wheels would be potentially a good idea and not so unrealistic as it has been done before. This is how they did in Bipod. Electric motors can be made lightweight, and adding one or two is very little (almost nothing) compared to the airframe or battery weight. Depending on these on takeoff though may not be very nice as the takeoff may be rather hairy in this kind of craft.
 
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plncraze

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Check out issue 26 of Contact Magazine. It talks about a 3/4 scale Douglas Skyhawk using a Corvette ZR-1 engine driving a Q-fan ducted fan. The references for the article might help you as well. Many years ago Kitplanes also ran a three article series on ducted fans.

My own advice when dealing with situations like this is to figure out what your design situation calls for. I am not an engineer but sometimes the inefficiencies in one part of a system can be overcome in another part. Like not having to use a reduction drive with a ducted fan.
 

orion

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The prop tip in a slot is discussed in the Periera's Thesis I referenced. I am suprised that AstroTurf has long enough threads to do what you are suggesting. What is the slot width and depth you are using? I am going to have the students try the slot with and without the AstroTurf type of sealing you are suggesting. I wanted to actually see the effect if it is measureable. Can you give me the title of the reference you said had done this before. Are you using counter rotating props or a single prop?
AstroTurf comes in various lengths up to I think somewhere around 2". I was assuming the 1" product line thus burying the tips about a half inch. The prop is a three blade conventional pusher. The baseline design uses a Hartzell unit but we are considering using either an MT or Avia as both are substantially lighter than the Hartzell model. The work for the grooved shroud was done in conjunction with the Rohr 2-175 program. To the best of my knowledge, the work was never published however I had the pleasure to work with Walt Mooney at GD - he was the principal designer on the program. Most of this is from conversations I had with him regarding the program and several of the details of the unique technologies developed for the airframe.
 

autoreply

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Therefore the disc loading of each fan will be low because instead of one little fan, there are 10 little fans doing the same as this one little prop.
The difference is that what I have been thinking, construction wise, the fans could be installed to a wing like that, but (pusher) props (behind the wing) would require an extension where they would be installed.
Just thinking out loud, but if you are going to pusher props, why not mount them to the flaps? Simpler structure and you have powered lift, so lower stall speeds as well.
 

Himat

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Just thinking out loud, but if you are going to pusher props, why not mount them to the flaps? Simpler structure and you have powered lift, so lower stall speeds as well.
Simple structure and powered lift. I like the idea.
One drawback, it would probably be loud. But I'm not sure the DF unit's would be any quieter, just a different pitch to the noise.
With the DF unit's I would be concerned about the aerodynamic effect if the motors are stopped. The propulsion system might turn into an effictive air brake.
 

karoliina.t.salminen

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Interesting idea to mount pusher props to flaps. How would you by the way mount them [to flaps]? Sounds like the flap mechanism would need to be engineered to take load to direction it normally doesn't.
 

WonderousMountain

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Guess we're throwing balanced flaps out the window...

The place to mount them would be directly behind one of the flap actuators, or pivots, so the load would have a path into the support structure. Doesn't seem too difficult from where I'm sitting, but a good question is are we using a two spar, trailing edge design, or do we need to carry the load to a main spar or D section
 

autoreply

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Interesting idea to mount pusher props to flaps. How would you by the way mount them [to flaps]? Sounds like the flap mechanism would need to be engineered to take load to direction it normally doesn't.
Assuming Fowler flaps, you could place them directly above the tracks, which would also solve the problem of balancing (since you retract them almost entirely into the wing). It also solves two inherent problems of powered lift at once, single engine failure and distributing the prop/jetwash over a major part of the wing.

I got inspired for this idea by the guys who're putting a dozen electric motors on the Sparrowhawk sailplane. Very different execution, but the philosophy is theirs.
 

genge0144

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Gordon Arnaut
I appreciated the detail in your response. Finally someone who appears to understand the basic science. I am trying to develop a fan that is half way between a ducted fan and a shrouded prop. I will be running the electric motor in a stationary position and want to push air against a 25 to 75 Pascal backpressure. I want 2000 cfm in 10 inch diameter and 8000 cfm in 22 inch diameter. We need someone to help us choose the motor, blade and control. We have our own controller that could put out a pulse modulated, voltage or potentiometer signal. Advice appreciated and would be willing to pay for someone to create a prototype. You can send me an e mail to [email protected]
Thanks, Colin Genge
 

kenkad

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Colin Genge,
What are the two different size (10 and 22 inch diameter) analyses for? If one is for full scale and one is for partial scale, ducted fans/shrouded propellers do not necessarily scale directly, even if by fan area. Smaller sizes have potentially greater losses. There are a very great number of factors in this type of analysis. I have tried locally (Redstone Arsenal UAV Program) to get reports that have been done by their contractors and they would not even engage in any discussion about what has been studied to date, claiming it is highly classified. Good luck with your project!
kenkad
 

kenkad

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I forgot to include the following link:

http://hal.archives-ouvertes.fr/docs/00/56/52/34/PDF/Article_preprint.pdf

This is what I believe is important about contra rotating propellers within a duct/shroud. I had suspected that the rear prop had to rotate faster than the front prop. This paper confirms that and shows the efficiency gain from using contra rotating props. I hope our school project, that I am pressing on with, will shed more light on this interestng configuration. One other point, hub diameter is important and thus a drive scheme that does not require the motor (with contra rotating gear box, etc.) to be on the duct/shroud centerline is (in my opinion) very critical. As I said in my earlier post, I am convinced that DARPA/Army UAV already have confirmed this information and are pressing on. It is really a shame that the public has to re-invent this again.
kenkad
 
M

Manticore

You might like a look at this old 1960 paper by Dr. August Raspet: http://4wings.com.phtemp.com/lib/files/Raspet_c.pdf about shrouded propellers and how he improved the static thrust by almost double over an open prop. Considering a lot of the drag & noise comes from the gap between the blade tips and the duct (or shroud), we could just make it all one piece & spin the duct as well as the fan (Whaddaya mean, 'Every time I turn left I go into a vertical dive?')
 

topspeed100

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Hello !

here is my question;

I need to adjust the incoming air flow to the tube and have an electic engine inside the cone as you can see in the enclosed pic. This arrangement is from my decompressing fan in the wing...the main compressor is supposed to be on the nose cone of the fuselage. I realised that I can use these compressor fans also for thrust.

The pic is really rudimentary and non scale etc.

Is this more effective than just a straight tube..will this give better performance at also under 200 mph speeds ? The dia of the intake is 800 mm and trailing edge hole has 600 mm tube.
 

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topspeed100

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For our 2 ft fan with the same power but with a swept area of about 0.3 m^2 our static thrust will be...also let's say our mechanical efficiency will be a bit higher at 0.9 due to reduction in tip losses compared to open prop...


Regards,

Gordon Arnaut.
Gordo,

Since you are such an expert in calculations and all..would it be possible for an electric engine ducted fan with right gearing and excessive power to reach super sonic speed ?

Another question...my hastily drawn illustration above has bigger incoming air volume than the exhaust...wouldn't the squeezing tube actually accelerate the flow speed in a tighter tube and thus enable higher speed for the craft too ? In theory ??

rgds,

juke
 
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