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Old Oct 12, 2012, 12:50 AM
It should fly at least once
clive45's Avatar
Australia, NSW, Grenfell
Joined Mar 2006
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I thought that with the extra blades (10) instead of 5 or 6 there would be less slippage in the airstream when the outlet was restricted compared with a 5 or 6 blade fan, this is just an idea that popped into my brain.
I'm not an engineer in fluid mechanics FYI
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Old Oct 12, 2012, 01:36 AM
Extreme CNC Alloy EDF
Extreme_RC's Avatar
Australia
Joined Mar 2006
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Hmm my F16 must be an illusion then with its tight 55mm exhaust nozzle and super compressor low velocity CS10 fan unit especially at 1 min 11 sec where its obviously a photoshop fake

Extreme RC F-16 with 10 blades Changesun rotor (2 min 22 sec)


My vampire has a 55mm exhaust nozzle coz I couldnt be bothered re-adjusting it after getting it all in there Still flies ok for 4-5 mins on some crusty old 4000 packs...

Vampire with EXTREME RC 70mm Alloy CS10 fan unit (2 min 49 sec)
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Old Oct 12, 2012, 01:44 AM
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Quote:
Originally Posted by clive45 View Post
I thought that with the extra blades (10) instead of 5 or 6 there would be less slippage in the airstream when the outlet was restricted compared with a 5 or 6 blade fan, this is just an idea that popped into my brain.
I'm not an engineer in fluid mechanics FYI
There is less slippage with more blades, and thats why restricting the outlet causes compression and loss of performance. Less blades dont get to compress the air so much.

The 10blades move more air CFM so if you restrict the outlet it starts compression and loss of efficiency

5 blades moves less air CFM and there is not enough air to get compressed so a smaller outlet can be used.

I hope all this makes sense.
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Old Oct 12, 2012, 01:46 AM
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FWIW I found on my F16 the best performance was with a 58mm outlet
here is my F16 with Extreme 4s alloy 10 blade. Jump to the 30sec mark to se the plane.
Phase 3 F16 CS10 4S (2 min 8 sec)
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Old Oct 12, 2012, 01:58 AM
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Quote:
Originally Posted by Extreme_RC View Post
Hmm my F16 must be an illusion then with its tight 55mm exhaust nozzle and super compressor low velocity CS10 fan unit especially at 1 min 11 sec where its obviously a photoshop fake
Obviously!! Really good photoshop job....
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Old Oct 12, 2012, 02:49 AM
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Yeah well that Mark post was really scientific, not... doesn't tell or prove a thing.
Especially the Vampire one.
No numbers, specs, facts.... means useless reference.

Drive the fan hard enough and it can do anything. But it can never do anything as well as a low blade fan can. Especially if a restricted exhaust.
If the Vampire runs "well" at 55mm... it will run better at 58mm. (unless it is underpowered)
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Old Oct 12, 2012, 02:56 AM
RA-TA-MA-HAT-TA
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Quote:
Originally Posted by PeterVRC View Post
Yeah well that Mark post was really scientific, not... doesn't tell or prove a thing.
Especially the Vampire one.
No numbers, specs, facts.... means useless reference.

Drive the fan hard enough and it can do anything. But it can never do anything as well as a low blade fan can. Especially if a restricted exhaust.
If the Vampire runs "well" at 55mm... it will run better at 58mm. (unless it is underpowered)
I thought pictures meant a thousand words. Obviously does not mean much to you. He posted spec up several times in this thread. And besides that it flies great.
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Old Oct 12, 2012, 07:04 AM
Life begins at transition
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Can't make a horse drink, but here's the water at least...

Stu's design methods: http://www.rcgroups.com/forums/showthread.php?t=360524
Klaus': http://southamptonmac.org.uk/doc/EDFpart1.pdf

If you want to do some research (remember, a lit survey is chapter 1 of a thesis...), look at any book on propulsive jets, be they turbofan or turbojet.

Industrial HVAC engineers have also done a lot of work relevant to us. Look into their work as well.

If someone wants to bring in new theories about propulsion and how it's achieved, go for it. Be prepared to explain it, and have it put up for review and scrutiny. I'll explain anything that I've put on paper...
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Old Oct 12, 2012, 08:28 AM
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thrust vs. diameter vs. efflux: why fans that simple?

I've been through similar discussions at least twice, and with all the well-meaning, intelligent, educated and curious people in this thread, maybe I will in the end understand what exactly makes fans completely different from props, and where and how this happens - and why air should be incompressible.

What I always get to read, endlessly repeated:

- thrust force is created (by props and fans alike) by accellerating a certain amount = mass of air (obviously true, simple high-school physics F = m x a)

- in a static thrust test you measure thrust. Period. Now if you know the exhaust diameter then you can calculate the efflux speed, they say. Hmmm, really? That requires the air being incompressible, like a fluid, and you can assume the volume of the air as being constant, AND that the air molecules have the same velocity. Then it's easy, if the diameter is restricted and the density is equal, the air molecules have to be faster. Very simple idea and true for water in a hose of varying thickness etc., you can use vectors or other highfalutin' notation in an attempt to hide it (as I've seen :-) but it's that simple: if you want to get a volume of water through a hole (in a certain period of time) it will have to be faster if the hole is smaller.

Sounds reasonable, also in our context? Well, let's take an example which should be familiar to all those who fly prop planes:
Setup A consists of some motor turning a 8x4 prop, thereby generating, say, 500g of thrust.
Setup B is running a 8x8 prop, and when we run it so that there is also 500g of thrust generated, we discover a shocking fact: We need a lot more power than in setup A!
This should not come as a surprise since props are most efficient when they accelerate air just a bit, in other words when the pitch is just sufficient to provide a positive angle to the incoming air. In a static setup, lower pitch will ALWAYS be better and give higher thrust - and bigger props are better because they accelerate more air by less. However some people like to fly their planes and in high-speed flight, and then things are very different, a higher-pitched prop will still be able to accelerate air, while a prop with two low pitch will even act as a brake. (There are still people who choose the prop which gives the best static thrust, but many know better and think about the desired airspeed too.)

Now let's assume we put some kind of ring, extending rearwards, around our two prop drives - we created two 2-blade fans. Now things are of course hardly different, still the 8x4 will provide good static thrust, while the 8x8 will have good pitch speed (now we call it efflux). What evidently happens is that, behind the props, the air is not a uniform volume consisting of air molecules moving at the same speed, but rather air which has been accelerated directly by the prop blades will be faster than the air which is dragged along.

Coming to "real" fans now (which however sometimes also only sport three blades) I wonder why suddenly the blade pitch should mean nothing, and static thrust giving all you ever need to know, as many "experts" want to make me believe. I do get it that fans ARE different, that there is not much "unloading" in flight (if at all), and that the air sucked in is already meeting the rotor blades with a certain speed, even in a static situation. But does the blade pitch speed, blade count etc. have ZERO influence?

There is just one thing I am absolutely certain: in a STATIC situation, the thrust really is all you need to know, since there is zero airspeed, and in the first moment when an EDF is starting to accelerate on the runway, nothing else matters. Later, with some airspeed, I would be very much surprised if pitch speed (blade pitch combined with blade speed) would not matter, and that all fans with equal exhaust diameter and equal static thrust would have the same efflux speed. Do they? Has it been measured?

I learned in school that fluids are incompressible (that's why you use them for hydraulics etc.), and gases aren't, which is why you should check the boiling temperature of your brake fluid regularly - once it's boiling you'll have gas in your brake lines and you know the rest... I have the equivalent of a masters degree in physics, but I say that only because I can say that I didn't learn ANYTHING of relevance about fluid- or aerodynamics at University (wasn't my field of specialisation) - and I think we should leave all that bickering about real and imagined engineers out of the discussion - this is neither philosophy nor psychoanalysis where you need to go to the "sources" and consult authorities but engineering and technology: Logic and empirical evidence are sufficient. An engineering background should rather prepare one for something else: Knowing that we are always dealing with models, and they are only so good as they can be applied to the problem.
Feynman said something like: If you really understand something, you can also explain it in simple terms - here you go, anyone feeling up to the challenge?
No need for links to jet propulsion theory for me though, thanks.

To sum it up: Using simple equations which effectively assume that air is incompressible and molecules moving along with the same velocity is a good assumption for water etc, but I would like to know why air in the EDF acts just like water and unlike air accelerated by prop blades (as opposed to fan blades).

Thanks in advance for arguments and facts
cheers
Clemens
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Old Oct 12, 2012, 08:46 AM
Life begins at transition
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Air is considered incompressible for our purposes because the pressure changes are small. same assumption applies for props. Until we start to deal with flows above about Mach 0.5, it's a good enough assumption that will only have errors of a few %

The reason a prop and fan act differently in terms of efflux, is the capture region of a prop constantly changes. As the capture region changes, the exhaust area also changes. Far downstream, the accelerated 'tube' of air behind a prop shrinks.

A fan doesn't have that. If designed properly, it is already fully expanded when it hits the tailpipe (which is why we squeeze them down to 85-90%, empirically). Once it's fully expanded, it's got no reason to contract further.

As for the example of an 8x5 vs an 8x8 static - could you explain how two props of the same diameter, producing the same thrust, are moving different amounts of air?
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Old Oct 12, 2012, 09:17 AM
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Different pitch speed at equal thrust (and diameter)

Quote:
Originally Posted by Odysis View Post
...
As for the example of an 8x5 vs an 8x8 static - could you explain how two props of the same diameter, producing the same thrust, are moving different amounts of air?
Thanks for the answer, I am afraid I only understand you last paragraph / question fully and that I can answer with a resounding YES, of course the two props are moving air completely differently: the higher-pitched prop will accelerate air much more compared to the lower-pitched prop, and evidently not all the air behind the props has the same velocity because not all the air is accelerated in the same way (air is not passing in the prop's vicinity is "dragged" along at much lower speed). How else would you account for the much higher pitch speed of the 8x8 prop? Now if you're thinking in terms of a uniform volume of air going the same speed behind a prop (or fan) you can't explain the difference in pitch speed...
Now that's obvious to anyone who has flown prop planes. If it isn't to you, how do you explain the different pitch speed at equal thrust?

I would also think that a 5-blade fan (with a lot of "empty space") between the blades will not accelerate the air as uniformly as the highly-loaded CS70 fan will With the CS70 there is not much chance for air to "escape" the blades, much like a turbine, and I would much rather think that simplifications of uniformly moving air will be applicable for a CS70 than the standard howler fan. I think it was at Daniel Schuebeler's site that I read the term "axial compressors" for the multi-blade fans...
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Old Oct 12, 2012, 09:28 AM
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I explain the different power for the props a bit differently.
Just a theory, as I haven't put any in a wind tunnel...

Static, they produce the same thrust, with the same diameter, so the velocity change is the same. Yes, the 8x8 will need more power. Here's a quick picture:



so, the vertical axis is efficiency. More is always good. The horizontal is the advance ratio. Think of it as speed. Right is fast, left is slow.

The curved lines are prop efficiency curves. Low blade angle = low pitch. As you can see, at low advance ratios, they are more efficient. Take for example J=0.6. A 15* prop is ~78% efficient, a 35* only 42. Almost twice the power for the low pitch prop, to do the same thing.

This applies too in fans. The reason for the change in efficiency is to do with the angle the airflow strikes the blades (be they prop or fan). They (high pitch / low pitch) both do the same work, but some are more efficient in different flow regimes (speeds) than others.

To get back on topic a bit, I've got a CS10 + ARC 2847-2 lined up to be tested tomorrow. Hoping for something that will work in a twin.
Planning on recording peak volts, min amps and sound level. Anything else I need?
(sorry, couldn't help. Gotta laugh sometimes!)
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Old Oct 12, 2012, 09:39 AM
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Not sure if the link from Klaus goes to the original thread on duct theory etc but this conversation is far from being new.

If memory serve Klaus entitled it the trouble with duct or fans or something like that. Once I find it I'll post the link.
The thread went on and was approached by many a learned people who are obvious all educated but didn't specialize in fluid dynamics and ducted systems.
Thus many of the assertion and concepts some are coming with, Klaus explained and dispelled.
While a fluid dynamics formula becomes just mathematics to some, it still discounts the concepts that we may think is happening.

That thread goes on and on and unless you are ready to high jack this thread from the CS70/10 thread to repeat that conversation, it may be beneficial to all to give it a read through. You'll see how others have had similar concepts and often why they don't apply or are inaccurate.

To quote a most renowned engineer, "you can't change the laws of physics captain", nothing has changed since ten years ago. The only thing that may have happened is with stronger more efficient motors and power sources, we are able to exploit the physics further.

If you haven't read that thread yet, set aside a few nights to give a good read.
Unless Klaus we know you're still a RCG member... Jump in any time! To explain it all over again to the next generation......
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Old Oct 12, 2012, 09:46 AM
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I should have known it - instead of explaing how props produce different pitch speed, you're "explaining that away" - I am now going to use shockflyer props on my warbids and hope for good efficiency and speed :-) Do you fly any prop planes?
An explanation I expected would have been: high-pitched props tend to stall when driven with higher RPM in static conditions (which is true), but not that EDF dogma again that thrust divided by diameter equals speed again - in a domain where it is clearly inapplicable. Not so clearly, obviously.

I give up and continue using the CS70 fans although they are terribly inefficient according to the old school guys who insist on small exhausts because it has always been that way. How do changes come about (in physics and elswhere)? When the proponents of the old theory have died out, so that they can no longer explain us that the world is a disc, or that cheap Chinese high-blade fans cannot work (the videos are fakes), and the quality of a fan is seen in how small an exhaust diameter it will support and still work (to end on a not so serious note myself).

PS: thanx Maxthrottle for the link, after reading lots of Klaus' (Winmodels) postings I have to say that it all sounds sensible and subtle, and there is no talk about THE correct exhaust diameter for ANY 70mm fan, and there is also not talk about doing away with efflux speed measurements altogether (as some claim, because you should just calculate it from the thrust and exhaust diameter...) - it's mainly in German forums where resistance to the CS70 is still strong, e.g., people refuse to open the rear ends of some models to make the CS70 work - and then it's the fan which is terribly inefficient, or it is claimed that a 70mm fan must be crap if it doesn't perform well (static) with a 55mm nozzle..You can also read that nacelle installations like in the Dynam Me-262 or ariliners are the most efficient setups (short, straight (!) ducting) - clearly seen by the superb performance of the big Dynam bird...
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Old Oct 12, 2012, 11:08 AM
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Actually its called "The truth about ducts". Klaus or winmodels had this lengthy discussion and even Stu and Klaus exchange and refine each others findings if you come across there many posts. They actually do specialized in this field. So give them a read.
Note the thread does appear to have been shortened some from its original. So you may have to follow Klaus and Stu conversations via their profiles.
Hopefully this helps some to better understand the various concepts.

I should add its important that you also read the PDF and side links to the conversations. Klaus wrote a number of white paper docs breaking down the various inlet, fan face, motor/energy in, exhaust, velocity, thrust, static or dynamic etc.
It not a short read. There are a lot of conversation converted to mathematics so in parts it may be a little thick for some. But as you go through the conversations you'll see where certain things apply and why and where they do not and why.
Its not a simple answer to be explained in a post other than by the boiled down mathematic formula. So if you really want to understand how much is involved... give it a read.
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Last edited by Maxthrottle; Oct 12, 2012 at 11:55 AM. Reason: additional explanation/Spelling
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