Oct 11, 2012, 08:55 PM
EDF rules... :)
Joined Nov 1999
13,517 Posts
Quote:
 Originally Posted by PeterVRC I hear people say EDF's are not a 'Compressor'... but just a pump. How can something that speeds up air flow, and feeds that into stationary air, NOT get back pressure... and thus increase pressure... like a Compressor? It must. It may not be DESIGNED to mimic what a jet engine compressor compresses air down to... but it still does compress it. And this air compression... density.... must come into play in the output... the maths... of it all... CFM doesn't state what density the fluid had. It tends to just be a measure of "you moved this much of that item" and assumes the item was the same. But denser air is not the same as less dense air....
Here again we are not breaking new ground, the laws of physics and specifically Bernoulis equation states that flow rate inversely changes proportional to the change in area if there is no other entry/exits available. This is basic pipe engineering when figuring flow rates of liquids, fluids and gases thru pipe.

Albeit there is a pressure component but at ,033 ft^2 for the outlet area it figures out to be less that .4 ounces. Do the math.

To get the mass flow rate of CFM there are three components Density, Area, and velocity so Density is definitely coupled into mass flow. Where did you get that it was nto a part of the equation?

Eric B.
 Oct 11, 2012, 09:03 PM Registered User Australia, VIC, Melbourne Joined Nov 2006 10,410 Posts It wasn't in your first listed equations..... You said thrust is a result of velocity times volume...... no mention of density. And I am sure that it is this density is what makes a CS10 different than a lower blade count fan. Not because they could not do the same, but because they don't - due to blade/area design. (I would expect a low blade count at much higher pitch could equal high blade count at lower pitch.... they just need to give equal velocity AND volume/density) And the proof is in that they are quite different results to each other. Irrespective of maths etc. (same as equal pitched 5 blade props are different results to 2 blade props of that same pitch, running at the same RPM - but in this case dead equal blade design still gave different output, again just because of the different density they each create - lower for less blades) You don't need to know the specific maths to know there is a difference. Last edited by PeterVRC; Oct 11, 2012 at 09:09 PM.
 Oct 11, 2012, 09:13 PM Registered User Australia, VIC, Melbourne Joined Nov 2006 10,410 Posts I think your 'flow rate through a pipe' is also amiss. It will still alter according to pressure. Very low pressure and the confines become negligible... very high pressure (especially getting to levels of 'forcing' it through under serious pressure) and you have added/increased back pressure restrictions. I doubt the formula is inversely linear. That formula would be a general formula not covering operations outside... umm, 'normal'. (a certain... who knows how wide.... range). You qualified with 'no extra entry/exits'... it should also have pressure included. If you really wanted... add in 'wall/surface friction', lol. (not worth worrying about, unless massive pressure) And temp and Alt are not all that alter density. They are just the common things. Just FORCE something through somewhere, and that is a far more significant density changer! And that is exactly what we are doing in an EDF. Latest blog entry: DG-808S - FlyFly 4metre Composite/Balsa...
Oct 11, 2012, 09:25 PM
EDF rules... :)
Joined Nov 1999
13,517 Posts
Quote:
 Originally Posted by PeterVRC It wasn't in your first listed equations..... You said thrust is a result of velocity times volume...... no mention of density. And I am sure that it is this density is what makes a CS10 different than a lower blade count fan. Not because they could not do the same, but because they don't - due to blade/area design. And the proof is in that they are quite different results to each other. Irrespective of maths etc. (same as equal pitched 5 blade props are different results to 2 blade props of that same pitch, running at the same RPM) You don't need to know the specific maths to know there is a difference.
Peter,

Whalleyboy asked about efflux velocity and thrust, I answered with the truth that the velocity and thrust are tied together.

Air density should be the same at anytime you test two different fans at the same time so calculation of both fans can be taken at the same density. The rotor moves air by virtue of blade angle of attack displacing air from the inlet side to the exit side of the fan. Number of blades has no effect other than a little friction on the blades surface area. Once air is moving the number of blades do not do anything different than a 3 blade rotor if the rotors have the same blade shape ie angle of attack and geometric twist.

My friend your an engineer who says math and physics do not define the abilities of a fan, does it also not define tires and road interaction? There are friction coefficients that give indication of how much taction the tires have both in dry conditions and wet conditions, these can be mathematically worked to give a good indication of how far it will take to stop a car moving at defined velocity rather well. Physics and math are in every part of engineering that I have been a part of and a part of everyday life.

Eric B.
Oct 11, 2012, 09:31 PM
EDF rules... :)
Joined Nov 1999
13,517 Posts
Quote:
 Originally Posted by PeterVRC I think your 'flow rate through a pipe' is also amiss. It will still alter according to pressure. Very low pressure and the confines become negligible... very high pressure (especially getting to levels of 'forcing' it through under serious pressure) and you have added/increased back pressure restrictions. I doubt the formula is inversely linear. That formula would be a general formula not covering operations outside... umm, 'normal'. (a certain... who knows how wide.... range). You qualified with 'no extra entry/exits'... it should also have pressure included. If you really wanted... add in 'wall/surface friction', lol. (not worth worrying about, unless massive pressure) And temp and Alt are not all that alter density. They are just the common things. Just FORCE something through somewhere, and that is a far more significant density changer! And that is exactly what we are doing in an EDF.
Peter,

Chew on this for a while.

http://en.wikipedia.org/wiki/Bernoulli%27s_principle

We are not working with more than a few tenths of an ounce in pressure in the 70mm fan at 600 watts.

Ok please school me on what variables define air being used in context in our fan?

Eric B.
 Oct 11, 2012, 09:41 PM Registered User Joined Jun 2012 417 Posts I know this is of topic but would some of you engineers, come over to the change sun 18blade fan thread and please explain why thrust numbers are to low and amps are so high.
 Oct 11, 2012, 09:49 PM Registered User Australia, VIC, Melbourne Joined Nov 2006 10,410 Posts There 'must' be more pressure in a CS!0... 1)They give less 'thrust' (LOL) than for equal power in a low blade count fan. 2) They 'hate' exhaust restriction, more than a low blade count fan (because the air density/compression is already higher and causing more strain) 3) They demonstrate more 'pulling power', but less speed. Even though their benched thrust is lower than the low blade count fan. So there must be a REASON for that. And those results gives clues as to where it fairly surely must come from. The pressure... and thus air density being higher in their exhaust, than in a low blade count fan. Low blade count is like putting your finger over a hose to get a faster flow.. narrower, and 'stronger' in that thin stream, but very weak (zero) outside that small stream. Thus the pressure in the housing/ducting is lower overall... but the airspeed is higher. High blade count is just the full hose sized stream.... especially a CS10 which covers over 90% of the swept area I don't really care about the MATHS, because you can twist and turn all kinds of formulas to suit any thoughts..... but often there are MORE aspects (formulas) to ALSO include, if you want to totally true answer - but someone truly knowing it ALL can use the correct maths and ALL the formulas that apply. Bernouli this, Newton's that, don't mean much if you LEAVE OUT other truly critical factors. But you don't need to 'know it all' to be able to see when there is something missing in maths supplied. Latest blog entry: DG-808S - FlyFly 4metre Composite/Balsa...
Oct 11, 2012, 09:52 PM
EDF rules... :)
Joined Nov 1999
13,517 Posts
Quote:
 Originally Posted by PAPE I know this is of topic but would some of you engineers, come over to the change sun 18blade fan thread and please explain why thrust numbers are to low and amps are so high.
I gave my assessment a few days ago when John tested it. The distance between the blades are to close at the root giving lower thrust in the 18 blade arrangement and the 9/12 blade arrangement seems to confirm that.
Amps being too high could mean the proper motor is not selected, too short or too high kv I have not really been following that but that seems to be the main problem. 1900kv is just a little too high kv to produce the torque needed on a fan of that size and pitched rotor.

Eric B.
Oct 11, 2012, 10:06 PM
EDF rules... :)
Joined Nov 1999
13,517 Posts
Quote:
 Originally Posted by PeterVRC There 'must' be more pressure in a CS!0... 1)They give less 'thrust' (LOL) than for equal power in a low blade count fan. 2) They 'hate' exhaust restriction, more than a low blade count fan (because the air density/compression is already higher and causing more strain) 3) They demonstrate more 'pulling power', but less speed. Even though their benched thrust is lower than the low blade count fan. So there must be a REASON for that. And those results gives clues as to where it fairly surely must come from. The pressure... and thus air density being higher in their exhaust, than in a low blade count fan. Low blade count is like putting your finger over a hose to get a faster flow.. narrower, and 'stronger' in that thin stream, but very weak (zero) outside that small stream. Thus the pressure in the housing/ducting is lower overall... but the airspeed is higher. High blade count is just the full hose sized stream.... especially a CS10 which covers over 90% of the swept area I don't really care about the MATHS, because you can twist and turn all kinds of formulas to suit any thoughts..... but often there are MORE aspects (formulas) to ALSO include, if you want to totally true answer - but someone truly knowing it ALL can use the correct maths and ALL the formulas that apply. Bernouli this, Newton's that, don't mean much if you LEAVE OUT other truly critical factors. But you don't need to 'know it all' to be able to see when there is something missing in maths supplied.
Peter,

Again blade count means absolutely nothing. Rotor blade shape is everything. The aspect ratio leads to flow problems at the blade as pressure rises and I am talking about slight pressures this is why the fan does not like outlets less than 85% FSA.

Peter if you dont care about math and physics you cant really give an intelligent answer to any real question as math and physics permiates all of what we do!

School me on what I left out?

Eric B.

PS: I look forward to seeing more of your posts, in fact I will give you the next 20 hours to come up with something in rebuttal but beware I will use math and physics in my defence.
Last edited by AirX; Oct 11, 2012 at 10:10 PM. Reason: Added PS
 Oct 11, 2012, 10:10 PM Registered User Australia, VIC, Melbourne Joined Nov 2006 10,410 Posts Pape... at a simple guess. Rotor/blade design. So it is likely to be the same base reason as the CS10 and CS12... but maybe they just got it all 'worse' in the 18. Do you mean benched results? Or in a plane. As they will get much worse in a plane.... more so for these high blade count fans. And I mean specifically CS 'high blade count'..... because Jetfan, or Stu, might have better designed blade and area covered. I suspect CS don't truly know what they are doing (or have copied etc). Latest blog entry: DG-808S - FlyFly 4metre Composite/Balsa...
 Oct 11, 2012, 10:15 PM Registered User Australia, VIC, Melbourne Joined Nov 2006 10,410 Posts I love maths... LOL (Did Pure Applied to HSC... but that was enough of it for me! hehe) But I hate its abuse.... Not saying YOU are. But all too often not ALL the maths is included in someone's use of it as support. And like I said, you can work all this stuff out WITHOUT maths even... not to truly understand the specifics, but to work out what is likely to be involved, because of the data store you have, and cross reference to other applicable knowledge base you have. Hehe, like you replay to Pate above... made as I was making mine. You list some maths/theoretical/tests..... I come up with the same without any. Just CS10/12 experience, without maths/formulas, still shows the probable sequence of what is going on. Latest blog entry: DG-808S - FlyFly 4metre Composite/Balsa...
Oct 11, 2012, 10:20 PM
Registered User
Joined Jun 2012
417 Posts
Quote:
 Originally Posted by PeterVRC Pape... at a simple guess. Rotor/blade design. So it is likely to be the same base reason as the CS10 and CS12... but maybe they just got it all 'worse' in the 18. Do you mean benched results? Or in a plane. As they will get much worse in a plane.... more so for these high blade count fans. And I mean specifically CS 'high blade count'..... because Jetfan, or Stu, might have better designed blade and area covered. I suspect CS don't truly know what they are doing (or have copied etc).
Benched results are low thrust high amps, Its just that with all this talk am thinking maybe in the air the CS18 fan will work differently than on a bench.
 Oct 11, 2012, 10:30 PM Registered User Australia, VIC, Melbourne Joined Nov 2006 10,410 Posts CS10 again: I believe it does not like low FSA because it is already compressing air 'excessively' with a large (or typical) FSA even. It is only a very small distance, but it might even already be excessive within the shroud! I wouldn't think so though. It more fits in that the shroud is ok still by then - not sure why, maybe as it is such a small time that it is within that distance - thus bench testing is still looking good, but in a plane, over the much longer ducting distance, it matters more.... and thus why their performance is worse than low blade count. I mention 'swept area'.... I guess when I say that it means the area covered, but I also mean the whole picture of what that fan 'grabs... blade profile, pitch etc too. The whole 'chunk of air' it grabs - for all those factors totalled. (There is probably a better word to call that total?) And it would seem they just made a fan total that is somewhat 'amiss' in design perfection.... thus having these spin-off issues. In part giving better torque.... but a bit less efficient... lower efflux.... too much back-pressure (and thus sensitivity to more) Which might all be totally FINE if that is what you need to do to get torque.... but it seems to me the level of it is excessive, and not required in most planes it will be used in. Most are more suited to having speed (efflux speed) instead. But this poor design can't cope with the typical 'convert flow to efflux by lowering FSA exhaust ratio' (again due to the pressure issue). And again, they probably never designed anything to do anything... they probably COPIED stuff, not specifically or accurately to be a perfect copy of something, without true understanding and just ended up with this resultant! Latest blog entry: DG-808S - FlyFly 4metre Composite/Balsa...
 Oct 11, 2012, 10:40 PM Registered User Australia, VIC, Melbourne Joined Nov 2006 10,410 Posts Well then it sounds like the CS18 is just a bit of a lemon! LOL I don't think it is possible to improve in a plane.... things can only be equal, or worse, there really. You can't beat a fan with an inlet lip, sitting alone in mid-air. (Though you convert efllux speed after the fact - and find out how much it likes that, or not). But anyway, I would have thought by now there is quite a bit of CS18 specific info in that thread. (I must remember not to buy one... LOL) Latest blog entry: DG-808S - FlyFly 4metre Composite/Balsa...
Oct 12, 2012, 12:39 AM
RA-TA-MA-HAT-TA
Melbourne, Australia.
Joined Feb 2010
3,263 Posts
Quote:
 Originally Posted by AirX Peter, Do tell us from your engineering background what is the proper way to calcuate the fan thrust? So far you have only touched on what you feel, let us know how you calculate thrust? Eric B.
Told you guys before. Troll bridge engineer.