Thread: New Product Change Sun 10 blade 70mm
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Old Oct 12, 2012, 01:10 PM
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Originally Posted by mopetista View Post
....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...
? What are you reading? The fact that you are looking for a set dia for 70mm tells me you missed something... actually most of what was said.
Klaus explains where and how that may vary. Even this main post where he start breaking it down into english he starts explaining why. And earlier he gives the link to the white papers that details the mathematical reasons for all of those. See also
The explanations are far longer for you to have already come back saying that its not there.
Post 147 "Thrust equals { mass flow times velocity difference } but the energy necessary to accelerate the air flow to this velocity difference is { mass flow times velocity difference squared }. Therefore it is sensible to use the maximum air flow possible with the least velocity difference. For the static case this leads to large diameter fans and exit areas, because the velocity difference here is the absolute air exit velocity. But in flight the velocity difference is measured against the flying speed, i.e. the fan air flow velocity difference is { air exit velocity minus flying velocity }. In many cases the air flow through the fan itself is not fast enough to have a sufficiently high velocity difference to produce any meaningful thrust at higher flying speeds. So the exit area is reduced to increase the velocity differential. The amount of reduction possible depends on the fan characteristic, which is what it is all about here."
Review of understanding
Diagram of Static flow and Dynamic flow regarding velocity momentum of air and pressure induced by the fan and duct
Volume/Mass Flow
As Klaus commented this is typical with many to have an aversion to the math and the details.

Most models do not give the attention to the inlet which can change the other factors greatly. But you've already discounted several videos, not just the ones posted here, of many following the practice of less than 100% FSA exhaust to great avail. If that can't convince you I'm not sure what will. Horse meet water, water horse. You can take it from there or not. Your disbelief won't make it any less true... I guess its just magic then.
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Last edited by Maxthrottle; Oct 12, 2012 at 03:56 PM.
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