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Elisa ili propeler

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Dule:
Kako to misliš?
Kako može da bude isti pritisak ispred i iza propelera?
Nisam rekao da se brod kreće zbog potpritiska već da je i ta mala sila korisna i da postoji. Uostalom, puno tekstova uporedjuje ponašanje avionskog krila i krila propelera. Ali stoji da voda ne može da se sabije kao vazduh.
Stoji i to da se kavitacija stvara, i da voda isparava na niskom pritisku koji nastaje ispred propelera tako da tvoja tvrdnja ne stoji nikako.

mossi:
Malo sam googlao ovih par minuta: http://physics.stackexchange.com/questions/61105/could-some-design-of-a-propeller-be-used-in-both-air-and-water

There's two important differences between air and water: Air is compressible, and the densities are about a factor of ~1000 apart - 1 kg/m³ vs 1 t/m³!

For most concerns where you use propellers, compression plays no role because the pressure diferences are very low. The densities, however play a large role.

The thrust can be described as F=m ˙ ∗Δv , with m ˙   beeing the mass flow - kg&/s or such - and Δv  the difference in velocity a volumeelement of fluid is accelerated.

So, to achieve a similiar thrust, the same propeller would have to move 1000 times more air than water by volume. Hence the often larger and faster spinning propellers for planes.

On the other hand, in a heavier medium each wing of the propeller is subject to stronger torque (all else beeing equal):


Q=ρV 2 a D 3 f q (ND V a   )


(Source) ρ  is density, V a   rate of advance (how much the propeller moves forward per revolution), D is diameter and N number of revolutions.

Without going into the math it can probably be shown that in a heavier medium the propeller will experience somewhat more torque for the same thrust - I'm to lazy to try now. The propeller will be built with more robust (and possibly heavier) material than would be the case id it's an only air propeller.

That said, I believe that a propeller for both media is entirely possible, though challenging.

However, a propeller for a both media will need a drivetrain that can accomodate speed roughly a factor 1000 apart (that's not trivial).

One other reason why don't see a propeller for both media is that there is no vehicle that could make use of one.

Dule:
E pa neću ni da čitam.  lkjhgfvh
Nisam napisao ni jednu reč a da je nerazumna i onima koji nemaju tehničku školu, neznaju latinski ili engleski. Ja bih to preveo i potrudio se da tekst razumem pa to prepričam svojim rečima. Ako ćemo da raspravljamo treba rasprava da bude razumljiva svima i da oni koji je prate i nauče nešto ili, ako su stručniji, daju svoj komentar.

mossi:
A čija je lopta?  rfcdvb

Vidi nisam ni ja razumeo sve iz "plavog" teksta.
Ali spominje se upravo gustina, stišnjivost i "ostali" zakoni fizike i formule koje su nam strane.

Moj tekst je bio replika na tekst od korisnika nijah, jer je njegovo tumačenje bilo po meni više za AVIO elise tj krila,
a ne za propelere brodskih i vanbrodskih pogona.

Da stvar bude gora, izgleda da postoje specijalni propeleri za super brze čamce koji nisu ni uronjeni u potpunosti u vodu.
Pa jedan deo puta prolaze kroz vodu, a onda jedan deo kroz vazduh.

 juhgfd frdxs

Mezija:

--- Citat: mossi  02. Oktobar. 2013. / 22:57:28 ---

One other reason why don't see a propeller for both media is that there is no vehicle that could make use of one.[/color]

--- Kraj citata ---

E ipak postoji....

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