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Attic ventilation from fans - to jet engines

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My nephew is an aircraft mechanic, frame and power plants, including jets.  He's tried to explain to me how a jet engine works, but I get stuck thinking there's nothing to thrust against.  My physics is almost perfect up to and including levers and pulleys, but anything dynamic is over my head.


Christopher Wright <chrisw(--nospam--at)> wrote:

On May 16, 2006, at 7:54 AM, Paul Ransom wrote:

> Manufacturer's data? website?
Canada has an exceptionally good residential energy code that Minnesota
has used to help with our, which is also pretty good, judging from the
amount of whining and bitching from the construction business about it.
Google around for attic ventilation references in local energy codes.

> I have these on my house installed by the previous owner. The throat
> size of the vent is limiting and the momentum of air passing through
> must also give up energy to spin the turbine. I do not believe that
> they
> provide additional ventilation but they look like they are doing
> something. Gives buyers a level of comfort to see activity.
I suspect the real limiting features are the small area between blades
and the fact that you need wind to turn the damn things. Unless they're
kept well lubricated they aren't going to turn fast enough to overcome
the velocity pressure of the wind on the upstream side or pump interior
air through the leeward side. When you think about it, it seems like a
violation of the Second Law: like a wind powered window fan. It can't
possibly be as effective as an unblocked open window.

Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at) | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania

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