>>How many RPM can a wood 12' --4 balde prop be ran safely? How about
>>a 10' 2 blade?
>>Thanks Dave
>
>A good rule is to try to prevent the tips exceeding 80m/s (about
>180mph). This should minimise wear and tear. On this rule, your 12 foot
blade
>should not exceed 425rpm and the smaller one can go to 500rpm.
>However this tip speed is often broken without hazard.

Dave,

I won't argue with Hugh's advice on rotor design; but, your question was
what RPM could various wood rotors take safely. It isn't a very simple
question because a number of forces enter into the engineering
calculation, and you haven't given such information as blade structure so
that one could know how much structure is available to take the load.

A good wood has a tensile strength of about 5,000 pounds per square inch.
But, you have centrifugal force throwing the blade outward, the thrust of
the wind trying to bend the blade back, aerodynamic moment which may try
to twist the blade, and quick loading changes and possible vibrations
which introduce fatigue considerations. The combined stresses eat away
safety factors quickly.

To go further than Hugh has in suggesting a "safe" operating speed for
your wood rotor, some serious engineering is required.

A time honored approach to this is "trial and error." If you make the
rotor and it explodes, then the engineering wasn't very good and you may
get clues from the failure as to how to change your design to work better
the next time. (This is like a doctor learning his trade from autopsies
of his patients) Just make sure that there isn't anything within striking
distance of your machine that could be damaged. (And striking distance
for a 40' tower could be about 200 yards or more as blades do fly pretty
well)

Another approach is to study the engineering yourself, or consult with a
knowledgeable engineer. This may reduce the time to get your machine
right, and avoid explosions.

Cheers,

Eric


___________________________________________________________________
You don't need to buy Internet access to use free Internet e-mail.
Get completely free e-mail from Juno at http://www.juno.com/getjuno.html
or call Juno at (800) 654-JUNO [654-5866]

------------------------------------------------------------------------
eGroup home: http://www.eGroups.com/list/awea-wind-home
Free Web-based e-mail groups by eGroups.com