Is there a minimum prop speed?

[Jim Carter]

Ok,
in another thread we've been talking about 2 blade vs. 3 blade props, maximum tip speed and so forth. Someone posted a comment about needing really long landing gear to handle the 2 blade prop for a 1350 SHP turbine. That got me to thinking about the prop chord length and what a minimum prop speed might be. So then comes the question, why not just go to a longer-chord 2 blade prop rather than a 3 blade to handle the higher power?

--
Jim Carter

[EDR]

In article >, Jim Carter
> wrote:

> Ok, in another thread we've been talking about 2 blade vs. 3 blade =
> props, maximum tip speed and so forth. Someone posted a comment about =
> needing really long landing gear to handle the 2 blade prop for a 1350 =
> SHP turbine. That got me to thinking about the prop chord length and =
> what a minimum prop speed might be. So then comes the question, why not =
> just go to a longer-chord 2 blade prop rather than a 3 blade to handle =
> the higher power?

Remember that the power produced is a function of torque and rpm.
So if you have a high torque engine, you can turn a large prop slowly
(think helicopters).

[dennis brown]

There's a favorite expression. up your aspect ratio.

The compromises are enough blade area to absorb the power.
Enough aspect ratio to make an efficient blade.
Enough blades to absorb the power (combine the first two items)
Enough diameter to keep the hub losses a lesser part of the total.
Short enough to clear the ground.
Short enough or slow enough to keep the tips from the mach drag rise.
Enough chord to keep away from low Reynold's number losses.

The list goes on.
Jim Carter wrote in message ...
Ok,
in another thread we've been talking about 2 blade vs. 3 blade props, maximum tip speed and so forth. Someone posted a comment about needing really long landing gear to handle the 2 blade prop for a 1350 SHP turbine. That got me to thinking about the prop chord length and what a minimum prop speed might be. So then comes the question, why not just go to a longer-chord 2 blade prop rather than a 3 blade to handle the higher power?

--
Jim Carter