Paul,
My comparison of efficiency of a 24 inch prop and a 72 inch prop is
comparing a freestream prop to a freestream prop, not a duct.
I explained that this was so we could compare just one factor, the
induced efficiency. In other words, we are concerned for the purpose of
that exercise only with how efficient, in terms of kinetic energy, a
small prop is versus a big prop.
I will get to the issue of duct efficiency and drag in my next post.
There is some very interesting stuff there. It's not just a matter of
the duct having a downside because of drag.
Also, your formatting scrambled all my math, taking out all the minus
signs and some other stuff as well. Anyone trying to work through it
will get nonsense results. Can we put up the math in proper form, in
case someone wants to do their own calculations?
Regards,
Gordon.
You can double check your calculations with the
single verses the twin with this chart as the twin has
more prop area. Note it only helps the climb.
The duct is the problem with its internal skin friction
drag. It is also a 3D problem as the air is whirling
around inside the duct. The density of the air on
the outer inner wall is higher than the density of
the air near the core.
Paul Lamar ...No rotor no motor.
Paul,
The twin has twice the prop disk area of the single. My comparison of a
72 inch prop and a 24 inch prop, the big prop has nine times the area of
the small prop.
Even so, this much smaller prop is is 83 percent as efficient as the big
prop in turning horsepoer into thrust. This is the surprising part and
it bodes well for the ducted fan in terms of overall efficiency, because
we will see that the duct has some real advantages in efficiency, in
comparison to a freestream prop of similar size.
Also you are bringing in the issue of drag on the inside of the duct. I
will get to that. That fact is the duct actually has an advantae there
too. A tracotr propeller plane has scrubbing drag losses from the
propstream blowing down the side of the airframe. Raymer estimates this
to be about five percent as a rule of thumb, but it rises with aircraft
speed. With a buried duct you eliminate this drag penalty completely, so
there's five percent better efficiency for the duct right off the bat.
Subtract that from our 16 percent that we calculated due to our smaller
size streamtube and you are down to about a 10 percent advantage for a prop.
This is a far cry from our original assumption that a duct can only
achieve about 60 percent efficiency, while a prop will see over 80 percent.
I will get to the drag issue shortly.
Regards,
Gordon.
.83 times .80 is .664.
The internal drag issue may get the other six point reduction in
eff. down to 60%. Just like K&W says.
The formatting does nothing unless you send non ASCII characters
or HTML. Determine what your email program format is set to. Set
it to plain ASCII text. Are you using a Mac?
Paul Lamar ...No rotor no motor.
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