Perry Mick wrote:

I have a theory about why the unducted thrust is so bad.
I've heard that when people run highly pitched props, the blades are
stalled during the early part of the takeoff run, then unstall and you
get a burst of acceleration. I've never experienced this. With my ducted
fan the thrust is always there and whatever that thrust curve looks like
vs. speed it is continuous - no discontinuities.

My fan has a pitch of 38 at the tip and 50 near the hub. That doesn't
sound high, but when you consider it is designed for 200 mph air to flow
through it at 5500 RPM. (160 mph aircraft airspeed => 200 mph airspeed
in fan plane). If you divide RPM by two you get 2750, a typical prop
RPM, and a comparable prop pitch would then be 76/100.

I'm guessing without the duct and the controlled flow it provides, the
blade is stalled, and I saw no indication it was unstalling even at 70
knots.

Perry

I think there is another factor Perry. I think your blades are too narrow.
I am waiting for Vance to come up with a direct drive, high RPM blade
design. In the mean time this is what the blade looks like from that Reno
Racer direct drive prop. Note the wide cord near the hub. Typical of
an unducted fan. What I think is happening is most of the available HP is
being consumed by blade tip drag. In other words the cord is too narrow down
around the root for an unducted fan blade.

The second and third jpg's are from a tech paper called Aerodynamic 
Performance of a Scale Model Counterrotating Unducted Fan. 
T.J. Sullivan
Journal of Turbomachinery October 1990 Vol. 112/579
Note the efficiency is well over 84% at advance ratios of over two.


Paul Lamar
 
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