Well that might explain it. The prop is too large in diameter
to to go 2995 RPM at 6500 engine RPM required to get 165 HP
from a 152 cubic inch piston engine without a turbo charger.
I think these guys tend to underestimate the PSRU ratio and RPM necessary
to get the 165 HP out of a small piston engine. I think it goes
against their instincts to scream them at 6500 RPM WOT for five or ten
minutes needed to take off and climb out with 165 HP.
It's just a habit of people to quote the manufacturers HP
for the engine. At Arlington 2000 for engine HP I wrote
120 HP on the data sheet taped to my plane. Several people
came up and asked "the engine puts out more HP than that
doesn't it?" I just reply that that is probably the realistic
HP that I'm seeing at my RPMs. I put 120HP on my website
also, and I'm sure that's fairly accurate.
If you ever experience a piston engine running on a dyno at a continuous
6500 RPM WOT full load you will be impressed and amazed. Actually
you get a feeling in the pit of your stomach that you don't want
to be in the same room with that thing. It is the only
reason I can think of why these guys with the 165 HP Subaru
cruise them at 70 HP 40% power or below.
BTW I would not be a bit surprised if the Subaru stock EFI cut
the power after a few seconds at 6500 RPM WOT. Car piston engines just
don't do those sorts of things well. When I rent a compact car down
in Florida I try to find roads where I can run it flat out for
awhile. Usually I can get the computer to shut the engine down
after just a minute or so at about top speed. Usually about 115 to 120
MPH. There is a road on the way from Tracy's house out to Cedar
Key that is pretty good for this sort of thing.
If you cut the prop diameter to allow the engine to rev more
with a 2.17:1 PSRU you lose a bit of prop efficiency. What it
really needs is a 2.5:1 PSRU.
I can get a better match with his data if I assume the Cozy
has more drag. Here it is.
BTW I am slowly coming to the conclusion that you would be better off
with less pitch, more twist and wider blades. Just a hunch.
I know how much you want to make yet another fan :-)
Paul Lamar
I can simulate that. I've been playing around on the computer for
years now trying to improve what I've got, so far nothing really
pops out that would give a significant enough improvement to
warrant building another fan. I can see less pitch, wider blades,
but not sure why more twist. My twist is already more than
any typical prop. My fan has 38 inch pitch at the tip and 50
inch pitch at the root. More angle of attack at the slower
moving blade elements to attempt to keep velocity across the
fan constant.
--
Perry Mick
Duckt N7XR
http://www.ductedfan.com
I was just going on what it looked like and what the latest small
turbo fans look like. Maybe you need a bit less diameter as well.
That would do two things. It would cut the drag of the duct and
increase the RPM and HP of the engine. Course I still think
a tighter and smaller engine cowl ahead of the duct is a
higher priority. Don't do anything till after OSH. We need
you there.
Paul Lamar
Especially for ducted fans (and shrouded props?), I wonder if it would
make sense to make prop blades similar to a submarine's modern screws?
The screws are designed for absolutely minimum turbulence which should
also equate to more effective power. Appropriate equation mods for the
differing viscosities between air and water would have to be developed,
as well as speed differences. I wonder if those 2 factors combined
reach the diminishing point of returns quite rapidly? Would be
interesting experiments.
AI Nut
(ducted fan fan, too)
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