Looks like very nice work Neil, glad you picked up the ball and ran. I had the same comment as Charlie but as long as you get a suitable prop the 3.12 will work. Only down side is I think the minimum BSFC 'sweet spot' of the rotary (about 5000 - 5500 rpm) may be below the usable cruise settings if propped for 8500 with a fixed pitch prop. Not a big deal though. When looking at my flying expenses, fuel turns out to be about the cheapest item :-)
I would urge you to investigate 'going naked' with the damper. My direct coupler version is at least as smooth as the rubber coupled damper version. Only downside is it may not be suitable for metal props but I doubt you could find a suitable metal one for 3.12 ratio anyway.
Keep up the good work, Tracy
Sent from my iPad
I understand about debugging what
you've got, before going off in a new direction. That's why I
hated to bring it up now. :-)
The prop doesn't know how fast the engine is turning; it just
knows how fast *it* is turning, and how fast it's moving through
the air. I think that the problems start when the prop carver gets
out of his comfort zone, which is 2700 design rpm and around 72"
maximum diameter (usually less). When diameter goes up to 74"-76"
and design rpm starts to drop, they seem to get a bit confused.
For those of us who never expect to exceed 7500 rpm, with cruise
down around 5500-6000, a 3.12 ratio would mean prop at 2400 rpm
max, and around 1900 at cruise. Nothing wrong with that if we
could swing a 7 foot diameter prop, but with small, short legged
a/c, you just can't do it.
I couldn't find anything in my old emails about the prop you had
problems with. What were its specs & mfgr? Was the 6000 rpm
static, or full throttle in flight at low altitude? Did you ever
get your engine on a dyno? I know what Lamar claimed for his
p-port Renesis, but if those were 'corrected' dyno numbers, they
are pretty useless for real world flying at higher than sea level
and anything above 'standard day' temperatures (which almost never
happens). 250 HP seems pretty optimistic for a normally aspirated
2 rotor actually flying in an a/c, but I hope you can get it
working. I've been very tempted to find another Renesis core &
send the housings out to get them p-ported, & if you're
successful, you just might push me over the edge...
Charlie
On 9/17/2015 11:17 PM, Neil Unger wrote:
Charlie, As usual anything is possible. The 2.85 ratio
is easier to alter the gear set , but by the time the
“little” mods are done the cost climbs yet again. May look
at the possibility as all the drawings are done and
alterations are now much simpler. Will get the bugs out of
this one first. Most prop makers that I went to just do not
want to hear “rotary”. The revs are there but the torque is
not compared to a lyc. Much to learn in this area.
Regards, neil.
Sent: Friday, September 18, 2015 12:44 AM
Subject: [FlyRotary] Re: New psru
Hi Neil,
Congrats on your progress; it looks nice. I like the
idea of being able to source some of the parts locally,
given that international shipping can cost more than a
lot of parts.
The damper issue is a real bummer. IIRC, Tracy's been
running a later version of his 2.85 directly coupled to
an aluminum racing flywheel. One or two others have used
light weight steel flywheels, but I think they were
still using Tracy's damper mechanism, which doesn't look
too complicated to build with precision machine tools
that you obviously have access to.
I know you'll hate to hear this question, but....are
there provisions to use the 2.85 ratio in your design?
The reason I ask is that many of us have no desire to
turn the engine beyond 7500 rpm, and that wide ratio
means cruise rpm at the prop of well below 2000 rpm.
Most of us are flying planes that can't swing the 7-8
foot diameter prop needed to maintain efficiency at
those low prop rpms. It also complicates getting a prop
made that will perform correctly, since it's outside the
area where most prop makers have worked. It's confused
some reputable prop makers here in the USA, even with
the 2.85 ratio.
Again, congrats on the new design,
Charlie
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