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Tracy, Have hear many whispers about your “inflexible”
coupling, but just the thought makes me nervous. Most of the money on a
coupling is in the splined block that the engine drives, the extra Flex part is
not a lot extra. Have al;ready has two made and destroyed both in testing
but the Poly and rubber (one each) survived. The poly is by far the
cheapest, but solid???? I just don’t feel right. Neil.
Sent: Saturday, September 19, 2015 10:09 PM
Subject: [FlyRotary] Re: New psru
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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