From: "Bill Bradburry" <flyrotary@lancaironline.net>
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To: "'Rotary motors in aircraft'" <flyrotary@lancaironline.net>
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Subject: RE: [FlyRotary] Re: 20B with RD-1C Prop Options
Date: Thu, 29 Oct 2015 23:33:18 -0500
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James,
There could be some slip involved, but it is mostly the change in angle of
attack as the prop disc is moved forward thru the air.

If you can stand the expense, I recommend you get some kind of in flight
adjustable prop.  The rotary power output is really affected by rpm and
being able to adjust it in flight is very helpful. 
I also recommend you set a max take off rpm target of 2500 prop and 7125
engine.  There are several good reasons for this not the least of which is
that the EC-2/3 has a cutoff at 7500 rpm. (Tracy can straighten me out here
if needed)
If you dyno the engine be sure and let us know what you get.  I don't think
you will be anywhere near those HP numbers you quote.  Also when you get up
to altitude and cut the rpm back to 48-5200, you will really be reducing the
output.  If you leave the engine at 7125 rpm and climb to 8000 ft, you will
be producing 75%, if you then reduce the rpm to 4800, I don't know what your
percent power might be...but a heck of a lot lower than 75%.

Do you plan to try and buy a completed airframe?  It sometimes takes a
surprising length of time to build one.  Ask me how I know.  :>)

Bill  

-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] 
Sent: Thursday, October 29, 2015 9:20 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: 20B with RD-1C Prop Options

Thanks for the input Tracy!  So is the difference between top speed rpm and
static rpm the "slip"?

> On Oct 29, 2015, at 6:32 PM, Tracy <flyrotary@lancaironline.net> wrote:
> 
> Hi James,
>   HP targets sound doable, but those rpm numbers are probably off.
Remember that with a fixed pitch prop, hp varies as the CUBE of rpm.  Would
recommend propping for a max power at top speed rpm of about 7000.  That
will give you a static rpm of around 6000.
> 
> Will get those instruction manuals you asked about to you next time I'm in
the office.
> 
> Tracy
> 
> Sent from my iPad
> 
>> On Oct 29, 2015, at 16:22, James R. Osborn <flyrotary@lancaironline.net>
wrote:
>> 
>> Hi guys,
>> 
>> So I am going to be building an RV-10.  I sourced Bobby's spare 20B /
RD-1C combo and my plan is to work on the motor first, kind of bass-ackward
but that is what I am doing.  My plan is to build a motor/mount test stand
and work everything out on the ground before I ever put it in an airframe.
I plan to set it up NA using the existing side port configuration.  I don't
know if it is ported - I am going to attempt to determine this by inspection
without cracking it open and doing a dynamic compression test to evaluate
the health of the motor.  If it is necessary to entirely rebuild the motor,
I would probably go PP.  So I am thinking the target HP as it is would be in
the 275 HP range and probably 325 HP if it ends up being a PP.  How do these
numbers sound to you all?
>> 
>> I will be looking for a way to dyno it, hopefully borrow one to get some
data.  But I was also thinking about just getting an appropriate fixed pitch
prop and working through the bugs based on static run ups.  I like the idea
of a three blade prop.  I like the idea of optimizing for cruise at 75%, say
5800 rpm which would put full throttle at 7733 rpm on the 20B.  At 5800 rpm,
the prop would be spinning 2035 rpm and at full throttle 2713 rpm (using the
2.85 ratio RD-1C PSRU).  Do all these numbers sound about right?
>> 
>> If so, then how do I choose a prop that will result in 2713 rpm static
run up at full throttle, but be tuned for efficiency at 2035 rpm cruise?  I
am assuming at these reasonable RPMs that it will be a larger swing and the
three blade makes sense - what do you think?  Is there a convenient way to
find props out there that will work at the target full throttle HP of 275 or
325?  Also if you guys have any good resources to learn about how to figure
these things out (books, web pages, online calculators, etc.) that would be
great!
>> 
>> Another option might be to go with something like the IVOPROP Magnum,
either ground adjustable or electric inflight adjustable.  What do you guys
think of these?  Would the adjustability really help me adapt to however my
power plant turns out?  Would the extra complexity be worth it?  For the
ground adjustable one, I am thinking the process would be:  1. select the
"correct" swing (how?) and go for three blades, 2) use ground adjustability
and work up to THE pitch while I tune the motor until I achieve full
throttle static run up around 7700 RPM.  Will this result in a reasonably
efficient setting for 75% cruise?  Am I thinking about this clearly?  All
opinions are welcome.
>> 
>> There's a lot for you to chew on!
>> 
>> - James
>> 
>> 
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