Mike, with the 2.17 gear box, I got a
nominal 5200-5400 so say 5300. On a cold day it would jump up to 5800 rpm
static.
The most I recall getting in level flight
was around 6400 rpm.
Just let me know if you want to try it,
however, 68x72 vs your 68 x80 might not make much difference - only 8”
difference in pitch
Ed
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Mike Wills
Sent: Sunday, March 07, 2010 12:18
PM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re:
Performance 68x72 prop. [FlyRotary] Re: TB size, Travel and Power
Thanks a bunch for the offer. I will likely take you up on
it at some point in the not to distant future. My guess is that It would
probably be very close to fitting my spinner since Clark's
props are very similar to Margie Warnke's. The other potential gotcha is prop
hub thickness. Per the data I got with my Warnke it’s a 68x80 but
seems like all of these wood prop manufacturers measure pitch differently so I
don’t know if a meaningful comparison can be made. Can you tell me what
sort of performance you were getting with this prop? Static RPM? WOT RPM in
flight?
On the MP gauge I'm going to need to go with an 2
1/4" due to panel space. The reason I havent gotten to this yet has
nothing to do with cost. I need to revamp my panel (in addition to the intake
manifold and cowl - I may never be finished with this thing). My current panel
has a bunch of redundant steam gauges in the right side of the panel. My vacuum
gauge and EC-2 controls are mounted on a small console in front of the stick. I
want to eliminate some of the redundant steam gauges, move the EC-2 controls up
to the panel, and replace the vacuum gauge with MP, also moved up to the panel.
Its going to be quite a lot of work so I've been stalling.
In spite of the weather this weekend, Spring and soaring
season are approaching. My time is starting to get diverted to getting my
sailplane ready for the coming season. I need to retire.
Sent: Saturday,
March 06, 2010 8:18 PM
Subject:
[FlyRotary] Performance 68x72 prop. [FlyRotary] Re: TB size, Travel and Power
Actually, Mike, I do have a 68x72
Performance prop that I used for my 2.17 gear box. Don’t know if
that’s sufficiently different from yours to make a difference or
not. Let me know if interested – for the paltry sum of shipping
costs, just might be able to get it to you {:>). Don’t know if
the blade profile will fit your spinner cut our or not – that’s
about the only problem I could see.
Ed
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Mike Wills
Sent: Saturday, March 06, 2010
10:13 PM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re: TB size,
Travel and Power
Yes I have confirmed that I get full ambient pressure at
2/3 (roughly) throttle near sea level. And yes I still have the POS vacuum
gauge. I know it needs replaced - just havent had the opportunity yet.
My point in bringing this up yet again was to emphasize
the point Ed made a couple of posts back - that this whole thing is a complex
problem with a lot of variables. And to not dive in and start whacking on the
prop until certain that is where the problem is. In my case I'm not sure it is.
Full ambient MP at the TB doesn’t indicate how well the intake is flowing
and it is entirely possible to have full MP but a very poor performing
manifold. Guys in racing make lots of $ porting and flow testing manifolds.
I need to make some changes to the intake anyway for other
reasons so I'm going to focus on that first. And once that's done, if I'm
still giving up some RPMs then I'll take a look at the prop. Unless of course I
can do it the easy way as Al suggested and borrow a prop. On that note,
are there any of you RV guys that have upgraded from the 2.17 gear ratio to the
2.85 that have a "wrong way" prop you'd be willing to lend for a
test?
Meanwhile I havent been able to fly for the past 3 weeks
due to this crappy weather. Why's it always have to rain on the weekends? Its
not supposed to rain in SoCal at all.
Sent: Saturday,
March 06, 2010 7:06 AM
Subject:
[FlyRotary] Re: TB size, Travel and Power
you are measuring the MP exactly where you
should, You can't do it properly anywhere else, at least not with a
simple instrument.
But are you at last confirming that you DO have full ambient MP at 2/3
throttle setting? That IS an important question if you want to know
the right answer to your question. BTW, ditch the vacuum
gauge (if that's what you are using) and get a proper MP gauge.
On Fri, Mar 5, 2010 at 9:30 PM, Mike Wills <rv-4mike@cox.net> wrote:
I hear what your saying (and I get it). Here's
the thing, and I admit I may have a misunderstanding here. Your
explanation applies if you are measuring manifold pressure essentially at
the rotor face. If you measure MP closer to the TB as I do unfortunately,
it is possible to see ambient at full throttle but have restrictions in the
intake tract that would result in less than ambient at the rotor face.
Conversely (and again theory - I'm open to being shown wrong here), a properly
designed DIE manifold would show ambient MP measured at the TB and greater than
ambient measured at the rotor face.
I accept that by trimming the prop I can lighten the load
and gain some HP (though some here seem to think that HP will increase linearly
with RPM to infinity and I don’t buy that either). But as you alluded to
in your previous, there's some potential for mistake in trimming any prop
until you are quite sure that you are not HP limited by something other than
load. And I'm not sure yet. Just a feeling based on the fact that I'm using a
cut down RX-7 TB that’s maxed out about 1/3 short of fully open. And
honestly I still havent dug any deeper because the airplane is flying well and
has good performance. But sooner or later I'm going to want to get more
than the 5700RPM I'm currently getting - I want all the performance
that’s there.
Sent: Friday,
March 05, 2010 6:36 PM
Subject:
[FlyRotary] TB size, Travel and Power
I am
apparently not doing a very good job making the point about the relationship
between throttle body size, throttle travel and engine power clear. So
here is another try at it - using extreme examples and no math.
Let’s assume your volumetric efficiency is 100% (no losses).
IF you put a
½ ” dia TB on you engine. You could have it wide open and your
engine (under normal prop load) probably wouldn’t turn 4000 rpm.
That is because even wide open, the ½” dia TB restricts airflow
sufficiently - that the manifold air density never approaches the ambient air
density. Since we know that the engine power is directly proportional to
the density of air in the combustion chamber – and this density is
limited in this case to less than ambient, you engine is not going to produce
much power, certainly not full power. So this bit of information tells us
“Bigger Lithium Crystals, Scotty!!” – i.e try a larger
throttle body.
Now if you
keep enlarging the diameter of the TB you would find that at fully open -your
engine would be producing more power than it was, but perhaps still not the maximum
power it is capable of. This is because the air density in the manifold
has increase due to the less restrictive flow, but is still below
ambient. This is due to the better, but still restrictive effect of the
TB size on the air flow. Now if you continued enlarging the TB size, you
would reach a point where with the TB just reaching fully open - your manifold
air density is exactly ambient and your engine is producing all the power it is
going to.
Now if you
enlarge the TB even further, you will simply find that you can cause the
manifold pressure (air density) to reach ambient without opening the TB
fully. Its simply large enough that all the air the engine can use
(ambient air density point) is met at partial throttle opening. In
fact , you can certainly continue to advance the throttle thereby opening the
throttle plate even more – but, you are not going to increase the air
density in the manifold and therefore you will not produce any additional power
for that additional throttle travel.
Bigger
Throttle body’s result in more power only up to the point the airflow
they permit causes manifold air density to reach ambient. Beyond that
point, the only thing they do is provide frustration – by having all that
throttle travel remaining which does nothing to produce more power {:>).
Now if you
can somehow lighten the load on your engine, then engine can turn faster
providing more “suction” on the manifold volume reducing the air
density below ambient, now opening your “oversize” TB a bit more will
produce more power because you are increasing the airflow again to the point
where the equilibrium point between ambient air density in the manifold and rpm
is again reached. Lighten the load further and you can again increase
engine power by opening your TB more. Etc, etc.
Ah,
ain’t this hobby wonderful {:>)
Ed
Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com
http://www.andersonee.com
http://www.dmack.net/mazda/index.html
http://www.flyrotary.com/
http://members.cox.net/rogersda/rotary/configs.htm#N494BW
http://www.rotaryaviation.com/Rotorhead%20Truth.htm
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