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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