As you already pointed out, it's likely that your low prop RPM are the issue
here. If I remember correctly you have an 84" prop, so knowing that I
converted your prop RPM to tip Mach numbers. Off the top of my head I seem
to remember that you want to reach at least 0.81 Mach minimum to achieve
good prop efficiency. Unfortunately, even at full power your not getting
there.
Max Power: 5000 RPM - no more than 5 minutes, prop speed 2370 RPM - 0.7780
Mach
Max Continuous Power: 4500 RPM, prop speed 2133 RPM - 0.7002 Mach
High Cruise (80%): 4000 RPM, prop speed 1896 RPM - 0.6225 Mach
Normal Cruise (70%): 3500 RPM, prop speed 1659 RPM - 0.5447 Mach
As I said, the 0.81 Mach is off the top of my head, but I'm pretty sure I
remember reading that. However... I just plugged in the numbers for my
C-150, and it's not getting Mach numbers as high as your airplane. In fact
my engine redlines at 2750, which with a 69" prop gives me 0.7416 Mach.
So I guess all I've managed to do is give you a few more numbers to mull
over. ;-)
Mike
----- Original Message -----
From: "Mike Kimball" <mkimball@gci.net>
To: "Rebel Builder's List" <rebel-builders@dcsol.com>
Sent: Sunday, July 20, 2008 12:29 AM
Subject: [rebel-builders] More on my Chevy V8 SR
I "talked" with Steve Bowley and Al Paxia via email recently and between
the
two of them I think I'm getting some more insight into some
characteristics
of my plane. I recently went on my longest flight to date which lasted
about an hour and a half. I had a chance to play around with power and
prop
settings and check out the cruise characteristics. I discovered something
that was pretty disappointing. Cruise speed. I have decided to use the
following engine RPMs for various phases of flight.
Max Power: 5000 RPM - no more than 5 minutes, prop speed 2370 RPM
Max Continuous Power: 4500 RPM, prop speed 2133 RPM
High Cruise (80%): 4000 RPM, prop speed 1896 RPM
Normal Cruise (70%): 3500 RPM, prop speed 1659 RPM
While cruising I only saw about 95 knots and couldn't get much better than
that with lots of playing around with the prop and power settings, between
3500 and 4000 engine RPM. One big problem I have is no manifold pressure
gauge. I don't know what I was thinking. I need that gauge and will soon
be adding it. I only guessed at power settings using the position of the
throttle. I left my prop in manual control, adjusting prop pitch and
observing the change in engine RPM without moving the throttle. The other
thing I noticed was that the plane showed a pitch up attitude, observed by
comparing the bottom of the wing with the distant horizon, while
maintaining
level flight between 3500 and 4000 engine RPM. That would certainly
contribute to a low cruise speed. Flaps were up which I'm told is the
reflex position even though it looks like no deflection to me. The top of
the flaps seems to be even with the cabin roof. Something to do with the
lower surface of the flaps or something. I was by myself with about 60
gallons of fuel and only about 15 pounds in the baggage area. CG was
slightly forward at about 13.5 inches aft of the datum. I also saw higher
oil temp after setting up in cruise for a while. It hovered around
250-260
degrees. At least coolant temp stayed right at 180 but the fan was on the
whole time. Made me a little nervous. An oil cooler looks like a must
have
item.
Al pretty much gave me the answer to my poor cruise speed with one simple
statement. "At lower RPM, say under 2000, my airplane is also nose up."
Steve can get great cruise speeds (140 knots) but that is using 23 squared
cruise settings and a pretty high fuel burn. His cowl is also much more
streamlined than mine. But I think my biggest problem is my prop isn't
turning fast enough. Dang those guys at Firewall Forward that convinced
me
the 2.11 ratio would be better than the 1.8 ratio that they had available.
Steve's gear reduction is 1.8. Good choice Steve. (Reminder, Steve is
using an LS1 Chevy conversion.) One solution would be to stick with my
low
prop RPMs and get a much bigger prop. Another solution would be to turn
my
engine faster. It is certainly capable of that since it's built with race
components, full roller rockers and lifters, etc. But I wanted to keep
the
engine RPMs as listed for reliability. Change out the gear box? Another
option but one I'd rather not do. My unit cost $12,500 and I cannot
simply
replace the internal gears. Firewall Forward tells me the entire unit is
different for the 1.8 ratio. Great!@#%
After I get the manifold pressure gauge installed and a few more flights
I'll report back. In the mean time I am hoping to fly in to our local EAA
chapter summer picnic tomorrow. 1700 foot grass strip on a ridge near the
Fairbanks International Airport. Looks a lot shorter than that from the
air. Hope I can get it down in the first quarter of the runway. There is
no over-run. Trees on both ends. At least I'm flying, eh?
Mike
044SR
P.S. Thanks Steve and Al.
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