I claim to know ABSOLUTELY NOTHING about torque curves, gearing and where
(what RPM) to make horsepower with your auto conversions (that's why I take
the easy/proven way out and stick with WWII technology!), but I can offer
that the reason a geared Lycoming outperforms (STOL, not cruise) a direct
drive Lycoming rated at the exact same Horsepower is the fact that it puts
out more torque at the propellor (were it matters) and can turn a larger
prop at a slower speed where it is more efficient, creating more THRUST.
This is also the secret to the radial engine plugging away at slow speed and
spinning a large prop.
If I'm full of sh@# let me know, I've been told before! Just my view on the
gearing issue!
For reference I provide the GSO 480 Lycoming. Take off power is 340 HP at
3400RPM. The prop is a 3 bladed Hartzell that is 96" (8 feet) in diameter
and is only turning 2182 RPM at full power due to the 77:120 gear ratio
(1:1.5584) and you want to see climb!!! Even in an aircraft that weighs
2875lbs empty I see close to 2000FPM, with 2 on board and 66 gallons of
fuel! Cruise is about 125 MPH at 60% (204 HP burning approx.17GPHUS).
Doesn't get much faster at 75%, with about 130 MPH, but really increases
the fuel burn on a supercharged engine.
This probably offers you nothing to go by with your 350 block, but you need
someone (that has been there before NOT THEORY) to determine a gearing that
lets you spin it to a reasonable engine speed (that will allow it to make
good power and remain dependable), while creating sufficient thrust at a low
enough prop speed to remain efficient, without bogging the engine down.
Maybe since you are going auto engine you could throw in the transmission
and shift gears after take off!!!! ;>)
I think you have already done so, but I strongly suggest trying to get Brian
Robinson to tell you the ratio on his drive in the LS1 Seabee. He derated
the Corvette engines HP, so it would last longer and turns it slower than it
would in the car also. The climb of this (pusher) aircraft is nothing short
of AMAZING!! Cruise is a lot better than the Franklin powered "bee also, but
of course it should with an extra 100 HP under the cowl!
Regards,
Wayne G. O'Shea
www.irishfield.on.ca
----- Original Message -----
From: "Peter Cowan/Lexy Cameron" <
cowcam@pipcom.com>
To: <
murphy-rebel@dcsol.com>
Sent: Saturday, December 01, 2001 8:36 PM
Subject: Re: Marcotte group purchase
I'm interested too. I'll need the Chevy V8 version rated for 350-400HP.
I'm assuming that the Dart Aluminum block 350 has a standard bell housing
pattern so that the Marcotte adapter will fit. Does John Worden happen
to
know? Oh, and I better make sure it has a provision for a constant speed
prop governor. My preferred ratio is 1.67:1 to keep max engine RPM at
4500,
but I have been toying with building my engine for a bit higher max RPM
to
maximize takeoff performance. The 1.96:1 drive would give me an engine
redline of about 5300 RPM assuming I still want to limit the prop to 2700
RPM. I'll have to do a little more investigation on how that may effect
reliability, knowing that I, as the engine operator, can manage the use
of
the engine near the redline to maximize reliability. I.e., only for
takeoff, limited to say, three continuous minutes, and such things. Does
the list have any comments about allowing the higher engine redline?
Mike Kimball
SR#044
Mike all of Marcotte's larger drives have provision for a constant speed
oil
supply.
Is there something magic about the 2700 rpm number? While you and I are
into different engines (and planes) we both will want good take off
performance with relatively slow cruise. To me that says go for as big a
prop as possible and if necessary run it slower. So what is wrong with,
say,
a 2400 max rpm? There have been lots of aircraft that had max prop rpm
less
than that. No one has offered any data that says it is "bad" to design for
lower prop rpm unless I've missed something.
Peter.
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