Date: Wed, 10 May 2000 00:21:17 -0700
From: fmoreno4@postoffice.pacbell.net
Subject: Still More on Engines
To: Lancair List <lancair.list@olsusa.com>
Reply-to: fmoreno4@pacbell.net
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I'm pleased to see that adding Avgas to the coals generated the desired
heat and light and stimulated more commentary.  Many excellent issues
were raised.  Let me comment to the best of my ability.

Bill Gradwhol raised the following issues:

Theorizing a new design?  Nope, not me.  Maybe if I were to start
Airplaneengine.com and do an IPO and raise LOTS of money.  Fortunately
we have entrepreneurs who continue to pursue the dream and put their
money where their mouths are.  We should all have such conviction.  I
hope they all succeed.  We need more choices.

PRSU (prop speed reduction units) - conspicuous by their absence.  How
right you are.  Virtually any V-8 implementation today will require one
since the V-8 of comparable power output will have smaller bore and
stroke than the flat 6 and thus operate at higher RPM which would be
inconsistent with propeller tip speed limitations.  Bill also notes that
a gearbox is required to get the prop up where you need it for ground
clearance.

Note that successful liquid cooled engines of W.W.II (Rolls-Royce and
Allisons in particular) all used speed reduction units.  In my mind the
design and execution of the PRSU is the key challenge to applying V-8
configurations to aircraft.  The issues of torsional natural
frequencies, gear profiles and loading, damping, bearings, cooling,
lubrication etc. are important and must draw upon good engineering
analysis, practice, and past experience.  Don't leave home without it.
Weight is an inescapable cost for the gear box.  But it is clear that
durable, reliable designs are well within the state of the art.

Key features for successful gearboxes must include smoothest possible
power impulses, preferably with no torque reversals (which means more
cylinders are better, and even firing patterns are better), and
selection of torsional stiffness of the system components to assure that
no natural frequencies are near expected excitation frequencies (such as
firing strokes per revolution, or multiples thereof).  Decoupling of the
crankshaft from the gearbox with low stiffness shafting or couplings can
eliminate most of the cyclic forces applied by pistons to the crankshaft
resulting in loads applied to the prop that are of near turbine-like
smoothness.  That's plus one for the prop since we can avid exciting
it's natural frequencies.  But it takes good analysis supported by good
testing to get the desired results.  That's minus one for development
cost (bring money).  The principles were well known in W.W.II, but with
computers we can now do a lot more inexpensive computer analysis and a
lot less expensive cut and try which was a primary development tool in
the past.

While on this topic I must express my amazement at the French diesel
undergoing certification.  This is a four cylinder flat four.  Four
cylinder engines generate large force reversals twice each revolution as
pistons rise during compression since there is no offsetting power
stroke.  With diesels the torque reversals are even larger.  These
torque reversals make it very tough on metal props which have much lower
damping than wood, and so can be made to flap and fail if not properly
matched to the engine.  (That is why metal props are illegal in Formula
1 aircraft using 0-200 Continental turning high RPM.)  Now add a speed
reduction gearbox (required on the higher horsepower versions) and you
would seem to have major torque reversals occurring through the gearbox
which I think would make the gear teeth, bearings, and such quite
unhappy.   There must be a special French solution to this seemingly
difficult problem.  Or maybe they have a big flywheel on the crankshaft,
and an isolation coupling between engine and gearbox  It's a puzzlement.

Belt drives are simple and perhaps adequate for lower horsepower
applications, but the discussion about torque reversals still apply, and
such reversals will eat belts if care is not taken.  Hooking a big,
toothed belt to the front of a crankshaft with nothing in between
strikes me as unwise.  Ditto for chains.  Show me the data.

I think the issue of gearbox size and location is much less an issue.
For minimum drag, the ideal radiator has large frontal area but is thin
in the air flow direction to minimize pressure drop.  Don't think of
radiator positioning like you would in a car.  The nose is NOT precisely
the place to put an aircraft radiator.  Large radiator frontal area
requirements (think nominally 1 square inch per horsepower more or less)
lead to inclined radiators much as we now see in Formula 1 and Indy
cars.  (I carefully inspected an Infinity-engined Indy car recently, and
the radiators were about 15 degrees from horizontal).  If the cooling
air stream is slowed in a properly designed diffuser, the turn to enter
the radiator has nearly negligible pressure loss because the velocity is
low.  Thus one can incline the radiator to nearly horizontal and put it
under the engine (I saw an RV with a Mazda using this configuration), or
incline vertical radiators on either side of the engine (Engineair), or
incline somewhere in the fuselage, or incline in a belly scoop.  This
last option (used in the Eagle 540, 500 HP V-8) adds some additional
frontal area, but is necessary if there is no other space to be had.
Here again one needs to use ample doses of engineering analysis,
experience, and a lot of testing.

V-6 applications: To get even firing patterns one needs to make the V-6
either a 60 degree V (which makes it very narrow between the heads) or
120 degrees which will be nearly as wide as a 180 degree engine (flat
opposed).  There are some secondary balance issues that make them less
desirable than V-8's but these can be managed or just tolerated.  And
volumetric efficiency improves as cylinders get smaller (scale effect;
surface area to volume) so I guess you might as well make it an eight.
Remember the Olds F-85 used a 215 cubic inch V-8 years ago still used
today in the Range Rover.  Some are even flying.  And I am told they are
very compact.

I know that small block 327 Chevrolet in my ancient Corvette
(predecessor to 350 and 400 inch versions later on) is quite small
compared to my Lycoming 540.  Jim Rahm's 420 horsepower Engineair V-8
uses virtually no GM parts, but being based on the small block Chevy
layout leads to a similarly compact package.  By extending the cowl
forward about 2 inches, they were able to get the radiators, tanks, and
all the engine hardware (but not the battery) inside the cowl.

Diesel Engines: In my opinion, the long term future of general aviation
depends on successful jet-fuel-burning diesels.  Leaded Avgas is an
endangered species.  I have already expressed my amazement at the French
geared four stroke opposed four cylinder engine.  There are also a
number of novel designs out there.  I tend to favor two stroke designs
because of the potential for smooth operation and light weight, but as
with all things engine associated, it is complicated and expensive to
come up with something truly worthwhile.

I have written about the Zoche air cooled radial two stroke eight
cylinder engine.  Stu Seffern asked about liquid cooled diesels.  Air
cooling would seem to make sense for diesels since exhaust temperatures
are MUCH lower with diesels than with gas engines thus (hopefully)
eliminating hot end distress.  But there are questions about lubrication
and localized heating above the exhaust port, the extreme light weight,
and most importantly, the development program that never ends.  I hope
we all live long enough to see Zoche manufacture production quantities
of certified engines.  As for other diesel programs, I wish them all
well - we need them.  The problem is that there is simply too small a
market for any new engine, gas, diesel, hydrogen, fairy dust, or
whatever, to justify the huge amounts of money required for a certified
power plant.

Marshall Michaelian rose to the bait :-), and is correct in his
observation that IF you have perfect mixture distribution between
cylinders, THEN you can successfully lean beyond peak, increase manifold
pressure, and achieve lower exhaust temperatures, cylinder head
temperatures, and improved specific fuel consumption.  This was the
basis of the original Malibu engine (Continental TSIO 520-BE, 310
horsepower) that was the predecessor to the TSIO 550.  The Malibu engine
used cross flow heads (intake manifold on top, exhaust manifold on
bottom) and made a valiant attempt to get equal flow distribution to
each cylinder with a serpentine intake manifold subsequently carried
over to the 550 we have today.  Make no mistake, these were big steps
forward, but the 520-BE lost out to the larger, thirstier Lycoming in
the Mirage for political as much as technical reasons.  But the intake
manifold design and fuel injection systems, while far better than
earlier designs, still fall far short of what has been achieved in the
automotive world where emissions control and fuel economy challenges
have been addressed with billions of dollars.

Marshall is right: if you spend the time and effort, design or use good
intake manifolds with good air distribution, and if you also take
advantage of electronic fuel injection at the intake ports, you can get
MUCH better mixture control, and thus take advantage of lean operation,
richening only if necessary for maximum possible power.  Remember, I
said my Skylane was carburated (boo!!!). I had 6 cylinder CHT/EGT
instrumentation, and the EGT exhaust spreads were always large.  Poor
mixture distribution is the norm, not the exception in aircraft, alas.
Most fuel injected aircraft engines are not much better.

So, as with all things associated with airplanes, we have to make
tradeoffs.  I still assert that if you burn gasoline, then even with the
complications of gearboxes and cooling systems, the liquid cooled V-8 is
the preferred configuration for making lots of horsepower at high power
settings at high altitudes.  It can provide lower fuel burn per
horsepower, lower engine repair costs (due to lower temperatures and
better controlled clearances and thus lower wear), and with care, lower
drag.   More speed for less money.  What's not to like?

Let the controversy continue......

Your humble reporter,

Fred Moreno


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