Some tough but interesting questions. I am certainly not an expect of this
particular matter, but I try to explain out the basics. There are competent
experts out there, who can give you much more precise advices.

Let's assume that direct drive concept is to be used. We should know, what
kind of propeller to use. An electric variable pitch propeller might work
well. I use the following as an example: Airmaster AP332 (
http://www.airmasterpropellers.com/w...idWebPage=3475 ). Note the maximum
speed 3300 rpm. This would give an opportunity to raise the take-off
engine/prop speed up to that 3300 rpm level. In VW Lupo 1.2 TDI engine case,
that would mean a take-off power of about 57-58 hp (standard version) or
about 82 hp tuned engine version. Of course the prop issue is more
complicated in a real life and slower prop speed would produce much less
noise.

That 3300 rpm produces the maximum propeller tip speed allowed by the
manufacturer. That is fine during a take-off, but instantly when the
aircraft starts to move forward, things change a bit. If we think only the
engine, it probably would run happily 3300 rpm hour after hour. That hardly
is an issue. But when the velocity of the aircraft increases, the propeller
tip speed will increase. That because the aircraft is now flying into a
"headwind". Aircraft forward speed & propeller rotational tip speed together
will produce so called propeller vector tip speed. That means, with a 3300
rpm cruise setting you are in a danger to run the blade tip speed into a
supersonic region, losing efficiency and possibly destroying the prop in a
very bad way! Not a way to go! That means, during the flight the rpm level
has to be brought down. How much? That can be calculated by someone (a
relative matter depending on cruise speed and probably also velocity never
exceed). You may already know this all, but I still explain it.

So during flight the rpm setting has to be lowered. Let's assume a figure
like 2500 rpm (about 76% of the max 3300). The VW engine still produces
about 48 hp (standard version) or about 61-62 hp (tuned version). In order
to get optimum efficiency out of the powerplant, a variable pitch propeller
is used in this example. By adding propeller pitch, the rpm level can be
maintained in preferred range and an optimum cylinder pressure level is
produced. [Note now, that it has to be mentioned, that probably all the
modern car diesel engines are controlled by an ECU (electronic control unit)
governing all the engine functions. It is a matter of its own to get such an
engine controller to function in an aircraft]. But presented simply you
could set a certain engine power level (by a throttle or actually amount of
injection in this case) and let an electronic propeller pitch control unit
(e.g. Airmaster AC 200:
http://www.airmasterpropellers.com/w...idWebPage=3473 ) to take care of
the prop rpm (e.g. that 2500). This 2500 rpm level is within the best torque
range of VW 1.2 TDI engines.

During a 2500 rpm cruise, the jet fuel (or light fuel oil - note there is
this issue of freezing point, it's cold up there!) consumption PER HOUR
might be something like 17+ pounds (2.5+ gallons) standard version, 22+
pounds (3.3+ gallons). That is just a rough estimation.

Direct drive concept in this case would provide a sort of WWI or 1920's look
where inline engine block is vertical and the propeller drive (power
take-off) is positioned relatively low. The propeller ground clearance must
to be checked carefully.

So it may be feasible to have a direct drive diesel engine configuration,
depending on the required power/weight/cost etc. This information presented
is not accurate and please do not use it for any exact planning. Get a
second opinion :-) I just present what I consider reasonable. Other people
may tackle possible propeller drive vibrations and many other issues etc.

JP


http://www.geocities.com/plane_diesel/index2.html
Little green and red VW Lupos (yellow graph on right side)

If you were plopping that diesel into a 750 lb. RV-3B/Sonex/whatever and
you
wanted to cruise 125 - 140 mph with great fuel gph rates, where would you
choose to cross the graph? Both red and blue options.

1800 torque (both)
2500 rpm's (red)?
3000 rpm's (blue)?

Is this in the realm of direct drive possibilities (assuming no vibration
issues, etc) or does this engine NEED a PSRU to take advantage of what the
engine has to offer? (Don't worry about the extra weight - if it needs
one,
it needs one! The plane will just have to be a single seater)

How important is torque vs. rpms vs. horsepower in determining where you
want to be on the graph - for engine cruise numbers?

With extra low end torque available (diesel), a known size for the engine
(110+ kg?), a top end cruise speed limit (140 mph), what kind of prop
would
you guess at? Big slow turning? Smaller for a higher revving engine?

Let's say today's VW or Toyota diesels can handle some higher rpms - 3000?
4000?

Thanks for helping me put the pieces together in my mind/imagination.


Montblack