Referring to that earlier message (below) I would like to add something.

When you look that VW Lupo 1.2 TDI engine performance diagram

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

You should know (perhaps you do) that the engine performance measurement
presented in such a diagram is always done by using wide open throttle
(WOT). That means, it is not possible to achieve the same performance level
in an aerial vehicle equipped with a fixed pitch propeller, because it is
always tuned to some specific speed range. By using variable pitch propeller
it is possible to get closer to the original automotive engine performance
specs.

When you look what I wrote earlier (below) about electronic propeller pitch
control unit, there is a big gain available. Basically you could give the
engine full throttle and let the electronic governor to regulate preferred
rpm level. For example:
http://www.airmasterpropellers.com/w...idWebPage=3473 &
http://www.airmasterpropellers.com/w...idWebPage=3476 &
http://www.airmasterpropellers.com/m...tion_sheet.pdf
show you some information. The controller functions: take-off, climb and
cruise are factory preset (rpm) settings that can be customized with a PC.
Then there is this HOLD function that gives you a possibility to manually
set any desired RPM during flight and hold that setting. As said, basically
you could just use full engine power setting and let the governor do its
work (maintain the set rpms, e.g. 2500) . This provides efficient engine
performance. There are some concerns still. There should be some kind of
warning indicator or actuator in case of engine over-rev situation = over
3300 rpm in this example (possible if the prop or controller fails). The
second matter is, that despite of all this (naturally) you yourself are the
regulator of airspeed, by manipulating throttle setting. It is of course
possible to violate aircraft structural speed limitations in some
situations.

JP


"JP" kirjoitti
...

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