I am all eyes & ears Dan on how to work out the design issues. One idea
is to feed the fluid into an accumulator(s) to be used as the spring

force(s).

Dan, every variable length system I have seen uses mechanical actuation

other than vacuum. The

reason is as Paul mentioned we are tuning rpm rather than vacuum. There

are some situations that

are particularly difficult to tune for. An easy example shows why they

invented fuel injection,

and accelerator pumps/"powervalves" for carbs. When the engine is running

steady-state at moderate

throttle, vacuum is high. If we then snap the throttle wide open with a

load, known as lugging the

engine to most people, the vacuum DROPS TO ALMOST NOTHING. Air flow due to

mass is still there,

but vacuum isn't. A large engine without acelerator pumps that is

carburetted will stumble so

badly that it will often stop. The problem is that there is no vacuum to

draw the fuel from the

float bowl, so some mechanical means is needed. Fuel injection handles the

problem with pumps to

deliver the fuel. Sorry to go so long, the short story is without complex

methods of accumulating

vacuum, like large chambers with valves etc...you just can't get it to

work when you want it to.

Bill Jepson

Dan Ruggirello wrote:

Man, I am really learning a bunch here, having little experience with
engines.  I can see your point about vacuum not correlating to RPM and I can
also see other variables that could influence the vacuum.  But, I am not so
sure it would be as exagerated in an aircraft where the engine is operated
at constant RPMs much of the time.  In addition, if airflow is a function of
RPM, then using a venturi in the system, the pressure drop is a function of
airflow.    I agree the vacuum system wouldn't work well in all cases, where
the pilot controlled design could.  I still maintain the system could be
developed, but with much effort.

The pilot controlled design is also susceptible to control issues as well as
a self regulating design.  For example, how accurately do you know what your
RPM will be prior to takeoff so you could dial in the highest HP?  When
using a variable pitch prop?  While taking off, will you want to have to
adjust the prop and the tuner while the 50' obstacle looms ahead.  (Am
thinking in terms of bush flying here).

I can see using a tuner as a way of tuning once you are in cruise mode, but
even so, once in cruise do you really care how much HP you can draw out of
your engine unless you are in a race and willing to burn an extra 2.5gph for
10 MPH?  Will tuning the intake increase your BSFC to make it beneficial in
cruise?

The more I think about it, the more I believe for my application where I
need high HP on takeoff,  I think a static system may be better and perhaps
the variable length could be used to develop the ideal fixed length for a
given application.

Dan

Most pilots flying fixed pitch props know pretty much what the take off RPM
is. Best BSFC setting will take some time to determine but a flow meter
would help. I set the mixture now using the flow meter in conjunction
with the EGT. The rate of climb indicator can be used to set best power
in a climb and top speed can be used to set best power flat out.
In this way you can optimize the engine for the prop you have.

Paul Lamar
 
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