Hello Paul-

As many know, we've had some issues getting full power from our PPort
install.  We've been working our way through reasons why we might not get
full power.  Today was "disassemble the slide throttle" day.  Here is what
we found.

As shown in the photos, at WOT, our slide position is about 3/16" shy of
being fully open.  Turns out that there is 1/8"+ play in the system
introduced by the vernier control.  With the throttle in the WOT position, I
can move the slide the additional 3/16" by hand to get it fully open, but I
cannot achieve this using the throttle knob.  Thus, we are assuming that
when we last set the slide distances, we accidentally moved the slide by
hand & thus set the wrong distance.

Whether this affects max power remains to be seen.  The restriction to
airflow does not seem like it should be enough to greatly effect power, but
add in turbulence & who knows.  We have now simulated flight conditions (I
pressed on the outboard side of the slide with my thumb while Terry moved
the knob to WOT) and set the position accordingly.  It will now move to the
true WOT position from in the cockpit.

We also investigated what selecting minimum throttle position would give.
See next 2 photos.  In this case we have the tiniest sliver of air gap for
each rotor.  Yet, the engine will (unhappily) run even with this small
amount of air.  We believe at this throttle setting, the engine is in fact
scavenging some/much/most of it's combustion air from the exhaust port vs.
the intake port.  Since the two ports are so close to each other, and the
rotor begins the intake cycle while the exhaust port is still partially
uncovered, we hypothesize that at these miniscule throttle openings already
combusted air in being ingested to make up the difference -- leading to the
really rough running at low rpm.

We think the current minimum throttle opening is too little -- Terry reports
engine out just before touchdown when he pulled power back to minimum on the
flare.  We can think of two ways to address this:

1) Install a set screw to prevent the slide from moving that far aft.
2) Change the port opening shape in the slide to be teardrop shaped instead
of round to ensure the proper minumum opening even when fully aft.

Our goal would be 2000rpm at fully aft throttle.  We can get the engine to
run reliably but somewhat roughly that rpm

Thoughts?

Mark & Terry (Rocky Mtn High)

** for those concerned about the liquid in the intakes.  It is not coolant.
It is excess fuel/oil mix from the last time I ran the engine.  I shutdown
by turning off the igniters; this resulted in the injectors continuing to
fire briefly & thus the excess buildup of fuel.  The excess fuel in the
photos is mostly oil -- the gas having evaporated in the week since last
engine run.

==================================================================

I doubt the 3/16" has much affect on full throttle power. For those that
may not be familiar with Mark's airport... density altitude is 8000 feet
where HP is down 70 to 80% fromn sea level anyway. Mark has demonstrated
a fuel burn of 17 GPH which is 106 pounds per hour. At a BSFC of .6 that
translates to 180 HP. At a BSFC of .5 it would be 212 HP.
If the engine had 250 HP at sea level 70% power would be 175 HP
at 8000 feet. At 80% power it would be 200 HP at 8000 feet so Mark is
not that far off.

Mark, Weigh down that tail wheel down with that steel hangar rafter
next door and run it up to static RPM and let me know what
that is.

I think the tips of your C172 prop are going negative
angle of attack at 150 MPH and destroying what little thrust it has
to begin with. We had a problem like that years ago with
the IVO prop roots going negative angle of attack on tractor
installations. In other words the IVO blade did not have enough twist.
In effect the airplanes run into a brick wall at 180 MPH.
Pusher airplanes were not affected as much because the fuselage
was blanking the blade roots.

Jarrett is doing an FEA on a new 3 blade 7075T6 billet hub design that
will take Sensenich or low cost $175 Warp Drive carbon
fiber blades. The Warp Drive blades themselves have inadequate twist
but that could be fixed at low cost with SS flaps bolted on the first
25% of the blade root. Crude but effective.

The new more costly  Sensenich carbon fiber blades Warp Drive hub
compatible are much better but still not enough twist IMHO. Saw some
at Sun n Fun. For 250 HP you may need four.

The Warp Drive ground adjustable pitch hub configuration is
becoming an industry standard :) The real Warp Drive made
hubs are cracking however hence the new 7075T6 billet hub design.

I think I would go with the idle set screw. Just about every carb in the
world
has an idle stop screw. The exhaust gases from the valve overlap causing
rough idle are well documented in the Mazda 13B technical literature but
little
can be done about it and it would be far worse if the throttle were further
away from the port. The p-port RX8 side exhaust port suffers far less.
Starting
with NSU this p-port rotary phenomena was well known and was why throttle
butterflies were sometimes installed in the rotor housings.

However a 2000 RPM idle fortunately is no problem with a 2.85:1 gear box as
prop idle RPM is 700 which is typical for a direct drive aircraft engine.
My O-470 does not idle all that smooth at 700 prop RPM either.


Paul Lamar



Vernier throttles make it more difficult to finely modulate power in a
timely fashion.  Some portion of the pilot's hand should be braced
against the panel or quadrant while power adjustments are being made.
Actuating the unlock control on the vernier (button on end) makes
bracing the hand properly difficult or impossible.  And that inhibits
dexterity.

I am fairly certain there have been airplanes bent because a vernier
throttle diminished the pilot's dexterity in giving a measured blast
of power at the appropriate time.  No big deal when the air is smooth,
but if it's gusty and bumpy, it's a headache you don't need.

Besides, I always find the hysteresis in the mechanism interferes with
any improved precision supposedly provided by the vernier.  Twist the
control one way.  Wait a minute or so and you end up with more
adjustment than you wanted, so you twist it back.  It doesn't respond
in the first half turn, so you twist it some more.  Finally the
reading on the panel starts to change.  Then vibration works out any
slack in the system, and so now it's misadjusted the other way.


Off soapbox..

Matt-