By the way, I don't recall hearing any comments
from you about any increase in acceleration on take-off. Was there
any? I know you indicated the P effect increase was quite noticeable -
but how about acceleration?
Oh
yeah, plenty of acceleration, but I don't think I ever tried full
throttle again after that first takeoff. When flying level at lower
power settings, the acceleration is quite noticeable compared to what it used
to be. I've been pretty severely limited by my restrictive intake,
so it will only get better.
BTW, I
finished the installation of the new oil cooler, and TB inlets. I only
managed a brief run to make sure nothing was leaking, but it seemed like the
mixture was way lean. Surely it didn't make that much
difference. Tomorrow, I'll tie it down, and check the tune, and static
rpm. I'm really happy with the way the inlets came out, and I'll be
even happier if they actually work.
Rusty
(thanks for letting me use your thread for my
update)
Ok, Rusty, I'll
record the static and map when I get to the full throttle runup
stage.
This got me to thinking about
manifold pressure and realized that while I associate high manifold pressure
with high power that it was not necessarily so. In fact, it can be
misleading.
Manifold pressure (in of itself) does not indicate power
unless other factors are considered
Example: At 2000 MSL you are drawing 20 "Hg Manifold
pressure at 5000 rpm (throttle is partially closed, so intake is restricted).
By using your mixture control (or prop control if variable pitch) reduce your
RPM to 4500 rpm without touching the throttle. You will see a sharp rise in
Manifold pressure. Does this mean more power, naturally not, you have simply
reduced the pumping speed and therefore the flow (suction) by the rotors which
permits the intake manifold pressure to increase without having changed the
throttle plate.
Example2: At 2000 MSL again drawing 20" Hg Manifold
pressure at 5000 rpm. Using your mixture control increase the engine speed to
5500 rpm. You will see a decrease in manifold pressure because the throttle
position has not change (therefore it still restricts the air flow), but the
pumping speed of the rotors have increased "sucking" more air out of the
intake manifold. So manifold pressure decreases, however, we are definitely
producing more power as attested to by the higher rpm and fuel
flow.
In the first example, increased manifold pressure actually
represented a power decrease and in the second example a decrease in manifold
pressure actually represented a power increase.
So Manifold pressure interpretation requires consideration
of other factors before it is very useful in telling us about our power
production.
Now for those of us with Fixed pitch props manifold
pressure AND rpm are a fair indication of power being produced. I say "fair"
because you really need to factor in fuel flow as well to get the best
indication of power.
Normally, in straight and level flight, if we advanced the
throttle (reducing the throttle plate restriction) our manifold pressure will
increase. Since our engine is a positive displacement pump - meaning it
displaces the same VOLUME of air each rotation (note. I said volume not mass)
this means an increase in manifold pressure will increase the MASS of air
pumped through the engine each revolution.
An increase in air MASS permits more fuel to be burned
producing more power, this increase in power will in turn enable the engine to
turn more RPM against the propeller load. This increased rpm will produce more
"suction" of the intake manifold until an equilibrium is again
reached.
Lets not forget that this all starts with the ambient air
pressure surrounding us whether at Sea Level or 10,000 MSL.
So Manifold pressure is a direct function of
- Ambient Air pressure
- Throttle plate position and
- Engine RPM
It may also be a good indirect indication of
power when other factors are properly considered.
Of the three parameters, throttle plate position is
probably the one we specific with the least accuracy – because unless you have
calibrated your throttle level – you really don’t know the throttle plate
position except perhaps for full open and fully closed.
FWIW
Ed
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC