Concerning
constant-speed-prop (henceforth known as
CSP) performance and function:
We first need to divorce ourselves of the idea that the pilot is controlling, or setting a blade-pitch. The pilot selects an
RPM, and then the CSP itself will continuously adjust the blade-pitch in order to maintain that RPM (
constant-speed).
For example: A takeoff-roll will begin with maximum manifold-pressure, and maximum RPM (
NOT finest pitch). As the airplane rolls down the runway and gains airspeed, the CSP will begin "
coarsening" the blade-pitch, in order to maintain the selected RPM. As the climb initiates, the blade-pitch will change accordingly, maintaining the selected RPM. As we level off and gain airspeed, the blade-pitch will "
coarsen" even further; maintaining a constant RPM... eventually becoming a relatively "
coarse pitch". And if you enter a descent, the blades will likely reach max "
coarseness". The blade-pitch has gone from its finest, to its coarsest, without the pilot ever touching the prop control.
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I'm going to paste part of a disscussion regarding CSP theory:
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Let's try a big-picture view of what a constant-speed prop tries to do (
on most light singles):
It's not an engine RPM governor directly.. it's a hydraulic/mechanical system using engine oil-pressure to modulate the prop blade pitch. As you advance the throttle, and the engine RPMs
try to increase; oil-pressure increases proportionally. This pressure forces the blade-pitch into a higher angle-of-attack, which in turn
keeps the RPMs from increasing. The increased power becomes increased thrust via the steeper blade AoA, while the RPMs remain
constant.
Of course the inverses is true. As you retard the throttle, the engine RPMs
try to decrease, but the proportionally reduced oil-pressure allows the blade-pitch to "
relax", which keeps the RPMs from decreasing.
The whole cylce is one-sided, in that an engine under load is always trying to spin faster. A constant-speed prop's governor, is more like a differential, by-pass valve, and the prop-knob is used to select the percentage of oil-pressure available to the prop mechanism. A higher percentage of pressure yields a relatively coarser pitch, and the whole setup is calibrated for the engine/prop/airframe, and its normal operating range.
The spinning prop-blade angle-of-attack itself, is what physically holds the engine (
always trying to spin faster) at a selected RPM. It's kinda like pitching for airspeed; where a wing's airpseed equates to a prop's RPM. The constant-speed prop is continuously "pitching" for a set RPM, like a pilot continuously pitches for a target airspeed. The prop doesn't govern engine RPM in a restrictive manner.. it takes the excess power and translates it into thrust via increased blade AoA, instead of increased RPM.
AoA is the key term here... not blade-pitch in degrees. That's where confusion sets in (
like relating max-RPM to finest-pitch). The blade-pitch in degrees can go from max to min and back again, regardless of selected RPM, without the pilot ever touching the prop-knob.
Quote
The sensation & sound of pulling the blue knob out for lower rpms.......is like shifting to a higher gear in cruise for highway driving.
True, in that that is what you
sense.. and that's what it
feels like.. but that's not what happens. A constant-speed prop is
always in the highest gear possible; given an airspeed and power-setting. That is what is "
holding" it at a set RPM. Doesn't matter if it's a takeoff roll, or a climb, or a cruise, or a descent.. there is no "
higher gear" to select.
In an automobile, the "
pilot" controls engine RPM with the throttle. You add/subract "
thrust" by increasing/decreasing engine RPMs, using a preset "gear". In an airplane, the pilot increases/decreases thrust via manifold-pressure, as RPMs remain
constant. As an automobile's speed changes (
or throttle position changes), the RPMs change too, regardless of the selected gear. As an airplane's speed changes (
or throttle position changes), the RPMs remain constant.
Ponder an automobile at 55mph on a level road. Pretend it has an airplane-like throttle, set to a fixed position maintaining that 55mph. If you select a higher gear; the RPMs will indeed decrease, but the the automobile will accelerate to a higher speed, as the RPMs increase again. In order to maintain the same 55mph in the more efficient, higher gear, you must also decrease the power, (controlling speed/RPM by throttle setting).
On the other hand; when you reduce airplane RPMs for cruise, and do not change the throttle setting; the airplane will not accelerate, and will likely DEcellerate a tad, because the new "highest gear", is holding the engine at a less powerful RPM. This theory applies to the go-around scenario too.. where the prop-knob is not pushed full-forward. It's not that a finest-pitch can't be achieved because of the prop-knob setting; it's that the constantly-changing, highest-gear holds the engine to a less powerful RPM. The finest-pitch can still be theoretically achieved at lower RPMs.. ie.. a steep, slow, full-power climb... theoretical, because that gets into "over-powering" the prop (
MP too high for a set RPM).
Quote
But, there are a few howevers, that are somewhat like a gear shift. For instance, if you shove the blue knob in, with power pulled back (no load on the engine)..........before it's time too, the engine and prop will react as if you downshifted a land vehicle while going too fast. You'll be thrown forward against the seat belt, and the engine will be spinning excessive rpms.
This scenario does fit in a way.. because it turns the tables on the prop. A constant-speed prop is designed to manage RPMs when the engine is the primarmy, prop spinning force. For a constant-speed prop, in a power-off descent; it's like trying to push something with a rope. But this is outside of its normal operating range.. kinda like either failure scenario (engine or prop itself). It depends on the specific setup, and their defaults. Singles, twins, and aerobatic setups all vary. Some twins are setup to default to coarsest (feathered), regardless of the failure.. some require the feathering to be done while the prop is still spinning.. some singles will default to finest pitch, regardless of the failures, and many singles don't even allow for feathering... All of this is outside the scope of this discussion, and beyond my expertise. I get involved in these discussions to help minimize the, pitch/rpm/gear, mis-conceptions.
