Mystery
The principle is the prop governor is rotated by the engine and is calibrated to send an exact amount of oil pressure to the prop based on RPM. The oil pressure (routed through a tube, bearing, hallow crank, into prop) changes the prop blade angle, from fine to course as needed. Once the pilot sets RPM, the prop governor knows if RPM increases or decreases by fly-weights. It will adjust oil pressure as needed to get maintain set fly weight position (that corresponds to that RPM). RPM will not change unless you move the prop control (with in limits).
The basic and main principle is RPM remains constant (with in limits) by varying the props blade angle or pitch. This is done automatically. If RPM is too high the pitch increases to slow the engine down by adding more load on the engine. If RPM is too low, pitch lowers to a finer setting angle to UNLOAD the engine and maintain RPM. A piston in the front of the prop is what the oil pressure works on. Usually its spring loaded to go to fine pitch (high rpm) and it takes oil pressure to make to blades go course (low rpm). Some props work different. Some props have accumulators and stops that allows the prop to feather. Some props will go to high pitch (low rpm) with out oil pressure. Regardless if you add or subtract power or increase or decrease airspeed (with elevator) you the rpm stays the same...beauty!
Operation wise its very easy, but it takes thought at first. After a few hours its second nature. The MAX rpm is limited automatically (by the prop governor) to 2.700 rpm, typical Lyc engine red line. The min RPM is limited by the high pitch stop.
Terminology
*Fine pitch, higher RPM, Prop IN, Prop control fwd (all synonyms)
*Course pitch, lower RPM, Prop out, Prop control aft (all synonyms)
*MAP (manifold pressure) affected by throttle position, idle = low MAP, full throttle = higher MAP
*MAP/RPM = power, both are needed to calculate power.
*MAP/RPM example: 25/25 = 25 in-hg MAP & 2,500 rpm
*Max MAP at sea level with wide open throttle, about 29"
*Max MAP goes down with altitude, about 1" per 1000' altitude
Typical Pilot action with Prop Control:
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Start engine (prop in, fwd)
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Run up (cycle prop control out & back in quickly, note slight
RPM drop, checks prop control & circulates warm oil to prop hub)
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Take off (Fwd - higher RPM the greater the hp)
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Initial Climb - reduce throttle first (MAP) than prop towards course
(RPM) typical "25 square" (25" & 25(00) rpm; rpm rounded to 100's)
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Enroute Climb add throttle to maintain MAP 25" (for altitude drop)
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Cruise - set throttle than RPM as desired. At high altitudes throttle
is wide open, MAP typ same/less than RPM to 2,300-2,500 rpm for efficiency.
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Descent you will typically reduce throttle (MAP) and leave prop alone
or even lower RPM slightly (for less drag). Avoid back driving engine.
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Landing - entering pattern, reduce MAP further, once slowed, go to
course pitch. RPM will not increase due to low power & speed. The prop is
"on the low pitch stop" and can't increase RPM. The reason for this step is,
if you go around, you will get full power (full RPM).
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Taxi in, do nothing with the prop should be fwd-high rpm-fine pitch.
So in a typical flight you might move the prop control as little as 4 or 5 times, including run-up.
General golden rule
When you decrease power you lead with throttle (MAP) than prop (RPM)
When you increase power you lead with prop (RPM) and than throttle (MAP)
There are no mysteries. It's like riding a bike, you can explain it all you want, but to learn you have to actually do it.
The advantage of constant speed props is you SET the RPM. With FIXED PITCH, RPM is indirectly related to airspeed and throttle position. With C/S you can set the most efficient RPM for takeoff, climb, cruise and even descent. Going to low RPM in a descent lowers drag, like up shifting on a down hill in a car. The opposite is also true, in the pattern a C/S prop can provide more drag in fine pitch/high RPM. This is also an advantage. You can make steeper approaches & land shorter with out slipping with a C/S prop verses fixed. Also its great for Acro. Set RPM and throttle and leave it alone. With fixed pitch prop you have to add power on the up-lines and pull power back when nose down to keep RPM below engine red line.
Typical RPM is from engine red line of 2,700, down to 2,300 or less. High RPM is not efficient in high speed cruise but great for take off. High rpm can make more HP, but also makes more noise and burns more fuel. Super low RPM may be restricted by prop/engine limitations for extended operations due to harmonic vibrations of prop/engine combo.
Power setting is not MAP/RPM
On take-off everything is FWD (throttle, prop, mixture) and you have about 29"/27; At 1000 feet you reduce to climb power 25/25; Cruise can be almost any combo of MAP/RPM or "square settings" such as: 25/25, 24/24 or 23/23. It does NOT have to be square, but more on that later. As you climb you can't maintain MAP. You lose about 1" MAP per 1000 feet due to lower air density, so by 8,500' you're going to be down to about 21" MAP. At 21" you can run at any RPM you want, but lower RPMs are more efficient. You do lose a little speed with lower RPM's, but in general gain MPG. So anything from 21/27 to 21/20 would be fine, if prop limits allow. Some props have min RPM limitations with different MAP's.
At lower altitudes where MAP is higher you can run wide open throttle (WOT) all day at +25/+25, but unless you're printing money, you want to get below 75% power so you can lean the heck out of the engine. You must be at 75% or less the lean. With practice you memorize a handful of typical MAP/RPM combos to set 75% or less power. A little laminated table of map/rpm is handy.
You could leave the prop full FWD all day long and fly it like a fixed prop plane, but that's like riding a 21-speed bike in 1st gear.
You can run OVER square where MAP is more than RPM. Not only can you do this, its a good idea with LIMITS. About 1" or 2" MAP over RPM is allowed. If MAP is real high and RPM low, its equivelant to LUGGING your car in high gear and slow speed. 24/23 would be fine, about 75% power at 1,500' std day. Where 23/24 is little less power and more noise. Above 5,000 feet, MAP is low, so generally you will not be able to run over square. There are some practical min RPM's allowed with props. You can drop down to below 2100 rpm, as low as 1900 or 1800 RPM and save gas while still flying fairly fast. Again prop limitations or MAP/RPM combo limits must be observed. In general the lower the RPM the better the prop efficiency. Efficiency does not mean speed, just better miles per gal. It also depends on altitude and airframe. The RV's max/best range speed is around TAS 130-140 mph, not too slow.
Bottom line, that blue knob gives you more control over the power settings and saves gas, while making takeoffs and landings shorter and climbs faster. With that said a Sensenich fixed prop is pretty awesome, and with the RV's excess performance you may not miss the difference if you are use to low performance planes. The Sensenich is set up pretty nicely for cruise so its efficient, but only at one condition and limited altitude range. The C/S prop again gives more range and control of conditions you can fly while keeping efficient.