It sounds like this post and some of the ones after it would be helpful:  http://www.simviation.com/cgi-bin/yabb2/YaBB.pl?num=1195085337/45#49

It has to do with true airspeed versus indicated airspeed at different altitudes.

He said it himself, it's at 100%.

Constant speed propellers are kind of like VORs, in that at first, they're counter-intuitive, and difficult to explain. The most common misunderstanding is that you're controlling the pitch, when all that you're really doing, is selecting an RPM.

If you leave the prop setting at max RPM and never touch it... and then takeoff from a dead start with full throtltle, climb steeply and then level for cruise, never touching the throttle either.... The prop blades will go through most of their pitch range, all on their own.

On takeoff, the blades will "flatten out", because at zero airspeed, that's what's required for max RPM. Then. as the takeoff roll ensues and airspeed builds, the blades will "coarsen" to maintain the selected RPM. As you beging the climb, the blades will change as needed, to maintain the selected RPM. A steeper climb means "flatter" blades, and of course a shallow climb will yield "coarser" blades. Not so much because of the different loads that different climbs put on the engine, but more about airspeed. The blades will find the pitch needed, to maintain a constant speed (RPM).  Pitch and power have more say over constant-speed prop-pitch, than the prop-control.

Another misconception (this might not apply as much to 4,000hp fighters) is that you select a prop pitch for higher cruise speed. There are certainly exceptions, but as a rule, cruise prop settings are about fuel efficiency and engine wear. As with a fixed-pitch prop, your speed is about pitch and power... and if you REALLY want to squeeze some extra airspeed, you'd go with max RPM, because the blades will find the "coarsest" setting that a given RPM-setting will allow.. they're allways seeking "high gear". So.. if you're going for flat-out, top speed, it's full throttle / highest (100%) RPM.

Again.. for a 4,000hp P-51, results may vary.. and there very well might be a less than 100% RPM sweet-spot, for top speed... but not for  most  constant-speed prop airplanes.

OK... here's something... I think this is just a matter of conflicting terminology... and the fact that some prop systems use a lever and a governor; sort of a "semi-automatic" arrangement.

There are fixed-pitch props, constant-speed props, and variable or controllable pitch props... my guess is that planes like the Mustang use a controllable-pitch arrangement.


See this excerpt from a good online article about non-fixed props:

"Within a given power setting, when using a constant-speed propeller, the pilot can set the propeller control to a given rpm and the propeller governor will automatically change the pitch (blade angle) to counteract any tendency for the engine to vary from this rpm.  For example, if manifold pressure or engine power is increased, the propeller governor automatically increases the pitch of the blade (more propeller drag) to maintain the same rpm.

A controllable-pitch propeller permits the pilot to select the blade angle that will result in the most efficient performance for a particular flight condition.  A low blade angle or decreased pitch, reduces the propeller drag and allows more engine power for takeoffs.  After airspeed is attained during cruising flight, the propeller blade is changed to a higher angle or increased pitch.  Consequently, the blade takes a larger bite of air at a lower power setting, and therefore increases the efficiency of the flight.  This process is similar to shifting gears in an automobile from low gear to high gear.


Note:
In modern aircraft, pitch control is achieved automatically, and the propellers are referred to as constant-speed propellers.  As power requirements vary, the pitch automatically changes, keeping the engine and the propeller operating at a constant rpm.   If the rpm rate increases, as in a dive, a governor on the hydraulic system changes the blade pitch to a higher angle.  This acts as a brake on the crankshaft.  If the rpm rate decreases, as in a climb, the blade pitch is lowered and the crankshaft rpm can increase."


Here's the full article:

http://www.oldbeacon.com/beacon/controllable-pitch_propeller.htm

Now, I could be wrong about the Mustang, and I have no RL experience with blue levers , but my research into properly flying, say, big radials  in MSFS indicates cruise prop rpm settings much lower than for takeoff and climb, with the "oversquare" rule in effect as well.

For example: with the big Wrights and P&Ws on the DC-3, METO settings are about 34" MAP and 2400 rpm, and after levelling off for cruise, power comes back to 30 or 32", and the prop lever comes way back to yield rpm of about 2100.

LOL... our posts are dove-tailing..  Yes.. it is a terminolgy thing. Constant-speed props and Controlable-pitch props are different animals.. and a P-51 might very well be the later ..

(reading your last post now..lol )

Brett_Henderson wrote on Jun 3^rd, 2008 at 11:06am:


Aye... makes sense.

I really think the initial problem of this post has less to do with prop pitch and more with indicated airspeed or possibly even kts to mph.  My suggestion is to take the Mustang to altitude, get up to speed, then pull up a GPS and see what your groundspeed is, given no wind.  Then you will see the 435mph you are looking for.