slow top speed, please help

[KnifeEdge]

I downloaded like 3 different P-51Ds and Hs and I havn't been able to reach the rated max speeds with any of them.

The D that I'm using came from

Warwick Carter
E-mail:  au_mav@hotmail.com
Garry J. Smith
E-Mail:  garry@gjsmith.net
Web:     http://www.gjsmith.net

I know the 51D can reach 437mph at altitude and somewhere around 400 at sealevel both in level flight, but in FSX i've only been able to hit 300-350 mph in level flight at any altitude. This plane doesn't have an autopilot so it may be my bad piloting skills and my constant corrections slowing me down. I am using full throttle and 100% prop with auto mix fuel mixture, and changing any of these settings seems to either make no difference or slow me down.

With the H I had a little better luck as the one I downloaded did have an auto pilot and it got a little closer to its rated speed but still not much in excess of 400mph at any given altitude.

Strangely if I go to the map or flight review page in game the speed given there is consistently slower than the reading from the dash of the airplane, this doesn't seem to happen with any of the default planes from FSX so I was wondering if tihs was a coding error.

If anyone has been able to hit the 437mph at 26700ft with the P51D I'm using or has any tips for what I should do please do tell, I've been searching through forums and googling the hell outta this topic with no avail. Thanks in advance everyone.

[Mobius]

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.

[ozzy72]

What pitch is your prop at?

[Anxyous]

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

[beaky]

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

Aha, that's the problem (although possibly also indicated vs. true airspeed)
Generally speaking, prop rpm settings for cruise are quite a bit lower than for takeoff.
If you think of a prop blade like a wing, having an angle of attack, it becomes clear why this is so:

On takeoff, you of course want full throttle,but you want the prop to "bit more air", that is, produce lots of lift (relative to the air streaming past  the nose, not relative to the ground like the wing). As with the wings on takeoff, you're not too worried about the extra drag.

But in cruise, if you don't "flatten out" the prop pitch by pulling the prop lever back to indicate a lower rpm, the drag produced by that high A of A will start to chip away at the advantage of producing more lift. Again, there's a similarity with the wing in cruise: many aircraft actually need a little "nose-down" trim to get the wing "flat" enough to produce enough lift without also producing extra drag at high speeds.

I'm sure you can find the correct prop/manifold pressure tables for the P51 somewhere... or try to find tables for any period fighter with the same engine and similar prop as a starting point.

In general, for high-altitude speed runs, you want to:

A: First reduce throttle slightly (as indicated by manifold pressure)... that gauge should have a green arc; start at the bottom of the green arc.
B: Then reduce prop rpm, again into the green arc, until you see an increase in indicated airspeed.

C:Then lean the mixture until you see an increase in manifold pressure, but without going past normal temperatures.

If you climb to the flight levels with everything redlined, you're not going to do well at all.

[Brett_Henderson]

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).

[beaky]

The most common misunderstanding is that you're controlling the pitch, when all that you're really doing, is selecting an RPM.

But doesn't the prop lever physically move the blades to adjust pitch? Unless the engine is geared, I don't see how you can select an rpm setting, then the blades adjust to match the selected rpm...

I know an Air-Matic type prop can self-adjust for max efficiency at a given power setting, but those don't have any prop control lever.

[beaky]

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/control ... peller.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.

[Brett_Henderson]

But doesn't the prop lever physically move the blades to adjust pitch?

No, the prop-control is like a valve that controls pressure differential. The position of the blade (pitch) is manipulated hydraulically. In most GA planes, that fluid is actually the engine oil, cycling through the prop hub.

The pressure differential (how much the hub is "allowed" force the blades "coarser") determines the pitch, and is engineered to work over the usaeble RPM range. Pulling the prop-control back simply increases the the ratio of oil pressure available to change the blade-pitch. That obviously changes with power settings.. and the whole thing is calibrated to RPM (and monitored by the tachometer).

A given prop-control setting can see the blades go through their entire pitch range.

[Brett_Henderson]

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]

[quote]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

[beaky]

But doesn't the prop lever physically move the blades to adjust pitch?

No, the prop-control is like a valve that controls pressure differential. The position of the blade (pitch) is manipulated hydraulically. In most GA planes, that fluid is actually the engine oil, cycling through the prop hub.

The pressure differential (how much the hub is "allowed" force the blades "coarser") determines the pitch, and is engineered to work over the usaeble RPM range. Pulling the prop-control back simply increases the the ratio of oil pressure available to change the blade-pitch. That obviously changes with power settings.. and the whole thing is calibrated to RPM (and monitored by the tachometer).

A given prop-control setting can see the blades go through their entire pitch range.

Aye... makes sense.

[Brett_Henderson]

The most common misunderstanding is that you're controlling the pitch, when all that you're really doing, is selecting an RPM.

But doesn't the prop lever physically move the blades to adjust pitch? Unless the engine is geared, I don't see how you can select an rpm setting, then the blades adjust to match the selected rpm...

I know an Air-Matic type prop can self-adjust for max efficiency at a given power setting, but those don't have any prop control lever.

Ok.. I can see a visualization proplem here. It confused me too; so I'll try to explain...

Scenario.. Level flight; 25" MP; 2500 RPM; 140kias

Action..

[DaveSims]

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.