72"

I agree with Talisman on this...

If the original test was at 3K RPM then I just have to ask how accurate was the measurement of both rpm and speed? How would we actually know what the wind speed was? Was the aircraft recently cleaned?

I don't mention this to say whether or not the aircraft needs 72" but simply because a max speed was mentioned as a max and it isn't. The speed can be moved upwards a very small amount based on criteria that aren't controlled in the experiment... Aside from the RPM... Apparently that one is controlled and I indeed was not running 3000. Who would do that when trying to go fast? It's not fast.

The question then becomes (on why RPM contributes some of that speed) exactly as Talisman says... Does our prop pitch act exactly like the real one at a given RPM and speed combo. My guess is that it does not and there's likely a very good reason for it. I have my suspicions as to what that reason is but again... I don't know if ours works the same way as the real one.

The speed can be moved upwards a very small amount based on criteria that aren't controlled in the experiment... Aside from the RPM... Apparently that one is controlled and I indeed was not running 3000. Who would do that when trying to go fast? It's not fast.

 

The question then becomes (on why RPM contributes some of that speed) exactly as Talisman says... Does our prop pitch act exactly like the real one at a given RPM and speed combo. My guess is that it does not and there's likely a very good reason for it.

I'm not sure I understand you. Are you saying that the real P-51 should go faster by reducing RPM, or that the P-51 does in the sim even though it shouldn't? I can't comment on whether or not the latter case is occurring, but if you mean the former, I must disagree. Yo-Yo and I both (independently) concluded that reducing RPM should not, in real life, increase the P-51's maximum speed. 3000 RPM is how they got max speed, IRL.

I'm not sure I understand you. Are you saying that the real P-51 should go faster by reducing RPM, or that the P-51 does in the sim even though it shouldn't? I can't comment on whether or not the latter case is occurring, but if you mean the former, I must disagree. Yo-Yo and I both (independently) concluded that reducing RPM should not, in real life, increase the P-51's maximum speed. 3000 RPM is how they got max speed, IRL.

Dropping the RPM while maintaining the same manifold pressure will not result in an increase of power. However when flying straight and level it might result in more efficient cooling of the engine, which would result in the radiators closing, which would lead to better aerodynamics and a faster speed.

I have no idea how, and I'm not concerned with, how the real aircraft operates in the context of my earlier statement.

I was referring to some of what I was doing in sim. I don't know what reducing engine RPM will do with a real P51.

Also...

What DOES the real aircraft do when you reduce RPM at max manifold pressure? I understand apprently it slows down for one but what exactly does it do mechanically and how do those things happen?

Yes, I used to be able to get these speeds when I flew the last 3 races of the last DCS Mustang racing series a couple of years back, but lately a quick test showed me that I can't get the same sort of speed anymore for some strange reason. However, it is important to note that the speeds stated above need manual control of radiators and extremely, repeat extremely, careful flying; the sort of flying that does not lend itself at all well to combat because to get these speeds in racing means flying on the extreme edge and the engine can blow in a heartbeat. Just lifting the nose a fraction for a second too long and a slight reduction in airspeed and resultant cooling effect can mean a lost engine very, very easily. Not only that, although these speeds can be sustained for a while, you need to take the word 'sustained' with a pinch of salt because the engine will not last indefinitely at the settings required for such speed and race winners have had engines blow just before the finish line but stayed in the air long enough to carry enough speed to win by a whisker; it is in fact a calculated risk to go for broke in the last seconds of a race to secure a win knowing that the engine will likely be lost (this is not real world realistic behaviour). I would suggest that we don't get side tracked by specialist racing settings, but concentrate on the real world speed tests for the DCS competitive combat environment and leave the racing settings and scenarios to one side.

 

P.S. You can't out run the bullets/cannons being fired at you from behind, so as soon as you manoeuvre to avoid them you won't be at optimum speed anyway.

 

Happy landings,

wellsaid :thumbup:

Also...

 

What DOES the real aircraft do when you reduce RPM at max manifold pressure? I understand apprently it slows down for one but what exactly does it do mechanically and how do those things happen?

if it slows down at a given weight, then your going too slow, and most likely the engine will overboost and/or carb temp will increase and cause detonation. If the A/C is going fast enough to support the loss of horsepower, as opposed to the gain in thrust from the increased prop pitch speed will increase

Yeah, as I mentioned in aforelinked post, lowering RPM while remaining at high MAP damages the engine, IRL, in addition to reducing thrust. So, if the P-51 can only reach its intended speed by using this bad method, that isn't right. Emphasis on "if" because we don't know exactly how the secretive sim-racers are doing it, which doesn't help confirm/deny the suspected bug.

by using this bad method

As I remember it, this method worked with the IL-2 P-51D as well. A mere coincidence?

If it worked in old IL-2: FB, then that isn't exactly a vote of confidence for the method's authenticity. : )

(Or doesn't anyone remember the slider trim-speed exploit and the manual prop pitch oscillation RoC exploit?)

Never said it was & am also fully aware of the grave discrepancies between RL and GL (game life) as it comes to that old sim. But there's a similarity regardless and I just pointed that out.