Quote:
Originally Posted by pandacat
horseback. I went back to re-read your thread on acceleration tests. In there, you noted that 310+mph, 2700 accelerates better than 3000 for P51. I just wonder that from your experience if that's the case for all other US planes, such as F4u, F6F and P-47. Because you mentioned 3000 works for all speeds for spitfire, that makes me wonder if acceleration physics are modelled differently for different planes. Also, when you reduced your RPM down to 2700, did you simotanously chop throttle or you just maintain the max power all time?
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Actually, there are a few variables for the Mustang; there's a point in the 10,000 ft tests where the acceleration at 2700 rpm catches up with and passes the acceleration rate at 3000rpm. At 500 ft and at 5000 ft (which I haven't gotten around to recording and posting yet), this didn't happen. I suspect that it
will happen at 15,000 ft and higher alts, where the air is thinner.
The Mustang's throttle control is kind of unique though (it controls manifold pressure directly, if I understand the manual correctly where most other aircraft's throttles just control how much fuel goes into the engine), and when testing the Spitfire Mk IX, which had essentially the same engine (and was lighter, but with a good bit more drag), I didn't get the same results at 2700 vs 3000 rpms.
The same is true with the P-38 and the American R-2800 powered fighters--they just don't react like a stick-shift car's transmission where you can put it in low gear (highest RPM) and gradually move up to higher gears (and progressively lower RPM) while accelerating at the same pace. Greatest constant state acceleration was obtained by shoving the throttle and prop pitch levers all the way forward and going all out until the engine overheated and then going for another 30 to 60 seconds or so, depending upon the aircraft (all the while mashing the nose down trim and adding rudder trim as required, and then in some cases, having to add nose up trim over certain higher speed ranges).
You CAN reach a desired speed and then lower prop pitch and get a temporary 'pop' in speed, but you can't keep the extra acceleration going by lowering your prop pitch some more--it's like putting too much strain on your engine, and making it work harder than it was designed to (like a bicyclist going uphill in too high a gear). What you want to do is get close to the speed you desire and then smoothly lower your prop pitch, allowing the engine to take advantage of the momentum you've built to reach that last bit and maintain your speed without overheating.
I also find that the better your power to weight ratio, the better your acceleration BUT drag is the ultimate limiter, and it increases on a steadily rising curve relative to speed. The Zero has a pretty good power to weight ratio and a spectacular acceleration at lower speeds, but it is larger than the Bf 109 and it has a really draggy high-lift wing, so the drag overpowers the engine fairly quickly once it reaches 370 to 400 kph or thereabouts.
At least that is what I found at 10,000ft/3000m and below.
I find that for the R-2800 powered fighters that the manual warns you not to keep high rpms during a dive; in fact, the recommended rpm for an extended dive is 2250, or close to 75% prop pitch. That will give you a pretty fast dive if you do it properly; you won't need much more than 60% throttle if you're taking it down over 3000 ft from your starting altitude.
The Corsair and even more, the Hellcat in this game exhibit an increasing nose-down attitude as the speed goes up. It seems to be a steady decrease in nose angle semi-proportional to speed, unlike the FW 190A, which has a sudden nose drop of a few degrees at (this is from memory, so don't bank on this) around 220 mph/ 360 kph and then stabilizes. These two USN birds just keep gradually tipping forwards, although the rate is greatest around 300-400 kph (160-220 knots) IAS. If you don't keep track of this, you can mess up your firing solution in a diving pass.
Your gunsight's center will not stay in the same spot if you maintain a straight line course even in a dive if your speed increases significantly; you have to 'aim' for a spot a bit farther ahead of your target than you would normally expect, and your diving gunnery pass will need to become shallower as you reach firing range.
Hope this helps.
cheers
horseback