Hi Claidemore,
>The climb rates for the F4F-3 were still bothering me, so as you suggested I looked for some other documents to shed some light on it. Found the attached pdf. (primary source document! woohoo!
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Wow, good find!
That illuminates the F4F better than what I had before!
>F4F-3 (no 1845) with two .30 fuselage guns and two .50s, weighing in at a very light 6262 lbs shows initial climb about 12.7 m/s while the heavier (no 1848 ) with 4 wing .50s @7300 lbs shows about 10.4 m/s.[/QUOTE]
Hehe, if I plug a constant 1000 HP and 7300 lbs weight into my calculation and then select the speed for best climb, I get 10.39 m/s at high gear full throttle height
But I won't claim my calculation is accurate to 0.1 %, the good fit owes a lot to coincedence.
Why was I comparing the climb rate at high gear full throttle height? Well, let's examine the four diagrams for No. 1848:
Upper left hand: Climb rate, supposed to be constant below high gear full throttle height, as can be cross-checked by the perfectly straight time to altitude graph in the same diagram. Note: The two-stage, two-speed R-1820-76 with its ability to bypass the auxiliary stage supercharger at low altitude would realistically have a three-step climb graph. Someone has added such a three-step chart with faint pencil lines, and though it's not really legible, it looks as if he has labelled the addition "BuAer", indicating that he was possibly taking his data from the Buereau of Aeronautics Aircraft Characteristics Sheet.
Upper right hand: Engine power curve in the vicinity of high gear full throttle height, with rated power indicated as 1000 HP. As the full throttle height (critical altitude in US terminology) is that given for the top speed case in the main text of the report, this is obviously with ram effect, and the difference to the upper left hand diagram for the climb case is thus explained. Note: The test aircraft did only indicate 981 HP instead of 1000 HP on the torque meter in flight, as pointed out in the main text of the report.
Lower left hand: Speed at high gear full throttle height as a function of power. The main text gives the top speed of No. 1848 as 330 mph, but here we can see that the test aircraft with 981 HP apparently achieved only 329 mph. That might look like cheating, but remember that the engineers were not trying to find out the peculiarities of No. 1848, but rather how a typical F4F-3 with the engine power exactly on specs (or perfectly average, as it was normal for engines to have a bit more or less power than specified) performed.
On the bottom right hand, we see the airspeed indicator calibration curve which is not terribly interesting for us right now.
Going back to the upper left hand diagram, we can conclude now that it is a generic diagram for the engine operating at a constant power of 1000 HP as measured by the torque meter, which would require the engine to be throttled back in supercharger low gear, and not be operated at constant manifold pressure even in high gear, as it was the practice in combat.
The only point where military power and test power coincede is at high-gear full throttle height, and that's where I get the deceptively accurate fit of 10.4 m/s you report versus the 10.39 m/s my calculation yields.
However, I still have to improve my F4F-3 model so that it reaches the 330 mph pointed out by the report. It is a bit short of this currently because I based it on the F4F-4's drag, and while the F4F-4 is often criticized for the extra weight it put on, the report you found shows that it also put on a bit of extra drag along with the weight - the extra gun barrel opening is an obvious source of some of that drag, and the small gaps in the wing skinning necessary for the folding wings are another. If I reduce the F4F-3 model's drag to make it represent the cleaner variant, that will increase climb rate a tiny bit so that my calculation will no longer be as close to the historical data, but 0.2 to 0.3 m/s higher.
My original F4F-3 graph was calculated for MIL power, based on the R-1830-86 data from the BuAer standard aircraft charactersistics chart for the F4F-4. The F4F-3 uses the R-1830-76, but as far as I can tell from the data on
AEHS Home, the two engines are identical with regard to the parameters that determine performance. MIL power is 1200 HP/2700 rpm @ sea level, 1150 HP/2700 rpm @ 11500 ft, and 1040 HP/2550 rpm @ 18400 ft. This does yield a contradiction to the F4F-3 report which shows a full throttle height of just 15500 ft for high gear, but that also seems to contradict the 21000 ft full throttle height in level flight. No idea how to resolve this one, and there is one like this in almost every flight report :-/
My original F4F-3 graph was also calculated for 7065 lbs instead of the 7300 lbs of the F4F-3 report, but as the F4F-3 report gives exactly these 7065 lbs as "full load weight", this was probably a good choice. The difference to the 7300 lbs in the report was responsible for some of the excess climb rate you were noticing, though. (I made it a habit to note the weight on the charts, but the F4F part of the chart was so old that I hadn't done that yet, and while assembling the various curves I did not go back into my calculation to look it up. Now you know anyway
Regards,
Henning (HoHun)