As the plane weighs 1565lb, with a lift to drag ratio of 7.7 it's
Thanks again to everyone for the thoughts and comments above. The stall speed, zero-lift drag coefficient, etc. usually quoted for the Camel seem to come from this very interesting source:
Quest for Performance: The Evolution of Modern Aircraft by Laurence K. Loftin, Jr. I don't have any particular reason to question the values given there, but the sources given there are three reference books from the 1960s--so written 50 years after the Camel was produced and (presumably) tested. So this is a at least a tertiary source and I haven't been able to trace it all the way back to the primary source. The value reported there is 41 kts (48 mph) for stall.
Meanwhile, I have at least 3 primary sources who report that the Camel stalls and/or takes off at 35-40 mph. (I realize the take off & stall speed are different, but it seems unlikely that an aircraft would lift off at 35 mph while stalling at 48 mph.)
I experimented by using the complete set of data given in Loftin (particularly zero-lift drag coefficient and wing surface area), and interestingly enough that really seemed to pull a lot of things together. Suddenly the stall speed was 48 mph, the multiplier I'd been using on the lift table wasn't needed, etc. I'm guessing all those values were worked out by some engineer using aircraft in some standard configuration and so in a sense they all 'go together'.
That was interesting, but why is the stall speed recorded there so wildly different from that reported by a number of pilots?
I re-read a few of these first-hand accounts, and realized that later pilots are often flying something of a stripped-down aircraft. They usually don't have machine guns, certainly not ammo, may be flying with less than a full tank of fuel for various reasons, may be flying with a different engine--perhaps one that weighs quite a bit less.
And of course it turns out if you knock a few hundred pounds off the loaded weight of the aircraft, stall speed and a bunch of other handling characteristics are quite different! Just for example,
here is a modern Camel replica, empty weight 943 lbs, useful load 300 lbs, stall speed 40 mph. So fully loaded, about 1250 lbs - whereas in the JSBSim Camel I'm modelling a fully loaded & combat-ready aircraft that masses out at 1567 lbs. Obviously, that extra 300+ lbs is going to make quite a difference, including for details such as stall speed. Losing 300 lbs weight is (more or less) equivalent to gaining 300 lbs lift . . .
Additionally, it explains (in my mind) some other discrepancies I've been trying to work out, like the different max turn rates reported, and the discrepancy between stall speed and reported max turn rate. Both of those are related to maximum angle of attack, so they should be closely related. But . . . the difference in mass of a fully loaded plane vs (say) one low on fuel and ammo is significant--and would allow quite a difference in stall speed and turn rate in those different configurations. Interestingly the difference we would expect to see from an aircraft in those different configurations, and the difference we do see in the JSBSim version of the aircraft in the different configurations, tracks pretty well with reported differences in performance.
Most reports on performance, even if first hand, don't necessarily give exact details about every aspect of the aircraft tested. So it seems as though you are getting precise first-hand data, but in reality it has quite a bit of wiggle room in it because of exact details of aircraft configuration, which are often un-reported. (One reason I would like to track down the original source of the basic characteristics of the Camel rather than relying on various tertiary sources--they original could/should have more of the specifics of the conditions tested.)
To make a long story short, I've spent the last few days rebalancing everything to these slightly different specs, which give a stall speed of 48 mph when fully laden and a lift/drag ratio close to the reported 7.7 (again, when fully loaded). I think this will be the best & most realistic version yet.
Another thing that makes me feel optimistic that we're settling in on something pretty close to the 'real thing' is that despite all the tweaks I've made since the 2.0gamma version of the Camel, they really are just tweaks. Flying it still feels like flying the same basic aircraft--just about the differences you might expect if you added or subtracted a few hundred pounds weight. Obviously it affects handling and specifics like stall speed noticeably, but it is still quite obviously the same basic aircraft.
I've tried tweaking some of the reported characteristics, where the behavior has been well documented but the exact values given in the FDM are by necessity something of a guess (ie, adverse yaw due to ailerons, exact gyroscopic moment due to rotary engine/prop). The interesting thing is you can tweak these things as much as +100% or -50% and, obviously, details of handling will different but what happens is that in a few minutes of flying you learn to adjust to the new situation and you just proceed. It is as though your particular aircraft happened to have slightly larger or smaller elevators, or you put a bigger/heavier prop on or whatever. Those things all do in fact affect aircraft performance and handling to some degree, but in real life, as long as the differences are within a certain range, you just adapt and proceed.
In fact we know that different variants of the Camel were in fact produced over time and undoubtedly those variants had differences in handling and performance to a certain degree. Yet it is still the same underlying aircraft.
Tweaking it to hit the reported max speeds at 6500, 10000, and 15000 ft, as well as the climb time to 6500, 10000, and 15000 feet (all within a reasonable margin of error) took some time and work, but the end result really was a fairly minor tweak to what it already does 'naturally' so to speak. And obviously those precise values all have an error bar around them in the first place and could be affected by something as small as carrying and extra 50 pounds of weight or outfitting a slightly different windscreen or gun mount.
All the above is a long-winded way of saying--new & improved FDM release coming soon! Different but not so different from the previous 2.0gamma version.