[Bill Walker]

Originally Posted by Johnny .45

Okay, I think I mostly follow you here..."mass distribution", i.e. how far apart the weight is along the wingspan.

Correct.

...with those heavy engines out on the wings, it took a moment for it to "accelerate" into it's full roll rate. Like a heavy car can have the same top speed as a lighter car, but for the same power, the lighter one will be far quicker "off the line".

Exactly. "P dot" is defined as the rate of change of roll rate with time. Think of the units: degrees per second per second

Cannon would have a similar effect...between a .303 and a 20mm-armed Hurricane: the wingspan is the same, the ailerons are the same, so the force the ailerons produce is the same, only it's trying to move a much heavier cannon.

Yup.

But part of what I was saying is if a pilot has to use slight OPPOSITE deflection to STOP the roll (the momentum will try to keep the roll going), it will be all the more sluggish on STOPPING the roll.

Today an airplane designer calls all this "agility", or the ability to point where you want, when you want, both accurately and quickly. It is a combination of speed of control response (both control input coming on and coming off), sustained rates (pitch, yaw and roll), and repeatability.

But I don't see how cannons could INCREASE the maximum roll rate. The maximum rate would be the same, it would just take longer to build up to that rate (and longer to stop).

The increased inertia can over power the ability to stop or even reduce the rate of roll. Some high speed aircraft have very strict restrictions on roll rate with wing stores, as the pilot soon looses the ability to stop the increasing roll rate, until things start breaking and falling off. Usually this is not desirable.

If heavy cannons increased any performance parameter, it'd be dive-rate! A little extra weight to increase the terminal velocity...although I'm not sure whether in that situation more weight would make a lot of difference, or if it's more about the power of the engine.

Terminal velocity occurs when accelerating down forces (gravity plus thrust) equals drag. Extra weight, anywhere, can increase terminal velocity. However, for most modern combat aircraft, and even for some slippery WW2 fighters, this classic definition of terminal velocity doesn't really mean anything. Drag is so low that very high speeds are needed to get drag up to equalling weight. Some other limit will be reached first: structural, aeroelasticity, etc. Today most fighters would start to melt, or just run out of altitude, long before terminal velocity is reached.

If a plane can dive at 500mph at full power, how much slower would it be in a glide? The Thunderbolts impressive diving ability makes me suspect that the weight of the plane is important, and I'm not even sure if a prop on full power at 500mph is helping to propel the plane, or if it's just contributing to drag at that point!

Propeller efficiency, and therefore thrust, does decrease as forward speed increases, but I suspect that a constant speed prop would break off long before it even produced zero net thrust. You can produce braking by operating at non-ideal blade angles - reverse thrust as often used on short landings by C-130s and others.

And anyway, speed is not primarily a function of engine throttle setting, but more a function of wing angle of attack. Throttle setting will mostly determine if you are climbing, level or descending at a given speed. The speed, within normal operating limits, is determined by elevator control position and pitch trim setting (if the airplane has this). That is why flying an airplane has been described as rubbing your stomach and patting your head at the same time.