Quote:
Originally Posted by Bill Walker
Correct.
Exactly. "P dot" is defined as the rate of change of roll rate with time. Think of the units: degrees per second per second
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.
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.
|
So what is the definition of "P dot"? I mean, what does it stand for? I can never remember all the jargon, but it's like "Vmax" is the maximum safe speed for whatever operating conditions apply..."Vmax = Velocity Max".
So is the "dot" supposed to be like a decimal or period? Or is it "d-o-t", and the letters stand for something?
And what I was saying about the cannons effect on "P dot"...your first post said:
"Cannon in the wings may slightly increase the maximum roll rate (P)"
...did you really mean to type "decrease"? Because in the next post, you say:
"
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."
The second makes more sense to me...I just don't see heavy cannons actually INCREASING rate of roll...they'd just make it a lot harder to STOP the roll, wouldn't they? It's funny how even a little weight in the wrong spot can alter an aircraft's handling so much!
What you said about "speed is not primarily a function of engine throttle setting"...that's less true with a powerful combat plane than with a small, docile plane, isn't it? With old biplanes, you climbed by increasing the throttle, and descended by lowering it. But with a warbird, anything over cruising speed is only maintained by high power levels...the second you "let off the gas" it begins to slow down until the drag matches the lower power setting. You CAN maintain speed, by only by loosing altitude. So I can't say that what you said is WRONG, because it's not...but the throttle did have other uses, or at least it was bit more complex than flying a Stearman!
I remember reading somewhere that a pilot could use the throttle to slow himself down and avoid overshooting...the prop acts as an airbrake, and early jet-fighter pilots had trouble adapting to the fact that they couldn't slow down and keep the sights on a target at the same time. You could nose up and slow down, but you wouldn't be able to shoot at the same time.
Of course, that was quite a while ago that I read that...it may be that the guy was referring to the fact that a Me 262 pilot couldn't "chop" his throttles, or he'd flame out the engines. In a prop fighter, even cutting the power while at high speeds will make you slow down quickly, but an Me 262 couldn't do that, and without an airbrake, he was stuck at the speed and power of the engine.
Interesting stuff; it makes me think! Thanks.