Originally Posted by
USMCFLYR
But in the EM diagram there is that one point where turn rate and rdius is optimized - correct? Or have I been out of the loop too long?
USMCFLYR
No you have not been out of the loop, immelmann, or Cuban 8 too long.
I made the statement earlier that for a
given angle of attack, the radius is the same, regardless of airspeed. Think of a 3/4 inch drill bit...it cuts a 3/4 inch hole regardless of how fast you drill it. You can think of a particular wing as being a certain-shaped drill-bit...it makes certain-sized holes in the sky.
For this relationship to work, however, we are assuming
all variables for that airplane have not changed. One that I cannot perfectly account for is tail-download. My best estimate is (for a fighter aircraft) the load is about 5% of the total lift in straight, unaccelerated flight, and as much as 10-15% when you are doing full-aft-stick, 9-g, "I've got to kill this SOB before he kills me!" ACM. I'm a mechanical engineer; Cubdriver is our resident aero, and could tell me if my ballparks estimates are about right.
Anyway, for me working backwards to calculate what speed equals the same AOA, I have to assume the tail-download into the equation, in order to figure the
total lift the wing is generating.
Second, this 'constant radius for a given AOA' assumes I have the
power to
sustain said AOA regardless of speed or altitude. As USMCFlyer knows , this is not true (although a big-mouth Block 50 F-16 comes close). Generally, form-drag and parasitic-drag will become more significant at higher speeds, changing the radius slightly. At very high speeds, mach-wave will affect both drag AND Max Coefficient of lift...also changing it.
If you look at an EM diagram along
lines of equal Ps, the radius is near-constant (which is what I am driving at).
Lastly, if I extrapolate these equations, I could (theoretically) pull 15 gs at 500 kts, and have the same radius of turn that I have at, say, 3 g and 300 kts. (I don't have a calculator in front of me, so don't hold me to those numbers....just for illustration).
But of course, my wings would have long departed the fuselage.
Point being, due to these minor variations caused by tail download and excess specific power, there is usually one point that is the best. But when I look at the airplanes I have flown, or fought against, as long as the configuration has not changed (ie, no flaps), the radius doesn't change much with changing speed.