Originally Posted By: Win
Astro, question:
Re: Finding of Fact #125 "Between the barometric altitudes of 3196 feet AGL ..... and 2856 feet AGL, the aircraft
reached its steepest nose down attitude of 86.8°, 26.6 AOA, ....
AOA is wing angle of attack? This would mean the aircraft was pointing almost straight down, but was moving in some other direction? I'm having trouble grasping this. Is AOA in an F-18 something different from AOA in a light aircraft?
Thanks in advance.
Win - AOA is AOA - but you're used to thinking of airplanes in level flight, where the relative wind comes from straight ahead...
Sure, he was pointed straight down, but the relative wind was impacting the mean chord line of the wing (the definition of AOA, right?) at 26.6 degrees. AOA is measured on the nose of the F/A-18 by an AOA vane - a pressure transducer that rotates to balance pressure between two slots. The position of the probe in that range of rotation defines the measured AOA.
So, the nose of the Hornet is pointed straight down, but the pilot has the stick pulled far aft. That means that the AOA is high. AOA and airspeed = lift, right? So, while the nose is pointed down, the airplane is pulling "G" and the lift vector is changing the velocity vector of the airplane.
Consider an airplane in a loop. While going straight up, the airplane is pulling positive "G" so that the path of the airplane is curved. The relative wind, created solely by the motion of the airplane, is from straight up. The AOA is positive, and dependent on elevator position. That AOA creates lift. The lift is pointed toward the center of the loop and curves the airplane's path.
Same thing when the airplane is going down. The only difference is this: when going straight up: the force of gravity is decelerating the airplane. When going straight down, the force of gravity is accelerating the airplane. But in both cases, the airplane "sees" the relative wind as coming from the exact opposite direction of its travel. And, in both cases, the airplane can pull "G" to change its path.
That help?