Hello everybody,
I was just reviewing my course "Performance" and there is 1 thing i just don't seem to understand:
-Why does the Vy (speed for best rate of climb) moves towards Vx (speed for best angle of climb) as altitude increases, while Vx remains constant during altitude increase???
Who can explain this??
thx
Gillespie
10th Oct 2005, 15:12
i'll do my best.
Vx is a function of the thrust and EAS. The point on the drag curve were the difference between thrust and drag is greatest gives you Vx, or the best angle of climb. For a jet this occurs at minimum drag speed, as thrust is fairly constant with increasing EAS. Remember 100kts EAS at sea level is the same as 100kts at 40,000ft. TAS however increases as you climb due to the air density decreasing. For a prop, Vx occurs at minimum power, as the thrust curve falloff quickly with increased EAS due to the prop blade's angle of attack decreasing with increasing TAS
Vy occurs at the greatest difference between power available and power required. Remember the power is the RATE at which work is done. So we look at the power/TAS curve. As we climb air density decreases therefore so does thrust. Power available = TAS x Trust. The power required curve goes up and to the right as we climb and for a jet, the straight thrust line starting at the origin moves down. Can you see that for increasing altitude your Vy increases? Vx remains the same as it's a function of EAS.
I think of EAS as a wind tunnel. It’s the air that’s physically moving around the aircraft regardless of air density, compressibility etc.
I'm no expert on this, nor am I an instructor so I hope I didn’t confuse you anymore.