If you're only looking at one chart - say, an excess power available over power required or an excess power curve - it can be hard to distinguish why Vx is at a lower speed than Vy. In fact, if you look only at a PA/
PR curve, it can be downright confusing.
In short:
- Vy occurs with maximum excess power
- Vx occurs with maximum excess thrust
Power and Thrust are not the same thing, even though we use the terms interchangeably.
Power is all about the engine. It's horsepower - work per unit time. We want to know how much power the engine delivers (power available) versus how much it has to deliver at any point in time (power required). Power available can be written as horsepower available and power required can be written as horsepower required. Power available tends to slightly increase with an increase in airspeed, and while initially decreasing, power required will increase. At some point between the power available curve and power required curve we will find the maximum distance between the two charts. This is where we find Vy. We have the most excess power available to us so that we can climb away at our best rate of climb.
Thrust, at least in this context, is a geometric consideration. It's the force in a climb that is required to overcome the drag (D) and the rearward component of the weight (RCW). Think of a van on a hill. When on level ground, all the weight of the van will act downwards to the centre of the earth. But move that van up a hill, and suddenly the weight is divided between that pressing down onto the road surface and that acting toward the centre of the earth. The angular difference between these two is the rearward component of weight (RCW). The RCW is what will cause the van to roll back down the hill if the car isn't travelling fast enough or you've not set the brake. While an airplane isn't on a road, it still has this rearward component of weight acting on it when in a climb. Much like a van, we have to ensure we have enough thrust to overcome both the RCW and the aerodynamic drag. However, for most pilot pilot studies this gets very complex, very quickly, so we normally just say it's thrust versus drag. And we can do that in steady flight, but not when talking about a climb.
Unlike power, we have the most thrust available to us when the airplane is stationary - there is no drag and there is no RCW, and the propeller is at its highest angle of attack. Then, as we accelerate the amount of thrust available decreases. The drag increases and the propeller angle of attack decreases (let's keep it simple and say we're dealing with a fixed pitch propeller right now). As we know from our drag curve, the amount of drag initially decreases before increasing again. This will change again in a climb where we would add in the effect of the RCW. At some point we will reach a maximum difference between the thrust available and the drag combined with RCW. This maximum difference is where we find the Vx. We have the most excess thrust available to us so we can climb away at our best angle of climb.
Now, let's take to your point about reducing the throttle. The point of Vy and Vx are that they occur at the maximum points of excess power thrust respectively. If you reduce the throttle you are no longer at the maximum points. What you're missing is that the charts assume you're operating at the maximum excess points. As Scotty would say, she's giving her all she's got.