Vx, Vy, Power Curve

Definitions

Thrust – The force which imparts a change in the velocity of a mass. This force is measured in pounds but has no element of time

Power – Implies work rate or units of work per unit of time, and as such, it is a function of the speed at which the force is developed.

Introduction

The backside of the power curve, or being behind the power curve, can mean a couple of things. When talking about general aviation, one of the common interpretations is the aircraft’s speed in relation to its drag. In order to make sense of this “curve”, we need to understand a few things.

Drag is present in two forms at all times an airplane is flying – induced drag and parasitic drag. If we graph these two forms of drag, we can see that induced drag is highest at slow airspeeds (when our AOA is high, and we are generating a lot of lift). Conversely, our parasitic drag is highest when we are moving the quickest through the air (lots of drag from moving the plane through the air, among other things).

Thrust Required

If we know the amount of drag, that means we know the amount of thrust required to counter that drug. If we plug a reciprocating engine’s thrust available line, we can compare the difference between thrust required vs. thrust available.

The biggest gap between the thrust available and required gives us our Vx (Best Angle) of climb. But we are here to talk about a power curve, not a thrust curve.

Thrust vs. Power

Thrust is the amount of power a propeller can generate at any given point. If we add an element of time to that data, we now have defined power. So how can graph power? We need to find both the amount of power required AND available. We can use our previous thrust required/available chart.

Multiplying the Thrust Available/Required * Airspeed = Power. This gives us a large number, and as a rough conversion we can divide this number by 300. If we did this with all of the points, we would get a thrust available/required chart like this.

We can see just like the thrust chart that we have a dip in our HP requirement. This is, again, due to the amount of drag being produced (induced on the left side, parasitic on the right side). This chart also shows us our Vy speed, because it is the speed where we have the greatest difference between horsepower required and available.

Putting it all together

We can finally look at the power curve, and we can see that as we get slower than the lowest dip in power required, it will take more power to go at a slower speed. This is what people call the “backside of the power curve” or “behind the power curve”. It’s simply when it takes more energy to go slower because of the amount of drag.