General Principles - Climb — Page 202, Lesson 235

We're now moving into the climb performance section of the book, and I want to start with the fundamental equation that governs everything we do when we point the nose up. The rate of climb formula is: Rate of Climb equals Power Available minus Power Required, divided by Weight. Let me unpack that carefully, because every symbol matters. Power Available is the power the engine can actually deliver to the aeroplane at a given speed and altitude. Power Required is the power needed to maintain steady, level flight at that same speed — it's the power needed to overcome drag. The difference between them, Power Available minus Power Required, is what we call Excess Power Available. That's the surplus power left over after we've paid the drag bill. And we divide that excess by Weight, the aeroplane's weight, to get the rate of climb. So the relationship is direct: for any given Weight, the greater the Excess Power Available, the greater the rate of climb. Conversely, the less the Excess Power Available, the smaller the rate of climb. That's the whole story in one sentence — climb performance is driven by how much spare power we have. So if we want to maximize the aeroplane's rate of climb, we need to maximize Excess Power. And to understand how to get the greatest amount of Excess Power Available, we need to look at some graphs. Let me set up the graph for a typical jet aeroplane. We plot Power Available and Power Required against speed. To give ourselves benchmarks, we locate two reference speeds we mentioned earlier: VMP and VMD. The speed found at the bottom of the Power Required curve is called the velocity for minimum power, or VMP. That's the speed where the power required to maintain level flight is at its absolute minimum. There's another speed, slightly faster than VMP, called VMD. This is the velocity for minimum drag, and it's found at the point of contact of the tangent from the origin to the Power Required curve. So VMD is where drag is minimized, and it sits a little faster than VMP. Now, having located those reference speeds, the object is to find where Excess Power Available is maximum. On the graph, the area between the two curves — Power Available and Power Required — represents the area of Excess Power Available. The greatest amount of Excess Power Available is found where the distance between the curves is at its maximum. And here's the key result for jets: that maximum occurs at a speed higher than VMD. At any other speed, the Excess Power is less, and therefore the rate of climb will be less. That speed, the one that gives the best rate of climb, is called VY. So for a jet aeroplane, VY occurs at a speed higher than VMD. VY is the airspeed to use to climb to the cruise or en route altitude, because it gives the greatest height gain per unit time. In a typical 737-400, this speed is about 275 knots, and it's usually published in the aeroplane flight manual as an indicated airspeed. So the takeaway is: we don't climb at the speed of minimum power or minimum drag — we climb at VY, which sits above VMD, because that's where the gap between power available and power required is widest, and that gap is what buys us altitude.