General Principles - Climb — Page 202, Lesson 235

Let's pick up with the climb. We've already established that rate of climb is the vertical speed of the aeroplane, and now I want to give you the precise formula that governs it. The rate of climb equals Power Available minus Power Required, divided by Weight. That whole numerator — Power Available minus Power Required — is what we call the Excess Power Available. So the formula reads: Excess Power Available divided by Weight. Let me unpack that. 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. So if we want to maximize the aeroplane's rate of climb, we need to maximize Excess Power. That's the whole game in climb performance. Now, to understand how we get the greatest amount of Excess Power Available, we need to look at some graphs. Let's start with a typical jet aeroplane. We plot Power Available and Power Required against speed. To give ourselves some 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. There's another speed, slightly faster than VMP, called VMD — the velocity for minimum drag. VMD is found at the point of contact of the tangent from the origin to the Power Required curve. So you draw a straight line from the origin of the graph, just touching the Power Required curve at one point — that point of tangency gives you VMD. 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. Here's the key result for a jet: 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. The speed for 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 chain of logic is: rate of climb equals Excess Power divided by Weight; for a fixed Weight, maximize Excess Power; for a jet, that happens at a speed above VMD; that speed is VY. That's your best-rate-of-climb speed.