General Principles - Climb — Page 202, Lesson 237

Let’s pick this up right where the climb performance story gets practical. We’ve already seen how excess power drives rate of climb for jets. Now I want to walk you through the propeller aeroplane case, because the picture is subtly different, and then we’ll look at what actually changes the rate of climb. First, the key graph. Figure 3.47 plots Power Available and Power Required against speed for a typical propeller aeroplane. Power Available is what the engine and propeller can deliver at a given speed. Power Required is what the aeroplane needs to maintain level flight at that speed — remember, it’s drag multiplied by velocity. The vertical gap between those two curves is the Excess Power, and that excess is what’s available to climb. So the greatest amount of Excess Power is found where the distance between the curves is at its maximum — that’s the point on the graph where the two lines are furthest apart vertically. Now here’s the crucial contrast with the jet. For a propeller aeroplane, that maximum Excess Power occurs at a speed higher than VMP. VMP is the minimum power speed — the speed at which the Power Required curve is at its lowest, where the aeroplane needs the least power to stay level. But the best climb doesn’t happen there. It happens a bit faster. At any other speed — either slower or faster than that maximum-gap point — the Excess Power is less, and therefore the rate of climb is less. And that brings us to VY. VY is the speed for the best rate of climb. For a propeller aeroplane, VY occurs at a speed higher than VMP. So if you’re climbing at exactly VMP and you select a slightly higher speed, the Excess Power increases and the rate of climb increases. That’s the direct consequence of where the maximum gap sits on the graph. It’s worth really picturing that graph — the Power Available curve for a propeller is fairly flat, and the Power Required curve dips down to VMP and then rises again. The widest vertical gap sits to the right of that dip, not at the dip itself. Now, what factors actually influence the rate of climb? Let’s start with weight. An increase in weight creates more weight apparent drag — that’s the drag component that acts along the flight path opposing the climb. That reduces the angle of climb. And here’s the link: for any given airspeed, if the angle of climb reduces, then so will the rate of climb, because they are fundamentally linked. The rate of climb is essentially the vertical component of your forward speed along the climb path. You can see this mathematically in the formula: Rate of Climb = (Power Available − Power Required) / Weight So the numerator is the Excess Power — the power left over after overcoming drag. Divide that by weight, and you get the rate of climb. Simply increasing the value of Weight in the denominator mathematically reduces the rate of climb. That’s the direct effect. But weight has a second, indirect effect we’ve already talked about. An increase in weight requires an increase in lift to support that heavier aeroplane. Increasing lift increases induced drag — that’s the drag generated as a by-product of producing lift. That induced drag increase causes the drag curve to move up and to the right. And since the Power Required curve is based upon drag — Power Required is drag multiplied by velocity — the Power Required curve shifts up and right as well. That means at any given speed, you need more power just to stay level, which shrinks your Excess Power and further reduces your rate of climb. So weight hits you twice: once directly through the denominator of that formula, and once indirectly by raising the Power Required curve through induced drag. Both effects push the rate of climb down. That’s the propeller picture and the weight effect. Next we’ll look at the other factors — altitude, temperature, and wind — that also shape the rate of climb for both jet and propeller aeroplanes.