General Principles - Climb — Page 202, Lesson 237

We're moving into the climb now, and I want to start with a graph that ties together power, speed, and climb performance for a propeller aeroplane. This is Figure 3.47, and it plots Power Available against Power Required across the speed range. Here's the key idea: the greatest amount of Excess Power available is found where the distance between the two curves is at its maximum. Excess Power, remember, is simply Power Available minus Power Required — the surplus you have left over after overcoming drag, and it's that surplus that drives your rate of climb. Now, the crucial point for a propeller aeroplane: this 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 maintain level flight. But the best climb isn't at that minimum-power point. It's higher. At any other speed, the Excess Power is less, and the rate of climb is less. And that connects directly to VY. VY is the speed for the best rate of climb. So for a propeller aeroplane, VY occurs at a speed higher than VMP. That's a distinction you must remember, because it's different for jets — for a jet aeroplane, VY sits at a different point, and I'll come back to that contrast. Let me make this concrete with the graph. If a propeller aeroplane were climbing at a speed equal to VMP and then selected a slightly higher speed, the Excess Power would increase, and the rate of climb would increase. So even though you're moving away from the minimum-power speed, you're moving toward the maximum Excess Power, and your climb improves. That's the counter-intuitive bit — climbing faster than VMP actually gives you a better climb rate for a propeller aeroplane. Now let's look at what factors influence the rate of climb, starting with weight. An increase in weight creates more weight apparent drag, which reduces the angle of climb. For any given airspeed, if the angle of climb reduces, then so will the rate of climb, because they're fundamentally linked. You can see this directly in the formula: Rate of Climb = (Power Available − Power Required) / Weight Simply by increasing the value of Weight in the denominator, mathematically the rate of climb reduces. That's the direct effect. But weight has a further effect we've already talked about. An increase in weight requires an increase in lift. Increasing lift increases induced drag, which causes the drag curve to move up and to the right. And here's the important link: the Power Required curve is actually based upon drag — Power Required is drag multiplied by velocity. So when induced drag rises, the Power Required curve shifts, and that reduces your Excess Power further, compounding the climb penalty. So weight hurts your climb twice: once directly through the denominator in the formula, and once indirectly by raising the Power Required curve through increased induced drag. Keep both effects in mind, and remember where VY sits for a propeller aeroplane — higher than VMP.