General Principles - Climb — Page 207, Lesson 239

We're now looking at the climb phase, and I want to walk you through the factors that change your rate of climb. We've already seen that the rate of climb depends on the excess power you have — the difference between power available and power required. Now let's look at what happens when we change the aircraft's weight. For a jet aeroplane, look at Figure 3.48, and for a propeller aeroplane, Figure 3.49. In both cases, if the drag curve moves up and to the right, the power required curve follows it — it moves up and to the right too. So what does that do to your excess power? It reduces it. Less excess power means your rate of climb decreases. That's the first effect. But here's the critical part — the speed for maximum excess power is no longer the same. It's now higher. So with a higher weight, your rate of climb is decreased, but your best rate of climb speed, which we call VY, is increased. Remember that — higher weight means you climb slower, but you need to fly faster to achieve that best rate of climb. Now let's move to the next factor — configuration. This is about the use of flaps and gear. When you deploy the gear and flaps, the profile drag of the aeroplane increases. That increases total drag, and the drag curve moves upwards and to the left. The power required curves follow the same movement as the drag curve — up and to the left. Look at Figure 3.50 to see this. Again, the reduction in excess power reduces your rate of climb. But the important effect here is that the speed for maximum excess power is now lower. So with the gear and/or flaps deployed, your rate of climb is decreased, and VY is decreased. That's the opposite of the weight effect. Now, a practical point — if you use flaps for takeoff, you should remove them in stages once you've attained a positive stable climb. Always check through the aeroplane flight manual for the correct actions for your specific aeroplane. As you retract the flaps, the rate of climb and the speed to attain the best rate of climb will increase, so you should accelerate to ensure you remain at VY. Finally, let's look at density. Density affects a lot of the variables in the rate of climb formula. Let me show you the expanded formula. Rate of Climb equals Power Available minus Power Required, all divided by Weight. And remember, Power Available is Thrust multiplied by true airspeed, and Power Required is Drag multiplied by true airspeed. So let's focus on Power Available first. When density decreases, the thrust decreases, but the true airspeed increases. The overall effect is that the thrust loss is more than the TAS gain, meaning overall, the Power Available decreases. Now looking at Power Required — decreased density will increase the true airspeed but have no effect on the drag. Therefore, the Power Required will increase. So you can see — decreased density reduces your power available and increases your power required. Both of those work against you, reducing your excess power and therefore your rate of climb. That's why on a hot day at high altitude, your climb performance suffers.