General Principles - Climb — Page 207, Lesson 239

Let's pick this up with the effect of weight on the climb. We've already seen how the power required curve behaves, so now we're applying it. When the aircraft gets heavier, the drag curve moves up and to the right. Since power required is drag multiplied by true airspeed, the power required curve follows that same movement — up and to the right. Look at Figure 3.48 for jet aeroplanes and Figure 3.49 for propeller aeroplanes. In both cases, the power required curve shifts up and right. Now, what does that do to the climb? The excess power available — that's the gap between power available and power required — gets smaller. Less excess power means the rate of climb decreases. That's the first effect: higher weight, lower rate of climb. But here's the important part. The speed at which you get maximum excess power is no longer the same. It's now higher. That speed is VY, the speed for best rate of climb. So with higher weight, the rate of climb is decreased, but VY is increased. That's a key contrast to remember — weight pushes VY up. Now let's move to configuration. This is about the use of flaps and gear. If 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 — up and to the left. Look at Figure 3.50. Again, the excess power available is reduced, as shown by the blue double-headed arrows. And a 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 gear and/or flaps deployed, the rate of climb is decreased, and VY is decreased. That's the opposite of weight — configuration pushes VY down. Now, a practical point. If you use flaps for takeoff, you remove them in stages once you have attained a positive stable climb. Always check the aeroplane flight manual for the correct actions for your 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. Now let's look at density. Density affects a lot of the variables in the rate of climb formula. Here's 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 the formula is: Rate of Climb = (Thrust × TAS) − (Drag × TAS), all over W. Let's focus on Power Available first. When density decreases — from high temperature, high altitude, or high humidity — the thrust decreases, but the true airspeed increases. The overall effect is that the thrust loss is more than the TAS gain. So overall, the Power Available decreases. Now for Power Required. When density decreases, the true airspeed increases, but there's no effect on the drag. So the Power Required increases. So decreased density reduces Power Available and increases Power Required. Both of those act to reduce the excess power, and therefore the rate of climb decreases. That's the density effect on climb.