Now I want to move into a new topic: aerofoil section lift characteristics — Page 83, Lesson 93

Let’s pick this up right where the density-altitude idea left off, because that last sentence is the key to the whole paragraph. We said that a high density altitude means the air is less dense, so for a given dynamic pressure — that is, for a given amount of ram air pressure felt by the pitot system — the aircraft needs a higher true airspeed, or TAS. And because the indicated airspeed, the IAS, is what the pilot actually reads, a higher TAS for the same IAS means the take-off run has to be longer to build up to that required IAS. So remember the phrase: think of “high density altitude” as “HIGH density ALTITUDE” — the altitude is high, the density is low. Now I want to move into a new topic: aerofoil section lift characteristics. Let’s look at Figure 5.5, which shows aerofoil sections with different thickness and camber combinations, and how each combination produces its own specific lift coefficient against angle of attack plot — that’s the CL against α curve. Two key relationships come out of that figure. First, an increase in the thickness of a symmetrical aerofoil gives a higher CLMAX — that’s the maximum lift coefficient, the peak of the lift curve. Second, the introduction of camber also has a beneficial effect on CLMAX. Camber is the curvature of the aerofoil’s mean line — the asymmetry between the top and bottom surfaces. Both thickness and camber push that maximum lift coefficient up. Why does CLMAX matter so much? Because the greater the CLMAX, the lower the minimum flight speed — that’s the stall speed. The lift equation ties them together: for a given weight, if you can generate more lift coefficient, you don’t need as much speed to hold the aircraft up. So a high CLMAX gives you a low minimum flight speed in terms of IAS. But here’s the catch, and this is the professional point. The thickness and camber you need for a high CLMAX will produce increased form drag — that’s the drag caused by the shape of the aerofoil pushing through the air — and large twisting moments at high speed. So a high CLMAX is only one of the requirements for an aerofoil section. You can’t just design for maximum lift and ignore everything else. Now think about the design trade-off. If you use an aerofoil section of greater camber to get a lower minimum flight speed, the efficient cruise speed will be lower, because the extra camber generates excessive drag in cruise. So the better solution is to use an aerofoil section that is efficient at high cruise speed, and then temporarily increase the camber of the wing when it is necessary to fly slowly. That temporary increase in camber is achieved by adjustable hinged sections of the wing’s leading and trailing edges — and those are the flaps. So the flaps let you have the best of both worlds: a clean, efficient aerofoil for cruise, and a high-camber, high-CLMAX configuration for slow flight and landing. That’s the whole logic chain: high density altitude means longer take-off run, and the aerofoil design — thickness, camber, and flaps — is how we manage the minimum flight speed without wrecking cruise performance.