Controlling Lift
Lift can be controlled by various means of which four are commonly used. Two ways of controlling lift have already been mentioned and consists of changing the coefficient of lift by changing the angle of attack or by varying the airspeed which will cause the amount of air molecules impacting on the wing to increase or decrease. Besides these two methods, the pilot is able to change the shape of the wing or vary the total wing surface area by using leading edge and trailing edge devices.

The most common method of controlling lift is to change the angle of attack. It not only allows you to actually control lift but airspeed and drag also to a certain extent. As we've discussed earlier, increasing the angle of attack will increase lift but only up to a specific point. Reaching this angle of attack in particular will cause the streamlined air flow to separate after which the amount of lift decreases rapidly. The angle at which this happens is called Clmax (figure 1.1) and is noticed by airframe buffeting. This point also tells the pilot that a stall is imminent. For each particular airfoil, a stall always occurs at the same angle of attack regardless of weight, altitude, or pitch attitude. From here it is obvious that a stall could occur at different indicated airspeeds and is never a fixed value.

Figure 1.1 - Angle of Attack vs Coefficient of Lift

Increasing the amount of lift also causes an increase in "induced drag". This can be demonstrated as the aircraft is configured for landing and flying just above minimum manoeuvring speed. In this case the airspeed is relatively low while the angle of attack is high in order to generate sufficient lift. However, the angle of attack can only be increased untill Clmax is reached. At this point you can no longer increase lift by simply increasing angle of attack as it would result in an abrupt loss of lift and possibly stalling the aircraft.

Clmax is normally reached as the indicated airspeed approaches the published stall speed. The danger you may encounter by flying near the stall speed is that any turbulence could cause changes in the direction of the relative air flow and eventually results in an increase of the speed at which the airplane stalls. Besides turbulence, G-loading could increase stall speed quite rapidly particularly during steep turns. During these kind of maneuvres the load factor multiplies the effective weight of the aircraft and should be balanced by an increase of angle of attack. Overal, stall speed depends on several factors like weight, center of gravity location, power and load factor.

The total lift over a particular airfoil can be represented by a single vector and can be determined by adding all pressure vectors for the entire wing surface (figure 1.2). This vector is commonly called the "center of lift" or "center of pressure". As said earlier, most airfoils have more curvature on their upper surface, called positive curvature, which results in a movement of the CoP forward as the angle of attack increases. The location of the center of pressure is of greatest importance when discussing aircraft stability later on.

Figure 1.2 - Component of lift and movement of CoP

 

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