ppl_05_e2

The stalling IAS for a given

aircraft and a given weight or load factor will be the same

at all altitudes.

And because, right up to the moment the aircraft stalls, lift must equal the aircraft’s weight, we may write, for the stall:

Weight = Lift = CL MAX Qstall S

Now, an aircraft’s straight-fight stall speed is always quoted in the Pilot’s Operating

Handbook for a given all-up weight. We may deduce, then, that:

Weight is a constant, at the point of stall.

And, as Weight = Lift

Lift is also a constant, at the point of stall

Likewise, as Weight = Lift = CL MAX Qstall S

The expression, CL MAX Qstall S must be a constant, at the stall:

Finally, CL MAX is, by defnition, a constant, and, likewise, if the pilot does not alter his fap setting, wing area, S, is also a constant. It follows then that:

Qstall is a constant

And Qstall is the Indicated Air Speed (IAS) at the stall.

We may conclude, then, that an aircraft always stalls at the same IAS from straight and level fight, whatever the altitude the aircraft is fying at, even though the stalling

IAS will represent different values of True Airspeed (TAS), depending on the prevailing value of air density, ρ.

By looking again at the original lift equation for the point of stall,

Lift = CL MAX ½ ρ(vstall)² S

Vortex generators

increase the kinetic energy

within the boundary layer.

Figure 13.13 Stall strips help to ensure that the wing root stalls first.

Vortex Generators.

Vortex generators are small aerofoil-shaped blades, normally attached in rows as depicted in Figure 13.14, which project about 2.5cms (1 inch) vertically from the upper surface of the wing. These devices are designed to generate a small vortex within the boundary layer.

Vortex generators cause swirling motions in the boundary layer. This greater turbulence energises the boundary layer and delays separation, thus increasing control effectiveness, especially at the ailerons.

The Effect of the Position of Centre of Gravity on the Stall.

You learnt in the Mass and Balance section of this series of text books that if the aircraft’s centre of gravity (C of G) is at its most forward limit, the tailplane or stabilator has to produce a greater downforce in order to balance the lift-weight couple (See Figure 13.16 overleaf). This greater tailplane force acting in the same direction as weight, effectively increases the weight of the aircraft. Consequently, for the reasons you have already learnt, the stalling speed increases. As the C of G moves aft, less tailplane downforce is required. This condition reduces the lift requirement for level fight, since lift must balance weight plus downforce, and consequently also decreases the stalling speed. (See Figure 13.17 overleaf). During fight testing, the stalling speed is calculated for the worst case scenario, i.e. maximum weight, with the C of G at its most forward position.

The downforce produced by the tailplane influences the stall speed.

When flaps are deployed

CL increases and stalling

speed will decrease.

Figure 13.16 A forward C of G position requires an increased tailplane down-force, leading to an increase in stall speed.

Figure 13.17 Stall speed decreases as the C of G moves aft, because the required tailplane down-force is less.

The Effect of Flaps on the Stall .

Chapter 10, Lift Augmentation, considers in detail the effect which the deployment of faps has on the stall. As you have learnt from Chapter 10, deploying faps alters the shape and camber of the wing’s aerofoil section and increases CL MAX. So, from the lift equation for the point of stall, we see that (vstall) must decrease to keep the equation balanced.

Weight = Lift = CL MAX ½ ρ (vstall)² S

You have also learnt that lowering faps, while increasing CL MAX, not only reduces the angle of attack at which the aircraft stalls, but also increases the angle of incidence of the wing by increasing its camber. This means that if the aircraft stalls from straight and level fight, the nose attitude, with faps down, will be lower than for an equivalent stall exercise with faps up.

The fact that lowering faps reduces the aircraft’s stalling speed is, of course, one of the main advantages of faps, allowing shorter take-off and landing distances to

be achieved, as well as low-speed cruising fight, should a pilot become lost or fnd himself in deteriorating weather conditions such as a lowering cloudbase.

Note, however, that if the aircraft stalls when faps are down, wing drop at the stall may be more marked than with faps up, because the inboard section of the wing will have a higher local CL than the outer section.

Wing Contamination.

Stalling speed may be increased signifcantlyifthewingiscontaminated

(See Figure 13.18). Snow, frost, and ice deposits on the wing, and even heavy rain, can reduce the value of CL MAX by as much as 30% because of their modifying effect on wing profle. Deposits on the airframe will also increase aircraft weight, causing a further increase in stalling speed. Any contamination (ice, raindrops, squashed insects, etc.) must be removed from the wings before fight,

especially from the leading edge.

Figure 13.18 Frost, snow and ice on the wings increase stall speed.

The Effect of Power on the Stall.

Figure 13.19 When stalling under power, thrust contributes to the Lift, and so the wing can fulfil the smaller lift requirement at a lower speed; stall speed is thus reduced.

When the propeller is producing signifcant thrust, the aircraft’s stalling speed will be lower than with the engine throttled back. The reasons for this are twofold:

•The propeller slipstream energises the airfow over the inner region of the wings and, thereby delays separation. This action increases the value of CL MAX and, thus, decreases the stalling speed.

Prior to flight, always ensure that the wing

is free of all contaminants, especially at the leading edge.

Figure 13.22 Washout is the name given to the reduction in a wing’s angle of incidence towards the wing tip.

Washout is the term used to describe the reduction in angle of incidence of the wing from root to tip. This feature helps ensure that the stall occurs at the wing root frst

(See Figure 13.22). (NB. The angle of incidence is the angle between the wing chord line and the aircraft’s longitudinal axis.)

Washout ensures that the wing root stalls first.

You must know

the signs of the approaching

stall so that your aircraft never stalls inadvertently.

SIGNS OF THE APPROACHING STALL.

During your PPL flying training, your instructor will demonstrate fully developed stalls to you, and teach you how to recover from the stall, using the standard stall recovery procedure. But while it is instructive to carry out stalls in the upper air under your instructor’s supervision, and to practise stall recovery, you must never allow your aircraft to stall unintentionally or inadvertently. Several hundred feet can be lost while recovering from the stall, so, if your aircraft were to stall inadvertently at low height, on the approach, say, you may not have time to recover before the aircraft hits the ground.

It is, therefore, of vital importance for the pilot that he should be able to recognise the signs and symptoms of an approaching stall so that he can intervene early to prevent the stall from occurring. By learning the symptoms of the approaching stall, you are unlikely to allow your aircraft to stall inadvertently.

If you bear in mind all you have learnt about the stall in this chapter, you should have little difficulty in recognising the warning signs that the aircraft is in danger of stalling. Those warning signs are listed below. They may be present individually or in combination.

•Possibly, a high nose attitude, though this is not an essential sign because, as you have learnt, a stall can occur at any attitude and at any speed. The stall occurs when the wing reaches the stalling angle of attack. The stalling speed will vary, among other things, with the load factor acting on the aircraft. If the stall occurs from straight and level flight, however, nose attitude will be high.

•Low and decreasing airspeed. When the stall is approached from level flight or the glide, the higher-than-normal nose attitude will be accompanied by low and decreasing airspeed.

•Sloppiness and decreasing effectiveness of the controls. Again, if the stall is approached from level flight, nose attitude will be high, speed will be decreasing and, consequently, the controls become noticeably less effective, feeling sloppy instead of firm.

•Stall warning device. At the point of the stall (at CL MAX) airframe buffeting becomes more severe. On some aircraft, such as the PA28 Warrior, a stall warning horn operates at a preset angle of attack, just before the onset of the stall, typically sounding 5-10 kts above the stall speed.

•Airframe buffeting. As you have learnt, as the angle of attack increases in the approach to the stall, the airflow begins to separate and break away from the wing surface. Airflow in this condition is turbulent and gives rise to airframe buffeting. As the turbulence strikes the elevator, it is felt through the control column as vibration.

Your flying instructor will teach you how to react to the signs of the approaching stall in order to prevent the stall from occurring. If you are operating normally in the climb, cruise or descent, in order to prevent the stall from occurring it will, generally, be sufficient for you to reselect the correct attitude for the phase of flight that you are in, and to confirm that you have the appropriate power setting.