UNLOADING MAY BE CRITICAL

By Johan Lottering

Aircraft owners and pilots are often the captains of industry. Having them as protégés cannot be easy, especially for young instructors. Instead of telling them to not indulge in extreme flying altogether, we can educate them and ourselves to look out for critical aerodynamic symptoms in those last moments before 'uncommanded roll' alias autorotation during unsolicited fly-pasts, beat-ups or 'victory rolls'.

The common 'problem' with modern light planes is their relative high wing loadings. From a design perspective high wing loading is desirable for speed and resilience in turbulence. High wing loading is made manageable with lift devices such as slats, slots and flaps for low-speed and hence shorter take-offs and landings. Sometimes the designers incorporate tail-wheels and relatively far forward main landing gears to obtain the necessary high nose attitudes and high plan-form drag situations during landings. Pilots do not always bear in mind the devil is in the details. Their engines are high-pressure and high-performance compared to older ones. If these high-performance engines experience mechanical breakages or fuel-starvation they stop altogether and do not continue wind-milling. We will look at these effects in a future article. The main fact to bear in mind during manoeuvres is that the lift distribution over the wings is not uniform.

During a tight turn the lower wing can have the front portion of the outside section of the wing providing negative lift with a positive pressure on the aft section, causing tortional twist, but producing an overall 'upward' total reaction. Meanwhile the upper wing, following a longer helix-pattern along the wing's individual flight path, can have a much larger total reaction corresponding with a much higher angle of attack. While the upper-wing is approaching the critical angle or stalling angle of attack the centre of pressure moves forward, towards the leading edge. At the same time the reversal of airflow will be causing a relatively severe difference in negative pressure gradient between the wings, translating in more drag on the higher wing an a yawing 'out' of the turn.

In a banked attitude the rate of change in the aerodynamic forces are also vastly different. In rudimentary terms 'wing loading' is essentially the ratio of wing lift over total weight distribution at that point. Wing loading is the killer. We ought to not forget from lessons on turning how the stalling speeds increase corresponding with increased bank angles.

Unbeknown to most inexperienced pilots in high wing loading manoeuvres like the high-speed beat-up 'with added bank' to add to showmanship is the critical moment at which the wing will 'flick', often with insufficient altitude to recover and far less spectacular results than hoped for.

Add to this whole mix the corresponding effects of torque and slipstream caused by the engine. Remember, the higher the wing loading and the lower the speed, the more these effects. The rate of roll, yaw and pitch may additionally cause precession in the gyroscopic forces. These vital aspects are often 'skimmed over' during pilot's licence courses. Unfortunately that does not render these forces and their intricate interactions absent.

Best is to never indulge in extreme forms of flying, especially not at low altitudes. Never attempt any unrehearsed manoeuvre not practiced to perfection at high altitudes. Should you sense warnings like stall hooters and buffeting, whatever you do, do not pull on the elevator control. Never increase the wing loading. Rather, ease forward, attempt to roll level and 'unload' the g-forces. Never attempt to force an aeroplane over an obstacle or through any manoeuvre. It's then when the surprises occur.

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