Modifying the Glide
Last time I introduced the concept of the L/D ratio and its corresponding importance to best glide speed after an engine failure. Setting, holding and trimming the glide attitude to fly this speed was emphasised in order to maximise ground distance flown for a given altitude.
What happens then, if you change something during the glide, such as configuration or flying beyond the recommended speed? This month I look at modifying the glide – both through the necessary flap selections for landing and also though a technique most students attempt at least once – “stretching” the glide with airspeed changes.
Engine out, in the glide, you decide to take flap stage 1. What does this do to the glide profile and why? Let’s assume our example is a plain or split flap, types likely to be found on basic trainers. The main effect produced by flap deflection is an increase in the effective camber of the wing, giving an increase in lift co-efficient for a given angle of attack. This means for that given angle of attack, the aerofoil produces more lift, stall speed is reduced and the attitude required to maintain a given speed will decrease. The aircraft will operate in a less nose-up attitude, improving pilot vision out the front. All good so far? However the increase in effective camber will also cause an increase in the drag co-efficient. More lift results in more induced drag. The flap itself, particularly at high flap settings, creates profile/parasite drag. The end result is an increase in the drag co-efficient which (without getting too aerodynamic) is in fact more than proportional to the increase in lift co-efficient. So the net effect is a decrease in the L/D ratio, resulting in a steeper flight path at a lower speed.
We need to take flap to get to our landing configuration but this will result in a shorter glide distance due to the steeper glide angle. Your instructor will demonstrate to you the effect of flap on the glide angle and importantly, the aim point. What is crucial is developing a feel for the point at which to select flap in order to avoid selecting too early, resulting in the aircraft undershooting the landing area. If this is done, with no engine to provide power to correct an undershoot, the aircraft will land short. Whilst having a height above the ground versus distance from landing provides a useful point, visually determining the aircraft is positively over-shooting the aim point when clean is required before selecting the next stage of flap. Positively determine the overshoot, select flap, hold the attitude to reduce speed to the new glide speed, anticipate this speed, select the new attitude, trim. Now allow the aircraft to settle at the new attitude and achieve the new rate of descent and glide angle. Assess your new aim point and adjust as necessary. Given a choice, being too high is preferable to being too low – the pilot can use more flap or techniques such as sideslip to increase rate of descent. When undershooting without an engine – options are extremely limited.
‘Stretching’ the Glide
I have seen many students when faced with the prospect of under shooting try the futile technique of raising the nose to stretch the glide. This gives a transient initial impression out the front window that the aim point can be reached…until the rate of descent increases as the glide path angle steepens. For a given configuration, a slower speed requires more lift through increased angle of attack and more lift creates more induced drag. The result? A reduction in the L/D and glide ratio. A look at any drag curve illustrates this effect of moving airspeed away from the minimum drag point. Another problem with attempting to stretch the glide through decreasing the airspeed is this: as induced drag increases with increasing angle of attack, the drag slows the aircraft more and with no power available to correct, lowering the nose to regain speed is the only option. Not good when close to the ground! This further increases glide path angle. Moral of the story? Don’t attempt to stretch the glide by lowering airspeed. Stick to the aircraft flight manual speeds, and never select flap too early: select it only when assured of achieving the landing point.
Glide descents and forced landings are a judgement exercise. Get in an aeroplane with an instructor and make sure you understand its capabilities in the glide. Don’t be afraid of practice forced landings, use each one you fly to learn how your aeroplane performs.