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The Glide

The Glide

We’ve previously looked at the descent, descent point and profile management. In this post I will expand on the basic descent and discuss the glide descent profile.

The most likely reason a student pilot will find themselves in a glide descent is an engine failure, either a real one or one proceeded by the ‘practice’ word. The initial actions of zoom to gain height, turn towards the field, set the glide attitude to achieve the best glide speed, trim and then deal with the engine issue have been covered in a previous article (June 2009). Aviating (trimming the aircraft out at the correct glide attitude, and hence speed), will allow you to concentrate upon the secondary, but no less important tasks, of navigating and communicating. So what speed should be flown? Obviously the actual number will differ from aircraft to aircraft, but let’s have a quick look at the theory and then the application.

The best glide speed in any aircraft aims to achieve the maximum distance over the ground given your height. With the engine running, getting the maximum range out of an aircraft requires it be flown at the best lift/drag ratio (L/D ratio or L/DMAX). The ratio is determined, at a given angle of attack, by dividing the resultant CL by the CD giving the L/D ratio. The higher the number, the more lift for drag the aerofoil creates. A glider may have a L/D ratio of up to 33, whereas a light aircraft may achieve a L/D ratio of only 6. This is also known as the glide ratio as for light aircraft it is approximately the same ratio as distance covered/height lost at glide speed. For example if L/DMAX equals 6 then the glide ratio is approximately 6:1, meaning the aircraft will travel 6000 feet for each 1000 feet of altitude lost.

The aerodynamics above are all well and good, but given you have only an ASI and not a ‘L/D o’meter,’ what speed should you fly? The angle of attack associated with L/DMAX determines the best engine-off glide speed (Vbg) given the aircraft weight (usually on light aircraft only one speed is given as the weight does not change much). In a larger aircraft with a greater range of operating weights, L/DMAX will still occur at a given angle of attack, but the speed will be higher than at a lighter aircraft weight. The flight manual Vbg provides L/DMAX giving the greatest air distance flown. This is the speed you should fly. In order to achieve that speed you must know the approximate attitude you need to set to achieve that speed. Remember, slower speed equals higher aircraft attitude. Your instructor will show you the attitude in order to achieve your best glide speed and it is important that you take the time to set this attitude and trim the forces out. Once this is done, your attention can then be taken by the engine issue and the requirement that a suitable landing area be found.

What effect will wind have? A headwind will decrease your gliding range and a tailwind will increase gliding range. Although some people may advocate an optimum glide speed for the conditions (i.e. increasing speed by one third or one half of the headwind and vice-versa for tailwind) bear in mind that any speed error away from the speed that gives L/DMAX will reduce air distance flown as drag increases, reducing the L/D ratio. My tip is to simply fly the manufacturer’s given speed. Why? Having been in a situation where the engine has stopped, the ability to trim the aircraft out at the correct attitude which gives a known speed and a corresponding known distance for height, unloads your mind to concentrate on other things. Yes understanding that if you turn into the headwind your range will decrease is important, but not enough to begin flying the aircraft at a speed which gives unknown results. Glider pilots may disagree, but they started without an engine in the first place and so are expecting to be gliding to a landing.

So, to sum up. Know your aircraft’s best glide speed and corresponding attitude to achieve it. Know that any speed away from this will reduce your air distance travelled for a given height. Finally, know how far your aeroplane is likely to glide per 1000 feet of altitude. It is useless trying to make a field which from a simple calculation given current height is out of reach.

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