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Bill,
Your question is not daft, it just sounds as though you've been listening to "experts" who tend to abbreviate the terminology in a way that might be confusing to outsiders. L/D is indeed the lift/drag ratio. It's possible to show by drawing the diagram of forces acting on the glider that this ratio is equal to the ratio of airspeed to sink rate. This is true for any speed since the drag changes as the airspeed changes. When you hear people talking about a glider's L/D or glide ratio, what they usually mean is the best or highest value of this ratio that can be achieved by a particular glider. An L/D or glide ratio of 33 is shorthand for 33:1, i.e. the glider will travel forwards 33 times as far as it descends in a given time. It's important to appreciate that this will occur at one specific speed, known as the best glide speed or Max Glide. What this speed actually is depends on the wing loading for any particular type, but is typically about 50-55 kts for most modern gliders and increases as the wing loading increases. The maximum achievable glide ratio is basically an inherent property of any particular type though you can achieve small alterations by moving the centre of gravity and other subtle modifications. The important thing to understand is that a particular L/D is only achieved at one single speed. In your example you are correct that the ASH25 achieves 60:1, but only at, say, 55 knots. At 100 knots its L/D might be, for example, 40:1 and will continue to decrease as the speed increases. If you consult the polar curve for any glider you can see how the glide ratio changes with speed. Lay a ruler from the origin to any point on the polar and its slope will represent the glide angle at that speed. Divide the IAS by the sink rate for that point on the polar to get the numerical value of glide ratio at that particular speed. It will be pretty obvious that as you move the ruler's point of contact along the polar, there will be a single position where it just makes a tangent to the polar, where the ruler's slope is at its most shallow. This represents the best achievable glide angle and you can read off the speed where this occurs. If the graph contain several polars at different wing loadings, you can see how the glide angle is affected as the wing loading changes. You will find that the actual best glide ratio is more or less the same for each wing loading, but will be achieved at higher speeds for the heavier glider. David Starer "Bill Gribble" wrote in message .. . Completely daft newbie question that I'm hoping will have a simple answer. Mention is frequently made of the term L/D, which I know is an abbreviation of Lift / Drag. For example, the L/D cited in an advert for a Ka6 recently was "L/d 33" If L/D is the Lift/Drag ratio, why are the figures accompanying it frequently not ratios? For instance, in the above, what is the "33"? Is L/D used as the best Lift/Drag ratio the glider in question can achieve? How does this translate in real terms? Does it describe the best glide speed, or relate to the glide ratio in any way? The glide ratio (eg. An ASH25 has a glide ratio of 60:1) seems a great way of describing certain aspects of the performance of a glider. Yet most the specifications I read describing gliders don't give this figure in quite such a straight forward way. Is this information somehow derived from other information given? Or is it too variable to generally provide as a generic statistic (eg. One ASH25 might have a 60:1 ratio, another less at 45:1)? Am I managing to make any sense, or am I completely confusing myself (in which case, no worries - more time, exposure and experience will eventually rattle all this out for me, I'm sure)? -- Bill Gribble /----------------------------------\ | http://www.cotswoldgliding.co.uk | | http://members.aol.com/annsweb | | http://www.shatteredkingdoms.org | \----------------------------------/ |
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