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#1
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Newbie question
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 | \----------------------------------/ |
#2
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People just tend to leave off the :1, e.g. 33 means 33:1. That's all.
Generally speaking the ratio applies to the model of sailplane and shouldn't be too hard to find, with a little digging. --- "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 | \----------------------------------/ |
#3
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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 | \----------------------------------/ |
#4
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Bill
Generally they are treated interchangeably so your LD 33 ( K6e on Gliderpilot ) is the same as 1:33 glide angle. A Nimbus 4 has an L/D of 1:60 or a glide angle of 1:60 ( 60 bits of lift for one bit of drag 60' forward for 1' of altitude lost ) Generally the figure quoted refers to the best glide angle that the aircraft can acheive, there is no standard speed at which this is quoted so its a bit misleading. What everyone works from is the gliders 'polar diagram'. A plot of speeds against sinkrate. A few examples can be found at the SSA site http://www.ssa.org/Magazines/Johnson.asp These days the manufacturers still aim to develop good L/D ratios ( around 1:50 seems to be the mark for todays 18m ships ) but the game seems to be to get the polar as flat as possible and to try to maintain good glide ratios into the 150 kph or 80 kts range and higher. Given two gliders with the same LD, say 1:42 ( 2nd generation glass ships ) one has best L/D at 90 kph the other at say 95 kph the theory is that the second must win all the tasks ( higher average speed ) Reichmans 'cross country soaring' is great for filling your head with the theory of all this. Well worth a copy even at the newbie end of your flying career! Ian "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 | \----------------------------------/ |
#5
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Peripheral, but pertinent to your understanding Bill. As part of his TLAR
exercise, Tom Knauff suggests this. You are approximately two (of your) paces tall. Mark a spot and take ten paces. Look back at the angle. That's 1/5 from your eyes to the spot. Take 30 more paces and you have 1/20. Now try 30, 40, and even 60/1 if you have the room. Gives a bit of meaning to the ratios. TLAR stands for "That Looks About Right". Frank Whiteley |
#6
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Thanks all. Much, much clearer now!
-- Bill Gribble /----------------------------------\ | http://www.cotswoldgliding.co.uk | | http://members.aol.com/annsweb | | http://www.shatteredkingdoms.org | \----------------------------------/ |
#7
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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 Perhaps it's good to know why this is important, too. In training and gliding, often in smooth air, you'll fly the min sink speed or the best L/D speed and think that is great. In soaring, there may be lift in places, and hellacious sink in places (caused by the ridge lee, rotor, divergence, a cruel God, etc...). When in sink, you'll want to get out of it in a shorter amount of time. To answer how well you can escape the sinking air, look at the L/D at the higher speeds (let's say at double the best L/D speed). A really bad L/D at high speeds means you'll be in bad sink, push the nose over to accelerate, and notice the airspeed indicator hardly increase, while the vario shows you going down a lot more. This is called "lack of penetration." Fancy gliders have negative flaps to reduce the sink rate at high speeds, and ballast to make the best L/D speed a higher speed. I noticed this most when flying a 2-33 with a heavy passenger, vs. flying a 1-26 solo. Boy, the 1-26 doesn't speed up much, but just goes down if I push the nose over. Fiberglass ships, especially with ballast and negative flaps, accelerate very quickly with little forward stick. The 1-26, the PW-2, the Quicksilver MXIII Sprint ultralight, and the Piper J-3 Cub are "floaters", meaning they require very little lift to stay aloft. On the other hand they "penetrate" poorly, meaning drag and sink rate build up quickly at higher airspeeds. The other place this is important is in winds. I got blown downwind from an airfield once at about 2000 ft, and didn't realize there was a 20-25 knot wind. I really had to push the nose over a lot to get back to the field and almost didn't make it back. Adding 15 knots over best L/D speed really increased the sink rate a lot. |
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