Usually, but not universally. Some high drag, low performance aircraft when at MAUW move so far along the basic polar curve that the ideal L/D can occur at above Vne. However for 99% of aircraft there will be a speed that will that will be best L/D at a higher all up weight that will match the line for best L/D at a lower weight.
Some modern high performance gliders actually achieve a better L/D with full ballast than at a lighter weight. |
henra
I could imagine it could potentially be related to the Reynolds Numbers. With some airfoils Cd decreases significantly with increasing Reynolds Numbers (and thus L/D increases) while others don't react much. abgd Is it still true that L/D remains constant for an aircraft with a lot of parasitic drag? e.g. hang-glider, biplane with lots of wires... |
Originally Posted by abgd
Is it still true that L/D remains constant for an aircraft with a lot of parasitic drag?
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Some modern high performance gliders actually achieve a better L/D with full ballast than at a lighter weight. I have seen slightly different L/Ds quoted for full ballast v no ballast, because the greater win-flex means the heavier glider does not have an identical wing shape, but thought mostly quoted a slightly LOWER L/D at heavy weights. |
Also, the heavy aircraft has more inertia to lose, so that adds to the earlier descent profile. I don't know by how much in percentage terms versus the l/d change but definately some, and it would be hard using the performance manuals to pick out one from the other.
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I assume that the ballast carried in a glider will normally shift the C. of G. aft, causing the tailplane to provide lift, hence lowering the total weight carried by the wings, the actual "wing loading". This is not the same as A.U.W/wing area. Think of a strain gauge fitted at the wing root, but only think !
The A.U.W. could remain constant with a forward shifting ballast tank - but the wing loading would be higher, to compensate for the downward or negative lift required from the tail. I am sure that there must be an abbreviation for this, but I have forgotten it. |
Anyone has an idea why the gross weight does not affect the glide performance? The extra weight contributes more to the "thrust" vector, so the down angle doens't need to be as much. http://i521.photobucket.com/albums/w...descending.jpg If you're descending at best glide speed, different story. Same distance, different time. |
Linktrained,
High-performance gliders carry their (water-) ballast in the wings and dump it when the thermals become weaker. Not much of a c.g. shift I think. Capn Bloggs, I like your diagram - very instructive illustration of the relation between lift-to-drag ratio and glide angle. Thanks!:ok: |
Bloggs.
Warped wings was right, as you say but I don't agree with your interpretation of your supplied diagram: If the total downwards resultant is increased, then the total upward vector must also. so you get increased lift (from the extra speed) and also increased drag, both increasing as a square, so we're back where we started, nicely in equilibrium, with the same l/d. |
HazelNuts, the great majority of gliders carry water in their wings only, but there are a few that have tail ballast as well. Tail ballast does affect the AoA and therefore the L/D.
Capn Bloggs, Please don't make the mistake of thinking that Professional Pilots only fly large jet aircraft. There are a large number of professional pilots around the world who only ever fly light aircraft, and even a few professional glider pilots. :ok: |
Originally Posted by 16024
If the total downwards resultant is increased, then the total upward vector must also. so you get increased lift (from the extra speed) and also increased drag, both increasing as a square, so we're back where we started, nicely in equilibrium, with the same l/d.
Originally Posted by Ka6crpe
Capn Bloggs, Please don't make the mistake of thinking that Professional Pilots only fly large jet aircraft. There are a large number of professional pilots around the world who only ever fly light aircraft, and even a few professional glider pilots.
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Originally Posted by Linktrained
I assume that the ballast carried in a glider will normally shift the C. of G. aft, causing the tailplane to provide lift, hence lowering the total weight carried by the wings, the actual "wing loading".
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Linktrained- No, glider ballast is not intended to shift CofG, it is carried ON the CofG and is specifically meant to raise the speed at which best L/D is achieved.
Some gliders do carry tail ballast, but this is in order to compensate for different pilot weights, and is usually not shiftable in flight. |
I did say " think."
And thank you all, for your thoughts. I last flew gliders in the 1950s when the most advanced available to me were the Olympia / N2000 / Weihe. ( All three had been made by different makers to much the same design IIRC. Some would creak a little to remind one that the LIFT had altered. Would this now be called " a feature " in the Sales literature, I wonder!) I saw one other at that time, which had an early jettisonable water ballast for flying at higher speeds between one strong thermal to get to the next one, perhaps earlier in the day. It was something like an enclosed water tank, I think. The thought of actually PUTTING moisture inside a wooden wing, perhaps melting some of the glued structure... Would have been avoided, then. |
I also agree with warped wings.
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Warped Wing's explanation is a good one. http://i.imgur.com/WaAipSJ.gif?1 |
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