Glide performance and gross weight
Join Date: Jul 2013
Location: New Zealand
Age: 71
Posts: 76
Likes: 0
Received 0 Likes
on
0 Posts
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.
Some modern high performance gliders actually achieve a better L/D with full ballast than at a lighter weight.
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 88
Posts: 476
Likes: 0
Received 0 Likes
on
0 Posts
henra
I see where you are coming from, but most of that improvement comes in the Reynolds Number range from wind tunnel to full scale. Once you get to full scale RN the variation is much slower, although there is always a reduction in skin friction over the whole aircraft as RN increases. So I suspect RN variations are not the answer.
abgd
I don't see any reason why it shouldn't be. That sort of aircraft has no Mach number variation to speak of; in fact they have not much Reynolds Number range either, so it would be difficult to see any L/D variation at different parts of their flight envelope.
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...
Join Date: Jul 2009
Location: France - mostly
Age: 84
Posts: 1,682
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by abgd
Is it still true that L/D remains constant for an aircraft with a lot of parasitic drag?
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.
Join Date: Feb 2002
Location: uk
Posts: 302
Likes: 0
Received 0 Likes
on
0 Posts
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.
Join Date: Jun 2011
Location: Devonshire
Age: 96
Posts: 297
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
If you're descending at best glide speed, different story. Same distance, different time.
Join Date: Jul 2009
Location: France - mostly
Age: 84
Posts: 1,682
Likes: 0
Received 0 Likes
on
0 Posts
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!
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!
Join Date: Feb 2002
Location: uk
Posts: 302
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jul 2013
Location: New Zealand
Age: 71
Posts: 76
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
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.
Last edited by Capn Bloggs; 21st Aug 2013 at 14:18.
Join Date: Jul 2009
Location: France - mostly
Age: 84
Posts: 1,682
Likes: 0
Received 0 Likes
on
0 Posts
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".
Last edited by HazelNuts39; 21st Aug 2013 at 15:46.
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.
Some gliders do carry tail ballast, but this is in order to compensate for different pilot weights, and is usually not shiftable in flight.
Join Date: Jun 2011
Location: Devonshire
Age: 96
Posts: 297
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
