Glide performance and gross weight

[Ka6crpe]

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.

[Owain Glyndwr]

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.

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

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...

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.

[HazelNuts39]

Originally Posted by abgd

Is it still true that L/D remains constant for an aircraft with a lot of parasitic drag?

A condition for L/D remaining constant is that the shape of the vehicle and its orientation to the airflow remain constant. Those conditions are probably not met by the hangglider and its pilot.

[Wizofoz]

Some modern high performance gliders actually achieve a better L/D with full ballast than at a lighter weight.

Are you sure about that?

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.

[16024]

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.

[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". 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.

[Capn Bloggs]

Anyone has an idea why the gross weight does not affect the glide performance?

If we're taking about a modern jet (this is Prune), then Warped Wings is on the money. Heavy goes much further. I assume Nitpicker is not thinking of his A330 now...

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.

[HazelNuts39]

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!

[16024]

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.

[Ka6crpe]

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]

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.

Fair enough, but I'm/Warped are talking about constant speed descents (above to well-above best L/D speed). That diagram "helps" understand why a heavy jet goes much further than a light one; it's the weight vector that pushes it along, and so the descent angle must be decreased to compensate, resulting in said heavier jet going further when at the same speed.

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.

My apologies, not offence intended. I made that comment as the original post didn't qualify what the scenario was; descending a jet at a constant cost index ie speed at various weights is obviously going to be quite different to another type that is being descended at best L/D.

[HazelNuts39]

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".

I'm not sure I can totally agree with that analysis. If the ballast tank is in the tailplane (as in the A330 for example) the weight of the ballast is carried by the tailplane. Yes, the C. of G. moves aft, but the lift provided by the wing does not change when adding ballast in the tailplane. Since the tailplane normally provides negative lift, the benefit of moving the C. of G. aft (e.g. by transfer of fuel from the wing tanks to the tailplane in the A330) lies in the reduction of that negative lift. If the ballast is between the wing and the tailplane, then part of its weight will be carried by the wing and the remainder by the tailplane.

[Wizofoz]

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.

[Linktrained]

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.

[thermostat]

I also agree with warped wings.

[HazelNuts39]

Warped Wing's explanation is a good one.

Agreed. To illustrate the point he is making (another airplane though):