Glide performance and gross weight

[Refindaria]

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

Cheers!

[Ka6crpe]

But it does. At a higher wing loading the same lift drag ratio (glide angle) is obtained at a higher airspeed than for a lower gross weight.

Think about the vectors involving thrust, weight, lift and drag. At a steady glide angle all vectors form an equilibrium. Without any thrust, the forward force to overcome drag is provided by gravity and the aircraft is moving downwards with respect to the air around it. The forward motion also provides the lift to overcome the weight. If the gross weight is increased then the amount of lift needed can only be obtained at a higher airspeed. This higher airspeed can only be obtained by descending faster, and results in more drag. Interestingly, for most low drag aircraft, the best Lift:drag ratio remains the same, but is obtained at a higher airspeed for a higher gross weight. This is why gliders carry water ballast in strong conditions. The best Lift:drag ratio is obtained at a higher speed and hence gives better penetration into the wind.

[Mr.Buzzy]

My understanding is the glide distance is unaffected, the lighter aeroplane will however give you a little more time to ponder the inevitable!

[nitpicker330]

Yes that's my recollection from 30 years ago.

Weight does not change the glide distance, only the time. :ok:

A heavy Aircraft will hit the earth in a shorter time but at around the same place as a light version......:)

[Ka6crpe]

Mr.Buzzy and Nitpicker330, you are both correct. :ok: The glide distance remains the same because the glide ratio remains the same, as long as the speed is increased. If you keep the speed the same as for the lighter weight then the glide distance will decrease.

I wish I could post my glider's polar curve up here to demonstrate.

[main_dog]

Spot on. In still air, both will travel the same distance along the same angle of descent (provided they are at the same angle of attack: assuming this AoA is the one that provides best L/D, then they will glide the greatest distance possible). The heavier a/c however will "slide down the slope" faster, as any given AoA will be obtained at a higher speed for a heavier aircraft. So in still air they will travel the same distance but the heavier one will reach the ground first.

Once wind kicks in it's a different story though: if it's a headwind, the light aircraft (which is taking longer to descend) will be exposed to the effects of the headwind for longer and travel a shorter distance. The heavier one will travel further (hence the ballast in gliders as they battle their way through headwinds towards the next updraft). The opposite occurs for a tailwind, with the light aircraft traveling further as it enjoys the benefit of the tailwind for longer.

[HazelNuts39]

Spot on. But have you considered that the speed for best glide angle is higher than the speed for best L/D in a headwind, and lower in a tailwind?

[Ka6crpe]

But have you considered that the speed for best glide angle is higher than
the speed for best L/D in a headwind


Unless you have the aircraft polar in front of you, a good rule of thumb is add half the head wind component onto the speed for best L/D to get maximum penetration.

[Wizofoz]

That's why gliders carry ballast in good lift conditions.

[Qantas94Heavy]

Does compressibility drag have a higher effect on the heavier aircraft than the lighter aircraft, and if so, would the heavier aircraft therefore travel a shorter distance? :confused: :ugh:

[HazelNuts39]

Does compressibility drag have a higher effect on the heavier aircraft than the lighter aircraft
Compressibility effects are a function of Mach number. At a given altitude the heavier aircraft will have its max L/D at a higher Mach number than the lighter aircraft. So the question really is: does max L/D reduce with Mach number? While the answer will probably depend on aircraft type, the following graph shows the variation of maximum L/D with Mach for one widebody twin transport airplane type.

http://i.imgur.com/CI7Z50V.gif?1

P.S.
Based on the above graph, when a heavier and a lighter aircraft descend side-by-side each at its max L/D from the same cruise altitude, the lighter aircraft will have a slight advantage until they get to lower altitudes where the heavier aircraft has the advantage.

[hawk37]

When a heavier and a lighter aircraft descend side-by-side from the same cruise altitude, the lighter aircraft will have a slight advantage until they get to lower altitudes where the heavier aircraft has the advantage.

HazelNuts, does the above explain the very slight variation of speed for max L/D between mach .2 and mach .8 shown in your graph? And if so, can you give an explanation of why max L/D occurs at about mach .5, and not mach .2 for example. I realize this is for a particular jet, however that the value peaks, then falls, as mach increases seems odd to me.

Secondly, I am amazed that over the same range of mach numbers, .2 to .8, the aoa for max L/D will vary from an initial value of 6 degrees (I'm assuming that degrees is the units), to a value of about 2.5 degrees, all while keeping a near constant max L/D of 22. Are you able to comment on this?

[HazelNuts39]

HazelNuts, does the above explain the very slight variation of speed for max L/D between mach .2 and mach .8 shown in your graph?The speed for max L/D varies with weight and altitude. I suppose you mean the very slight variation of max L/D between mach .2 and mach .8 ? No, its the other way round. the P.S. is based on the variation shown in the graph. At typical cruise altitudes for this airplane the max L/D will occur in the mach range of 0.7 to 0.75, shifting to the left as the altitude reduces.

I cannot explain why the effect of Mach is as shown, except that I would expect the max. L/D to reduce near the aerodynamic ceiling beyond M.8. Compressibility changes the pressure distribution around the airplane. I would be surprised if the effect on drag would be identical to the effect on lift at any given angle of attack.

[henra]

Mr.Buzzy and Nitpicker330, you are both correct. :ok: The glide distance remains the same because the glide ratio remains the same, as long as the speed is increased. If you keep the speed the same as for the lighter weight then the glide distance will decrease.

I wish I could post my glider's polar curve up here to demonstrate.

Yup.
Suffices to say that L/D is primarily a function of AoA. Weight itself is not a factor influencing the max L/D value. As Long as you keep Alpha at max L/D the Lift/Drag Ratio (=Glide Ratio) and thus the distance achieved from a given altitude will remain unchanged.
The only things which we ignore in this case are the Re- Number effect on L/D when the Speed changes due to different weight and as has been mentioned compressibility effects once >M0,8.
In reality these two factors can be ignored since the relatively small speed difference will have no appreciable effect on L/D and thus distance achieved.

[Owain Glyndwr]

hawk37

As you say, HN39's chart is type specific, and I am not sure the small increase in L/D between 0.2M and 0.5 is at all typical - certainly for the aircraft with which I am familiar the drag and L/D are virtually constant up to about 0.6M where compressibility starts to kick in.

I can offer an explanation for the AoA effect though. When compressibility starts to be significant it increases the effective lift dependent drag contribution so the profile/lift dependent drag balance is shifted, pushing the lift coefficient for L/Dmax down to a slightly lower value - say from 0.65 down to about 0.6. This by itself cannot explain the reduction in AoA for best L/D.

The main reason for the AoA reduction is that the lift curve slope of the wing increases dramatically with increasing Mach No. - typically from about 5.0 at 0.2M up to 8.0 or higher at 0.8M. The lift coefficient at zero AoA will be about 0.15, so the AoA for L/Dmax would typically be about 5.7deg at 0.2M falling to about 3.2deg at 0.8M. I'm using a different aircraft to HN39 of course, but the numbers are not very different.

[Warped Wings]

An interesting discussion.

On a more practical level with regards to everyday descent performance, I find new jet trainees have a little trouble explaining the effect weight on a typical jet descent. ie. Why does a heavier jet require more track miles for descent compared to the same jet at a lighter weight, assuming a flight idle descent at typical jet descent speeds (280kt/250kt below 10,000)?

The answer of course lies in how close an aircraft’s descent speed is to it’s current best L/D (driftdown) speed for the current weight.

Example;
B737 at 41,000’ – track miles required for an idle descent in nil wind;

70,000kg -142nm Best L/D speed: 247kts
50,000kg - 119nm Best L/D speed: 209kts

Descending at 280kts or 250kts below 10,000’, the heavier 737 will be descending at a speed closer to it’s current best L/D speed compared to the lighter example. At 250kts, the heavy 737 is descending at almost it’s best L/D speed and the lighter version is descending much faster than best L/D. At 280kts the lighter jet is even further from best L/D.

So the bottom line to remember? If you are held high on descent (and unable to increase your descent speed), a heavy jet is going to have more trouble getting down compared to a lighter one. At lighter weights and the same descent speed, you will have a much steeper descent path (higher rate of descent) compared to a descent at a heavier weight.

[HazelNuts39]

I've edited the graph in post #11 to show more clearly what it represents.

I removed the AoA curve because it is not relevant to the question being addressed, and also because the AoA's seem to be less well defined than the L/D values.

The two curves in red show the variation of L/D vs Mach at a particular weight at two altitudes, FL150 for the left curve and FL350 for that on the right. Many of those curves could be drawn to cover all altitudes and weights. The blue line connects the maxima of all the 'red' curves that could be drawn. So any of those curves, for whatever weight and altitude, will have a maximum L/D that lies on the blue line.

[Desert185]

Warped Wing's explanation is a good one.

With 747 actual landing weights anywhere from 380,000 to 630,000 (-200), descent planning involved beginning descent late (light) or early (heavy) when descending at the same Mach/IAS profile. With that weight range, the effect is very noticeable.

[henra]

hawk37

As you say, HN39's chart is type specific, and I am not sure the small increase in L/D between 0.2M and 0.5 is at all typical - certainly for the aircraft with which I am familiar the drag and L/D are virtually constant up to about 0.6M where compressibility starts to kick in.


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...

[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]

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.

http://i521.photobucket.com/albums/w334/capnbloggs/descending.jpg (http://s521.photobucket.com/user/capnbloggs/media/descending.jpg.html)

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!:ok:

[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. :ok:

[Capn Bloggs]

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.

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. :ok:

[HazelNuts39]

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):
http://i.imgur.com/WaAipSJ.gif?1